Advanced Foundation Design Module
December 23, 2016 | Author: Yeoh Gim Heng | Category: N/A
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LECTURE MODULE
COURSE FASCILITATOR: ASSOC. PROF IR. DR. RAMLI NAZIR PROF. DR. KHAIRUL ANUAR KASSIM
SECTION A
COURSE FASCILITATOR: ASSOC. PROF IR. DR. RAMLI NAZIR
LECTURE 1
FOUNDATION ENG. DESIGN PRINCIPLES
By ASSOC. PROF. Ir. DR. HJ. RAMLI NAZIR UNIVERSITI TEKNOLOGI MALAYSIA
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There is no glory as a Geotechnical Engineer - Terzaghi
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GEOTECHNICAL BRAIN FUNCTION
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A PROFESSIONAL COMPARISON
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What is Value Engineering in Foundation Design???
Challenge The Norm Thru Innovation To Excel
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VALUE ENGINEERING. FOUNDATION ENG. DESIGN PRINCIPLES
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Stage of Design •
Normally there are 3 stages of design i.e
1.
PRE DESIGN STAGE
2.
CONSTRUCTION STAGE
3.
POST DESIGN STAGE
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PRE DESIGN STAGE • Accurate and reliable SI data is vital. • Type of foundation use for the structure is based from the above. • An overall aspect and anticipation during construction has to be considered especially practical and economics consideration. • During this stage, loading, foundation arrangement and location, bearing capacity and other related practice has been identified. • Anticipation of the problem in foundation construction work should be recognised and overcoming the problem should be readily available.
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DESIGN ANALYSES • Which one to use??? • TOTAL STRESS ANALYSIS Or • EFFECTIVE STRESS ANALYSIS
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TOTAL STRESS ANALYSES • This type of analysis uses the undrained shear strength of the cohesive soil and also known as short term analysis. • The undrained shear strength, cu can be obtained from field such as vane shear and laboratory such as unconfined compression test. If the undrained shear strength is constant throughout the depth then cu = c and =0o. The use of unconsolidated undrained triaxial compression test is also applicable provided that it is saturated plastic soil. • The groundwater does not have an effect in the use of total stress parameters.
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EFFECTIVE STRESS ANALYSIS • This type of analysis uses the drained shear strength, c’ and ’ of the plastic soil. • The drained shear strength could be obtained from triaxial compression test with pore pressure measurement tested on a fully saturated specimen of the plastic soil. • Also known as long term analysis since the shear-induced pore water pressure (positive or negative) from the loading has dissipated and the hydrostatic pore pressure conditions now prevail in the field. • Thus the location of the water table is significant in considering in the analysis.
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GENESIS OF FOUNDATION DESIGN
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PRINCIPLE IN GEOTECHNICAL ENGINEERING DESIGN • ENGINEERING PROPERTIES • CHEMICAL PROPERTIES
SI
SOIL PROPERTIES INTERPRETATION JUDGEMENT
GROUND CHARACTERIZATION MODELLING PREDICTION CODE OF PRACTICES:• FOUNDATION BS 8004 •ANCHORS BS8081 •EARTHWORKS BS6031 •REINFORCED FILLS BS8006 •GEOGUIDES
GROUND BEHAVIOUR DEFORMATION DISPLACEMENT STABILITY
ENGINEERING PERFORMANCE
• BASIC & INDEX PROPERTIES
• MASS PROPERTIES • TYPICAL & GENERALISED SUBSOIL PROFILE & PROPERTIES OF TYPICAL GEOLOGICAL FORMATIONS, MAN MADE FILL etc.. • ENGINEERING GEOLOGY
SOIL & ROCK MECHANICS • EFFECTIVE STRESS THEORY • SEEPAGE THEORY • STRESS DISTRIBUTION • LATERAL PRESSURE •BEARING CAPACITY • COMPRESSIBILITY
INSTRUMENTATION FOR • PORE WATER PRESSURE • EARTH PRESSURE • DISPLACEMENT(SURFACE & SUBSURFACE • INTERNAL STRESSES
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THE IMPORTANCE OF SI • To study the general suitability of the site for an engineering project. (FEED Program)- FRONTIER EVALUATION ENGINEERING DEVELOPMENT. • To enable a safe, practical and economic design to be prepared. • To determine the possible difficulties that may be encountered by a specific construction method. • To study the suitability of construction material (soil or rock).
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Cont… • SI nowdays has become contracting exercise and we tend to forget that SI is an INVESTIGATION. • As in many INVESTIGATION it is an itterative process. • For information to be reliable, adhere to the procedure is very important. • SI is the most procedure oriented operation within Civil Engineering Discipline. • This is due to the variability of the soil formation millions of years ago • The properties of oil assessment or test carried out is affected by the latter. • Accuracy and correct procedure is of vital important.
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The Facts Why SI is needed • This is a part of geotechnical processes. • Lack of geotechnical processes will lead to a:• •
Failures where many case histories are available. Significant delay and increase in construction costs when the design has to be revised or ammended.
• Generally the elimination of the SI will not safe the cost of the project thus it only comprises from only 0.1% to 5% of the project cost. • In fact most frequent claims in civil engineering contracts are on the basis of inadequate SI or obstructions resulting in extra costs which could not reasonably have been forseen by an experience contractor. FOUNDATION ENG. DESIGN PRINCIPLES
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YOU HAVE TO PAY FOR THE S.I WHETHER YOU LIKE IT OR NOT!!
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Method of Site Investigation • • • • • • • • • • •
JKR PROBE/MACKINTOSH PROBE HAND AUGERING (HA) MOTORISED HAND BORING (MHB) DEEP BORING (DB) TRIAL PITS AND PLATE BEARING TEST DEEP SOUNDING (DS) INSITU VANE SHEAR TEST (IVST) STANDARD PENETRATION TEST (SPT) PRESSUREMETER TEST GROUND WATER INVESTIGATION ROCK CORING
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HAND AUGERING
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ROTARY WASH BORING
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BEARING PLATE
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CONE PENETRATION TEST/ DEEP SOUNDING
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IN SITU VANE SHEAR TEST
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STANDARD PENETRATION TEST (SPT)
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STANDARD PENETRATION TEST (SPT) • This a dynamic field test usually carried out in boreholes. • Test consists of driving a standard split barrel sampler 50.8mm in diameter. • The SPT is read from a 65kg drop hammer fall at a vertical height of 75cm. • The sampler is driven to a total of 45cm into the soil and the number of blows recorded for the last 30cm of penetration (SPT, N-value)
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Numbers of BH, POSITION and Depth
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STANDARD PENETRATION TEST VALUE FOR DESIGN
• • •
Developed in 1927 and currently the most popular method and economical means to obtain subsurface information. Currently 85% - 90% of usage in conventional foundation design. Test consist of :• • •
•
Driving the split barrel sample at a distance of 460mm into the soil at the bottom of boring. Counting the number of blows to drive sample at last two 150mm distances to obtain N value Using 63.5kg driving mass falling free from a height of 760mm.
The boring log shows refusal and the test is halted if:• • •
50 blows are required for any 150mm increment 100 blows are obtained to drive the required 300mm 10 successive blows produce no advance.
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• When full test depth cannot be obtained, boring log will show a ratio as 70/100 or 50/100 indicating that 70 or 50 blows resulted in a penetration of 100mm. • The blow count is directly related to the driving energy:• Substituting Both Equations : W= weight of mass or hammer H = height of fall
m = v= 2 2gh = Wh
For standard test:‐ E = 63.5 x 9.81 x 0.762 = 474.5 ~ 475 kJ
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• •
Kovac and Salomone ( 1982) found that the actual energy impact to the sampler range about 30% to 80% while Riggs (1983) obtained energy input from 70% to 100% The discrepancies arises from:• • • • •
•
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Equipment from different manufacturers Driving hammer configuration Usage of liner inside the barrel Overburden pressure Length of drill rod
Therefore SPT can be standardised to some energy ratio Er such that:-
Er= (Actual hammer energy to sampler (Ea)/ Input Energy (E)) x 100
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• Energy input of 70% is normally use since observation is close to the actual energy ratio (Er) • Therefore the standard blow count N’70 is measure from N as follows: N’70 = CN x N x x x x Where i = adjustment factor from table N’70 = Adjusted N CN = Adjustment for effective overburden pressure
CN FOUNDATION ENG. DESIGN PRINCIPLES
95.76 po '
po in kPa
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• Note that larger Er decrease the blow count nearly linearly i.e Er45 gives N=20 Er 1xN1 Er 2 xN2 Er 1 Er90 gives N = 10 N2 xN1 Er 2 With Er70 gives N = 13 • Energy ratio x blow count should be constant thus :Say Er1 = 70 thus gives N2 = (70/Er2)xN1 Say N2 for Er45 = 20 = Er2 We obtain N1 = 13 If we convert N70 to N60 than N2 = N60 = (70/60)x13 = 15 • Using the equation we can readily convert any energy ratio to any other base. FOUNDATION ENG. DESIGN PRINCIPLES
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SPT CORRELATIONS • It can be used in correlation for unit weight relative density, Dr, angle of internal friction angle , undrained compressive strength, qu, bearing capacity and stress-strain modulus. • Angle of internal friction:Base from Japanese Railway Standard: 4.5N 70 20 • Relative Density N'70 32 0.288p' o Base from Meyerhof(1957) : 2 Dr where p’o is in kPa • For OCR > 1 Skempton suggest the following adjustment has been made:N '70 A BC OCR p' o 2 Dr FOUNDATION ENG. DESIGN PRINCIPLES
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Where A range between 15 to 54 B range between 0.306 to 0.204 And
COCR
p ' onc p ' oOCR
For COCR=1 it relates to normally consolidated clay o
o
Thus Meyerhof estimate:- 28 15 Dr • A correlation for N versus qu in general form of:qu = kN Where k tend to be site dependant. However k = 12 has been used i.e for N’70 = 10, qu = 120kPa FOUNDATION ENG. DESIGN PRINCIPLES
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DESIGN N-values
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Relationship between Angle of Internal Friction and N-Value (Sandy Soil)
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SPT (Standard Penetration Test) SPT
Hammer Type
N-SPT = Total No. of Blows for spoon sampler to penetrate at a depth of 30cm c (t/m2) = 2/3 N FOUNDATION ENG. DESIGN PRINCIPLES
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Relationship between Cohesion and N-Value (Cohesive soil)
2/3 N
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PRESSUREMETER TEST
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ROCK CORING • To determine the soundness of rock. •
Sound rock : Rock which ring when struck with a pick or bar. Does not integrate after exposure to air or water, breaks with a sharp, fresh fracture, in which cracks are unweathered and less than 3mm wide and generally not closer than 1m apart. Core recovery is normally 85%.
•
Medium rock : Characteristic as for sound rock but the cracks maybe 6mm wide and slightly weathered, generally no closer than 60cm. Core recovery is 50% or more.
•
Intermediate rock : Give dull sound when hit by pick or bar. Does not integrate after exposure to air or water. Broken pieces may show weathered faces. Fractures up to 25mm wide and space no closer than 30cm. Core recovery generally is 35% or greater.
•
Soft rock : Any rock which flakes on exposure to air or water. Give a very dull sound when struck with pick or bar. Core recovery generally is less than 35% or greater but SPT more than 50.
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Strength of Rock Materials Term
Uniaxial Compressive Strength (MN/m2)
Very Weak
< 1.25
Weak
1.25 – 5.0
Moderately Weak
5.0 – 12.5
Moderately Strong
12.5 – 50.0
Strong
50 - 100
Very Strong
100 - 200
Extremely strong
> 200
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Depending on moisture , anisotrophy and test procedure
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SOIL SAMPLING TECHNIQUE • 2 TYPES OF SAMPLE :•
Undisturbed : To determine properties such as strength parameters, consolidation, permeability and parameters which need to observed as per site condition.
•
Disturbed : Do determine physical properties such as grain size, colour, texture, compaction properties, remoulded properties and for testing etc.
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FIELD IDENTIFICATION AND DESCRIPTION OF SOIL • Soil descriptions are made from washed and disturbed samples recovered from the boreholes. • The soil name is based on particle size distribution and plasticity, which can be readily estimated and measured at the laboratory.
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• According to BS 5930, soil samples are described with each element of the descriptions having a fixed position within the overall description:• • • • • • •
a) b) c) d) e) f) g)
Consistency (cohesive) or RD (non cohesive) Fabric and Fissuring, if distinguishable Colour Subsidiary constituent Angularity or grading of principal soil type (for coarse grained soil) Principal soil type (in capital letter) More detailed comments on constituents or fabric.
EG. Very Stiff (a)
Dark Grey (c)
Dense (a) Brown (c)
Fine to Coarse (e)
Very Stiff (a) Greenish blue (c)
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CLAY (f) Angular (e) GRAVEL (f)
Sandy (d) CLAY (f) With some rounded gravel (g)
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• When soils are desribed at field, it is important to learn how to distinguish between clay and non cohesive soils on the basis of estimated engineering behaviour. (10% of clay can impart an essentially cohesive behaviour. Eg. •
A soil containing 50% of silt, 30% of clay and 20% of sand is described as sandy silty CLAY because the soil behaves more like a clay.
•
Clayey SAND – not cohesive, but contains clay
•
Very clayey SAND or Very sandy CLAY – borderline
•
Sandy CLAY – cohesive, but sand may be the major constituents by weight.
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CONSTRUCTION STAGE • Engineers should allow or apt with changes during construction of foundation at site. • Alternative design need to be in hand whenever there are changes during this stage. • At this stage a critical, fast and accurate decision need to be done as the delay in making decision will hold or retarding the process of construction. • This is a stage where foundation engineers are really tested in their knowledge integrity. • This is also a stage where reliability of SI data is known.
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POST DESIGN STAGE • To validate the design, load test need to be carried out. The designer may choose to have them conducted either before or after the bids are taken. • The first alternative permits development or revision of design and specifications to fit the actual conditions. • The second saves expenses on mobilisation but may lead to delay if the results is unsatisfactorily.
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PILE LOAD TEST AND INTERPRETATION
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LOAD TEST To ensure the pile workability before and after construction. It is also as a method to determine settlement and ensuring that it does not exceed allowable limit. Failure of load test according to JKR specification:1. Residual settlement at working load exceed 6.5mm 2. Total settlement at working load exceed 12.5mm 3. Total settlement exceed 38mm or 10% of pile diameter or width whichever is lower at twice working load. Methods of statement shall be refer to JKR Specification or BS8004. Pile in granular soil are often tested 24 to 48 hrs when load arrangement have been made. FOUNDATION ENG. DESIGN PRINCIPLES
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• The time lapse is sufficient for excess pore water pressure to dissipates. • Pile in cohesive soils should be tested after sufficient lapse for excess pore water pressure to dissipates. • This time lapse is commonly in the order of 30 to 90 days giving also some additional strength gain from thixotropic effects.
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NEW FAILURE INTERPRETATION i)
The total residual settlement after removal of the test load at working load exceeds ((diameter of pile or diagonal width for non-circular pile / 120) + 4) mm or 12.50 mm whichever is the lower value.
ii)
The total settlement under twice the Working Load exceeds 38.0 mm, or 10% of pile diameter / width whichever is the lower value.
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DL
38mm
settlement
12.5mm
6.5mm
LOAD
2DL
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Failure Load Definition
1. 2. 3. 4. 5.
NAVFAC Method Van Weele Chin Fung Kee Method DeBeer Method Mazurkiewicz Method
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NAVFAC Method
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Van Weele Method
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• • •
250k N
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From point O to ‘a’ the capacity is based on the skin resistance plus any small point contribution. From point ‘a’ to ‘b’ the load capacity is the sum of the limiting skin resistance plus the point capacity. From point ‘b’ the curves becomes vertical as the ultimate point capacity is reached. Often the vertical asymptote is anticipated and the test terminated before a vertical curve branch is established.
1600-250 = 1350kN
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Chin Fung Kee Method
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De Beer Method
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Load (Log Scale)
Settlement (Log Scale)
The load settlement curve is plotted in log-log plot and the point of intersection of the two straight lines thus obtained is the failure load.
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Mazurkiewicz Method
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45o
Settlement
Load
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He assumed that the load settlement curve is parabolic after an initial straight portion . The ultimate load can be obtained by geometric construction. After the initial straight portion, draw sets of equal settlement lines to intersect the load settlement curve. Draw vertical line loads from this intersection to intersect the load axis. Draw 45o line to intersect the next load line. The intersection fall in a line which cuts the load axis at the ultimate load. 99
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STARTING POINT OF FOUNDATION DESIGN • 1. 2. 3. 4. 5.
Following steps are the minimum requirement for designing a foundation. Locate the site and the position of the load Physical inspect the site for any geological or other evidence that may indicate potential design problems Establish the field exploration program for design parameters Determine necessary design parameters base on integration of test data, scientific principles and engineering judgement. Design the foundation using the latter and it should be economical and be able to be built by the available construction personnel.
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GENERAL REQUIREMENT TWO MOST IMPORTANT QUESTION FOR DESIGNER!!! • WHAT LOADS ARE TO BE SUPPORTED. • HOW FAR MAY THE FOUNDATION SETTLE IN RESPONSE TO THESE LOAD.
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• Generally the proper design requires the following:1. Determine the building purpose, probable service life loading, type of framing, soil profile, construction methods and construction cost. 2. Determine the client owner and client needs. 3. Making the design, but ensuring that it does not successively degrade the environment and provide a margin of safety that produces a tolerable risk level to all parties, the public, the owner and the engineer.
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ADDITIONAL CONSIDERATION IN FOUNDATION DESIGN • • • • • • • •
Adequate depth Depth of foundation to be below seasonal change Considering problematic soil Compressive strength consideration Protection of foundation against natural causes Sustainable to changes Buildable or limitation. Apt to local environment standard.
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CHOICE OF FOUNDATION TYPE • 1. 2. 3. 4. 5.
Based from Neoh C.A, the choice of the foundation designs are considered from: Loads per column Bearing type either end or skin Bearing layer Type of Intermediate layer Location of water level.
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PROCEDURE FOR THE CHOICE OF FOUNDATION TYPE FOR A SITE
Assess Foundation Base
Assess Ground Conditions and Type of Structures
YES Are pile necessary Technical Considerations for Different Pile Types:1.
Ground Condition
2.
Loading Condition
3.
Environmental Considerations
4.
Site and Plant Considerations
5.
Safety
List all technically feasible pile types and rank them in order of suitability based on technical consideration.
NO Choose Shallow Foundation Types
Assess construction programme for each suitable pile type and rank them based on program consideration
Make overall ranking of each pile type based on technical, cost and programme considerations
Submit individual and overall rankings of each pile type to client and make recommendation on most suitable pile type.
Assess cost of each suitable pile type and rank them based on cost consideration.
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Myths in Piling • Myth • Dynamic Formulae such as Hiley’s Formula Tells us the Capacity of the Pile The Truth • Pile Capacity can only be verified by using: • (i) Maintained (Static) Load Tests • (ii)Pile Dynamic Analyser (PDA) Tests
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Continue
Myth: Pile Achieves Capacity When It is Set. Truth: Pile May Only “Set” on Intermediate Hard Layer BUT May Still Not Achieve Required Capacity within Allowable Settlement. Myth: • Pile settlement at 2 times working load must be less than certain magnitude (e.g. 38mm) • Truth: • Pile designed to Factor of Safety of 2.0. Therefore, at 2 times working load: - Pile expected to fail unless capacity under- predicted significantly • • • •
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Continue • • • •
Myth Load test can opt not to be done since the pile has all set. Truth Load test need to be done since it is part of Geotechnical Design process i.e to verify. Pile set does not mean that it has reach its allowable capacity at designated settlement.
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THE END
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
ASSOC. PROF. Ir. DR. HJ. RAMLI NAZIR DEPT. OF GEOTECHNIC AND TRANSPORTATION, UNIVERSITI TEKNOLOGI MALAYSIA
INSPIRING CREATIVE AND INNOVATIVE MINDS
Lecture 2
UNDERSTANDING THE DESIGN USING EUROCODE (EN-7 (MALAYSIAN ANNEXE))
INSPIRING CREATIVE AND INNOVATIVE MINDS
INTRODUCTION • The Eurocode system consists of : 1. 2. 3. 4. 5.
EN1990 Eurocode 0 EN1991 Eurocode 1 EN1992 Eurocode 2 EN1993 Eurocode 3 EN1994 Eurocode 4
6. 7. 8. 9.
EN1995 Eurocode 5 EN1996 Eurocode 6 EN1997 Eurocode 7 EN1998 Eurocode 8
10.
EN1999 Eurocode 9 Structures. Other related documents : CEN and ISO
Basis of Design Actions on Structure Design of Concrete Structures Design of Steel Structures Design of Composite Steel and Concrete Structures Design of Timber Structures. Design of Masonry Structures Geotechnical Design Design of Structure for Earthquake Resistance Design of Aluminium Alloy
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
OBJECTIVES OF THE EUROCODES • As a mean to prove compliance of building and civil engineering works with the essential requirements of mechanical resistance and stability and safety in case of fire. • A basis for specifying contracts for construction works and related engineering services. • A framework for drawing up harmonised technical specs for construction products. • Improve the functioning of a single market for products and engineering services by removing obstacles arising from different nationality codified practices for the assessment of structural liabilities. • Improve the competitiveness of the European construction industry and its professionals and industries, in countries outside the European Union.
Eurocode Design Method
• All the Eurocodes are all based on a common design method • The common design method is presented in EN 1990 • A common loading code for all the Eurocodes is presented in EN1991- Actions • The Eurocodes share a common terminology and symbols • The common design method for the verification of safety and serviceability involves – The limit state design method – Partial factors – Characteristic actions and material parameters or resistances – Reliability based
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
Q: Why do we need a change?
• Eurocode 7 draws geotechnical design into a framework common to other aspects of civil and structural engineering. • In the past, differences in design approach have arisen due to the properties of soil and rock being fundamentally different and more difficult to predict than other engineering materials. • In order to overcome difficulties in prediction and uncertainty of material behaviour, designers have often adopted large factors of safety under working loads to ensure serviceability. • However, to avoid problems, designers need to grasp fundamental geotechnical principles, including overall stability, hydraulic uplift and piping.
Contd… • Eurocode 7 may be seen by some as an unnecessary complication, it introduces the concepts of limit state design to geotechnical calculations. • This will be second nature to most structural engineers who will not find any difficulty with the concepts. • The currently accepted methods of analysis of geotechnical problems remain largely unchanged. • The real advantage in its application lies in a common framework for design, including overall stability, and uplift. • The Eurocodes adopt, for all civil and building engineering materials and structures, a common design philosophy based on the use of separate limit states and partial factors rather than global factor of safety. • The intended are to ensure safe structures, so they will be use both by the designers and the checkers of the design.
COMPARISON BETWEEN CONVENTIONAL DESIGN AND EUROCODES
• Advantages :
Conventional Design
Eurocode
Using Global FOS and simple applications
Using PFOS and harmonic design
Accustomed to use
Type of load has different levels of uncertainty Uniform Level of Safety Risk Assessment
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
COMPARISON BETWEEN CONVENTIONAL DESIGN AND EUROCODES
• Disadvantages : Conventional Design
Eurocode
Inadequate amount of variability
More Complex
Stress is not a good measure of resistance
Old Habits
FOS is subjective
Requires availability of statistical data
No risk assessment
Resistance Factor varies
Whereabout in Eurocodes ?
The suite of primary structural Eurocodes
Numbers
Name
EN1990
Subject Basis of structural design
EN1991
Eurocode 1
Action on structures
EN1992
Eurocode 2
Design of concrete stuctures
EN1993
Eurocode 3
Design of steel structures
EN1994
Eurocode 4
Design of composite steel and concrete structures
EN1995
Eurocode 5
Design of timber structures
EN1996
Eurocode 6
Design of masonry structures
EN1997
Eurocode 7
Geotechnical Design
EN1998
Eurocode 8
Design of structures for earthquake resistance
EN1999
Eurocode 9
Design of aluminium sructures.
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
EN1990
EN1990
• EN 1990 describes the Principles and requirements for safety, serviceability and durability of structures. • It is based on the limit state concept used in conjunction with a partial factor method. • For the design of new structures, EN 1990 is intended to be used, for direct application, together with Eurocodes EN 1991 to 1999. • EN 1990 also gives guidelines for the aspects of structural reliability relating to safety, serviceability and durability: – for design cases not covered by EN 1991 to EN 1999 (other actions, structures not treated, other materials) ; – to serve as a reference document for other CEN TCs concerning structural matters.
EN1990
•
EN 1990 is intended for use by : – committees drafting standards for structural design and related product, testing and execution standards ; – clients (e.g. for the formulation of their specific requirements on reliability levels and durability) ; – designers and constructors ; – relevant authorities.
• EN 1990 may be used, when relevant, as a guidance document for the design of structures outside the scope of the Eurocodes EN 1991 to EN 1999, for : – assessing other actions and their combinations ; – modelling material and structural behaviour ; – assessing numerical values of the reliability format
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
EN1990 • This standard gives alternative procedures, values and recommendations for classes with notes indicating where national choices may have to be made. • Therefore the National Standard implementing EN 1990 should have a National annex containing all Nationally Determined Parameters to be used for the design of buildings and civil engineering works to be constructed in the relevant country. • National choice is allowed in EN 1990 through : • – A1.1(1) • – A1.2.1(1) • – A1.2.2 (Table A1.1) • – A1.3.1(1) (Tables A1.2(A) to (C)) • – A1.3.1(5) • – A1.3.2 (Table A1.3) • – A1.4.2(2)
ASSUMPTIONS • The general assumptions of EN 1990 are : • the choice of the structural system and the design of the structure is made by appropriately qualified and experienced personnel; • execution is carried out by personnel having the appropriate skill and experience; • adequate supervision and quality control is provided during execution of the work, i.e. in design offices, factories, plants, and on site; • the construction materials and products are used as specified in EN 1990 or in EN 1991 to EN 1999 or in the relevant execution standards, or reference material or product specifications; • the structure will be adequately maintained; • the structure will be used in accordance with the design assumptions.
TERMS USED
• ‘Principles’ are mandatory (‘Normative’) requirements; ‘Principle’ clauses in the Code are identified by a ‘P’ after the clause number and contain the word ‘shall’. • All other clauses are ‘Application Rules’ that indicate the manner in which the design may be shown to comply with the Principles. • Application Rules are ‘Informative’ (i.e. not mandatory and for Information only) and use words such as ‘should’ and ‘may’.
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
EN1997
What is the structure of the new code? • Eurocode 7 consists of two Parts: Part 1 (EN 1997-1) – Geotechnical design – General rules and Part 2 (EN 1997-2) - Ground investigation and testing. • It is important to appreciate that EN 1997-1 is not a detailed geotechnical design manual but is intended to provide a framework for design and for checking that a design will perform satisfactorily; that is, that the structure will not reach a ‘limiting condition’ in prescribed ‘design situations’. • The Code therefore provides, in outline, all the general requirements for conducting and checking design. • It provides only limited assistance or information on how to perform design calculations and further detail may be required from other texts, such as standard soil mechanics books and industry publications.
EN1997
• Part 2 covers Ground Investigation and Testing. • The application of the code in the Malaysia requires reference to the Malaysia National Annexes which provide the partial factors prescribed for use in the Malaysia. • The Malaysia National Annex for Part 1 will be available in 2012 and the National Annex for Part 2 is expected to be published after that since it is still in progress. • A series of geotechnical execution standards covering geotechnical processes such as piling works and grouting also exist; these are primarily of interest to construction, but are also of general interest to designers.
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
EN1997 • It describes the general ‘Principles’ and ‘Application Rules’ for geotechnical design, primarily to ensure ‘safety’ (adequate strength and stability), ‘serviceability’ (acceptable movement and deformation) and ‘durability’ of supported structures, that is of buildings and civil engineering works , founded on soil or rock. • ‘Principles’ are mandatory (‘Normative’) requirements; ‘Principle’ clauses in the Code are identified by a ‘P’ after the clause number and contain the word ‘shall’. • All other clauses are ‘Application Rules’ that indicate the manner in which the design may be shown to comply with the Principles. • Application Rules are ‘Informative’ (i.e. not mandatory and for information only) and use words such as ‘should’ and ‘may’.
Content of EN1997-1 • • • • • • • • • • • • •
BS EN 1997-1 contains the following Sections: Section 1 General Section 2 Basis of geotechnical design Section 3 Geotechnical data Section 4 Supervision of construction, monitoring and maintenance Section 5 Fill, dewatering, ground improvement and reinforcement Section 6 Spread foundations Section 7 Pile foundations Section 8 Anchorages (Still not Apply in Malaysia) Section 9 Retaining structures Section 10 Hydraulic failure Section 11 Site stability Section 12 Embankments.
Annexes
• The Annex is ‘informative’ which means that the partial factors listed must be used; however, the values of these factors are a matter for national determination and the values shown in the Annex are thus only ‘recommended’ • Annex A – Annex A is used with Sections 6 to 12, as it gives the relevant partial and correlation factors, and their recommended values, for ultimate limit state design. – Annex A is normative , which means that it is an integral part of the standard and the factors in it must be used, although their values are informative and may therefore be modified in the National Annex.
• Annex B – Annex B gives some background information on the three alternative Design Approaches permitted by EN 1990 and given in EN 1997-1
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
• Annex C - H – Annexes C to G provide examples of internationally recognised calculation methods for the design of foundations or retaining structures; – Annexes C to J are informative , which means that in principle, they may be superseded in the National Annex
SUMMARY OF ANNEXES
DESIGN PHILOSOPHY IN EN1997-1
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
Traditional Design Philosophies
• FOS on the materials is applied in the choice of the stresses used in the design of the piles and pile caps as structural members. • When pile considered single, the working load shall not exceed the allowable bearing capacity. The ultimate value shall be obtain from load tests whenever practicable. In general a value of 2 to 3 is normally used. • Settlement or differential settlement at working load shall not be greater than can be tolerated by the structure. • When settlement is not critical a smaller FOS can be employed. • The basis of design will be use allowable value and check the settlement.
Limit state design
• An understanding of limit state design can be obtained by contrasting it with “working state design” • Working state design : Analyse the expected, working state, then apply margin of safety. • Limit state design : Analyse the unexpected states at which the structure has reach an unacceptable limit. • Make sure the limit states are unrealistic or at least unlikely.
DESIGN PHILOSOPHY IN EN1997-1
• EN 1997-1 is a ‘limit state design’ code; this means that a design that complies with it will prevent the occurrence of a limit state • A limit state could, for example, be: – an unsafe situation – damage to the structure – economic loss.
• While there are, in theory, many limit states that can be envisaged, it has been found convenient to identify two fundamentally different types of limit state, each of them having its own design requirements: – ultimate limit states (ULS); – serviceability limit states (SLS).
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
Ultimate Limit Stress • ULSs are defined as states associated with collapse or with other similar forms of structural failure (e.g. failure of the foundation due to insufficient bearing resistance). • In geotechnical design, ULSs include: – failure by excessive deformation, – loss of stability of the structure or any part of it. • Hence, a state in which part of a structure becomes unsafe because of foundation settlement or other ground movements should be regarded as a ULS even if the ground itself has not reached the limit of its strength.
Contd….
• Ultimate limit states of full ‘collapse’ or ‘failure’ of geotechnical structures are fortunately quite rare. • However, an ultimate state may develop in the supported structure because of large displacement of a foundation, which has itself not ‘failed’. • This means, for example, that a foundation may be stable, after initially settling (it hasn’t ‘exceeded a ULS’ or ‘failed’), but part of the supported structure may have failed (for example, a beam has lost its bearing and collapsed owing to substantial deformation in the structure).
What is the general approach to design? • The principal emphasis of Eurocode 7 is in the definition and application of partial factors of safety. • Factors are applied to characteristic actions, nominal dimensions and characteristic material properties. • These are considered through calculation with a view to ensuring that the design effects are less than or equal to the design resistances. • Where relevant, the code requires a total of five different ultimate states to be considered.
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
Contd… • EQU: the loss of equilibrium of the structure or the ground, considered as a rigid body, in which the strengths of structural materials and the ground are insignificant in providing resistance; • STR: internal failure or excessive deformation of the structure or structural elements, including footings, piles, basement walls, etc, in which the strength of structural materials is significant in providing resistance; • GEO: failure or excessive deformation of the ground, in which the strength of soil or rock is significant in providing resistance (e.g. overall stability, bearing resistance of spread foundations or pile foundations); • UPL: loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy) or other vertical actions; • HYD: hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients.
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
Contd…
• An exception to the application of partial factors is made in relation to water pressure. • It is recognized that the application of partial factors to water pressure can, in some circumstances, lead to unrealistically high water pressure. In this case, it is suggested that a suitable margin of safety be applied to characteristic water levels. • Three basic design approaches are permitted in the assessment of ultimate limit states and are applied according to local practice.
What limit states need to be considered? • For most simple geotechnical design situations, the GEO limit state will be critical to the sizing of foundations and structural members. • The sections of the code covering specific design issues, such as pile foundations and spread footings etc., give advice on the limit states that need to be considered. • Where groundwater is present in excavations or cuttings, the UPL and HYD limit states need to be considered. • The STR limit state is less well defined, but is nevertheless very important in some design situations. • The STR case might become critical where imposed loading causes deformation of some part of the structure or deformation of the ground imposes deformation on a structural member.
Which frequently used ?
• For most of the design problems likely to be encountered the STR and GEO ultimate limit states are the ones that will apply, as they cover the routine design of shallow and pile foundations and other ‘common’ geotechnical structures. •
The EQU ULS is intended to cater for the rare occasion when, for example, a rigid retaining wall, bearing on a rigid rock foundation, could rotate about one edge of its base.
• The UPL and HYD ULSs, while more common than EQU, are generally beyond the ‘routine’
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LECTURE 2
APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
Unrealistic possibility
Serviceability Limit States
• SLSs are defined as states that correspond to conditions beyond which specified service requirements for a structure or structural member are no longer met (e.g. settlement that is excessive for the purposes of the structure). • It is a non technical statement • (i)P The limit states that concern :-
The functioning of the structures or structural members under normal use; The comfort of people; The appearance of the construction works Shall be classified as serviceability limit states (SLS)
Inconvenience, disappointments and more manageable costs. Should be rare, but it might be uneconomic to eliminate them completely.
SERVICEABILITY FAILURE
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
Who should carry out geotechnical design? • Part 1 provides a useful, although optional, definition of categories of geotechnical structures. • Geotechnical Category 1 (GC1) includes relatively straightforward structures in which routine methods, including prescriptive methods, may be used. • While the code makes no attempt to define levels of competency, experienced civil and structural engineers should be capable of preparing the geotechnical design basis for Category 1 structures. • A designer should be capable of judging whether a design situation is not more complex than allowed within the Geotechnical Category.
Contd… • Structures that involve excavation below the water table, but otherwise conventional structures without unusual risk, are defined as Geotechnical Category 2 (CG2). • Such structures normally require some form of geotechnical characterisation based on field or laboratory testing. • The terms ‘geotechnical engineer’, ‘geotechnical specialist’ and ‘geotechnical advisor’ are defined. • It was suggested that design work on CG2 structures should be carried out by an experienced civil or structural engineer.
Contd…
• Geotechnical Category 3 (GC3) covers situations that are considered unusual or are associated with high risk. • GC3 projects will typically involve advanced field or laboratory testing and numerical analysis. • The Association of Geotechnical Specialists advocates the role of Geotechnical Advisors in establishing the design strategy of large projects, and this would seem to be appropriate to GC3 structures.
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
En1990 fundamental equation for ULSs
Fundamental limit states requirement
Design values of action
6.3.1 (1)
Design values of actions The design value Fd of an action F can be expressed in general terms as: Fd=FFrep with
Where F = Frep= FK =
Frep=FK
Partial Factor of Safety for the action which takes account the possibility of unfavourable deviations of the action values from the representatives value. The relevant representative values for the action The characteristic values of the action is either 1.0 or
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
Contd…
Contd..
Design values of material or product properties
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
Contd..
Design values of geometrical data
Basis of geotechnical design
• Refer details to EN7: Geotechnical Design Part 1 : General Rules Section 2 page 19 onwards. 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8
Design requirement Design situations Durability Geotechnical Design by Calculation Design by prescriptive measures Load tests and tests on experimental models Observational method Geotechnical Design Report.
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
DESIGN APPROACH
• Generally EN1997 provides 3 Design Approach for the application of partial factor of Safety. • The Design Approach is know as DA-1/1, DA-1/2 (Design Approach 1), DA – 2 (Design Approach 2) and DA-3(Design Approach 3) • MALAYSIA PRACTICE USE ONLY DESIGN APPROACH 1 FOR STR and EQU IN THE DESIGN.
What combinations of partial factors to use?
• Combination 1 involves the consideration of factored actions and unfactored material properties and resistances. • Combination 2 considers unfactored actions, except unfavourable variable actions, and factored material properties. • Difficulties arise with the application of numerical methods, such as finite element, in the assessment of ultimate state. • In this case, the factoring of soil strength or stiffness can lead to the generation of inappropriate mechanisms in the analysis.
Contd…
• Uncertainty can also be experienced in assessing slope stability, where it can be difficult to separate favourable and unfavourable actions, and in the design of ground anchors where the design and execution codes provide conflicting advice. • Serviceability states are usually assessed by adopting unfactored actions and material properties. • In this area, numerical analysis provides a useful tool for GC2 and GC3 projects.
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
DESIGN APPROACH 1 (M’SIA PRACTICE)
• National choice is permitted in the use of a Design Approach for the STR and GEO limit states (see MS EN 1997-1:2012, 2.4.7.3.4.1(1)P). • As indicated in Table NA1, only Design Approach 1 is to be used in Malaysia. • Table NA1 of this national annex lists the clauses in MS EN 1997-1:2012 where national choice may be exercised in respect of factor values for design in Malaysia. • Where choice applies, Table NA1 indicates where values are given, or states a value to be used, or describes the procedure for specifying the factor. • The values given in the Tables in Annex A of this national annex replace the recommended values in Annex A of MS EN 1997-1:2012.
DESIGN APPROACH 1 (M’SIA PRACTICE) ONLY FOR DESIGN APPROACH 1 - STR AND GEO Clause 2.4.7.3.4.2 • Other than pile and anchor use Combination 1 : A1 + M1 + R1 Combination 2 : A2 + M2 + R1 • For Axially loaded Pile and Anchor Combination 1 : A1 + M1 + R1 Combination 2 : A2 + (M1 or M2) + R4 where M2 is for calculating any unfavourable actions such as negative skin or transverse loading. INSPIRING CREATIVE AND INNOVATIVE MINDS
SUMMARY FOR FACTOR OF SAFETY
• Refer only to Design Approach 1
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APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
SUMMARY OF GEOTECHNICAL DESIGN BY CALCULATION CHARACTERISTIC MATERIALS PROPERTIES
REPRESENTATIVE ACTION, Fk
THE DESIGN IS ALL ABOUT DIVIDED BY M VALUES
MULTIPLIED BY F VALUES
Actions:(loads, forces etc.)
and
DESIGN ACTION, Fd
Material Properties (c, tan , etc) DESIGN MATERIALS PROPERTIES
Geotechnical Design Analysis DESIGN RESISTANCE, Rd
DESIGN EFFECT ANALYSIS, Ed
VERIFY Ed≤ Rd
DESIGN VALUES OF ACTIONS
DESIGN ACTION, Fd CHARACTERISTIC ACTIONS, Fk DESIGN EFFECT OF ACTION, Ed
REPRESENTATIVE ACTION, Frep
Correlation Factor, rep
Partial Factor of Safety, rep
ENGINEERING STUDENT
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LECTURE 2
APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
GEOTECHNICAL ENGINEERING STUDENT
CIVIL ENGINEERS
DESIGN ENGINEER’S RATIONAL
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LECTURE 2
APPLICATION OF EUROCODE IN GEOTECHNICAL DESIGN
FINALLY - HOW TO LOOK SMART FOR ENGINEERS
THE END
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LECTURE 3
Shallow Foundation
DESIGN OF SHALLOW FOUNDATION ASSOC. PROF. Ir. DR. RAMLI NAZIR TEL : 013 7927925 OFF: 07 5531722
INSPIRING CREATIVE AND INNOVATIVE MINDS
Lecture 3 DESIGN OF SHALLOW FOUNDATION
INSPIRING CREATIVE AND INNOVATIVE MINDS
Brief Revision
1
LECTURE 3
Shallow Foundation
•
Basic Consideration in designing the shallow footings are:-
1.
Significance and use
2. Settlement limitations 3. Total Settlement 4. Differential settlement 5. Bearing Capacity
Stability Problem Bearing Capacity Failure
• How do we estimate the maximum bearing pressure that the soil can withstand before failure occurs?
DESIGN REQUIREMENT
• The design must meet two principle requirement of the Limit State:1. Capacity is sufficient to support loads 2. Avoiding excess settlement which might lead t a loss of function. • • • •
This limit state is known as Ultimate Limit State and Serviceability Limit State. Both states must always be considered in the design. This philosophies is the basis of Eurocode 7. The concept related to shallow foundation design can be shown in the figure below.
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LECTURE 3
Shallow Foundation
Bearing Capacity and Limit Analysis Types/Modes of Failure • general shear failure • local shear failure • punching shear failure
Typical Mode of Failure
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LECTURE 3
Shallow Foundation
Mode (a)
• • • • •
As the pressure increase towards failure value, qf, a state of plastic equilibrium is reached initially in the soil around the edges of footing. As the soil is not perfectly level , the soil movement will accompany with tilting and heaving to one side of the footing. This mode is typical for low compressibility soil where the peak value is significant. Ultimately the state of plastic equilibrium is fully developed throughout the soil above the failure surface. This type of failure is called a general shear failure.
Mode (b)
• • • • •
There is a significant compression of the soil under the footing and only partial development of the state of plastic equilibrium. The failure surfaces does not reach the ground surface and only slight heaving occurs. Tilting of foundation will less been expected. The ultimate bearing capacity is not well defined. This mode is associated with high compressibility and is called Local Shear Failure.
Mode (c)
• • • • • •
Relatively to high compression of soil under the footing. This will accompanied by shearing in a vertical direction around the footing. No heaving occurs on the ground surface away from the edges of footing and no tilting occurs. Large settlement is the main characteristic of this mode. The bearing capacity is not well defined. In general, he mode of failure depend on the compressibility of the soil and the depth of foundation related to the breadth.
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LECTURE 3
Shallow Foundation
Model Tests by Vesic (1973)
General Guidelines • Footings in clays - general shear • Footings in Dense sands ( D> r 67%)-general shear • Footings in Loose to Medium dense (30%< D < r 67%) - Local Shear • Footings in Very Loose Sand (D< 30%)- punching shear r
•
Shear Stress
•
The bearing capacity problem can be considered in terms of plastic theory. It can be assumed that the stress-strain behaviour of the soil can be represented by the rigid-perfectly plastic idealization.
Y’
Shear Strain
• Both yielding and shear failure occur at the same state of stress. • Unrestricted plastic flow takes place at this stress level. • A soil mass is said to be in a state of plastic equilibrium if the shear stress at every point within the mass reaches the value represented by Y’.
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LECTURE 3
• • • •
Shallow Foundation
The plastic collapse will occur after plastic equilibrium has reach in part of the soil mass. This will result in the formation of unstable mechanism ( The par of the soil mass slip) The applied load including body forces is called collapse load. Determination of the collapse load is achieved using the limit theorem of plasticity known as limit analysis to calculate LOWER and UPPER BOUND to the true collapse load.
LOWER BOUND THEOREM
•
•
If the state of stress can be found which at no point exceeds the failure criterion for the soil and is in equilibrium with the external load system, than there will be no collapse. Therefore the external load system constitute a lower bound to the true collapse since a more efficient stress distribution may exit, which would be in equilibrium with higher external loads.
UPPER BOUND THEOREM
•
•
If a kinematically admissible mechanism ( the motion of a sliding mass must remain continuous and be compatible with any boundary restriction) of plastic collapse is postulated and if, in an increment of displacement, the work done by the system of external loads is equal to the dissipation of energy by the internal stresses, then collapse will occurs. The external load system constitute an upper bound to the true collapse loads since more efficient mechanism may exist resulting in collapse under lower external loads.
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LECTURE 3
Shallow Foundation
BEARING CAPACITY IN UNDRAINED MATERIALS UPPER BOUND APPROACH MECHANISM UB-1 For undrained condition the failure mechanism within the soil mass should be a slip lines which are either a straight line or a circular arcs or both. For simplification a straight line is used to identify the three sliding block of a soil under vertical loading. The load will push downwards and the blocks will have to move to form a mechanism and therefore be kinematically admissible. As a result a slip line shown OA, OB, OC,AB and BC which are the results of energy dissipation along this line.
•
•
•
•
• • •
•
•
The energy line is shown as in the velocity diagram known a hodograph. It is use to determine the velocities along the slip line Starting with the known vertical displacement (v) of a footing, the point f is known. Block A must move 45o horizontal to the stationary soil. The vertical component of this motion must equal to v so the soil and footing will remain contact. Two construction line may be added to the hodograph to represent the two limiting conditions. The crossing line will meet at point a and form velocity vOA. Similar to Block B where it moves horizontally with respect to O and at 45o with respect to A. the process continuously move which is therefore a kinematically admissible.
•
The energy dissipated (Ei) due to shearing at relative velocity vi along a slip line of Length Li is given by :
•
Total energy dissipated in the soil can then be found by summing Ei for all slip lines. Slip Line
Stress, f
OA
cu
OB
cu
OC
cu
AB
cu
BC
cu
Length, Li
2 B 2 2 2
Relative velocity, vi
Energy Dissipated, Ei
2
cuBv
2v
2cuBv
2
cuBv
2
cuBv
2
Total Energy, Ei
cuBv
6cuBv
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LECTURE 3
Shallow Foundation
•
The work done Wi by a pressure qi acting over an area per unit length Bi moving at velocity vi is given by :-
•
For qf, the pressure acting downward while for Block C as the motion move upwards, the surcharge pressure will tend to move against gravity. This is negative work. Therefore the work done for surcharge (q) will be:
•
Summing W for all component:
•
Therefore, for mechanism UB-1, for undrained materials the bearing capacity qf is : =
UPPER BOUND APPROACH, MECHANISM UB-2 •
• •
• •
•
Another mechanism approach is by replacing Block B with a number of smaller wedges. These wedges describe a circular arc of Radius, R between the rigid block A and block C which is known as shear fan. Block A and C will move in the same direction and b the same magnitude. The velocity around the edge of the circular arc will be constant as its rotates around point X. Since Li is circular length then Li = R Thus giving :
The next energy dissipated due to the shearing occurrence between each wedge is similarlyfound and given as:,
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LECTURE 3
• •
Shallow Foundation
The total amount of energy disipated is by summing all amount of energy across all wedges. If the wedges angle is small, this summation becomes an integral over a full internal angle of the zone ().
,
Stress, f
Slip Line OA
Length, Li
cu
Fan Zone (/2)
Energy Dissipated, Ei
2
cuBv
2 vfan = 2
R=
cu
OC
Relative velocity, vi
2
cu
cuBv
2 Total Energy, Ei =
•
cuBv
Applying the same equation as previous for UB-1 it yields :
The results in UB-2 is lower than UB-1, so UB-2 present the true collapse load by upper bound theorem.
LOWER BOUND APPROACH – STRESS STATE LB-1 •
• •
In undrained condition the yield criterion are satisfied without considering mode of deformation thus f = cu. For equilibrium purposes, 1 in zone 2 must be equal to 3 in zone 1. The major principal stress at any point in zone 1 is :
•
The minor principal stress in zone 2 is smilarly :
•
If the soil is undrained with shear strength cu, it is in the state of plastic yielding and the diameter of each circle is 2cu. At the point where the circle meet :
•
=
∴
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LECTURE 3
Shallow Foundation
Lower Bound Approach, Stress State LB-2 •
A more realistic stress state forming a fan zone which gradually rotate the major principle stress from vertical beneath the footing to horizontal outside.
•
The change in direction of major principle stress across a frictional discontinuity depend on the frictional strength along the discontinuity , d.
•
In crossing the discontinuity, the major principle stress will rotate by an amount ∆
And the radius of the Mohr circles are cu; ∆ → ; →
=
•
For a fan zone of frictional discontinuities substended to an angle , the equation can be integrated as follow across the fan angle fan:-
The principal stress rotation required in the fan is : ,
giving :
∴ This value is higher than for LB-1 so LB-2 represent a better estimate of the true collapse load by the lower bound theorem.
BEARING CAPACITY FACTOR (Undrained Materials) • •
•
General Equation : (refer pg. 157, App. D, EN7-1) For the case of footing surrounded by surcharge pressure q, Nc = 5.14 where Nc is bearing capacity factor for strip footing under undrained conditions (f = cu) Skempton (1951) provided figure by the side with included value(solid line) suggested by Salgado et.al. (2004) given that :.
(Eqn 8.18)
Where d : footing depth and B : footing width. For general rectangular footing dimension B x L, Eurocode 7 recommends that shape factor : .
(Eqn 8.19)
Nc for circular may be obtain by taking square footing (B/L = 1) and should not exceed 9 for deeply embedded square (sc=1) or circular (sc=2)foundation.
10
LECTURE 3
Shallow Foundation
Footing in Layered Undrained Soil •
Values of Nc obtained previously may be used for stratified deposits, provided the value of cu for a particular stratum is not greater than the average value for all strata within the significant depth by more than 50% of the average value. Merifield et al. (1999) presented upper and lower bound values for Nc for strip footing resting on a two cohesive layer as a function of thickness H on upper layer of strength cu1 overlying deep deposit materials with strength cu2. Proposed design value Nc as suggested in general terms from Figure (a) is valid if the undrained shear strngth of the upper layer is used in the latter equation. (cu = cu1) The resulting shape factor for square footing B/L=1 is given as in Figure (b).
•
•
•
Footing Associated With Slopes • •
•
• • •
For foundation constructed close to the slope, inevitably the bearing capacity will reduced. Georgiaids (2010), proposed charts for Nc for strip footing set back from the crest of the slope with angle by a multiple of the foundation width. These are based on upper bound analyses in which an optimal failure mechanism was found giving the lowest upper bound. Thus it is important to include both local and global failure mechanism. The value of Nc reduces with the slope increment. If the foundation is set far enough back from the crest of the slope (l>2B), then the slope will have no effect on the bearing capacity and consider as a level ground (Nc = 2 + )
Variation of cu with Depth •
Davies and Booker(1973) conducted upper and lower bound plasticity analyses for soil with linear variation of undrained shear strength with depth z below founding plane :-
Where cu0 is the undrained shear strength at z = 0 and C is the gradient of the cu-z relationship. The general expression is the given as:
If C=0, then Fz=1 giving Nc=5.14.
11
LECTURE 3
Shallow Foundation
BEARING CAPACITY IN DRAINED MATERIALS
Upper Bound Theorem •
The slip surface within the kinematically admissible failure mechanism is either straight lines or spiral log curves or both.
•
Normally for drained materials it is a cohesionless soil where c’ = 0 and will exhibit some amount of dilatancy ( ’).
•
In special case where ( ’), the direction of movement will be perpendicular to resultant force, Rs.
•
The condition is known as normality principle and it represent an associative flow rules.
•
Figure (a) shows a failure mechanism in a weightless cohesionless soil (=c’=0) with a friction angle ’ which is similar to UB-2 but log spiral replacing circular fan. To determine the geometry of the mechanism, the equation describe the log spiral must be first found. Thus :-
•
•
Which may be integrated from ro at =0 to r at .
12
LECTURE 3
•
Shallow Foundation
Appling r where ro = LAB, r = LBC and = /2, for associative rule = ’ ;
Length of the spiral log with known foundation width B and wedge angle of
∅
are :-
∅
∅
∅ •
The area per unit length over which the surcharge acts on the mechanism L can be define as :∅ ∅ .
•
As a results of normality principle, there is no energy dissipated by shearing within soil mass which gives ∑ = 0.
•
As for undrained case, the footing and surcharges pressure still do work and the computations for the drain case are as shown:-
∅
∅
∅
∅
Lower Bound Analysis •
•
The change in the direction of the major principle stresses across a frictional discontinuity depends on the frictional strength along the discontinuity as before (td). For the drained case, the envelope bounding the Mohr circle in zone 1 and zone 2 form:∅
and ′ ∅′ where ∅′ is the mobilised friction angle along the discontinuity. •
The major principle stress will rotate at an amount of whereas the mean effective stress in each zone is represented by s’ which gives:
2 ∆
∆
∅′
∆ ∆ ∆
∅′
∅′ ∅′
13
LECTURE 3
Shallow Foundation
•
The radii of the Mohr circles (tA and tB) for cohesionless soil can be describe by : thus it means that .
•
Substitute into the latter equations gives :′ ′
∅
∅′ ∅′
′ ′
Setting s’B = s’ as ’mob approach ’ the above equation can be written as:′
∅′
′ ′
∅′
′ For small
•
For a fan zone of frictional stress discontinuities subtending an angle qfan, the latter equation can be integrated from zone 1 to zone 2:′
∅′
′ ′ ′ Since ′ required if the fan is
∅
∅ in zone 1 and ′ ′ ∅ in zone 2 the principle stress rotaion or 90o will give the above equation as:-
∅′
.
∅′
∅′ ∅′
∴
∅
′ ∅
′
Bearing Capacity Factor
•
Bearing capacity in drained materials is generally expressed as:′
′
N : Bearing Capacity Factor related to self weight Nc : Bearing Capacity Factor related to cohesion s and sc : Shape factor Value of Nq is found by limit analysis and given in closed-form by :∅′ ∅′
∅
14
LECTURE 3
Shallow Foundation
•
Parameter Nc can be derived for soil with non-zero c’ to give :
•
The final bearing capacity factor N is difficult to determine analytically as it is influence by the roughness of the base and soil interaction. In MSEN7, supersedes by N given in Annex D the following expression is proposed :-
•
The sample method given in MS EN 1997-1:2012, Annex D omits depth and ground inclination factors which are commonly found in bearing resistance formulations. The omission of the depth factor errs on the side of safety, but the omission of the ground inclination factor does not. To determine the ground inclination factor, one of the methods which may be considered is described in Foundations and Earth Structures Design Manual [NAVFAC DM 7.02 pp 7.2-135] which will be mentioned in the next topic.
∅′
. ∅′
• •
Bearing Capacity Factor Chart (MSEN7)
Bearing Capacity Factor
1000
100
Nc 10
Nq
N (MSEN7)
1 0
5
10
15
20
25
30
35
40
45
50
' (DEGREE)
•
•
Lyamin et al. (2007) present rectangular shape factor derived from rigorous limit analyses. The results is as shown in Fig. a. However, sq recommended by EC7 are: ∅′ for a rectangular shape ∅ for a square and circular shape
•
s recommended by EC7 are: .
for a rectangular shape
. for a square and circular shape •
sc recommended by EC7 is :-
15
LECTURE 3
Shallow Foundation
Water Condition
′ • 1.
′
It is vital that the appropriate values of unit weight are used in the bearing capacity equation. In an effective stress analysis, three different situation must be considered:If the water table is below the foundation plane, the bulk unit weight is to be used in the first and second terms of the equation.
2.
If the water table is at the foundation plane, the buoyant unit weight (’) must be used in the second term of the equation. The bulk unit weight shall be used in the first term of the equation.
3.
If the water table is at ground surface or above, the effective unit weight must be used in the first and second term of the equation.
Partial Factor of Safety for MSEN7-2012
DESIGN APPROACH 1 COMBINATION 2 A2
M2
1.00
M2*
R4
R4
Slopes and embankment
R1
tan '
'
1.00
1.25
1.35
Effective cohesion
c'
1.00
1.25
1.35
Undrained strength
cu
1.00
1.40
1.50
Unconfined Strength
qu
1.00
1.40
1.50
Weight density
'
1.00
1.00
1.00
Bearing
R;v
1.00
Sliding
R;h
1.00
1.35
Unfavourable Permanent Favourable ACTION (F , E)
G
Unfavourable Variables Favourable
SOIL (M)
M1
Other than slopes and embankment
A1
With explixit verification of SLS(A)
COMBINATION 1
Symbol
Without explicit verification of SLS(A)
ULS PARTIAL FOS FOR STR AND GEO
Q
1.00
1.00
1.50
1.30
0.00
0.00
Spread Footing
Calculation Procedure for Shallow Foundation on Undrained and Drained Materials Using Limit State Design
• Total Stress or Effective Stress analysis : Determine the shear strength parameter or effective stress parameters and unit weight of the underlying soil to determine the bearing capacity factors. • Groundwater Table: For an effective stress analysis, the groundwater table will give an impact to bearing capacity. • Ultimate Limit State Evaluation : Depend on type of footing and analysis using Design Approach 1 only. Determine the size of footing initially. • Check the respond of action against the effect of action together with the model factor of 1.4 or 1.2. • Combination 1 : A1 + M1 + R1 • Combination 2 : A2 + M2 + R1 • Check on Serviceability Limit States : The allowable bearing capacity may have to be downgraded due to local building code of practice or lower bearing pressure to avoid excessive settlement allowed.
16
LECTURE 3
Shallow Foundation
Shallow Foundation Under Combined Loads
Undrained Materials
•
In some cases apart from vertical loads, foundation can experience horizontal loads and moments. • If the horizontal loads is small in comparison to vertical load, than the horizontal load and moment may be disregard. • For the foundation loaded by action V, H and M, the following limits state must be met: 1. The resultant vertical action must not exceed the bearing resistance of supported soil. 2. Sliding must not occurs due to the resultant of H 3. Overturning must not occurs due to resultant action of M. • The foundation movement due to any settlement must not cause undue distress or lost of function in the supported structures.
Foundation Stability from ULS •
Similar to lower bound limit analysis techniques, the addition of horizontal load, H will induced an additional stress f=H/Af at the footing surface as shown. • It is assume that the surface of footing is rough and rotate the major principle stress direction in zone 1. • In undrained materials, the rotation will be = /2. from the vertical. • Overall rotation of principal stresses across the fan zone is now fan = /2 – /2. • Therefore ∆ From Figure : ∆ •
In zone 2, zone1
•
Thus giving:
, while in ∆ ∆
∆
17
LECTURE 3
Shallow Foundation
Eqn (b)
•
For all possible value of H (0 H/Afcu it can be found from ∆
;
and V/Afcu ( = bearing capacity Nc) can be found from •
When H/Afcu = 0 (purely vertical load) , =0 and V/Afcu= 2 + . When H/Afcu , the shear stress u = cu. The footing will slide horizontally, irrespective of V. The resulting curves representing the yield surface for the foundation under V-H loading. Combination of V-H which lie within the yield surface will be stable while those lying outside the yield surface will be unstable. If V>>H, failure will be in bearing while if VV,H. The yield surface assumes that tension cannot be sustained along the soil footing interface. This will due to uplift if the overturning effect is strong. Provided that the combination of V, H and M is within the yield surface, the foundation will satisfy in terms of bearing, sliding and overturning.
•
• •
•
Drained Materials •
Using a lower bound analysis, it gives:
•
From the Mohr circle, the rotation of the major principal stress direction in zone 1 is =(+)/2 from vertical. The stress condition in zone 2 are unchanged as shown in Fig.(a). Overall rotation of principal stresses across the an zone is now : ∆
• •
Thus
∆
∅
∆
From Fig. (b) :
∅
In zone 2, s’2 = q + s’2sin ’ while in zone 1, s’1 = qf + s’1sin ’ cos() as in Figure (b).
•
Substituting the equation previously gives: ∅
• • • •
∆ ∅′
∆
∅
Value of can be found in any combination of V and H with and Nq from the latter equations. The value is plotted as in the Figure shown. If the footing is perfectly rough (’=’), sliding will occur if H/V tan ’ Similar to undrained case the EC7 apply additional inclinaion factor in the equation by :
Valid when H/V ∅′ to account for sliding Fot the case of strip footing on cohesionless soil (c’=0) ∅′ ∅′
∅
19
LECTURE 3
•
Shallow Foundation
Butterfield and Gottardi (1994) presented a yield surface of a general case of V-H-M loading on drained soil: .
•
.
.
For value of V/R at any value of H/V the value of Nq from the latter equation will be divided by the value of Nq at H/V=0; ∅ ∅
∅
and that H/R=(H/V) x
(V/R) thus the value of qf and Nq in latter equation can be expressed in terms of V/R and H/R for case M=0. •
•
• •
•
Figure (a) compares the lower bound solution , EC7 and the full yield stress surface for the case of V-H loading (M=0)
When M the yield surface will become 3 dimensiona surface which shows a contour of V/R for combinations of H and M under general loading for uae in ULS design from equation by Butterfield and Gottardi (1994). In EC7 also accounted for the moments effect through the use of B’=B-2e where e = M/V. For strip footing, the footing soil contact area is B’ per meter length under V-H-M loading and: ′ Under pure ;oading V (where V = R at bearing capacity failure)
Dividing the above equations and substituting for iq, B’ and e gives: ′
Foundation in Two way Eccentricity
20
LECTURE 3
• • • •
Shallow Foundation
Considering the foundation is subjected to Vertical Loading and Moment M. The moment component is determine in 2 direction namely Mx and My. This condition is equivalent to the load Qult placed eccentrically on the foundation with x = eB and y = eL Since then :
eB =
and eL =
• R = q’f A’ • A’ = effective area B’ x L’
When determine effective area (A’) four possible case may arise.
Case 1 :
Where
. . L’ is the larger of two dimension that is B1 or L1. B’ = A’/L’
Case II : eL/L 1 : acts as individual pile
•
In practise if = 1
Qgu = Qu
•
Another Equation use is a CONVERSE – LABARRE equation:-
(n1 1)n 2 (n 2 1)n1 1 90n1n 2 Where deg) = tan-1(D/d) Conclusion: 1. For driven pile in sand (group) d > = 3D Qgu = Qu 2. For Bore Piles (group) in sand d~ 3D; Qgu = 2/3 to ¾ Qu
23
LECTURE 4
DEEP FOUNDATION
GROUP PILES IN CLAY •
The ultimate load bearing capacity of group piles in clay can be estimated in the following manner:-
1. Determine Qu = n1n2(Qp + Qs) Nc = 9 for H/Bg > 5 Therefore : Qu = n1n2(9Apcu + cupL) 2. Determine the ultimate capacity assuming that the piles in the group act as a block with dimension Lg x Bg x L Skin resistance of block = pgcuL = 2(Lg + Bg)cuNc Point bearing : Apqp = ApcuNc = (LgBg)cuNc Qu = LgBgcu(p)Nc + 2(LgBg)cuL 3. Compare 1 and 2. The lowest value will govern the design criteria.
Lg/Bg
L
g
LOAD DISTRIBUTION ON GROUP PILE
• It is very difficult if not possible to determine the true distribution of load of a pile group. • Generally it is reliable to use methods that are simple but logical. • A vertical load on a group of vertical piles with an axis of symmetry is considered to be distributed according to the following equation which is similar to that of an eccentric load on pad footing.
24
LECTURE 4
DEEP FOUNDATION
Y
exx
X
Where : Axial load on an individual pile Pn N : Vertical load on group pile eyy n : Number of pile exx and eyy: Eccentricity of the load N about the centroidal axes XX and YY of pile group. : Second moment area of the pile group about axes XX and YY. Ixx and Iyy : Distance of the individual pile from axes YY and XX respectively. xn and yn ∑ ∑
n
n
with respect to XX axes with respect to YY axis
SETTLEMENT OF GROUP PILE AND BLOCK FAILURE
•
Similar to any cases of settlement, it can be classified into 2 types:-
a. Elastic settlement b. Consolidation settlement.
25
LECTURE 4
DEEP FOUNDATION
26
LECTURE 4
DEEP FOUNDATION
Elastic Settlement of Piles Groups The simplest relation for settlement of groups piles is given by Vesic(1969):-
Sg ( e )
Bg Ds
Where Bg = Width or pile group section D = Width or diameter of each pile in group s = elastic settlement of each pile at comparable working load.
For pile group in sand and gravel, Meyerhof (1976) suggested :-
Sg (e )(mm )
0.92q Bg I Ncor
Where q(kN/m2) = Qg/(LgBg) Lg and Bg : Length and width of the pile group section in m Ncor : Average corrected SPT within seat of settlement ( ~ Bg deep below the tip of the piles). I = Influence factor = 1 – L/8Bg >= 0.5 L = Length of pile embedment
• In similar manner, the pile group settlement can be related to the cone penetration resistance as:-
Sg ( e )
qBgI 2qc
Where qc = average cone resistance within the seat of settlement.
27
LECTURE 4
DEEP FOUNDATION
Elastic Settlement of Piles Under Working Load Caused by three factors:
s = s1 + s2 + s3 Where s : Total pile settlement s1 : Settlement of pile shaft s2 : settlement of pile caused by the load at pile point. s3 : settlement of pile caused by the load transmitted along the pile shaft.
Qwp Qws s1 L ApEp • Where : Qwp = load carried at the pile point under working load condition. Qws : load carried by skin resistance under working load condition Ap = Area of pile cross section L = Length of pile Ep = Young Modulus of the pile material.
•
Magnitude of depend on the skin resistance distribution as below:
f
f f
28
LECTURE 4
DEEP FOUNDATION
s2
qwpD 2 1 Iwp
s Where:Es D = width or pile diameter qwp = point load per unit area at the pile point Es
= Young Modulus of soil
s
= Poisson’s ratio of soil
= Qwp/Ap
Iwp = Influence factor
Iwp can be taken as shown while s is obtained from table given.
29
LECTURE 4
DEEP FOUNDATION
Vesic also proposed semiempirical method to obtain s2 as :-
s2
QwpCp Dqp
Where qp = ultimate point resistance of pile Cp = empirical coefficient. Cp is as given below(Vesic-1977) Soil Type
Driven Pile
Bored Pile
Sand(Dense to Loose) Clay(Stiff to Soft) Silt (Dense to Loose)
0.02-0.04 0.02-0.03 0.03-0.05
0.09-0.18 0.03-0.06 0.09-0.12
• S3
Qws D 2 1 Iws s 3 s pL E s
Where L Iws 2 0.35 P : perimeter of pile D L : embedded length of pile Iws : Influence factor Vesic (1977) proposed simple empirical relation for s3 as: s3
QwsCs Lqp
where L Cp Cs 0.93 0.16 D
Consolidation Settlement of Group Pile.
30
LECTURE 4
DEEP FOUNDATION
END BEARING
D
Bearing Stratum
SKIN FRICTION
2/3D
General Consolidation Settlement of Group Piles.
31
LECTURE 4
DEEP FOUNDATION
Estimation can be made using a 2:1 stress distribution methods as shown.
Step 1:
Let depth of embedment as L with the pile group subjected to load Qg. If pile cap is below the OGL, then Qg = Load from superstructure – effective weight of soil remove.
Step 2:
Assuming load Qg is transfer to the soil beginning at a depth of 2L/3 from the top of the pile. This is considered as depth z = 0. From then Qg is spreaded out in 1:2 direction to the below of the pile tip.
Contd… Step 3 :
Calculate the stress increase cause at the middle of each soil layer cause by load Qg:
Step 4 :
Calculate the settlement of each layer caused by the stress increase
Step 5 :
Total Consolidation of pile settlement is calculated by
Sc(g) = Sc(i) It should also be noted that the settlement can be initiated by fills nearby, adjacent floor load and lowering of water table.
32
LECTURE 4
DEEP FOUNDATION
END
33
LECTURE 5
SPECIAL CASES PILES
SPECIAL CASES PILES (LECTURE 5) ASSOC. PROF. Ir. DR. RAMLI NAZIR TEL : 013 7927925 OFF: 07 5531722
INSPIRING CREATIVE AND INNOVATIVE MINDS
Ultimate Lateral Resistance Of Pile
•
Normally pile resist vertical load. However in some cases horizontal load is more prominent such as gantries, quay, trnsmission tower, unipole structure etc.
•
In designing such foundation, two criteria must be satisfied:-
1. Adequate factor of safety against ultimate failure. 2. Acceptable deflection at working loads. •
In many practical cases, the design of piles for lateral loading will be dependant on satisfying a limiting lateral-deflection requirement that may result in the specification of allowable lateral loads much less than the ultimate lateral capacity of the piles.
1
LECTURE 5
SPECIAL CASES PILES
ULTIMATE SOIL RESISTANCE
•
For purely cohesive soil, the ultimate lateral resistance, Pu increases from the surface down to the depth of about 3 pile diameters and remain constant for a greater depth.
•
When Pu becomes constant, lateral failure involves plastic flow of the soil around the pile in horizontal plane only and the value Pu can be determine by plastic theory.
•
For more general cases of a c – soil, an alternative derivation of the ultimate lateral soil resistance, based essentially on earth pressure theory has been given by Brinch Hansen (1961)
2Cu
3D
8 – 12 CuD
2
LECTURE 5
•
SPECIAL CASES PILES
He considers the variation of resistance with depth along the pile. The ultimate resistance at any depth, z below the surface is expressed as :Pu = qKq + c Kc
Where
q = vertical overburden pressure c = soil cohesion value Kc and Kq = Factors that are a function of and z/d
BROMS’ THEORY
•
Essentially similar as Brinch Hansen method except that simplifications are made to the ultimate soil resistance distribution along the pile.
•
Full consideration is given to restrained or fixed head piles as well as unrestrained or free head pile
3
LECTURE 5
SPECIAL CASES PILES
Unrestrained or Free Headed Pile
•
For Short Pile (Rigid Pile) •
•
For Long Pile (Flexible Pile) •
•
Lateral resistant is dependant wholly on the soil resistance.
Lateral resistance is primarily dependant on the yield moment of the pile itself.
f defines the location of the maximum moment, and since the shear there is zero,
f
•
Hu 9cud
Eqn 1
Also taking moments about the maximum moment location, Mmax = Hu(e + 1.5d + 0.5f) ………………(Eqn 2)
•
Also, Mmax = 2.25dg2cu …………………………( Eqn 3)
• • • • •
Since L = 1.5d + f + g , eqn (1) and (2) can be solved for the ultimate lateral load, Hu. The solution is plotted in terms of L/d and H/cud2 and applies for short piles in which the yield moment My > Mmax For long piles, Eqn (3) no longer holds. Hu is obtained from Eqn(1) and (2) by setting Mmax equal to the known value of yield moment (My) The solution is plotted in terms of : Hu/cud2 and My/cud2
4
LECTURE 5
SPECIAL CASES PILES
RESTRAINED / FIXED HEADED PILE • • •
The change over points from one failure mode to another depend again on the yield moment of the pile. It is assumed that moment restrained equal to the moment in the pile just below the cap is available. For short piles: Hu = 9cud(L – 1.5d)
………….Eqn(4)
Mmax = Hu(0.5L + 0.75d) ……………..Eqn(5) •
For intermediate piles Eqn(3) and Eqn(1) holds and taking moments about the surface. My = 2.25cudg2 – 9cudf(1.5d + 0.5f)………………Eqn(6)
5
LECTURE 5
SPECIAL CASES PILES
•
This equation together with the relationship L = 1.5d + f + g May be solved for Hu. • • •
It is necessary to check the maximum positive moment, at depth f + 1.5d is less than My. Otherwise the failure mechanism for long piles holds. For the later mechanism, the following relationship applies :
Hu
2M y 1.5d 0.5f
…………..Eqn(7)
PILES IN COHESIONLESS SOIL • Following assumptions are made by Broms 1. Active pressure acting on the back of the pile is neglected. 2. Distribution of passive pressure along the front of the pile is equal to three times the Rankine passive pressure. 3. The shape of the pile section has no influence on the distribution of ultimate soil pressure or the ultimate lateral resistance. 4. The full lateral resistance is mobilised at the movement considered. •
The distribution of soil resistance is : Pu = 3’vKp
Where
’v: effective vertical overburden pressure Kp: Rankine passive earth pressure coefficient
6
LECTURE 5
SPECIAL CASES PILES
UNRESTRAINED/FREE HEAD • •
•
Pile will act as a short pile if the maximum moment is less than the yield moment of the section. The rotation is assumed to be about a point close to the tip and a high pressure acting near this point are replaced by a single concentrated force at the tip. Taking moment about the toe, 3
Hu • •
0.5 dL Kp
……………….Eqn.(9)
eL
The relationship can be plotted using L/d and Hu/Kpd3. Maximum moment occurs at a distance of below the surface where:Hu = (3/2) dKpf2 …………………………..Eqn (10)
Where
f 0.82
Hu dKp
……………………….Eqn(11)
The maximum moment is : Mmax = Hu(e + 2/3(f)) ……………….Eqn(12) •
If after using Eqn(9), the calculated Hu results in Mmax>Myield, then the pile is treated as long pile.
•
Hu may then be calculated from Eqn(10) and (12) putting Mmax=Myield.
•
Solution of the long pile can be plotted as : Hu/Kpd3 and My/d4Kp
7
LECTURE 5
SPECIAL CASES PILES
OTHER METHODS •
Dickin and Wei’s Method.
•
Limitation for pile with L/D less than 3.
M D
3
L L 1 Km D
where e Km 0.24 0.08 ln D • • •
L : pile embedment length D : pile diameter M : Moment at ground level
•
Dickin’s and Ramli’s method
• • • •
Using the integration of plain strain condition to three dimensional projection. Apply shape factor as integration coefficient. Limitation is for L/D < 7 If L/D more than 7, Brom’s equation is applicable.
2
4eL D M
eD
2
Sfm Sfm : Shape factor
where Sfm
M : moment at ground Level
2 7
L D
1
Thus
M = Hu x e
8
LECTURE 5
SPECIAL CASES PILES
RESTRAINED OR FIXED HEAD PILE •
For short piles horizontal equilibrium it gives:Hu = 1.5L2dKp ……………………………………………..Eqn(13)
•
Maximum moment is :Mmax = 2/3 (HuL) ……………………………..Eqn(14).
•
If Mmax>Myield, the intermediate pile is relevant by giving ,f (horizontal equilibrium) as:f = (3/2)dL2Kp – Hu ………………………..Eqn(15)
•
Taking moments about the top of the pile and substituting F from Eqn(15) Myield = 0.5dL3Kp – HuL …………………….Eqn(16) Hence Hu can be obtained.
•
The equations only holds if the maximum moment at depth f is less than Myield. f is calculated from the earlier equations.
•
For long pile where maximum moment reaches Myield at two locations, it is readily found that :-
2Myield Hu (e
2 f) 3
Pile Behaviour Predictions • Apart from determining using Mmax and Myield to determine long pile or short pile, Matlock and Reese employed Stiffness Factors R and T to predict whether the pile acts as long or short pile. • Both units are in m. • For clay :
EpIp 4 R khB
• For sand :
EpIp 5 T nh
1
1
• Where , E : Pile Young Modulus, I : moment of inertia of pile cross section, B : Width or pile diameter, kh and nh as given.
9
LECTURE 5
SPECIAL CASES PILES
Criteria in Pile Classification • If L≤ 2T or ≤ 2R , the pile is short or rigid • If L≥ 4T or ≥ 3.5R, the pile is long and elastic • For the intermediate case the value is in between. • The following length are considered sufficient to develop the maximum resistance in the soil and there will be not much benefit in taking piles deeper than these values. 1. 2.
For constant soil modulus, free head L = 3.5R and fixed head L = 2R For linearly increase soil modulus, free head L = 4T and fixed head, L = 2T.
•
Typical value of nh and kh nh (MN/m3) for granular soil Loose
Medium
Dense
Dry or moist
2.2
6.6
17.5
Submerged
1.25
4.4
10.5
kh (MN/m2) for cohesive soil Soft Medium Stiff
8.0 16.0 32.0
DETERMINATION OF DISPLACEMENT AT TOP OF PILE •
Displacement at top of the pile can be obtained from Figure A for cohesive soil and Figure B for cohesionless soil.
• •
Figure A Deflection has been plotted as a function of the dimensionless length L where:………………………Eqn (1) khB 1
4 EpIp
Ep : Modulus of elasticity of pile materials Ip : Moment of inertia of pile in bending plane B : Pile diameter or width kh : coefficient of horizontal subgrade reaction
10
LECTURE 5
SPECIAL CASES PILES
•
Much of the accuracy depend on kh value. • In absence of such information kh can be estimated by : kh = nhz/B Where z is the point considered nh is constant of horizontal subgrade reaction for piles in soil
• •
Figure B For cohesionless soil and the relative stiffness of the pile and the soil are contained in the parameter h where :-
nh 1 5 EpIp
….. Eqn(2)
EXAMPLE I II
11
LECTURE 5
SPECIAL CASES PILES
DETERMINATION OF DISPLACEMENT AT TOP OF PILE •
Displacement at top of the pile can be obtained from Figure A for cohesive soil and Figure B for cohesionless soil.
• •
Figure A Deflection has been plotted as a function of the dimensionless length L where:………………………Eqn (1) khB 1
4 EpIp
Ep : Modulus of elasticity of pile materials Ip : Moment of inertia of pile in bending plane B : Pile diameter or width kh : coefficient of horizontal subgrade reaction
•
Much of the accuracy depend on kh value. In absence of such information kh can be estimated by : kh = nhz/B Where z is the point considered nh is constant of horizontal subgrade reaction for piles in soil
•
• •
Figure B For cohesionless soil and the relative stiffness of the pile and the soil are contained in the parameter where :-
nh 1 5 E p Ip
….. Eqn(2)
12
LECTURE 5
•
SPECIAL CASES PILES
Typical value of nh and kh nh (MN/m3) for granular soil Loose
Medium
Dense
Dry or moist
2.2
6.6
17.5
Submerged
1.25
4.4
10.5
kh (MN/m2) for cohesive soil Soft Medium Stiff
8.0 16.0 32.0
PULL OUT RESISTANCE OF PILE
• The gross uplift resistance of the pile subjected to uplift forces:-
Tug
Tug = Tun + W Where Tug Tun W
: Gross uplift capacity : Net uplift capacity : Effective weight of pile
Tun
L W
D
13
LECTURE 5
SPECIAL CASES PILES
Pile Embedded in Saturated Clay
•
Das and Seeley (1982) Tun = Lp’cu
L P ’ cu
: Length of pile : Pile perimeter : adhesion coefficient at soil-pile interface : Undrained cohesion of clay
• For cast insitu piles (concrete) ’ = 0.9 – 0.00625cu (for cu 80 kPa) • For pipe piles ’ = 0.715 – 0.0191cu (for cu 27 kPa)
Pile Embedded in Granular Soil
L
Tun (fup )dz 0
• fu = Unit Skin Resistance during uplift p = Perimeter of pile cross section • The unit skin friction varies with depth. It increase linearly up to a depth of z = Lcr. Beyond that it remain constant. •
For z
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