Drilled Shaft in Rock Analysis and Design_Part1

Drilled Shafts in Rock Analysis and Design

LIANYANG ZHANG ICF Consulting, Lexington, MA, USA

A.A.BALKEMA PUBLISHERS LEIDEN/LONDON/NEW YORK/PHILADELPHIA/SINGAPORE

Library of Congress Cataloging-in-Publication Data A Catalogue record for the book is available from the Library of Congress Copyright © 2004 Taylor & Francis Group plc, London, UK All rights reserved. No part of this publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publishers. Although all care is taken to ensure the integrity and quality of this publication and the information herein, no responsibility is assumed by the publishers nor the author for any damage to property or persons as a result of operation or use of this publication and/or the information contained herein. Published by: A.A.Balkema Publishers, a member of Taylor & Francis Group plc. http://www.balkema.nl/, www.tandf.co.uk/books This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to http://www.ebookstore.tandf.co.uk/.” ISBN 0-203-02442-7 Master e-book ISBN

ISBN 90 5809 650 5 (Print Edition)

Contents Preface

vi

1 Introduction

1

2 Intact rock and rock mass

9

3 Characterization of discontinuities in rock

36

4 Deformability and strength of rock

80

5 Site investigation and rock testing

173

6 Axial load capacity of drilled shafts in rock

214

7 Axial deformation of drilled shafts in rock

258

8 Lateral load capacity of drilled shafts in rock

285

9 Lateral deformation of drilled shafts in rock

299

10 Stability of drilled shaft foundations in rock

349

11 Drilled shafts in karstic formations

359

12 Loading test of drilled shafts in rock

372

References

404

Index

423

Preface Drilled shafts are widely used to transfer heavy structural loads (both axial and lateral) through the overburden soil to the underlying rock mass. The last three decades have seen sustained research and development efforts around the world to improve the technology of drilled shafts in rock. Although much has been learned on the analysis and design of drilled shafts in rock, all the major findings are reported in the form of reports and articles published in technical journals and conference proceedings. The main purpose of this book is to assist the reader in the analysis and design of drilled shafts in rock by summarizing and presenting the latest information in one volume. The primary difference between foundations in soil and those in rock is that rock masses contain discontinuities. Compared to intact rock, jointed rock masses have increased deformability and reduced strength. Also, the existence of discontinuities in a rock mass creates anisotropy in its response to loading and unloading. To analyze and design drilled shafts in rock, a geotechnical engineer has to know the properties of rock. Chapters 2–5 are devoted to the discussion of rocks: • Chapter 2 Intact rock and rock mass • Chapter 3 Characterization of discontinuities in rock • Chapter 4 Deformability and strength of rock • Chapter 5 Site investigation and rock testing The details of analysis and design procedures for drilled shafts in rock are presented in Chapters 6–11, specifically • Chapter 6 Axial load capacity of drilled shafts in rock • Chapter 7 Axial deformation of drilled shafts in rock • Chapter 8 Lateral load capacity of drilled shafts in rock • Chapter 9 Lateral deformation of drilled shafts in rock • Chapter 10 Stability of drilled shaft foundations in rock • Chapter 11 Drilled shafts in karstic formations These chapters contain worked examples illustrating the practical application of the analysis and design methods. Load tests are a key method in the study of drilled shafts in rock. Chapter 12 describes the various techniques used in testing drilled shafts in rock, with special treatment of the subject of interpretation. The anticipated audience for this book is the design professional of drilled shafts in rock. It may also be used as a reference text for courses of geological engineering, rock mechanics and foundation engineering. Portions of Chapters 3, 4, 6, 8 and 9 are based on the author’s doctoral research conducted at the Massachusetts Institute of Technology. The author acknowledges the support and advice given by Professor Herbert Einstein.

Mr. Ralph Grismala of ICF Consulting, on the author’s request, found time to look through the manuscript and made suggestions for improvement. His suggestions as an experienced geotechnical engineer have been particularly valuable and the author is grateful to him. Finally, the author wants to thank Dr. Francisco Silva of ICF Consulting for his support during the preparation of this book. Lianyang Zhang Lexington, MA, USA

1 Introduction 1.1 DEFINITION OF DRILLED SHAFTS A drilled shaft is a deep foundation that is constructed by placing concrete in an excavated hole. Reinforcing steel can be installed in the excavation, if desired, prior to placing the concrete. A schematic example of a typical drilled shaft socketed into rock is shown in Figure 1.1. The drilled shaft can carry both axial and lateral loads. Drilled shafts are sometimes called bored piles, piers, drilled piers, caissons, drilled caissons, or cast-inplace piles. To increase the bearing capacity, drilled shafts are commonly socketed into rock. The portion of the shaft drilled into rock is referred to as a rock socket. In many cases where there is no overburden soil, drilled shafts entirely embedded in rock are also used. Because of the high bearing capacity of rock sockets, the analysis and design of them are extremely important. This book discusses both rock sockets and drilled shafts entirely embedded in rock.

1.2 HISTORICAL DEVELOPMENT OF DRILLED SHAFTS The ancient “well foundation” can be considered the earliest version of drilled shafts. Such foundations were stone masonry pedestals built in hand-excavated holes, long before hydraulic cements came into common use. During the late nineteenth and early twentieth centuries when taller and heavier buildings began to appear, high-capacity foundations became necessary in large cities such as Chicago, Cleveland and Detroit. These cities are underlain by relatively thick deposits of medium to soft clays overlying deep glacial till or bedrock. Because traditional spread foot foundations settled excessively under the heavier building load, engineers began to use shafts such as the hand-dug “Chicago” and “Gow” caissons. The shafts were constructed by making the excavation and by placing sections of permanent liners (wooden lagging or steel rings) to retain the soil by hand. Hand excavation methods were slow and tedious, so machine-drilled shafts soon superseded the hand-dug caissons. A few examples of horse and engine-driven augers appeared between 1900 and 1930, but they had limited capabilities. By the late 1920s, manufacturers were building practical truck-mounted engine-driven augers, thus bringing drilled shaft construction into its maturity.

Drilled shafts in rock

2

Fig. 1.1 A drilled shaft socketed into rock. During the next three decades, manufacturers and contractors developed larger and more powerful drilling equipment, which allowed more economical and faster construction of drilled shafts. In the late 1940s and early 1950s, drilling contractors introduced techniques for drilling in rock. By introducing casing and drilling mud into boreholes, a process long established by the oil industry, boreholes could be drilled through difficult soils economically. By the 1960s, drilled shafts had become a strong competitor to driven piles. In the past decade, the use of drilled shafts has increased dramatically. In 1997, the value of drilled shaft construction in the United States reached more than one billion US dollars (O’Neill, 1998). Today, drilled shafts support different structures including one-

Introduction

3

story wood frame buildings to the largest skyscrapers, highway bridges, and retaining structures.

1.3 USE OF DRILLED SHAFTS Compared to other types of deep foundations, drilled shafts have the following major advantages: 1. The costs of mobilizing and demobilizing a drill rig are much less than those for a pile driver. 2. The construction process generates less vibration and noise, making drilled shafts appropriate for urban construction. 3. The quality of the bearing material can be inspected visually and tests can be run to determine its physical properties. For end-bearing designs, the soil/rock beneath the base can be probed for cavities or weak layers if desirable. 4. The diameter or length of the drilled shaft can be easily changed during construction to compensate for unanticipated soil/rock conditions. 5. The drilled shafts can penetrate through soils with cobbles or boulders. They can also be socketed into rock. 6. It is usually possible to support very large loads with one large drilled shaft instead of several piles, thus eliminating the need for a pile cap. 7. Large-diameter drilled shafts are particularly well-suited as foundations for structures that must resist extreme events that produce large lateral loads (e.g. earthquake and vessel impact loading) because of the very large moments of inertia. Drilled shafts also have the following major disadvantages: 1. The quality and performance of drilled shafts is very dependent on the contractor’s skills. Poor workmanship can produce weak foundations that may not be able to support the design load. 2. Since shaft construction removes soil/rock from the ground, it may decrease the competency of the bearing stratum. 3. The construction of drilled shafts through contaminated soils/rocks is problematic because of the expenses associated with disposing of the spoil. Because of the above advantages, drilled shafts have become an appropriate and economical foundation system for heavily loaded structures. When deep foundations are required, drilled shafts should always be considered as an option. An application example of drilled shafts in rock O’Neill and Reese (1999) presented an application example of drilled shafts in rock. It clearly shows the advantages of drilled shafts over pile-footings for the foundations of the interior bents of a river bridge in the United States. The bridge is a two-lane bridge with four spans. Siltstone near the surface at one end dips to a depth of about 6.1 m (20 ft) near the other end of the bridge. Mixed fine sediments exist above the siltstone. Two alternate foundation designs were considered by the design agency before the project bid. The first one called for the construction of one

Drilled shafts in rock

4

spread footing and two capped groups of steel H-piles for the three interior bents that were required to be placed in the river. Both the spread footing and driven piles (with pile caps) were to be constructed within cofferdams because of the need to construct footings/caps. The second one called for the replacement of the spread footing and driven pile groups by three large-diameter drilled shafts. The drilled shafts could be drilled during low water using a crane-mounted drill rig positioned on timber mats within the river and pouring the concrete for the shafts to an elevation above the water level, eliminating the need for cofferdams. Comparison of the pile-footing alternate with the drilled shaft alternate is shown in Table 1.1. The cost savings realized by using drilled shafts were $422,000 (50%).

Table 1.1 Comparison of the pile-footing alternate with the drilled shaft alternate—Queens River Bridge, Olympic Peninsula, Washington, USA (after O’Neill & Reese, 1999). Details

Pile-Footings

Drilled Shafts

25 capped H-piles driven into the soft siltstone for each of the two interior bents and a spread-footing at the other interior bent. All pile driving, cap construction and spread footing construction were within cofferdams. A single-bent column was formed on top of the spread footing or the pile cap prior to removal of the cofferdams. The construction of work trestle was required so that cofferdams could be constructed prior to installing the foundations. Because of the length of time required to construct the trestle and cofferdams, construction of pile groups, caps and footing could not proceed until the following working season, since operations in the river had to be suspended during the salmon runs.

Three 3.2 m (10.5 ft) diameter drilled shafts socketed about 10m (30 ft) into the siltstone, with casing extending from the top of the siltstone to high water level. The casing was used as a form, and the drilled shaft concrete was poured directly up to the top of the casing. The single columns for the bents were formed on top of the extended sections of drilled shafts, with no requirement to construct cofferdams.

Estimated $842,000 Cost

$420,000

1.4 CHARACTERISTICS OF DRILLED SHAFTS IN ROCK The characteristics of drilled shafts in rock are closely related to the special properties of rock masses. The following briefly describes some of the special rock mass properties that will affect the performance of drilled shafts.

Introduction

5

1.4.1 Effect of discontinuities The primary difference between drilled shafts in rock and those in soil is that rock masses contain discontinuities. The intact rock may have a high strength but the presence of discontinuities in the rock may result in very low strength of the rock mass. Wedges or blocks formed by sets of unfavorably orientated discontinuities may fail by sliding or toppling, causing excessive movement or failure of drilled shaft foundations. Figure 1.2 shows the drilled shaft foundations of a bridge across a river. The rock at this site consists of two sets of discontinuities with about the same dip angles; but set A is discontinuous and more widely spaced than set B. At the East side, the drilled shaft foundation would be stable because the discontinuities approximately parallel to the rock slope face are not continuous. In contrast, at the West side, the discontinuities approximately parallel to and dipping out of the slope face are continuous and movement of the entire foundation along these discontinuities is possible.

Fig. 1.2 Effect of discontinuities on the stability of drilled shafts. 1.4.2 Effect of groundwater Groundwater may affect the performance of drilled shafts in the following ways: 1. The most obvious is through the operation of the effective stress law. Water under pressure in the discontinuities defining rock blocks reduces the normal effective stress between the rock surfaces, and thus reduces the potential shear resistance which can be mobilized by friction. In porous rocks, such as sandstones, the effective stress law is obeyed as in granular soils. In both cases, the effect of fissure or pore water pressure

Drilled shafts in rock

6

is to reduce the ultimate strength of the rock mass, and thus decrease the bearing capacity of the drilled shaft foundation. 2. Groundwater affects rock mechanical properties due to the deleterious action of water on particular rocks and minerals. For example, clay seams may soften in the presence of groundwater, reducing the strength and increasing the deformability of the rock mass. Argillaceous rocks, such as shales and argillitic sandstones, also demonstrate marked reductions in material strength following infusion with water. According to Hoek and Brown (1997), strength losses of 30–100% may occur in many rocks as a result of chemical deterioration of the cement or clay binder. 3. Groundwater flow into the excavation of a drilled shaft can make cleaning and inspection of bearing surfaces difficult and result in decreased bearing capacity for the drilled shaft. 1.4.3 Effect of karstic formations A number of problems may arise when drilled shafts are built in karstic formations (Brown, 1990; Goodman, 1993; Sowers, 1996): 1. An existing cavity may underlie the base of the drilled shaft and collapse when the building is under construction or in service. The collapse of the cavity may be caused by excessive construction loading or erosion by acid groundwater [Fig. 1.3(a)]. 2. The tip of the shaft may slide along a steeply inclined rock instead of penetrating into bedrock, especially when part of the tip is located on top of a pinnacle with existing joints or cracks [Fig. 1.3(b)]. 3. The drilled shaft is placed on a cantilever rock over cavities or soft clay, so that excessive loads or continuing water erosion may cause rock collapse [Fig. 1.3(c)]. 4. A shifting rock slab or rock block floating in the residual soil may lead people to mistakenly believe that the bedrock has been reached and the bearing stratum has been located (d)]. Chapter 11 will discuss the performance of drilled shafts in karstic formations in more detail.

1.5 CONSIDERATIONS IN THE DESIGN OF DRILLED SHAFTS As for the design of any foundations, the design of drilled shafts must satisfy criteria related to strength, deformation and durability. For the strength, criteria are applied to both the structural strength of the shaft itself and the geotechnical strength, i.e., the load carrying capacity of the soil/rock. The structural and geotechnical strength criteria depend on the basis of the design method. The traditional working stress design method, sometimes referred to as the allowable stress design (ASD) method, relies on an overall safety factor against ultimate failure and the corresponding design criteria can be expressed as (1.1)

Introduction

7

where Qu is the ultimate load bearing capacity; FS is the global factor of safety; and Q is the allowable working load or the allowable design load. Equation (1.1) applies to both axial and lateral loadings. Typical factors of safety for the geotechnical strength of drilled shafts range between two and three, depending on the method of capacity calculation, the extent of the designer’s experience and knowledge of the site and the geotechnical conditions, and the likely consequences of failure. In cases where there is extensive experience of the site and field shaft load tests have been carried out, values of safety factor as low as 1.5 may be appropriate. On the other hand, where knowledge of the site is limited, and the consequences of failure may be extreme, safety factors of three or higher may be appropriate.

Fig. 1.3 Failure modes of drilled shafts in karstic formations (after Tang, 1995).

Drilled shafts in rock

8

In recent years, there has been an increasing tendency to use load and resistance factor design (LRFD) for drilled shafts and other structural components (AASHTO, 1994; FHWA, 1996a). With this method, various factors, with values of 1 or above, are applied to the individual components of load. Other factors, with values of 1 or less, are applied to the total resistance, or individual components of resistance, in such a way to assure a margin of safety consistent with historical practice using global factors of safety. The design criterion for the LRFD approach can be written as (1.2) where η is factor varying from 0.95 to 1.05 to reflect ductility, redundancy and operational importance of the structure; γi is the load factor for load type i; Qi is the nominal value of load type i; is the resistance factor for resistance component j; Quj is the estimated (nominal) value of ultimate resistance component j. Equation (1.2) applies to both axial and lateral loadings, and to structural and geotechnical strengths. The LRFD approach to foundation design has the advantages that (a) foundations are easier to design if the superstructure is designed using LRFD and (b) it offers a means to incorporate reliability into the design process in a rational manner. For the serviceability limit state, the design criteria for deformations may be stated generally as: Estimated deformation≤Allowable deformation (1.3) Estimated differential deformation≤Allowable differential deformation (1.4) Equations (1.3) and (1.4) apply to both axial and lateral deformations. The allowable deformations and differential deformations depend primarily on the nature of the structure. Grant et al. (1982) and Moulton et al. (1985) listed typical values of allowable deformations and differential deformations for different structures. For the durability, the usual design criterion is the drilled shafts shall have a design life that exceeds the design life of the structure to be supported; this is usually 50 years or more for permanent structures. In recent years, the influence of environmental factors on the design and construction of drilled shaft foundations has become more and more important. Requirements that impact the excavation, handling, and disposal of river bottom sediments are continually more restrictive. Consequently, design and construction techniques are being developed and modified to lessen the need for excavation.

2 Intact rock and rock mass 2.1 INTRODUCTION Rock differs from most other engineering materials in that it contains discontinuities of one type or another which render its structure discontinuous. Thus a clear distinction must be made between the intact rock or rock material on the one hand and the rock mass on the other. The intact rock may be considered as a continuum or polycrystalline solid between discontinuities consisting of an aggregate of minerals or grains. The rock mass is the in situ medium comprised of intact rock blocks separated by discontinuities such as joints, bedding planes, folds, sheared zones and faults. The properties of the intact rock are governed by the physical properties of the materials of which it is composed and the manner in which they are bonded to each other. The parameters which may be used in a description of intact rock include petrological name, color, texture, grain size, minor lithological characteristics, degree of weathering or alternation, density, porosity, strength, hardness and deformability. Rock masses are discontinuous and often have heterogeneous and anisotropic properties. Since the behavior of a rock mass is, to a large extent, determined by the type, spacing, orientation and characteristics of the discontinuities present, the parameters used to describe a rock mass include the nature and geometry of discontinuities as well as its overall strength and deformability. This chapter describes the types and important properties of intact rocks and different rock mass classification systems that will be useful in the analysis and design of drilled shafts in rock. Chapter 3 will discuss the characterization of discontinuities in rock masses.

2.2 INTACT ROCK Intact rocks may be classified from a geological or an engineering point of view. In the first case the mineral content of the rock is of prime importance, as is its texture and any change which has occurred since its formation. Although geological classifications of intact rocks usually have a genetic basis, they may provide little information relating to the engineering behavior of the rocks concerned since intact rocks of the same geological category may show a large scatter in strength and deformability, say of the order of 10 times. Therefore, engineering classifications of intact rocks are more related to the analysis and design of foundations in rock.

Drilled shafts in rock

10

2.2.1 Geological classification (a) Rock-forming minerals Rocks are composed of minerals, which are formed by the combination of naturally occurring elements. Although there are hundreds of recognized minerals, only a few are common. Table 2.1 summarizes the common rock-forming minerals and their properties. Moh’s scale, used in the table, is a standard of ten minerals by which the hardness of a mineral may be determined. Hardness is defined as the ability of a mineral to scratch another. The scale is one for the softest mineral (talc) and ten for the hardest (diamond).

Table 2.1 Common rock-forming minerals and their properties. Mineral

Hardness (Moh’s scale, 1–10)

Relative Density

Fracture

Structure

Orthoclase feldspar

6

2.6

Good cleavage at right angles

Monoclinic. Commonly occurs as crystals

Plagioclase feldspar

6

2.7

Cleavage nearly at right angles—very marked

Triclinic. Showing distinct cleavage lamellae

Quartz

7

2.65

No cleavage. Choncoidal fracture

hexagonal

Muscovite

2.5

2.8

Perfect single cleavage Monoclinic. Exhibiting into thin easily separated strong cleavage plates lamellae

Biotite

3

3

Perfect single cleavage Monoclinic. Exhibiting into thin easily separated strong cleavage plates lamellae

Hornblende

5–6

3.05

Good cleavage at 120°

Hexagonal—normally in elongated prisms

Augite

5–6

3.05

Cleavage nearly at right angles

Monoclinic

Olivine

6–7

3.5

No cleavage

No distinctive structure

Calcite

3

2.7

Three perfect cleavages. Hexagonal Rhomboids formed

Dolomite

4

2.8

Three perfect cleavages

Hexagonal

Kaolinite

1

2.6

No cleavage

No distinctive structure (altered feldspar)

Hematite

6

5

No cleavage

Hexagonal

Intact rock and rock mass

11

(b) Elementary rock classification Intact rocks are classified into three main groups according to the process by which they are formed: igneous, sedimentary and metamorphic. Igneous rocks are formed by crystallization of molten magma. The mode of crystallization of the magma, at depth in the Earth’s crust or by extrusion, and the rate of cooling affect the rock texture or crystal size. The igneous rocks are subdivided into plutonic, hypabyssal and extrusive (volcanic), according to their texture. They are further subdivided into acid, intermediate, basic and ultrabasic, according to their silica content. Table 2.2 shows a schematic classification of igneous rocks. Sedimentary rocks are formed from the consolidation of sediments. Sedimentary rocks cover three-quarters of the continental areas and most of the sea floor. In the process of erosion, rocks weather and are broken down into small particles or totally dissolved. These detritic particles may be carried away by water, wind or glaciers, and deposited far from their original position. When these sediments start to form thick deposits, they consolidate under their own weight and eventually turn into solid rock through chemical or biochemical precipitation or organic process. As a result of this process, sedimentary rocks almost invariably possess a distinct stratified, or bedded, structure. Table 2.3 shows the classification of sedimentary rocks. Metamorphic rocks are derived from pre-existing rocks by temperature, pressure and/or chemical changes. Table 2.4 shows a classification of the metamorphic rocks according to their physical structure, i.e., massive or foliated. (c) Weathering of rock Weathering is the disintegration and decomposition of the in situ rock, which is generally depth controlled, that is, the degree of weathering decreases with increasing depth below the surface. The engineering properties of a rock as discussed in next section can be, and often are, altered to varying degrees by weathering of the rock material. Intact rocks can be divided into 5 groups according to the degree of weathering (see Table 2.5).

Table 2.2 Geological classification of igneous rocks. Type Grain size

Acid >65% silica

Intermediate 55– 65% silica

Basic 45–55% silica

Ultrabasic 10

Fresh basalt, chert, diabase, gneiss, granite, quartzite

1)

Point load tests on rocks with unconfined compressive strength below 25 MPa are likely to yield highly ambiguous results.

2.2.2 Engineering classification The engineering classification of intact rocks is based on strength and/or deformation properties of the rock. Table 2.6 shows the classification system of the International Society of Rock Mechanics (ISRM, 1978). The ISRM classification is also recommended in the Canadian Foundation Engineering Manual (CGS, 1985). In this classification, the rock may range from extremely weak to extremely strong depending on the unconfined compressive strength or the approximate field identification. Based on laboratory measurements of strength and deformation properties of rocks, Deere and Miller (1966) established a classification system based on the ultimate strength (unconfined compressive strength) and the tangent modulus Et of elasticity at 50% of the ultimate strength. Figure 2.1 summarizes the engineering classification of igneous, sedimentary and metamorphic rocks, respectively, as given in Deere and Miller (1966). The modulus ratio in these figures is that of the elastic modulus to the unconfined compressive strength. A rock may be classified as AM, BH, BL, etc. Voight (1968), however, argued that the elastic properties of intact rock could be omitted from practical classification since the elastic moduli as determined in the laboratory are seldom those required for engineering analysis.

Intact rock and rock mass

15

2.2.3 Typical values of intact rock properties This section lists the typical values of intact rock properties, including density (Table 2.7), unconfined compressive strength (Table 2.8), elastic modulus (Table 2.9) and Poisson’s ratio (Table 2.10). These values are listed only for reference and should not be used directly in design.

2.3 ROCK MASS Numerous rock mass classification systems have been developed, including Terzaghi’s Rock Load Height Classification (Terzaghi, 1946); Lauffer’s Classification (Lauffer, 1958); Deere’s Rock Quality Designation (RQD) (Deere, 1964); RSR Concept (Wickham et al., 1972); the Rock Mass Rating (RMR) system (Bieniawski, 1973, 1976, 1989); the Q-System (Barton et al., 1974), and the Geological Strength Index (GSI) system (Hoek & Brown, 1997). Most of the above systems were primarily developed for the design of underground excavations. However, four of the above classification systems have been used extensively in correlation with parameters applicable to the design of rock foundations. These four classification systems are the Rock Quality Designation (RQD), the Rock Mass Rating (RMR), the Q-System, and the Geological Strength Index (GSI). 2.3.1 Rock quality designation (RQD) Rock Quality Designation (RQD) was introduced by Deere (1964) as an index assessing rock quality quantitatively. The RQD is defined as the ratio (in percent) of the total length of sound core pieces 4 in. (10.16 cm) in length or longer to the length of the core run. RQD is perhaps the most commonly used method for characterizing the jointing in borehole cores, although this parameter may also implicitly include other rock mass features such as weathering and core loss. Deere (1964) proposed the relationship between the RQD index and the rock mass quality as shown in Table 2.11. (a) Direct method for determining RQD For RQD determination, the International Society for Rock Mechanics (ISRM) recommends a core size of at least NX (size 54.7 mm) drilled with double-tube core barrel using a diamond bit. Artificial fractures can be identified by close fitting of cores and unstained surfaces. All the artificial fractures should be ignored while counting the core length for RQD. A slow rate of drilling will also give better RQD. The correct procedure for measuring RQD is shown in Figure 2.2.

Drilled shafts in rock

16

Fig. 2.1 Engineering classification of intact rocks (Et is the tangent modulus at 50% ultimate strength) (after Deere & Miller, 1966).

Intact rock and rock mass

17

Table 2.7 Typical values of density of intact rocks (after Lama & Vutukuri, 1978). Range of density (kg/m3)

Mean density (kg/m3)

Granite

2516–2809

2667

Granodiorite

2668–2785

2716

Syenite

2630–2899

2757

Quartz diorite

2680–2960

2806

Diorite

2721–2960

2839

Norite

2720–3020

2984

Gabbro

2850–3120

2976

Diabase

2804–110

2965

Peridotite

3152–3276

3234

Dunite

3204–3314

3277

Pyroxenite

3100–3318

3231

Anorthosite

2640–2920

2734

Sandstone

2170–2700

–

Limestone

2370–750

–

Dolomite

2750–2800

–

Chalk

2230

–

Marble

2750

–

Shale

2060–2660

–

Sand

1920–1930

–

Gneiss

2590–3060

2703

Schist

2700–3030

2790

Slate

2720–840

2810

Amphibolite

2790–3140

2990

Granulite

2630–3100

2830

Eclogite

3338–3452

3392

Rock type Igneous rocks

Sedimentary rocks

Metamorphic rocks

Note: The values listed in the table are for the bulk density determined at natural water content.

Drilled shafts in rock

18

Table 2.8 Typical range of unconfined compressive strength of intact rocks (AASHTO, 1989). Rock category

General description

Rock

Unconfined compressive strength, σc(1) (MPa)

A

Carbonate rocks with welldeveloped crystal cleavage

Dolostone Limestone Carbonatite Marble Tactite-Skarn

33–310 24–290 38–69 38–241 131–338

B

Lithified argillaceous rock

Argillite Claystone Marlstone Phyllite Siltstone Shale(2) Slate

29–145 1–8 52–193 24–241 10–117 7–35 145–207

C

Arenaceous rocks with strong crystals and poor cleavage

Conglomerate Sandstone Quartzite

33–221 67–172 62–379

D

Fine-grained igneous crystalline rock

Andesite Diabase

97–179 21–572

E

Coarse-grained igneous and metamorphic crystalline rock

Amphibolite Gabbro Gneiss Granite Quartz diorite Quartz monozonite Schist Syenite

117–276 124–310 24–310 14–338 10–97 131–159 10–145 179–427

(1)

Range of unconfined compressive strength reported by various investigations. Not including oil shale.

(2)

Table 2.9 Typical values of elastic modulus of intact rocks (AASHTO, 1989). Rock type

No. of values

Elastic modulus (GPa)

No. of rock types

Maximum Minimum Mean

Standard Deviation

Granite

26

26

100

6.41

52.7

24.5

Diorite

3

3

112

17.1

51.4

42.7

Gabbro

3

3

84.1

67.6

75.8

6.69

Intact rock and rock mass

Diabase

19

7

7

104

69.0

88.3

12.3

12

12

84.1

29.0

56.1

17.9

7

7

88.3

36.5

66.1

16.0

Marble

14

13

73.8

4.00

42.6

17.2

Gneiss

13

13

82.1

28.5

61.1

15.9

Slate

11

2

26.1

2.41

9.58

6.62

Schist

13

12

69.0

5.93

34.3

21.9

3

3

17.3

8.62

11.8

3.93

27

19

39.2

0.62

14.7

8.21

5

5

32.8

2.62

16.5

11.4

Shale

30

14

38.6

0.007

9.79

10.0

Limestone

30

30

89.6

4.48

39.3

25.7

Dolostone

17

16

78.6

5.72

29.1

23.7

Basalt Quartzite

Phyllite Sandstone Siltstone

Table 2.10 Typical values of Poisson’s ratio of intact rocks (AASHTO, 1989). No. of values

Poisson’s ratio

No. of rock types

Rock type

Standard Deviation

Maximum Minimum Mean

Granite

22

22

0.39

0.09

0.20

0.08

Gabbro

3

3

0.20

0.16

0.18

0.02

Diabase

6

6

0.38

0.20

0.29

0.06

11

11

0.32

0.16

0.23

0.05

Quartzite

6

6

0.22

0.08

0.14

0.05

Marble

5

5

0.40

0.17

0.28

0.08

Gneiss

11

11

0.40

0.09

0.22

0.09

Schist

12

11

0.31

0.02

0.12

0.08

Sandstone

12

9

0.46

0.08

0.20

0.11

Siltstone

3

3

0.23

0.09

0.18

0.06

Shale

3

3

0.18

0.03

0.09

0.06

Basalt

Drilled shafts in rock

20

Table 2.11 Correlation between RQD and rock mass quality. RQD (%)

Rock Mass Quality

10

4–10

2–4

1–2

Unconfined compressive strength (MPa)

>250

100–250

50–100

25–50

5– 25

1–