1991_Recommendations Clouterre - English Translation
Short Description
couterre traducida al inglés!...
Description
FRENCH NATIONAL RESEARCH PROJECT CLOUTERRE
1111111111111111111111111111111
RECOMMENDATIONS
CLOUTERRE 1991 Soil Nailing Recommendations - 1991 For Designing, Calculating, Constructing and Inspecting Earth Support Systems Using Soil Nailing
PROTECTED UNDER INTERNATIONAL COPYRIGHT ALL RIGHTS RESERVED. NATIONAL TECHNICAL INFORMATION SERVICE U.S. DEPARTMENT OF COMMERCE
PB94-109980
Presses de l'ENPC, 28 Rue des Saints-Peres 75343 Paris Cedex 07, France Tel. 33 (1) 44 58 28 30
Original Document: Recommandations CLOUTERRE -1991 ISBN 2-85978-170-6
English Language Translation: Soil Nailing Recommendations - 1991 Printed by the Federal Highway Administration (FHWA) with the permission of the Presses de l'ENPC. All copyrights to future reproduction are retained by the Presses de l'ENPC. Neither the FHWA nor the National Technical Information Service (NTIS) will authorize or expressly permit any other party to distribute copies of this document in France.
Technical Report Documentation Page
2.
1. Report No.
1111111111111111111111111111111
FHWA-SA-93-026
3. Recipient's Catalog No.
PB94-109980 5. Report Date Auqust 1993
4. Title and Subtitle RECOMMANDATIONS CLOUTERRE 1991 (ENGLISH TRANSLATION) Soil Nailing Recommendations-1991
~
6. Performing Organization Code 8. Performing Organization Report No.
7. Author(s) See Introduction for further information on authors of original French version and translation of the English version. 10. Work Unit No. (TRAIS)
9. Performing Organization Name and Address See Introduction
11. Contract or Grant No. 13. Type of Report and Period Covered 12. Sponsoring Agency Name and Address English Translation Published by Federal Highway Administration 400 Seventh Street,m SW. Washington, D.C. 20590
14. Sponsoring Agency Code
15. Supplementary Notes See introduction for further information on authors of original French version and translation of the English version.
16. Abstract The Recommandations CLOUTERRE 1991 - constitute the culmination of the French National Project CLOUTERRE, which was conducted from 1986 to 1990 with a total bUdget of 22 million French francs. These "Soiling Nailing Recommendations" represent a major contribution·to the engineering community interested in the cost effective earth excavation support system known as soil nailing. Nailing, a recent technique of reinforcing in-place soils, started in France in the first wall built in Versailles in 1972. Since then, the French experience has continued to progress to the point that in 1990 more than 100,000 square meters of walls were built on highway, railway, and building construction projects. This remarkable development is due to the two principal advantages of soil nailing: its financial competitiveness compared to other earth support systems and its speed of construction. However, one must also credit the National Project CLOUTERRE that has greatly eased the dissemination of the technology to National and international audiences. Soil nailing techniques have been used since the 1970s and technical papers have been published at geotechnical conferences around the world since its inception. However, as late as 1992 no document that summarizes the whole design and construction process, from geotechnical investigation to field quality control, as this one does was available. Shortly after publication by the Presses de IEcole Nationale des Ponts et Chaussees (EN PC), bilingual engineers recognized the value of producing an English translation of the document. This document is a faithful translation of the original book published by the Presses of ENPC. It was prepared under the general supervision of Mr. Francois Schlosser, the Scientific Director of the French National Project CLOUTERRE and President of Terrasol in Paris.
18. Distribution Statement
17. Key Words Nails, soil nails, soil nailing, soil-nailed wall, anchored wall, reinforced earth, ground modification, retaining wall, French National Project Clouterre 19. Security Classif. (of this report) Unclassified
Form DOT F 1700.7
No restrictions. This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161
20. Security Classif. (of this page) Unclassified
(8-72)
Reproduction of completed page authorized
21. No. of Pages 321
22. Price
--
I"
in
miles
inches feet yards
fluid ounces gallons cubic feet cubic yards
square square square acres square
inches feet yards miles
When You Know
29.57 3.785 0.028 0.765
VOLUME
645.2 0.093 0.836 0.405 2.59
AREA
25.4 0.305 0.914 1.61
LENGTH
MUltiply By
milliliters liters cubic meters cubic meters
square millimeters square meters square meters hectares square kilometers
millimeters meters meters kilometers
To Find
foot-candles foot-Lamberts
fc fl 10.76 3.426
ILLUMINATION
5(F-32)/9 or (F-32)/1.8
lux candelalm 2
Celcius temperature
grams kilograms megagrams
4.45 6.89
newtons kilo pascals
• SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380.
poundforce poundforce per square inch
FORCE and PRESSURE or STRESS
Fahrenheit temperature
Ibf Ibf/in2
28.35 0.454 0.907
TEMPERATURE (exact)
ounces pounds short tons (2000 Ib)
of
oz Ib T
MASS
NOTE: Volumes greater than 1000 I shall be shown in m3 .
yd'
It'
II oz gal
yd2 ac mi 2
ft2
2
yd mi
ft
in
Symbol
••
APPROXIMATE CONVERSIONS TO 51 UNITS
N kPa
Ix cdlm 2
°C
g kg Mg
ml I m3 m3
mm 2 m2 m2 ha km 2
mm m m km
Symbol
N kPa
Ix cdlm 2
°C
g kg Mg
ml I m3 m3
mm 2 m2 m2 ha km 2
mm m m km
mSymbol
Celcius temperature
0.0929 0.2919
ILLUMINATION
1.8C + 32
newtons kilo pascals
0.225 0.145
miles
inches feet yards
ft3 yd'
II oz gal
yd 2 ac mi 2
ft2
in 2
yd mi
ft
in
Symbol
foot-candles foot -Lamberts
Fahrenheit temperature
Ibl Ibl/in2
(Revised June 1993)
pOllndforce poundlorce per square inch
Ic II
of
ounces oz pounds Ib short tons (2000 Ib) T
fluid ounces gallons cubic feet cubic yards
square square square acres square
inches feet yards miles
To Find
FORCE and PRESSURE or STRESS
lux candelalm 2
0.035 2.202 1.103
MASS
0.034 0.264 35.71 1.307
VOLUME
0.0016 10.764 1.195 2.47 0.386
AREA
0.039 3.28 1.09 0.621
LENGTH
MUltiply By
TEMPERATURE (exact)
grams kilograms megagrams
milliliters liters cubic meters cubic meters
square millimeters square meters square meters hectares square kilometers
millimeters meters meters kilometers
When You Know
•
APPROXIMATE CONVERSIONS FROM 51 UNITS
Preface
PREFACE (TO THE ENGLISH TRANSLATION)
The French National Research Project CLOUTERRE and the resulting manual Recommandations CLOUTERRE -1991 are important contributions to the engineering
community interested in the cost effective earth excavation support system known as Soil Nailing. Soil nailing techniques have been used since the 1970s and technical papers have been published at geotechnical conferences around the world since its inception. However, there is at this time (1992) no document that summarizes the whole design and construction process, from geotechnical investigation to field quality control, as this one does. Shortly after publication by the Presses de l'Ecole Nationale des Ponts et Chaussees (ENPC), Paris, France, bilingual engineers recognized the value of producing an English translation of the document. This document is a faithful translation of the original book published by the Presses of ENPC. It was also prepared under the general supervision of Mr. Fran~ois Schlosser, the Scientific Director of the French National Project CLOUTERRE, Professor at the ENPC, and President of Terrasol, Geotechnical Consultants, Paris. In this book, the reader will find many references to French publications and to agencies that produce documents in France relevant to the subject. Titles of French publications are listed as originally referenced, along with an English translation of the title. French abbreviations for the various agencies involved have been maintained in the text. A list of abbreviations is included showing both the proper title and a translation. This is to provide the reader with a reasonably good indication of what organization in his or her own country would produce similar publications, guidelines, specifications, or regulations. The translators also found the need to develop a list of the agreed translations of various terms and expressions. These are presented in the Lexicon. This is not a true dictionary in the technical sense, but represents the translators' experience in dealing with soil nailing terms in common use in the U.S.A. Some additional comments are in order about the translation, which has been a truly international team effort. The first translation of the document was made by Mr. Bernard Myles and associates at Soil Nailing Limited, Cardiff, UK, together with contributions from the Transport Research Laboratories of the UK Department of Transport. Various chapters from that document were then distributed to Messrs. Claude Plumelle, Professor (CNAM), Consultant at the CEBTP; Daniel Raynoud, Engineer at the CEBTP; Philippe Unterreiner, Assistant Professor (ENPC), Research Engineer (CERMES); and John
v
Soil Nailing Recommendations-1991
Walkinshaw, P.E., G.E., Regional Geotechnical Engineer for the Federal Highway Administration (FHWA) in San Francisco, California. These corrected chapters were retyped and redistributed for final review and further corrections. In this process, all chapters received at least a double review by Messrs. Unterreiner and Walkinshaw for technical accuracy of the translation. In the process some minor typographical errors in the original document were corrected. The final document was then submitted to an American editor prior to printing. The whole effort would not have been possible without the patient and tireless assistance of Ms. Pat Thoburn (Soil Nailing Limited, Cardiff, UK) and Mr. Philippe Unterreiner (CERMESENPC/LCPC, Paris, France), who coordinated the effort and kept the translators on schedule. Financial support for redrafting the figures (done at Terrasol using the originals) and publication of the manual was given by Mr. Douglas Bernard, Director of the FHWA Office of Technology Applications, Washington, D.C., under the guidance of Mr. John Hooks, Chief of the Structures and Soils Application Branch. In France, the translation was sponsored by the Direction des Affaires Economiques et Internationales (DAEI), Ministere de l'Equipment, du Logement, des Transports et de L'Espace (MELTE).
In the United Kingdom, Soil Nailing Limited has received financial support from its parent company, Ryan Group Limited, for the preparation of the prepublication document. Participants were sponsored by their respective companies or agencies with much time donated by each individuaL
vi
Preface
PREFACE (TO THE ORIGINAL FRENCH DOCUMENT)
The Soil Nailing Recommandations 91 constitute the culmination of the French National Project CLOUTERRE, which was conducted from 1986 to 1990 with a total budget of 22 million French francs. Nailing, a recent technique of reinforcing in-place soils, started in France with the first wall built in Versailles in 1972. Since then, the French experience has continued to progress to the point that in 1990 more than 100,000 square meters of walls were built on highway, railway, and building construction projects. This remarkable development is due to the two principal advantages of soil nailing: its financial competitiveness compared to other earth support systems and its speed of construction. However, one must also credit the National Project CLOUTERRE that has greatly eased the dissemination of this technology to National and International audiences. The Soil Nailing Recommandations 91 are the result of an important team effort of reflections and synthesis and represent well the five years of research, studies, and tests of the National Project. They should allow for a large development of soil nailing and, notably, its use in permanent structures in Geotechnical Engineering; this is the second application of limit state concepts to ground reinforcement, after the Recommandations sur la Terre Armee (Reinforced Earth Recommendations) published by the Direction des Routes (French Highway Administration) in 1979. The present stage of knowledge has allowed us to develop only design concepts for dimensioning at ultimate limit state; the design at service limit state is at the present time an area of research and study for future years.
vii
Soil Nailing Recommendations-1991
Please allow me to thank all the participants of the French National Project CLOUTERRE and particularly those who have participated in the preparation and editing of these recommendations. C. MARTINAND
Directeur des Affaires Economiques et Internationales Ministere de l'Equipement, du Logement des Transports et de l'Espace
viii
Introduction
INTRODUCTION
These recommendations have been compiled from studies of the French National Project "CLOUTERRE" (dou =: nail, terre =: soil) carried out from 1986 to 1990 by a group of contracting authorities, prime contractors, research centers and laboratories, consulting firms, and construction companies under the auspices of The Economic and International Affairs Division (DAEI - Direction des Affaires Economiques et Internationales) of The Ministry of Public Works, Housing, Transport and Space (MELTE Ministere de l'Equipement, du Logement, des Transports et de l'Espace), and the National Federation of Public Works (FNTP - Federation Nationale des Travaux Publics).
ix
Soil Nailing Recommendations-1991
This study and research program has been financed by members of the National Project: Bachy, Ballot, Bouygues, CEBTP, ENPC-CERMES, Cofiroute and Socaso, DDE de la Moselle et de Savoie, DDST de la Martinique, EMCC, FNTP, Fougerolle, Gie Semed Dumez, IMG, Intrafor, Forezienne d'Entreprises, LCPC, Laboratoires Regionaux de l'Est Parisien, de Lorraine et du Rhone, Ministere des Transport du Quebec, Sade, Scetauroute, Sefi, SEMALY, SETRA, Societe du Metro de Marseille, Soletanche, SpieBatignolles, Terrasol, with the support of the DAE!. The French National Project CLOUTERRE included a management committee presided over by R. Soulas and a scientific committee presided over by F. Schlosser. The running of the Project was guaranteed by R. Soulas, R. Aris, F. Schlosser, and C. Plumelle. These recommendations comprise seven chapters compiled by six working groups under the direction of the scientific committee and finalized by the editing committee.
x
Introduction
The following have taken part in drawing up and editing these recommendations:
Scientific Committee of the French National Project CLOUTERRE President: F. Schlosser Members M. Boucherie (Socotec) P. de Buhan (LMS) J.M. Forestier (DAEI) N. Goulesco (Bouygues) G. Haiun (SETRA) M. Lenoire (FNTP) G. Maraficaud (Intrafor) P. Unterreiner (CERMES-ENPC/LCPC)
J.R. Brulois (Bachy) P. Delage (CERMES-ENPC/LCPC) J.-P. Gigan (LREP) D. Gouvenot (Soh~tanche) L. Hurpin (Bouygues) I.-P. Magnan (LCPC) C. Plumelle (CEBTP) P. Vezole (Forezienne d'Entereprises)
Members of the Editing Committee D. Allagnat (Scetauroute) F. Blondeau (Terrasol and Blondeau Consultants) G. Bolle (Spie-Batignolles) J.R. Brulois (Bachy) J.-P. Clautour (Sefi) P. de Buhan (LMS) B. Gicquel (Bouygues) N. Goulesco (Bouygues) Y. Guerpillon (Scetauroute) C. Heurtebis (SETRA) M. Khizardjian (LRR) Y. Matichard (LRL) A. Morbois (Scetauroute) A. Raharinaivo (LCPC) B. Simon (Terrasol)
M. Besson (Intrafor) M. Boucherie (Socotec) S. Buzet (Bouygues) P. Clement (Bachy) P. Delmas (LCPC) J.-P. Gigan (LREP) D. Gouvenot (Soletanche) G. Haiun (SETRA) L. Hurpin (Bouygues) J. Marchal (LRR) G. Mercieca (CEBTP) C. Plumelle (CEBTP) M. Salomon (CEBTP)
Spokesmen M. Boucherie (Socotec) A. Guilloux (Terrasol) C. Plumelle (CEBTP)
J.-P. Gigan (LREP) G. Haiun (SETRA) F. Schlosser (Terrasol)
Editing Committee: J. P. Magnan (LCPC) J. Salen.. = H [1 - ton
JK
L
Displacement of facing
H
Figure 10. Use of soil nailing technique on an urban site with existing structures nearby.
Generally speaking, the attenuation of the displacements away from the facing is not linear (Peck, 1969) and can sometimes be concentrated at a fissure that forms at the end of the upper nails. Where the existing structure is unable to tolerate these distortions, the design may have to include prestressed anchors and whalers at the top of the nail. Even with these measures it is important to check again during construction that displacements are kept within a tolerable level for the existing structures. The building of soil nailed walls below a water table should not be undertaken without prior permanent lowering of the water table to protect the structure against pore water pressures. These precautions are taken in order to resolve the problems associated with building the structure, as well as the problems of its long-term stability. The limitations posed by the type of soil found impose several constraints. In cohesionless sands, it is not possible to ensure the stability of a near-vertical excavation, even one that is limited in both length and height. Caving sands and water pockets may lead to instability of the structure and have serious repercussions. In order to try to avoid this situation, the soil will need to be drained as the excavation work progresses. Certain precautions must be taken where clay soils are found to prevent the ingress of water, that will lead to a loss of strength in the soil and consequently a significant reduction in the soil/nail friction. Finally, recent
12
Chapter 1: The Technique Used for Soil Nailed Structures
experiences with soil nailed walls built in mountainous districts have shown that frostsusceptible soils create problems as a result of the swelling that occurs under the influence of frost and its effect on the soil nailed. Appropriate measures should therefore be taken, either by increasing the size and number of nails to absorb the effects of the pressure caused by this swelling or by installing an insulating structure next to the facing (a Texsol wall, prefabricated wall panels) to prevent the freezing front from penetrating the soil.
3. THE FRENCH NATIONAL PROJECT "CLOUTERRE"
3.1.
General background
The technique of reinforcing soil by the use of "nails" is relatively recent, as was noted in the previous section. From the very beginning, France has lead in this field, both practically and theoretically. However, following the initial spurt of using soil nailing for short-term earth support structures at the beginning of the eighties, developments for medium- or long-term structures were still being held up by the lack of recommendations and regulations. To respond to this need, the Project CLOUTERRE (Memorandum of Presentation - April 1986) was set up in 1985 at the initiative of the DAEI and the FNTP. This national project conforms with the structure of all other national projects. Thanks to its original financing, more than 21 organizations from different backgrounds have been able to participate together in a research project that resulted in 1991 with the writing of the recommendations that are presented here.
3.2.
The participants and financing of the project
Besides the DAEI, which initiated the project and which financed approximately 15 percent of the 21 million francs budget, 21 other participants from widely varying backgrounds also took part. These have included public and semi-public organizations (7), public and private contracting authorities (3), and building contractors (11), who, among them, have financed more than 85 percent of the Project CLOUTERRE either by direct contributions or by supplying materials and manpower.
3.3.
The research program
The aim was to promote the use of soil nailing, both for short-, medium-, and long-term structures, based on the recommendations issued. The areas of application needed to be welldefined and the level of knowledge improved. It was on this basis that the research program was launched in 1986 and finished in 1991. The Project CLOUTERRE did not set out to develop or research new construction techniques, which are the responsibility of the contractors; it sought only to conduct research targeted at improving understanding of the behavior and design of the structures built.
13
Soil Nailing Recommendations-1991
Four central areas of research were therefore defined and studied: 1) To better understand the behavior of soil nailed walls.
2) To define the limitations of the process. 3) To improve methods for designing structures. 4) How to use soil nailing for long-term structures. The first area involved not only the overall behavior of a soil nailed wall when it fails (internal and external stability) and under service loads (deformations and movements of the soil nailed mass), but also the local behavior where soil and nail interact, which is a vital element for the internal stability of any soil nailed structure. Studies into the internal stability of a soil nailed wall during the excavation phase were carried out with the full-scale experimental soil nailed wall CEBTP No.2 (CLOUTERRE, CEBTP, December 1989) and using centrifuge models of walls under construction (French National Project CLOUTERRE, LCPC, October 1987 and December 1989). The deformations and movements of soil nailed walls under service loads have themselves been studied with measurements taken on several inservice structures together with numerical analysis (French National Project CLOUTERRE, CERMES, October 1986, October 1988, and December 1989).
In addition to the full scale soil nailed wall of the CEBTP Experiment No.1, which was instrumented and monitored from the time of its construction until it failed (French National Project CLOUTERRE, CEBTP, May 1986), five other soil nailed structures built by different owners in France have been instrumented within the Project "CLOUTERRE": The A 71 Vierzon-Bourges Highway (French National Project CLOUTERRE, CEBTP, October 1987 and December 1989); the RN 90 by-pass of Aigueblanche, Esserts cut section (French National Project CLOUTERRE, CETE Rhone-Alpes, March 1988 and French National Project CLOUTERRE, LRPC, December 1989); the A 30, Knutange-Hayange highway section, Bois des Chenes Tunnel (French National Project CLOUTERRE, CETE de l'Est, November 1987 and March 1988); cut section of the terminus for Line D of the Metro-Lyon, Venissieux (French National Project CLOUTERRE, INSA Lyon, October 1988); sloping wall at the A6-A40 (split) at Macon (French National Project CLOUTERRE, Scetauroute, March 1988). As far as soil/nail interaction is concerned, a principal input in the soil nailing design, it has been possible to significantly extend our knowledge of the factors involved, thanks to the numerous experimental, numerical, and theoretical studies conducted as part of the Project CLOUTERRE. The experimental studies comprised not only original laboratory research, development of a local normal pressure gauge on a bar 20 mm in diameter and housed in a sample tested in a triaxial testing chamber (French National Project CLOUTERRE, CERMES, December 1989); the shearing of sand samples reinforced by rods or metal piates (French National Project CLOUTERRE, CERMES, September 1987, June 1988, and December 1989); direct shear tests between sand and steel with a normal stiffness imposed (French National Project CLOUTERRE, IMG, October 1989); pull-out tests on nails in a calibrated chamber (French National Project CLOUTERRE, IMG, May 1987) but also full scale tests conducted by the CEBTP; pull-out tests on several types of nails (French National Project CLOUTERRE,
14
Chapter 1: The Technique Used for Soil Nailed Structures
CEBTP, June 1987, June 1988, and December 1989); and shearing of a sand mass reinforced with vertical nails (French National Project CLOUTERRE, CEBTP, June 1988). Data have also been compiled on more than 450 pull-out tests carried out by contractors and this has made it possible to set up the only data bank that exists in this field (CLOUTERRE, CEBTP, December 1987). This data bank makes it possible to estimate for the preliminary design the soil/nail interaction parameters by using charts (chapter 3, appendix 1). The theoretical and numerical aspects of the soil/nail interaction were developed in two directions: 1) Development of the interface behavior law of skin friction from pull-out tests results (French National Project CLOUTERRE, CEBTP, June 1988 and December 1989). 2) Development and use of interface elements in computer modelling of soil nailed walls (French National Project CLOUTERRE, CERMES, December 1989, and French National Project CLOUTERRE, IMG, May 1988 and March 1989). For the second direction of the research, the objective was to determine the limitations of the soil nailing technique by bringing together all the participants with their experiences. This has allowed the definition of the soils for which nailing is well-suited, those presenting some risk, and those to exclude. The third direction of the research-improved design methods---eonsisted of comparing the various design methods used in France, studying the influence of the various parameters, notably the soil/nail interaction, and calibrating the design methods against real cases. The fourth direction of the research was to study the corrosion and durability of nails (French National Project CLOUTERRE, TERRASOL, December 1989).
3.4.
Full-scale tests on soil nailed walls at the CEBTP
The originality of the Project CLOUTERRE lies in the fact that three fully instrumented experimental soil nailed walls were built by the CEBTP and monitored from construction to failure. Each of these experimental walls was conceived to study a different failure mode. The CEBTP No.1 experimental wall failed through breakage of the nails after partial saturation of the soil from the top of the wall (French National Project CLOUTERRE, CEBTP, May 1986). The CEBTP No.2 experimental soil nailed wall, on the other hand, failed by increasing the height of the excavation phase (French National Project CLOUTERRE, CEBTP, December 1989). The CEBTP No.3 experimental soil nailed wall was failed through progressive shortening of the lengths of the nails (French National Project CLOUTERRE, CEBTP, December 1989).
15
Soil Nailing Recommendations-1991
To provide the best experimental conditions, each of the three walls was built in Fontainebleau sand, which had been excavated, replaced, and compacted under strict compaction control. Like many natural sands, it contained a small percentage of fines, which when combined with the water used with compaction, possessed sufficient apparent cohesion to ensure the stability of the various excavation phases during the course of construction. •
First experiment CLOUTERRE at the CEBTP (1986)
The soil nailed wall built as part of this experiment was constructed in excavation phases of one meter in height, and used grouted nails 6 to 8 meters long. Its total height was 7 meters. The structure had been designed with a sufficiently low safety factor for failure by breakage of the nails (F = 1.1) so that it would be possible to break it easily by gradually saturating the soil, starting at the top of the wall. The effect of saturation was to reduce the apparent cohesion and to increase the overall weight. The total failure of the structure was prevented because the shotcrete embedded itself in the foundation soil. Thanks to the instrumentation developed, it has been possible to carry out a number of measurements (tensions in the nails, displacements of the facing and distortion of the soil nailed mass, etc.) both during and after construction and up to the point of failure. Moreover, the excavation of the soil nailed wall, once collapsed, allowed a detailed and more fruitful investigation to be conducted into the behavior of the structure at failure (figure 11).
5m
i
__ ---l
Water filled basin
~~:_~_9_e---1mif---_----" 27em
Observed
H=7m
27em
Figure 11. Post-failure observations of the first full-scale experimental soil nailed wall (CEBTP-French National Project CLOUTERRE 1986).
16
Chapter 1: The Technique Used for Soil Nailed Structures
•
Second experiment CLOUTERRE at the CEBTP (1989)
The aim of the second CEBTP experimental wall was to look into the stability, both local and global, of the soil nailed mass during the excavation phase. For this, a soil nailed wall 6 meters high was built then brought to failure point through extending the height of excavation at the foot of the wall from 1 to 3 meters. During the first excavation phase (1 meter high excavation), the excavation - like the wall - was stable. During the second excavation phase (2 meters high), local failure occurred and stabilized itself through arching. In overall terms, the wall remained stable. During the third pass (3 meters high), the effectiveness of the arch was destroyed and local failure propagated to the surface level. This in turn led to overall and internal failure of the wall (figure 12).
h (m) 6
1-
1
fT-------
!I
II
5
1.80 m
:1
I
. ---nails
Failure surface 3
2
Figure 12. Post-failure observation of the second full-scale experimental soil nailed wall (CEBTP - French National Project CLOUTERRE, 1989).
•
Third experiment CLOUTERRE at the CEBTP (1989)
The third CEBTP experiment with a soil nailed wall sought to study the type of failure caused by the nails being too short. A soil nailed wall 6 meters high was therefore constructed and brought to failure point by gradually reducing the length of the telescopic
17
Soil Nailing Recommendations-1991
nails (comprising nails slid into tubes). After reducing the length of the nails to a minimum, the whole of the soil nailed mass sank 0.27 cm and slid along a well-defined failure surface, which was demarcated by the nails (figure 13). The uniform lengths of the nails used at the outset of the trial were gradually reduced during the course of the experiment until finally very short nails were used at the base of the wall (0.50 m), increasing, as a function of height, up to 2.30 meters at the top. This layout imposed the shape of the failure surface which corresponded to a failure limit between a failure due to lack of adherence and the external failure mode.
Protective frame
Shotcrete facing Crack ___::-j"oj,-~
I.---;~:ltl=:::.:--~
Extracted parts of telescopic nails
.---
25.5"cni
Struts in contact with frame after failure
E o o r 1 m), and the overall behavior is considered to be similar to that of the previous method as long as 5 v 5 h :::; 6 m 2• The two main differences between the two methods concern: The stresses taken up by the facing (tension To at the head of the nails, local pressure p of the soil); those stresses are much lower in the method called "method of Hurpin," which makes possible the use of thinner facing. The forces and bending moment mobilized in the nails; in the "method of Hurpin," neither shear force nor bending moments are mobilized in the nails because of their small moments of inertia.
2. SOIUNAIL INTERACTION
Two types of interaction develop in nailing used in retaining structures: The most important interaction is the shear stress (skin friction) applied by the soil along the nail, which induces tension in the nails. A second, less important interaction is the passive pressure of the earth along the nail during the displacement of the latter. The passive earth pressure mobilized makes possible the bending moment and shear force to be mobilized in the nails; this mobilization occurs only if a shear zone develops in the soil nailed mass.
2.1. Soil/nail friction 2.1.1. Similarity between skin friction in a fill and an in situ soil
Experience with Reinforced Earth has shown that friction along a linear reinforcement placed within a soil and subject to tension was affected by the three-dimensional nature of the contact surfaces. In dense granular soil, under the effect of shear stresses 't applied by the reinforcement, the tendency of the zone of soil surrounding the reinforcement to increase its volume is restrained by the low compressibility of the neighboring soil; this results in an increase L1a of the normal initial stress a o applied to the surface of the reinforcement. This is
29
Soil Nailing Recommendations-1991
the phenomenon of restrained dilatancy (Schlosser and Elias, 1978) that, in the case of Reinforced Earth, led to the definition of an apparent coefficient of friction 11' defined by:
't
J1* =
which can be significantly higher than the real coefficient of friction:
J1
This phenomenon was measured in situ for the first time by Plumelle (1979) at the CEBTP during pull-out tests of passive ground anchors that had been buried in an embankment consisting of Fontainebleau sand. Figure 4 shows that, in the immediate vicinity of the ground anchor, the increase .1.0' can reach four times the value of the initial normal stress 0'0'
cr (kPa)
"
,
'.'
. '.:", :'" :,', .......
.' .. ".: '::., : SAND
"',: "', . :>:... . ',: "
~
100
t::.cr
i>
d[=
PRESSURE CELLS
.
~ - - GROUND ANCHOR
-H-
20cm
50
Distance d (an) 2816
70
100
200
Figure 4. Increase of normal stress due to restrained dilatancy around an inclusion that is in tension.
Within the framework of the Project CLOUTERRE (French National Project CLOUTERRE, CERMES, December 1989), this phenomenon was also observed during the pull-out tests of small-scale nails in a minicalibration chamber, while the additional normal stress generated was measured locally. Figure 5 shows, in the case of a smooth nail, the variations of fl' in function of the initial stress and the density of the sand.
30
Chapter 2: Soil Nailing in Retaining Structures: Mechanisms and Behavior
Concerning soil nailing and in situ soils, the same phenomenon of restrained dilatancy for friction on nails was observed by Cartier and Gigan, 1983. It was also shown by Schlosser (1983), that the soil nail unit skin friction qs was practically independent of the depth; the decrease of the apparent friction coefficient fl * with depth, due to the decrease of dilatancy, is compensated by the increase of the normal vertical stress crv = y z; that is to say: (
qs = J.1* (z) 'Y z = constant
1,0
0,8
0,6
\ 0.._
O-("S:::g Smooth nail
0,4
Smooth nail (High density sandi I = 85%) (Low density sand, D I D= 45% to 60%)
0,2
(J (kRJ)
o
01----,---,----.,----,-----,-_
100
Figure 5. Variation of
J.1*
500
Tmax
as a function of 0"0 (CERMES, CLOUTERRE, 1989).
Figure 6, taken from observations on the A86 freeway experimental soil nailed wall with driven steel angles (Cartier and Gigan, 1983), illustrates this point. There is therefore a certain similarity between friction in piles and in nails that justifies the use of correlations between the results of in situ tests and the soil nail unit skin friction qs along the nails.
31
Soil Nailing Recommendations-1991
9~
( kPa ) + Square o Square
200
angle nails 50x50x5 mm
0::,
angle nails 60x60x6 mm
b
(Values of qA adjusted to equivalent size of square angle nails 50x50x5 mm)
150
+ +
100
o
+
Average value 90 kPa
o
o
50 Fontainebleau sand Depth (m)
o Figure 6. Variations of unit skin friction with depth (after Cartier and Gigan, 1983).
Boulon et al., (1986) have carried out theoretical and experimental studies on the influence of the compressibility of the soil around the reinforcement on the value of qs' using an analogy with a direct shear box with controlled normal stiffness (k = a/iI) Assuming that the thickness e of the zone of sheared soil is small compared to the radius R of the inclusion, stiffness can be expressed as a function of the pressuremeter modulus EM using the formula:
k
= 2
where R is the radius of the nail. Graphs (figure 7) enable qs to be estimated as a function of the normal initial stress the value k in the case of sands.
0'0
and of
2.1.2. Mobilization of skin friction along a nail
Numerous experimental studies were carried out on the mobilization of skin friction in nails within the Project CLOUTERRE that make it possible to predict accurately and completely the results of previous studies (Plumelle, 1979, 1984).
32
Chapter 2: Soil Nailing in Retaining Structures: Mechanisms and Behavior
Un
R
Rough
9.6
surface
~
ao
00 15
~ R
EL
Nail
20
=k:::
u
'k=OMPa/m • k= 1000
• k =5CXX> " 6. k= IOCXX) " D k=20000 " ok=oo "
10
----.
k = 500 _.- • k = 1000
, 5
A k =0 MPo/m
I I I
--- --
--
~o \
¢
.k
= 25(X)
•okk== 5000 00
\-\*
\~>"
1500
o trA":>~ t_-..-_-__ .A __ o 300 6(X) 9:X)
__ A______
~
1200
1500
0-0 (kPa)
Dense sand. Rough surface.
Loose sand. Rough surface.
Figure 7. Study of the influence of the stiffness of the soil k on the unit skin friction (Boulon et aL, 1986).
As in the case of piles, the mobilization of skin friction requires only a very small relative displacement of the nail in relation to the soil, of the order of a few millimeters, as was confirmed by the pull-out tests in the minicalibration chamber previously mentioned. The mobilization of the local unit skin friction between soil and reinforcement can be validly represented by a bilinear law of the Frank and Zhao type (1982), as shown by the comparison between the theoretical and experimental pull-out curves for the tests carried out in the Fountainbleau sand at the CEBTP (French National Project CLOUTERRE, CEBTP, June 1988 and December 1989) (figure 8). This law is represented in the plane ('1:, y) by a limiting value at qs and two straight lines having slopes in the ratio of 1 to 5 that intersect at a co-ordinate equal to qJ2. Skin friction can therefore be characterized by two parameters: k~, the slope of the first segment, and qs the ultimate unit skin friction.
33
Soil Nailing Recommendations-1991
Figure 9 shows the stress-displacement experimental curves obtained in pull-out tests conducted by loading increments, at the CEBTP in Fountainebleau sand (Deguillaume, 1991). It can be observed that, for the same type of nail the displacement necessary to reach the limiting value on the loading curve is all the greater as the nail is longer. This is due to the deformation of the nail, which behaves as a rigid nail for the shorter lengths and as an extensible nail for longer lengths. These results are very similar to the results obtained, particularly by the LCPC, regarding the friction along piles.
Tension load at the head of the nail To (kN) 30
L=4m
Theoretical curve {kJ3= 25 M Palm
)7
2
I~
20
Ex erimental curve
'l
q6 - - - - - - -
15
LJ kJ3/5 q..b
10
""2
1
y
61
T
c;:=1--.
FRANK and ZHAo'S LAW y
O~----.--
2
__r----r---r---,-------.--.-------'--'---10 12 14 16 18 8 6 4 Displacement at head (Yo) (mm)
Figure 8. Modelling of an experimental pull-out curve using the Frank and Zhao's law (CEBTP, CLOUTERRE, 1988).
34
Chapter 2: Soil Nailing in Retaining Structures: Mechanisms and Behavior
'l"o (kPo)
Average unit skin friction 80 L=2m L =3m __-=-=~~---=--=~L =4 m L=6m
60
L =9m
40
20 Yo (mm) 2
4
6
8
12
14
16
18
Figure 9. Pull-out tests on prefabricated nails installed in a backfill: Average unit skin friction curves versus displacement at head (Deguillaume, 1981).
2.1.3. Influence of the type of nail
Within the Project CLOUTERRE, a study was carried out to investigate the friction of several types of nails placed in some Fontainebleau sand: Driven bars (steel angle). Tubes driven and grouted. Bars grouted under gravity in predrilled boreholes. Bars grouted under low pressure in predrilled boreholes. Bars grouted under high pressure in predrilled boreholes. Results (French National Project CLOUTERRE, CEBTP, June 1988 and December 1989) have shown that nails grouted under gravity presented a wide variability in the parameters k~ and qs of the pull-out curve compared to the other types of nails. In particular, it seems the smoother and more even the walls of the borehole, the lower the values of k~ and qs. This seems to be due to the fact that drilling reduces normal stress to zero and because of that the initial stress aD after installation of the nail is, very low. Besides, drilling irregularities cause important effects of restrained dilatancy leading to high values of the increase of normal stress L\a.
35
Soil Nailing Recommendations-1991
2.1.4. Correlations between parameters
(k~,
qs) and PI
During the Project CLOUTERRE, charts were developed for estimating qs (chapter 3, appendix 1) as a function of the various types of nails mentioned in the previous paragraph and as a function of main soil types (sand, gravel, clay, marl and weathered rock). These charts are based on a database containing more than 450 pull-out tests on nails, obtained from the various members participating in the project. Correlations obtained are different from those obtained by the DTU 13.2, the SETRA 1985 and TA 86, but they are not fundamentally different, as shown in figure 10. It is also possible to estimate the value of the parameter k~ from the following formula, adapted from the work carried out by Frank:
k~
where R is the radius of the nail, EM the pressuremeter modulus, and m a factor, which depends on the nature of the soil and which can vary from 1 to 5. Figure 11 compares the values thus obtained with experimental values for different types of nails.
NAIL GROUTED UNDER GRAVITY FLOW IN SAND 0.25
0.20
- - - Clouterre
- - - - DTU
1991
13.2
- - - SETRA 1985
0.15
0.10
0.05
a
a
2
3
Pe
(MPo )
Figure 10. Comparisons between the charts from CLOUTERRE, DTU 13.2, SETRA 1985, developed to estimate the unit skin friction qs'
36
Chapter 2: Soil Nailing in Retaining Structures: Mechanisms and Behavior
(kPa Imm)
•••
400
•
•
o
m. kJ3 et.
0
• Clays o Sands A
Marl-limestone
100
50
(X)
150
2C()
~ ( kPa Imm) Figure 11. Determination of the coefficient m giving the value of k p to be used in the law (1:, y) (CEBTP, CLOUTERRE, 1989).
2.1.5. Influence of moisture content on skin friction
In a soil with frictional characteristics that, nevertheless contains a nonnegligible fraction of fines, the short-term soil nail skin friction can be greatly influenced by the degree of saturation Sr' Figure 12, which illustrates the influence of this parameter, is taken from experiments carried out during studies performed by the Reinforced Earth Company; the maximum pull-out force is divided by a factor greater than 2 when the moisture content is increased from the optimum water content (Proctor) to the saturation moisture content. The displacement corresponding to this maximum force is divided by 3. In fine-grained soils (clay and silt), the degree of saturation is an essential parameter since, in rapid shear of a saturated soil, the soil nail friction can be reduced to the undrained adhesion (a fraction of the undrained cohesion).
37
Soil Nailing Recommendations-1991
To (kN) Tension at head Very clayey grovel
(34%
of weight from particles smaller than 80p.m,PI=IS)
30
I Ribbed .
.
striP Height of overlaying soil, Sm L=2m
20
10
-L
Satured
" --------- ---------------yo 0.05 Figure 12.
0.10
0.15
(m)
0.20
Pull-out tests of a Reinforced Earth ribbed strip in a very clayey gravel. Influence of the degree of saturation.
2.1.6. Mobilization of skin friction with deformation It is the internal deformations of the soil nailed wall, and especially the horizontal extension,
that induce the mobilization of friction along the nails and the tension of the latter. These wall deformations are due to the lateral decompression of the soil as excavation proceeds. Figure 13-which relates to the first full-scale experimented soil nailed wall at the CEBTPshows that, in the case of a soil nailed wall in sand with a ratio of the length of the nails to the height H of the wall (LIH) equal to 1.1, the extension zone is situated mainly at the front of the nailed mass at a distance of 0 to 4 m from the facing. However, in the case of the soil nailed wall built in a clayey soil (M4 wall; Vierzon-Bourges A 71 freeway; French National Project CLOUTERRE; COFIROUTE-SOCASO-CEBTP, December 1989), horizontal extension appears far more uniformly distributed within the soil nailed mass (figure 14).
38
Chapter 2: Soil Nailing in Retaining Structures: Mechanisms and Behavior
,/" Crocks at failure
8m
1i-'--------=::...r·--
1.;--
Z----;T"'----:
4m :-- / -
Shotcrete facing
At failure
End oi e'J-co\Joflon
50
(rrm)
50
100
150 Displacement (mm)
Figure 13. Horizontal displacements in the first full-scale experimental soil nailed wall at the CEBTP (CLOUTERRE, 1986).
Inclinometer
Depth (m) 0
Inclinometer
n02 !
n03
Inclinometer Facing
(mm) 5 10 15 20
j o n ". . . . . . . .. " , . . . . . . . . . . , . . . . . . .. . . . .
E
2
0
"" .... ".::. ,'.,': ~.~.~. .' .~~~~ ~~ -~
~E
{~'.~',:. ;);.~h~.~r·~;ane
11-'-I--:-'-1+-:-~G;~utednails: borehole q, 63 mm
T (kPa)
30
20
o With nails tJ. Without nails
a-(kPa) 10
20
30
b. CLOUTERRE CEBTP (1988). Figure 16. Direct shear tests on soil reinforced with vertical bars.
where To is the shear force in a bar on the shear plane and S the total cross section of sheared bars. This is explained by the fact that the presence of the nails significantly modifies the distribution of the normal stress cr and of the tangential stress 't along the shear plane. Finite elements studies showed how complex the behavior was (Juran et a1., 1981). The tests carried out by the CEBTP within the framework of the Project CLOUTERRE, on an in situ sand mass reinforced with grouted bars (see figure 16), have shown that stressdeformation curves in shear tests with and without nails have the same initial modulus of deformation and that the peak was reached for practically the same displacement value. 2.2.3. Influence of the direction of the nails on the mobilization of stresses
All experimental studies carried out, in particular those of Marchal (1984), show that the direction of the nails with respect to the potential failure surface plays a role in the mobilization of tension and shear (figure 17) and, more generally, in the overall shear strength of the reinforced soi1.
43
Soil Nailing Recommendations-1991
Jewell (1980) did verify experimentally that optimum directions for the mobilization of tension in flexible nails corresponded to the directions of maximum extension in nonreinforced soil (that is to say, approximately 30° in relation to the normal at the shear surface) (figure 18), as had been demonstrated theoretically by Basset et aL, (see paragraph I, chapter 2). Therefore, for example, a marked inclination of the nails toward the bottom of a vertically faced wall reduces the tensile forces mobilized in favor of the shear force and bending, as shown by the results of theoretical finite element studies (Shaffie, 1986) (figure 19).
3O-+------Q----,-------.-------, 20+----+-~------'~~-___t---__t
,..- ~"-20
30
40
50
60
70
80
~.~
"...L=-21°
(mm)
-IO+-----t-----::~--+----+-------i
... ----. Rigid bars -30+-----7"t------1f----
1.:
angle of inclination of the bars with respect to the normal to the failure sur face
Figure 17. Variations of the ratio between the tensile force and the shear force as a function of the displacement on the failure surface (Marchal, 1984).
44
Chapter 2: Soil Nailing in Retaining Structures: Mechanisms and Behavior
COMPRESSION
I
\
COMPRESSION \ I
EXTENSION
\
E
l-re = 0"..
tang
'('I + 6:( inclu.
I
max
EFFICIENT - FLEXIBLE NAIL Jz~::6::::~=
THEORY
• -30
0
30
I__~I~NC::::L~T~",,~;.;;I~~I~;"'N,.;.I.:..:N
60
90 ..
120
I. INCLI~SIO"i
I
INEFFICIENT FLEXIBLE NAIL
COMPRESSION
Figure 18. Influence of the orientation of flexible nails on the tensions mobilized and the increase of the shear stress at failure (Jewell, 1980).
Displacements: h;',:;,j
2 10
X
Plostified soil
r-- -- ~ ~ --.~.-:~~~ -;.~- -.-:-:: •
"
\
: ~
SOIL NAILED WALL (Horizontal nails)
•
•
.. ' .,~. A......
-.;.'
". ,':::. ;F·:.. ,·
' .. ;.'
: ....
':.~ .,'.r
..... , ......:: ".;"- .~;'" K o)' This decrease in coefficient K with depth, from K o to K a, is far more marked in soil nailed walls than in Reinforced Earth walls. This is due to the construction method going from top
50
---",..
Chapter 2: Soil Nailing in Retaining Structures: Mechanisms and Behavior
to bottom and from the successive construction stages that, once the top of the wall is built, mobilize the arch effects between the top of the wall and its base.
=::::::::-' =30°
100
T L /L=40kN/ml F=1.5
80
60 +---1----I---f--+---+--t....
--
40 +--f=-'j'-...:::-:..::-~ .....=!==~~~--j20 -I---1----I---f--+---+--t--+
~+-10--+---0l----I---+-110f-----1---+-12-0----4-8 (degree)
a
-\ ---'
8=lo2J-
(m/ml)
60 I--
--- -- -
-L
40
--- 1'-------
t- ______
20
o
0.1
0.2
... ------
0.3
0.4
tan
0.4
tan /3
7)
/3
L:L{m/ml}
8=10 0 I
I
80
60
-- ---
40
".'"
I
I
,-
'"
20 0 0
0.1
0.2
0.3
-- .... -- Calculations for nails of constant length (Sv variable -0-
Calculations with constant horizontal nail spacing (Lvariable I Sh=1.5m)
Figure 8. Influence of parameters
e, U,
~
I
L=9.15m)
Sh
on the total length of the nails.
87
Soil Nailing Recommendations-1991
More precisely, one can distinguish between the maximum tensile strength T G of the nail, which can be mobilized when the reinforcing bar fails, and the maximum force that can be mobilized at the head of the nail T L ' which results from the soil/nail skin friction. With the help of the latter, one can define a new nondimensional parameter, d, which is characteristic of the soil/nail skin friction (Gigan, 1987). In order to distinguish it from the former, it is referred to as "nailing density."
d
The ratio TJL represents the friction force t per meter of nail. The interesting point of defining the nailing density d consists of not having to consider the tensile resistance T G of the bar.
1.6.4. Preliminary design charts The preliminary design phase of a soil nailed wall seeks to define in approximate terms, the lengths, spacings, and resistance values of the nails in order to ensure both the internal and the external stability of the structure. At this stage, except in special cases, one will be looking only at walls in which all the nails are identical and evenly distributed. One will also assume the soil to be homogeneous. The wall to be designed is, then, characterized by a uniform reinforcement density X or nailing density d. At this stage calculations can also be made based on pure tension, Le., by assuming that, whatever the angle of incidence on the potential failure surface, the nails are only working in tension and their bending stiffness can be neglected. Based on limit equilibrium methods, several charts have been published, the most important being those by Gigan (1986) and Juran (1990). Only the Gigan data will be looked at here. Gigan's work was developed on the basis of a calculation program that uses the classic method of vertical slices with circular potential failure surfaces. Any bending in the nails was disregarded. The resistant cross sections of the reinforcing nails were determined in such a way that the tensile strength of nails T G is always greater than the pull-out resistance T L of the nail, calculated for the total length of the nail. The use of this conservative rule can be justified for preliminary design and sometimes has the advantage of considerably simplifying the analysis of the stability of soil nailed walls. In fact, in this case, the only failure criterion to be considered is failure by pull-out of the nails. The charts proposed by Gigan are based on a system of coordinates (tan , N = e/y If) where H is the height of the soil nailed wall, and which characterizes the shear resistance of the soil in question - assumed to be homogeneous. Here one finds isovalue curves for the nailing
88
Chapter 3: Conception and Design
density d corresponding to the stability limit of the wall. Each chart is presented for one determined L I H value. Figure 9 shows examples of this type of diagram and how they are used. Having located the joint M (tan A .
Sv
= Sh'
fm,e' 1.0
1m
f m,Q5'4.3
,--£!..J-
fmp'I.O • !
.-£!-l-
CD
fm,r =1.0
~
f 53 '1.0
~
..-£!-2~ ~
,
1.
DESIGN / BREAKAGE OF NAILS
Scale
Bishop's method
2
I
Soil N y
a
16.6 kN 1m 19.3. kN/m
f st
1.00
c
3 kPa
1.00 o kPa
'f
38"
38"
qs
80 kPa
80 kPo
1200 kPa
1200 kPa
Pt
27500 kPa 27500 kPa
Ks'S
I"
Sv = Sh '1 m
I
5.00m
.
2.BOm
I
fm,'f= 1.0
R~'2.:'·:'(I'
I
.....---:
------
f min , 1.0
1«1,1\ -- \.cf)
fm,e' 1.0
..E-!.
f m,Q5'4.3 fm,p, =1.0
..-£!-2-
CD
f 53 =1.0
~
~
®
f m,. =1.0
..-£!-2-
b) STABILITY
~~ ~ ~ura h=225m soil ~.
ANALYSIS
ACTUAL
OF THE
FAILURE
Scale ,
•
~
TALREN
V2.0 du 12/03/91
TERRASOL Figure 23. Design and stability analysis of the actual failure of the first full-scale experimental soil nailed wall (CEBTP, French National Project CLOUTERRE, 1986).
134
Chapter 3: Conception and Design
h tml
I
....
...
3
I
'-,I ..... ..... I
: I
2 Without
...... ~With bending in - '.." the nails
-Y
""',
1
bending: in the nails :
-', "
I
,
I
o
1.10
1.05
0.95
fmin
( a )
w t%l
Average water content
t w = 29% ~
'''''...
I
Sr
= 100 1
~ With bending
....... ~
..... ....
20
__________
%
yt:J~_%
in the nails
....... .....
.........
, I
~
.... ....... : fmin =0,98:,
I
Without bending in the nai Is ..........
.......... ...
(Wo=10.7%,Sr=37 % ) : -.-.-.-.-.---·----------.. ------------r·---I---------------10 L-._ _---r ---r_ _-'---,-_._ _
'......
-._----~...
r
_ min _
-,-~loo.__.....,::~
0.85
0.90
1.05
0.95 (
~
1.10
b)
Figure 24. Variations of the factor r min with the average water content wof backfill (CEBTP wall NO.1 - calculations using the TALREN software program).
135
Soil Nailing Recommendations-1991
By assuming that the cohesion varies linearly as a function of the degree of saturation Sr , and the water content (where c = 0 kPa, Sr = 100 percent) one can trace two curves of the coefficient r min (figure 24b), one taking into account the bending stiffness of the nails, and the other not. It is noted that the value of the coefficient r min upon failure thus falls within 0.95 and 0.98. On the whole, and in spite of the uncertainties inherent in the type of loading used in this full-scale experiment, the results showed that the values r min calculated using classical limit equilibrium methods are a correct approach to reality.
3.2.4.3.
Example of a mixed soil nailed wall with surcharges and partial drainage in a layer system
Here we are looking at the case of a soil nailed wall with a vertical facing, a total height of H = 17 m, comprising five rows of nails and a row of prestressed ground anchors at the top. The nails are shorter at the base but are also placed closer together (figure 25). Prestressed ground anchors have been used in view of the sensitive nature of the structure (buildings and roadway to be constructed at the upper part of the wall) and to reduce the deformations undergone at the top of the structure to acceptable levels. Since the structure was situated in a slope with a water table, drainage of the soil nailed structure was provided. This drainage was effective for the prestressed anchors but only partly effective in the zone of the lowest nails. Figure 25 shows the results from calculations made using the TALREN software package. The typical features of the soils, the surcharges, the nails, and the ground anchors are shown in table V. When designing the structure, the characteristic parameters of the nails and the ground anchors were taken to be such that the factor r min linked to the required values for both the load factors and the partial safety factors had a value of 1.00. It will be noted (figure 25) that this value corresponds to two potential failure surfaces: a circle affecting mainly the nailed structure, and another encircling a large part of the nailed structure and passing through the centers of the prestressed ground anchors, and therefore more representative of the overall stability of the structure.
3.2.5. Simplified methods 3.2.5.1. Assumptions
These methods are referred to as "simplified" because they formulate the assumption that the nails work only in tension.
136
......
-...J
W
46000.0
26000.0
17000.0
0.0
0.0
!P1
Ks. B
0.01
5500.0
Sr 3
/
/
0990-INF-S
Fich:
0990infsy
SOIL NAILED WALL WITH GROUND ANCHORS SURCHAGES AND PARTIAL DRAINAGE
··~~·.I
hl
0.0 0.0
500.0
1+ 17
1,10
I.;: 07
It 08
1. 00
1,11
1+07
If 04
1,04
Nails
TERRASOL
I .. 10
I+. 07
1+ 06
1,02
1+. 01
1.(.01
'
f
"
.i?ni.: ..
= 125
fm'R = 1.15
f"'Q =150
fm,r =1.15
f m,Qs=1.50
fm,pe=z.OO
fm,c= 1.65
fm,'f= 1.30
fmin= 1.00
1.+08
l-i- 03
1,01
G:OO)
.:-: /ca~e.: ..... :...
\ l
\
~
1.; 04
1.;:02
1,17~,02
1,10
1,06
Ground anchors
If 06
1
1,07
I.;: 04
1,06
Calculation done by
C1 5
C1 4
Sl2..
C1 2
Ti 1
1 f 09
If 11
1,02
1.;: 02
1.(.03
Figure 25. Ultimate limit state design of a soil nailed wall with surcharges and partial drainage.
TERRASOL
Doted (12 March 1931
TALREN
»:-:~
..
Sr 4
Units in kN 1 meters, and degrees Calculation method' PERTURBATIONS
0.0
0.0
2000.0
1500.0
1000.0
80.0
0.0
0.0
30.0
40.0
36.0 200.0
35.0 160.0
35.0
0.0
20.0
200.0
30.0
20.0
10.0
10.0
120.0
0.0 1. 000
17 .0 1. 050
20.0
1. 050
19.0 1.050
19.0
1. 050
7
19.0
6
1. 050
4,' .
19.0
·3·
1. 050
2.
0.0
I
i 1-
'
J---.. I
(l)
Cf)
o 5
10
15
20
25
Displacement
y (mm)
Figure 10. Force/displacement curves at the head and tip of both a short and a long nail.
184
o
E
Chapter 4: Investigations and Tests
To (kN)
Load at head of nail
L L= 12 m
250
=
12m
l
~;r
,,
x
,X l
x
200
"
,,x.
,x
-x--x--x-
,
,X
150
~ Calculation of
the force-displacement at the head of the nail assuming that the nail tip is fixed and T'= constant
-+-+-T-? curve
,,
'i
• \ Calculation of the force-displacement
•
f
-0-
o-? curve at the head of the nail ta king into accunt the unit skin friction mobilization low
100
50 I I
: -0
/1 /
:,
---- ---
L=2m --------
~---~
I I I
I I
5
I
K)
Displacement of the head of nail
I
15
y 2
2b
Yo
(mm)
...
Figure 11. Theoretical comparison of force-displacement curves at the nail head (based on two different sets of assumptions).
2.7.2.3.
Behavior of nail at failure To
=T
L
Skin friction is fully mobilized (figures 8 and 9). At failure, the theoretical displacements at the head and tip of the nail will be equal to:
where Yo YLs
displacement of the nail head at failure, displacement of the nail tip at failure,
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Soil Nailing Recommendations-1991
p
qs ES Ls
nail perimeter, unit skin friction, nail stiffness, length in contact with the soil.
The experimental measurements are in agreement with the theoretical calculations (see appendix to this chapter). At failure, every nail has a displacement at its tip interaction law.
2.7.2.4.
YLS
equal to the value Yz of the soil nail
Calculations of the unit skin friction qs
The objective of a pull-out test is to find the maximum pull-out force TL in order to determine the unit skin friction qs' The unit skin friction qs will be calculated using the TL value given in paragraph 2.7.1. and figure 6.
where p Ls
nail perimeter, length in contact with the soil.
NOTE: Even as a first approximation, the comparison of the real force-displacement curve with the theoretical one based on the following two assumptions: The nail tip is fixed and the shear stress is constant along the nail, whatever the pulling force, is wrong. The real forcedisplacement curve can be well-approximated using the skin friction mobilization law. Figure 11 shows simulations for both models and clearly indicates that the first approach is inappropriate, Le., the shorter the nail, the greater the degree of inaccuracy.
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Chapter 4: Investigations and Tests
Yo (mm)
Cumulated displacements
8
7 6
0.9 T max
4
3l-_------------- 0 .8 T max
L----------------=-:~-0 6 max 2 l--l - - _ - - - - - - - - - - - - - 0.5. TTmax ~--------------0.4
~============== I10
max
0.7 T
TmlJ.( max
0.3 T 0.2 Tma Y2 0< Yl < Yl
The displacement at the head is on the residual part of the curve, the skin friction at the head is fully mobilized, the displacement at the tip is located on the first linear section.
3:
Yl < Yo < Y2 Yl < Yl < Y2
The displacements at both head and tip are located on the second linear part of the curve.
4:
Yo> Y2 Yl < Yz < Y2
The displacement at the head is on the residual part of the curve; the skin friction at the head is fully mobilized and the displacement at the base is located on the second linear part.
5:
2.
The displacement at the head is already on the residual part and the displacement at the tip has reached the residual part. The limit pull-out force is reached, but the nail fails through lack of friction.
CALCULATION OF DISPLACEMENTS
Among all the phases of mobilization of the skin friction along the nail, two are of particular interest. The first is the stage at which the displacement at the head is still lower than Yl' This can be regarded as the "elastic" phase. The second is the limit pull-out force, which is reached when the nail tip reaches Y2'
2.1.
First phase (Yo < Yt)
Assumption: Small displacements and small strains are considered. The reference state will be the initial state. The example is based on a nail driven so that the effects of the grout will not be considered.
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Soil Nailing Recommendations-1991
e(x)
cr(x)
=
small strains
dy dx
= E e(x)
Hooke's elasticity law of the bar
T(x) = E e(x) ·5
dy
T
dx
5 E
2
5: Section of bar
d y
dT
1
2
dx
5 E
dx
dT
- 't P dx
p: Perimeter of bar in friction
- k~ Y
't 2
d y dx
E: Young's modulus of bar
2
P
5 E
k~ Y
Using the parameter a =
k
~ __~ 5 E
y
= M 1 ch(a x) + N 1 shea x)
T
= a E 5 [M1 shea x) + N1 ch(a x)]
The boundary conditions are:
as homogeneous to
t 1, we can express y(x) and T(x):
x=O x =1
The displacements and the forces can be computed at any point x in the function of the pullout force To and the soil and bar characteristics:
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Chapter 4: Investigations and Tests
Y
T
ch (a (L - x)) shea I)
To E 5 a
To sh
(a (L - x))
shea l)
displacement at the head (under the conventions adopted, Yo is negative). To E 5 a thea I)
-
displacement at the tip:
To E 5 a shea l)
The modelling shows that, in theory, there is some displacement at the tip when a force is applied to the head; this corresponds well with the results of tests performed on inclusion of different lengths (from 2 to 12 m). However, the displacement at the tip of a short nail starts to occur at the beginning of the pull-out test (figure lOa of this chapter), whereas in a long nail, the displacement of the tip can only be measured when the force is close to the limit pull-out force (figure lIb of this chapter). It should be noticed that if only the "elastic" phase 0 < Yo < Yl is to be used, there is an
anchor limit length beyond which the nail does not transmit any force to the soil.
If Yo = Yl is considered as the limit of the "elastic domain"
where, th(2)
= 0.964 # 1 then: a I I = 2 II = 2/a
I "2 I
J
E S P k~
II increases with the stiffness E 5 of the bar and decreases with the stiffness of the soil represents the length of transfer of the tension force from the nail to the soil.
k~.
It
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Soil Nailing Recommendations-1991
NOTE: The present simulation has been done for a test conducted up to failure, therefore there is no reason to limit the pull-out capacity of the nail to its "elastic" phase. There is equilibrium as long as the displacement at the tip of the nail does not reach the residual part of the skin friction mobilization curve.
2.2.
Second phase: at failure
This phase is reached when displacement at the tip is equal to Y2:
At this stage the unit skin friction value becomes constant along the whole length of the nail and is equal to qs: 't
= qs
T=T a - P qs x and Ta dy dx
p qs 1
T 5 E
(according to the conventions adopted, y is negative) Taking y at its absolute value:
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Chapter 4: Investigations and Tests
In the first equation, the expression Tal /2 E 5 represents the lengthening of the nail, for which the tip is clamped, and the unit skin friction is mobilized and constant along the nail. The value Yl' representing the displacement at its tip, must be added to obtain the exact displacement at the nail head. For the same type of nail and the same type of soil, at failure, the tip displacement is obviously the same whatever the nail length. On the other hand, the lengthening of the nail corresponding to the difference between the head and tip displacements is proportional to the square of the length. The theoretical results have been verified by full-scale tests conducted with 2 to 12 m long nails (Deguillaume 1981, Plumelle 1984). The results shown in figure 3 give the theoretical and experimental displacements at the head and tip of the nail at the critical creep tension Te •
2.3.
Example of a full calculation
Accurate calculations at each phase of mobilization were made using a steel angle of cross section 70 mm x 70 mm x 7 mm. The mobilization of the skin friction was studied without taking into account the possible yielding of the steel above the elastic limit. The skin friction mobilization law is shown in figure 4. Results from theoretical calculations are given for two lengths - 2 m and 12 m - in figures 5 and 6. For each phase, the respective positions of the displacements that occurred at the head and the tip of the nail are shown on the force versus displacement curves.
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Soil Nailing Recommendations-1991
Nail displacement (mm)
10
Yo
theoretical displacement of nail head
• 5
• e:theoretical displacement
Y {
•
0--0'--0---_
o
2
•
3
4
6
of nail tip
_
o
----0
9
12
Length of nail (m)
Experimental displacement at head of nail
o Experimental displacement
at
tip of nail
Figure 3. Comparisons between the experimental and theoretical displacements of both the head and the tip of 2- to 12-m long nails at the critical creep tension Te .
The rigid behavior of the short (2 m) steel angle will be evident in comparison to the more "elastic" behavior of the long steel angle (12 m).
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Chapter 4: Investigations and Tests
To (kN)
280 Experimental Curve
240 7; (kPo)
200
160 k,t3=25kPo/mm
120
8 60
1,8
10,8
Yf
Y2
Y (mm)
SKIN FRICTION MOBILIZATION CURVE
Linear part of curve
Displacement at head of nail 5
10
15
20
Y (mm) o
Figure 4. Theoretical force versus displacement curve at the head of the nail.
In both cases the "elastic" phase is clearly limited to a 1.8 mm soil-nail relative displacement, relative to the value of Yl' For instance, a straight line has been plotted on each graph that corresponds to a simulated head displacement where the tip is deemed to be fixed and a unit skin friction fully mobilized and constant along the nail. Comparisons with the exact curves clearly show that it is wrong and dangerous to try to use this type of simulation.
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Soil Nailing Recommendations-1991
Load at head
150
.-------------------,
1
Steel angle nails : 70mm x 70mm x 7rrrn L=2m
100 at head yo and tip
DisPlacements
YC
of the nail are about equal
50
~~~-:----
\
y ( mm
Displacement
)
O~--+----'-----r----------,----l------.--------
15
5
Figure 5. Comparison between two simulated force versus displacement curves for both the head and foot of a short nail. To (kN)
Load at head
1
To
Yo = z'Es-'
e ~ ,x '/
250
00-0--0
I
."..0 ............
j'k==y
'/ 1/
,;0
/,;.
/.
2
/
/0 o
150
/
I I
cf )'' "
/
I-
I I
1
./
/
I I I
I 1
/
I
f
/
1
o
10
/
I
Steel angle nails: 70mm x 70mm x 7 mm
" 1
I/
;:.
o
-0---0-
1
Displacement Ye at tip
of nail
I 1
Displacement yo at head of nail
Displacement y (mm)
o
5
101
15
20
25
1 I
YZ
18.8
Figure 6. Comparison between two simulated force versus displacement curves for both head and tip of a long nail.
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C HAP T E R
5
_
WALL STRUCTURES CONSTRUCTION
1.
GENERAL PROVISIONS
1.1. General Aspects -
Principle governing the construction of the structures
The construction of a soil nailed structure appears as a succession of earthworks, between which operations for inserting reinforcement bars in the in situ soil and installing a reinforced concrete facing - generally shotcrete - takes place. With current technology, the inclusions are generally made of metal and installed by driving straight into the ground or inside a borehole. In the latter case, they are always sealed to the ground by means of cement grout or mortar. Earthworks operations are carried out in phases (generally horizontally) of sufficiently low height to ensure general and local stability of the soil mass in conditions acceptable for the structure itself and for its environment (figure 1). The need for permanently ensuring local stability, especially after each earthworks phase, means that the use of the technique is, in principle, only applicable to soils with sufficient cohesion - at least in the short-term - and without the presence of a water table (eventually after lowering the water table, if possible). Attention must be drawn to the fact that the soil nailing technique discussed here is passive nailing, even if, for various reasons (to ensure the stability of the facing before starting earthworks, for example), the inclusions can be pressed against the facing with some tension. The association of prestressed anchors with soil nailing is not excluded and is a commonly used technique, especially when environmental constraints impose a fairly strict limit on the displacements of the structure. However, the installation of this type of anchor, which is extensively described in the Recornrnandations TA 86 on ground anchors, will not be discussed below.
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Soil Nailing Recommendations-1991
Part of structure completed
/
Adjoining excavation
/
/ Genera I excavation
----
/
./
/
/
Figure 1. Excavation and stability.
1.2. Earthworks As work progresses, it is imperative that the conditions of construction of the structures, as set up and taken into account in the justifications by the consulting engineer, be strictly observed. It is essential that good coordination exists during construction; this coordination must also apply to general earthwork operations, or at least part of them, since they can influence structural stability. It is therefore recommended that earthwork operations, including a certain part of the general earthworks, while not carried out by the company responsible for the construction of the nailed structure, must be under its direct control and guidance.
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Chapter 5: Wall Structures Construction
1.3. Choice of an installation method for the inclusions The techniques most generally used for installing the inclusions in the ground are driving and drilling. There are, however, some special methods that have not yet been widely developed, but in any case, the rate at which nailing techniques are being developed does not exclude possible improvements to existing techniques, even the introduction of new methods in this field. Generally, the selection of the method of installation is practically one of design data. Frequently, this selection is imposed by certain constraints linked to the nature and characteristics of the ground or to certain requirements relating to the structure itself (geometric characteristics, short-, medium- or long-term nature of the structure and length/characteristics of the inclusions). In other cases it is not uncommon for the design of the structure to be the result of the selection of the technique beforehand. •
Driving
Normally, when driving is used to install the inclusions, the latter are not secured by grout injection, the pull-out strength of the inclusions being obtained by adhesion of the inclusions to the ground. However, some special techniques do allow the sealing of an inclusion installed by driving to be carried out with cement, grout, or mortar. Driving is a technique that can be particularly effective when used in loose, noncohesive soil that contains neither hard obstacles nor too many large blocks. Theoretically, driving is best adapted to small- or medium-sized inclusions (not exceeding approximately 8 meters) mainly because of the space taken up by the equipment when the metal device is in one piece, as often is the case, and because of the power of the driving equipment with regard to the rigidity of the inclusions. In any case, it is recommended that the latter be correctly guided during driving. The main disadvantage of driving exists in its limited uses. Its advantages, when used judicially, are speed of installation, flexibility of use, and, especially, that the efficiency of the inclusion is immediate when pull-out strength is obtained directly from adhesion of the inclusion to the ground (no securing with grout is required), as is often the case. •
Drilling
When the inclusions are installed in a borehole, they are always sealed to the ground, generally by means of a cement grout or mortar. The main advantages of drilling are the wide field of use of the technique, which covers practically any type of ground as long as the means used are well-adapted. The possibility of always obtaining the required length and, very often, of obtaining the pull-out resistance sought, using, if necessary, sealing injection under pressure.
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Soil Nailing Recommendations-1991
Among the disadvantages of the technique are the necessity of very often having to adapt the drilling technique as a function of variation in the nature and characteristics of the ground, and the fact that this can make the technique somewhat cumbersome. The pull-out strength of the inclusions sealed with cement grout can be fully mobilized only when the grout has reached a certain strength. This can be awkward, particularly in fairly loose soils or for small sites (slowing down the rate of progress of the earthworks). Special sealing conditions can partially remedy this disadvantage. Because of construction requirements (drilling and injection of the sealing grout), inclusions installed in a borehole are inclined on the horizontal (toward the bottom). As a general rule, this inclination must not be smaller than approximately 100. If the inclination of the bars is smaller than this value, particular care must be taken when carrying out sealing operations.
1.4. Selection of the method to be used to lay the concrete of the facing As a general rule, facings of soil nailed structures are made of reinforced concrete: The concrete can be cast-in-place or it can be sprayed with a gun (shotcrete). In some cases, precast concrete or metal elements have been used, but they are very special applications that have not been really developed. Cast-in-place concrete can be suitable when no short-term stability problems exist (ground of sufficient cohesion or the previous placing of a shotcrete protective curtain, for example). This placing technique is adapted when a large amount of reinforcement is used or when the facing must look like shuttered concrete. Generally, this technique is carried out using excavations in slots. Shotcrete is well-adapted to the nailing technique; it is flexible and permits rapid "protection" of the facing built by earthworks. Initially, only dry spraying was recommended for direct application in situ (see AFTES Recommandations). Recent experience has shown that wet shotcrete can be equally effective. However, dry shotcrete is more flexible to use; it allows more immediate use when it is needed, for instance, when local collapse of the ground is likely to evolve rapidly.
1.5. Resources used -
Equipment
One of the main conditions for the successful construction of soil nailed structures is the ability of the construction company to respond quickly to eventual adaptations of the design caused by risks linked to construction conditions of the structures in ground where the heterogeneity has not always been recognized. This means the use of qualified and experienced staff, who are capable of evaluating certain foreseeable risks and of reacting promptly and efficiently in case of need. It also means the use of well-adapted equipment capable of intervening rapidly (partial refilling, rapid spraying of concrete, pumping) or the possibility of eventually adapting working conditions
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Chapter 5: Wall Structures Construction
if the need arises, e.g., change in drilling technique or even method of inserting the inclusions. For fairly large structures, or in the case of highly heterogeneous ground, it is recommended that the possibility of changing the technique of installation of the inclusions be allowed during construction and, in particular, the ability to switch to drilling when driving had been planned. Such an arrangement must be foreseen from the moment construction work is planned (see paragraph 1.7. of this chapter) insofar as late improvisation would have serious consequences, at least on completion time.
1.6. Controls -
Monitoring deformations of the structures
The construction of soil nailed structures - be they short-, medium-, or long-term - is accompanied by controls that, in practice, concern nearly all construction stages: Controls are therefore needed before work begins to check that the specifications concerning materials and products have been respected. Control of the conformity of the methods and conditions of construction decided upon to obtain the required results, specially for grout, concrete, and pull-out strength of the inclusions. Controls during construction address the nature and characteristics of the ground encountered (earthworks, drilling or driving investigations), the quality of the grout and concrete applied, pull-out strength of the inclusions, and respect of tolerances. The behavior of soil nailed walls is also controlled in all cases during construction and service (particularly the monitoring of the displacements and deformations of the wall and, eventually, of the tension in some of the inclusions). The nature of these controls, their consistency and the frequency with which they must be carried out, are defined in chapters 4 and 7. However, it is necessary to recall that provisions relating to these controls must be defined before work begins and stated in the specifications or in quality assurance plans.
1.7. Program for the construction of the structures For the reasons already mentioned, and because of the special constraints imposed by the conditions of laying the materials used in the structures to structural stability during construction, the design and installation of the latter are closely linked. It is therefore important for conditions of construction to be set up in close cooperation with the consulting engineer at the same time as the construction drawings are prepared.
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Soil Nailing Recommendations-1991
For short-, medium-, and long-term structures, it is important to establish a program and method statement for the work before the installation begins. Precise data should be given on: The location and the conditions of installation of the inclusions for preliminary tests or conformity tests and technology of these inclusions. The selection of the method(s) of installing the inclusions as a function of the nature and characteristics of the ground concerned. General conditions of construction of the structure and, in particular, different phases of earthworks and the succession of operations carried out at each of these phases. Particular conditions of installation (reinforcement, concrete, phases planned, special zones) and methods envisioned as a function of the constructional provisions appearing on the plans showing the conduct of the work. Controls (resistance of inclusions, sealing grout, concrete), special recordings and measurements (monitoring of the movements of the wall), their frequencies, construction techniques, and names of the people (and eventually organizations) responsible for these different tasks (see paragraph 1.6. of this chapter). Names and qualifications of the managerial staff and construction staff allocated to these different tasks, and the means planned for the execution of the work.
2.
CONDUCT OF THE WORK
2.1. Earthworks .It is imperative to strictly observe the conditions laid out for the conduct of the earthworks
(phasing, height of earthworks at each phase, length of sections, possible delay before the start of new earthworks phase), and the way in which they were planned and taken into account in the justifications provided by the consulting engineer. They should only be modified with the agreement of the consulting engineer who will ensure beforehand their relevance to the stability of the structure and define, if necessary, the provisions to be adopted to ensure that this stability fulfills the normal conditions of safety. Experience has shown that these provisions concern both earthworks contiguous to the structure and general earthworks, the influence of which on the stability of the mass often appears less obvious. 2.1.1. Height of earthworks
Earthworks contiguous to the structure are carried out in phases. The selection of the maximum height of earthworks at each phase is dependent on the justifications during the
208
Chapter 5: Wall Structures Construction
phases of the work and on the evaluation of the local stability of the ground. It depends on many factors, especially on the nature and characteristics of the ground, on the more or less thorough knowledge of the latter, on the eventual presence of water (preliminary site investigations, similar work already completed on site or in the vicinity in the same type of formation, previous general earthworks) on environmental conditions, and on the inclination of the facing. At present there exists no general rule for evaluating the maximum height of earthworks. As an example is the soil nailed wall of the CEBTP experiment No.1 (French National Project CLOUTERRE 1986) where earthworks/1m high, in Fontainebleau sand, were stable with an apparent cohesion in the order of 4 kPa. Height must be adapted as a function of the risks of local collapse (figure 2). Thus, in terrain subject to collapse (loose granular soil without apparent cohesion), it could be necessary to use another technique. However, some particular provisions can limit these risks of local collapse (see below). It is recommended that the height of the initial phases of earthworks be reduced (in
particular of the first) if eventual difficulties have not been properly evaluated at the beginning of the work, as it is often difficult to react to them quickly. It is also recommended to lay at least one row of inclusions during each works phase.
Height of excavation
Loca Iized failurG
_ and its possible development over time
Figure 2. Height of earthworks and local stability.
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Soil Nailing Recommendations-1991
2.1.2. Special provisions
Certain special provisions can offer a solution to problems of stability of the mass or local stability:
•
Construction of the structure in small continuous or alternate slots (photograph of figure 3).
Such provision, more especially adapted to the solution of structural stability problems during construction, is generally decided upon before work begins, but it is constraining as it does not allow a rapid rate of progress and necessitates very good organization of the site. However, in good quality ground, it makes it possible to plan greater height of earthworks (figure 3).
Figure 3. Building a soil nailed wall by alternating excavation sections.
•
Installation of the inclusions before earthworks operations (a berm must be reserved)
The inclusions can be either inclusions already decided upon or additional inclusions, generally of shorter length. Such an arrangement enables the local behavior of the soil to be strengthened and reduces the time between earthworks and the placing of concrete. It can only prove really effective if the inclusions are placed sufficiently close to each other, and it appears better adapted and easier to install if the inclusions are driven directly into the soil (figure 4).
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Chapter 5: Wall Structures Construction
•
Spreading a thin protective concrete layer immediately after earthworks are completed
The goal is mainly to limit the risks of local collapse by reducing decompaction and weathering of the ground, which is at the origin of this collapse (figure 4).
Protective berm
Protective shotcrete 1f"";;;;~~~~~;;;;;;;;;;::,. ski n ----- 40%), muds or sludge for example, are analyzed in the condition in which they are found. All the soils must have a fluid consistency. Standard equipment is used to determine the pH level - see the following standards for details of sample taking and measuring: NF T 01-012: pH metrie - Solutions etalon pour l'etalonnage d'un pH metre. Measuring pH levels - standard solutions to be used when calibrating a pH. NF T 01-013: pH metrie - Mesure electrochimique au moyen d'une electrode de verre. Vocabulaire et methode de mesure. Measuring pH levels - electrochemical measurements using a glass electrode. Glossary and measuring methodology.
DETERMINING THE MOISTURE CONTENT OF A SOIL SAMPLE •
In a laboratory
Weigh out about five grams of the soil sample in a porcelain crucible or refractory dish. Heat in an oven for two hours at 105°C. Leave to cool in a desiccator, then weigh and reheat to 105°C for an hour.
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Soil Nailing Recommendations-1991
Continue the drying process until a constant weight is achieved. The moisture content is calculated as follows: p -p
Moisture content
Po
= weight of sample before drying (g)
P
= constant weight after drying (g)
•
On site
=
100
_ 0_
p
Weigh out about 100 g of the soil sample, which should be crumbled into a metal receptacle. Sprinkle the sample with alcohol (about 20 cm3) and ignite the mixture. Reweigh the sample after the alcohol has burned. The approximate moisture content is calculated in the same way as in the laboratory test:
P - P Moisture content = 100 _o-=-_ P Po
= weight of sample before drying (g)
P
= constant weight after drying (g)
ON-SITE EVALUATION OF THE RESISTIVITY OF A SOIL •
The Wenner method with four probes (see figure 1)
This measurement is taken from the surface of the soil. Four metal electrodes are arranged in a straight line at equidistant intervals (length 50 cm, diameter 1 cm). These are then connected by conductors to a quadripolar resistance bridge (alternating current is used).
-
I
a
a
a
Figure 1. Wenner method of measuring resistivity of the soil.
248
Chapter 6: Durability of Structures
The depth of soil encompassed in the measurement roughly corresponds to the distances between the electrodes "a". The apparent specific resistance of the soil can be calculated using the formula:
p =21taR
where R is the resistance measured between the two middle electrodes.
•
Cane probe method (see figure 2)
This method involves pushing into the ground a probe fitted with either two or four electrodes and linked to an alternating current resistance bridge (Kohlrausch, etc.). The method tests only a small volume of soil in the vicinity of the probe tip. For more details, see Neveux, 1968.
Sounding probe
Koh Irausch bridge Ewphone
o
Figure 2. Cane probe for measuring soil resistivity.
DETERMINING THE RESISTIVITY OF A SOIL SAMPLE -
ON-SITE TEST
The retrieved soil sample is cleaned of any gravel or stones it might contain. Resistivity is measured in a specific cell (with two or four electrodes). The naturally moist soil is densely compacted into the cell to duplicate as closely as possible the in situ density of the soil. To determine the minimum potential ground resistivity, water without minerals is added in volumes of 10, 20, and 30 percent - and mixed with the soil sample. Make sure that proper homogeneity is achieved and that any soluble salts are put into the solution before resistivity level is measured. The minimum resistivity value found after the water has been added will be used when evaluating absolute corrosiveness levels from tables.
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Soil Nailing Recommendations-1991
This resistivity value will have been corrected to reflect the effects of temperature as shown in the following ratio:
P (t)
where: t taken.
[1
+
x (to -
= the temperature of the soil (in degrees Celsius) to
x x
250
p (t) /
= = =
18°C 0.03 where t < 18°C 0.02 where t > 18°C
t) ]
at the time the measurement was
C HAP T E R
7
_
SPECIFICATIONS AND INSPECTIONS
1.
SPECIFICATIONS AND INSPECTIONS OF MATERIALS
INTRODUCTORY REMARKS Passive soil nailing is often associated with the use of prestressed ground anchors, particularly on urban sites, so that the magnitude of displacements in the structures can be limited. It is important to remember that the difference between passive nails and prestressed ground
anchors is not limited simply to the fact that the latter are put into tension. This is of particular importance because it is not uncommon, for a variety of reasons, for so-called passive nails to be tightened against the facing after they have been put into partial tension (or traction). The difference between passive nails and prestressed ground anchors extends well beyond this particular aspect of their installation. The technology involved, the nature and qualities of the steels used, the durability of the steels and the way corrosion protection is addressed, the field inspections during construction, and even justification for their use during design are all differences worthy of discussion. For prestressed ground anchors, these aspects are dealt with in the Recommandations TA 86 developed by SECURITAS, which should be consulted. Only the recommendations concerning passive soil nail inclusions are dealt with here.
1.1. Materials used for nail reinforcements 1.1.1. Metal reinforcements The metal reinforcements must be manufactured from the following types of steel: High adherence bars made from a steel included on the list of accepted or authorized bars compiled by the Acceptance and Inspection Committee for Reinforced Concrete
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Soil Nailing Recommendations-1991
(Commission d'agrement et de contrale des armatures pour beton arme); the specified elastic limit must be lower than or equal to 500 MPa. Nonalloyed steels that are not the subject of an acceptance procedure (profiles or rolled merchant bars, rods, extruded bars, etc.) but that conform to the relevant French standards: "Shades and Qualities," "Dimensions and Tolerances" ("Nuances et Qualites," "Dimensions et Tolerances").· The specified elasticity limit must be lower than or equal to 400 MPa. Nonalloyed "soft" steels used in the oil industry. The use of high-strength steels (elastic limit higher than 500 MPa) for passive inclusions is not recommended mainly because of their low resistance to bending. It must also be remembered that because they are highly prone to stress corrosion, the use of
high strength steels, without exception, requires the same protection as prestressed ground anchors given in the Recommandations TA 86. The heads of the anchoring devices must be protected the same as prestressed bars, and the anchoring devices themselves (nuts, couplings, support plates, etc.) must be made of nonalloyed steel so that no galvanic corrosion cell is formed. In all cases, prestressing steel must come from the approved list prepared by the Inter-Ministerial Commission for Prestressing (Commission Interministerielle de la Precontrainte). NOTE: For the reasons explained in chapter 6, passive and alloy steels must not be used in
soil nailing. 1.1.2. Nonmetal bars
As a general rule, bars are made of metal; however, in certain cases where special requirements need to be met, other materials have sometimes been used. Fiberglass is a case in point when it was used as a short-term measure to facilitate the subsequent destruction of the reinforcements. Materials other than steel are not used except in short-term soil nailed structures or as part of an experiment. This is because our current level of understanding into the way these materials work is still not sufficiently advanced, particularly their long-term behavior and durability. For the reasons given above, and until such time as we have more sophisticated understanding of these materials, only metal inclusions (Le., steel) will figure in the specifications.
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Chapter 7: Specification and Inspections
1.2. Procedures and materials for protecting nail bars against corrosion 1.2.1. Protection for standard steels
See recommendations contained in chapter 6. 1.2.2. Protection for prestressing steels
Types of protection and the conditions for implementing them must conform with the Recommandations TA 86 developed by SECURITAS (January 1986).
1.3. Grout used for securing the nails This paragraph deals only with cement-based grouts, which are the types used in the majority of cases. 1.3.1. Specifications for grout components 1.3.1.1.
Cements
Cements must conform to the stipulations of NFP 15-301 and the NF-VP lists, as issued by AFNOR. The type of cement used must be chosen in keeping with the aggressivity of the local environment (ground water), the type of structure (long-term or short-term) and the duration of the excavation phases. In aggressive settings (see chapter 6), the type of cement used must be chosen from the relevant COPLA lists. Other criteria might have a bearing on the type of cement chosen, such as its short-term resistance or how long it takes to set. If there are plans to use prestressing steel, the specifications contained in the Recommandations TA 86 relating to chlorine and sulphur levels must be respected:
Total chlorine Total sulphurs/sulphides 1.3.1.2.
~ ~
0.05% of the weight of the cement 0.15% of the weight of the cement
Water
Water in the grout must conform to standard NFP 18-303.
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Soil Nailing Recommendations-1991
1.3.1.3.
Specific ingredients and additives
Any additives used must conform to NFP 18-103 and NFP 18-331 to 338 standards, and be stamped with a mark signifying that they comply with the relevant French standards, otherwise they can be chosen from the CaPLA list of approved additives.
In the case of medium-term or long-term soil nailed walls, additives must not contain any substance that would prove harmful to steel. A compatibility study must be made if more than one additive is used. 1.3.2. Specifications relating to grout mix design
Grouts are mixtures of cement and water, possibly stabilized by a small quantity of bentonite. The cement/water ratio is generally between 1.5 and 2.2. The bentonite dosage should not exceed 10 to 15 kg/m3 • Special grouts can be used in fissured, karstic, or very porous ground in order to limit the quantities needed. These can be: Grouts with fillers (fine sands, fly ashes, etc.). Stiff grouts in which the set time is speeded up by the use of additives (sodium silicate is the most commonly used). Special grouts, etc. 1.3.3. Checking the quality of the grout 1.3.3.1.
Checking that the grout components conform
Care should be taken to ensure that the grout components comply with the stipulations of paragraph 1.3.1. and to the contract documents. 1.3.3.2.
Conformity and control tests
The usual tests check such things as the composition of the grout, uniformity of production, the conformity to the design, the mechanical resistance of the grout, etc. Some of these tests are carried out either before work starts on the building site (conformity tests and preliminary pull-out tests) or during the course of construction. The following factors are measured: Density. Viscosity.
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Chapter 7: Specification and Inspections
-
Resistance (simple compression).
In most cases these tests will be the only ones required. However, in special circumstances, additional tests can be carried out (measuring the time needed for the grout to set, temperature measurement, test tube settling measurements and spin-drying using a filter press). It is not anticipated, however, that these additional tests will be systematically carried out. • Density Measurements Measurements are taken using either: BAROID scales with liquid grout, or Hydrostatic (submerged) weighing using hardened grout. These measurements can then be used to check the composition of the grout. Indeed, by looking at the volumic weights of: the cement the bentonite the water
Yc = 29.5kN/m3 (CLK) to 31 kN/m3 (CPA)
= 25.5 to 26.5 kN/m3 Yw = 10 kN/m3 Yb
and the respective proportional weights of the cement (C), the bentonite (B) and the water (E), the density of the grout, using the weight ratios C/E and B/C, are expressed using the formula:
1
C
+-
E
d 1
C
+-
E
B
+-
C
Yw
Yw +-
Yc
Yb
"-
1
B C /
• Viscosity Measurements Viscosity is traditionally measured using a Marsh flow cone with a 5 mm-diameter nozzle. The viscosity v is expressed in seconds and corresponds to the flow of a grout volume of 946 cm3 (standard API 13B). The initial viscosity of the grouting is generally regulated, whatever its composition, to ensure it is stable (only low sweating rate or none at all). Experience demonstrates that this result is obtained for values of around 50 seconds. In this range of values, the simple-to-use Marsh cone test is perfectly suitable.
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Soil Nailing Recommendations-1991
However, with high viscosity levels, the values rapidly become meaningless (v > 80 seconds). Here, the modified Marsh cone - also called the LCPC cone with adjustable nozzle should be used.
• Simple compression resistance At the moment, there is no standardized method for this type of test; the sample sizes are not standardized. The method recommended involves putting the grouting sample into hermetically sealed cylindrical moulds with a cross sectional area of about 12 m 2• The samples should have a height to width ratio of 2. However, other sampling modes are also acceptable, particularly cube-shaped samples. The shape of the samples should then be specified. Simple compression resistance is measured in grouts that are 7 days and 28 days old, and wherever possible after 24 or 48 hours. The need to know the short-term resistance of the grout stems from the fact that the nails are subjected to tensile stresses from the time of the excavation layer following the one in which they were installed (often involving a period of less than 24 hours). Since it is not always possible to carry out short-term compression tests on site, the following diagrams (figure 1) can be consulted to help with the choice of grouts. These give data on the way the grouts' resistances develop over time depending on: The classification and type of cement. The C/E ratio. These diagrams reveal the following: The dispersion of the test results is relatively high for class 55 CPA grouts where the C/E ratio is equal to 1.5. This high performance cement is not especially recommended in situations where the grout can become diluted. Temperature also influences the way its resistance develops. The dispersion noted when testing CLK cements is explained primarily by differences in the origin of the cements (their compositions are therefore different). Where the nails are subjected to tensile stresses within 24 hours, a minimum indicative resistance of 5 MPa after 24 hours of the mix design is required. A higher resistance can be achieved using special grouts. Attention is drawn to the fact that simple compression resistance can show strong dispersions over fairly short periods of time.
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Chapter 7: Specification and Inspections
Portland cement
Portland cement
CLASS 45
CLASS 55
(MPo)
Slag cement (elK 45) CL ASS 45
(MPo)
C "[=2
0
a.. ~
w
40
40
u z e:t
~=2 E
l~
40
30
30
w
0::
z
C
'E=I,5
2
C_
["-1,5
.f..=15 E I
(f)
20
20
10
10
0
(f) (f)
w
0::
a.. ~
0
u
0
0
20
10
:y) (d)
0 0
TIME
k)
20 TIME
:y) (d)
0
0
10
;n
30 (d)
TIME
Figure 1. Simple compression resistance of grout versus time (Document Soletanche).
If the need exists, traction tests on young grouts can be used to verify proper adhesion between the grout and the reinforcement. 1.3.3.3. Test frequency
Density and viscosity measurements must be taken during conformity tests and inspection tests, as well as once per work shift. Simple compression resistance measurements can be taken at the same time as the conformity tests. However, inspection tests need not be taken systematically since the time-scales needed for the test results to become available are often incompatible with work schedules.
1.4. Concrete bars for the facing Facing bars can be made from high-adherence reinforcing bars or welded mesh. The types of steel used must meet the specifications of NFA 35-015 to NFA 35-022. Furthermore, the high-adherence reinforcing bars and welded mesh chosen must have been approved by the Inter-Ministerial Commission on the Approval and Inspection of Reinforced Bars for Reinforced Concrete (Commission interministerielle d'homologation et de contrale des armatures pour beton armee). Certification documents must be supplied before work starts.
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Soil Nailing Recommendations-1991
1.5. Concrete to be used for the facing (shotcrete or cast-in-place) 1.5.1. Shotcrete
At the moment there are two methods for placing shotcrete:
• Dry method A mixture of small aggregates and cement is transported in a flow of compressed air to the placement nozzle. Water is added as the mix exits the nozzle and is being placed.
• Wet method The fresh concrete, having first been blended with the mixing water, is pumped to the placement nozzle. This can be done either pneumatically (diluted flow), or using a concrete pump (dense flow).
1.5.1.1. Specifications relating to the concrete mix design
• Small aggregates The aggregates used for either dry or wet methods must conform with current French standards NFP 18-301 or NFP 18-302. The granulometric curves must be well-graded so that rates on the shooting will be achieved. For guidance the AFTES or AFB Recommendations are reproduced (see figure 2).
a ) Ma x i murn grain size
ep 8 mm
.. w
(%) 100
Passing
L.« Y
00 6
°
4
°
~
K y-
< ~V
k r'-.: y
,/
~
...« "-.: y'? "
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