EAG 345 – GEOTECHNICAL ANALYSIS

EAG 345 – GEOTECHNICAL ANALYSIS
(iv) Determination of shear strength parameters of soils By: Dr Mohd Ashraf Mohamad Ismail

Determination of shear strength parameters of soils (c, f or c’, f’)
Laboratory tests on specimens taken from representative undisturbed samples Field tests Most common laboratory tests to determine the shear strength parameters are: Direct shear test Triaxial shear test Vane shear test Torvane Pocket penetrometer Fall cone Pressure meter Static cone penetrometer Standard penetration test Other laboratory tests include, Direct simple shear test, torsional ring shear test, plane strain triaxial test, laboratory vane shear test, laboratory fall cone test

Laboratory tests Field conditions z z svc svc + Ds shc shc
After and during construction Field conditions z svc shc Before construction A representative soil sample

How to take undisturbed samples
Laboratory tests How to take undisturbed samples

Laboratory tests Simulating field conditions in the laboratory shc
svc + Ds shc Traxial test Laboratory tests Simulating field conditions in the laboratory Representative soil sample taken from the site Step 1 Set the specimen in the apparatus and apply the initial stress condition svc shc svc t Direct shear test Step 2 Apply the corresponding field stress conditions

Schematic diagram of the direct shear apparatus
Direct shear test Schematic diagram of the direct shear apparatus

Preparation of a sand specimen
Direct shear test Direct shear test is most suitable for consolidated drained tests specially on granular soils (e.g.: sand) or stiff clays Preparation of a sand specimen Components of the shear box Preparation of a sand specimen Porous plates

Preparation of a sand specimen
Direct shear test Preparation of a sand specimen Specimen preparation completed Pressure plate Leveling the top surface of specimen

Direct shear test Steel ball P Test procedure Pressure plate
Step 1: Apply a vertical load to the specimen and wait for consolidation P Pressure plate Porous plates Proving ring to measure shear force S

Direct shear test Steel ball P Test procedure Pressure plate
Step 1: Apply a vertical load to the specimen and wait for consolidation P Test procedure Pressure plate Steel ball Proving ring to measure shear force S Porous plates Step 2: Lower box is subjected to a horizontal displacement at a constant rate

Direct shear test Shear box Loading frame to apply vertical load
Dial gauge to measure vertical displacement Shear box Proving ring to measure shear force Dial gauge to measure horizontal displacement Loading frame to apply vertical load

Analysis of test results
Direct shear test Analysis of test results Note: Cross-sectional area of the sample changes with the horizontal displacement

Direct shear tests on sands
Stress-strain relationship Shear stress, t Shear displacement Dense sand/ OC clay tf Loose sand/ NC clay tf Change in height of the sample Expansion Compression Shear displacement Dense sand/OC Clay Loose sand/NC Clay

How to determine strength parameters c and f Shear stress, t Shear displacement tf3 Normal stress = s3 tf2 Normal stress = s2 tf1 Normal stress = s1 Shear stress at failure, tf Normal stress, s f Mohr – Coulomb failure envelope

Some important facts on strength parameters c and f of sand: Sand is cohesionless hence c = 0 Direct shear tests are drained and pore water pressures are dissipated, hence u = 0 Therefore, f’ = f and c’ = c = 0

Direct shear tests on clays
In case of clay, horizontal displacement should be applied at a very slow rate to allow dissipation of pore water pressure (therefore, one test would take several days to finish) Failure envelopes for clay from drained direct shear tests Shear stress at failure, tf Normal force, s f’ Normally consolidated clay (c’ = 0) Overconsolidated clay (c’ ≠ 0)

Interface tests on direct shear apparatus
In many foundation design problems and retaining wall problems, it is required to determine the angle of internal friction between soil and the structural material (concrete, steel or wood) Where, ca = adhesion, d = angle of internal friction

Advantages of direct shear apparatus
Due to the smaller thickness of the sample, rapid drainage can be achieved Can be used to determine interface strength parameters Clay samples can be oriented along the plane of weakness or an identified failure plane Disadvantages of direct shear apparatus Failure occurs along a predetermined failure plane Area of the sliding surface changes as the test progresses Non-uniform distribution of shear stress along the failure surface

Triaxial Shear Test Soil sample at failure Failure plane Soil sample
Porous stone impervious membrane Piston (to apply deviatoric stress) O-ring pedestal Perspex cell Cell pressure Back pressure Pore pressure or volume change Water Soil sample

Triaxial Shear Test Specimen preparation (undisturbed sample)
Sample extruder Sampling tubes

Triaxial Shear Test Specimen preparation (undisturbed sample)
Edges of the sample are carefully trimmed Setting up the sample in the triaxial cell

Specimen preparation (undisturbed sample)
Triaxial Shear Test Specimen preparation (undisturbed sample) Sample is covered with a rubber membrane and sealed Cell is completely filled with water

Triaxial Shear Test Specimen preparation (undisturbed sample)

Triaxial Shear Test Specimen preparation (undisturbed sample) Proving ring to measure the deviator load Dial gauge to measure vertical displacement

Types of Triaxial Tests
deviatoric stress ( = q) Shearing (loading) Step 2 c c+ q Under all-around cell pressure c c Step 1 Is the drainage valve open? Is the drainage valve open? yes no Consolidated sample Unconsolidated sample yes no Drained loading Undrained loading

Types of Triaxial Tests
Is the drainage valve open? yes no Consolidated sample Unconsolidated sample Under all-around cell pressure c Step 1 Is the drainage valve open? yes no Drained loading Undrained loading Shearing (loading) Step 2 CU test CD test UU test

CD, CU and UU Triaxial Tests
Consolidated Drained (CD) Test no excess pore pressure throughout the test very slow shearing to avoid build-up of pore pressure Can be days!  not desirable gives c’ and ’ Use c’ and ’ for analysing fully drained situations (e.g., long term stability, very slow loading)

Consolidated Undrained (CU) Test pore pressure develops during shear Measure  ’ gives c’ and ’ faster than CD (preferred way to find c’ and ’)

Unconsolidated Undrained (UU) Test pore pressure develops during shear = 0; i.e., failure envelope is horizontal Not measured ’ unknown analyse in terms of   gives cu and u very quick test Use cu and u for analysing undrained situations (e.g., short term stability, quick loading)

Consolidated- drained test (CD Test)
Total, s = Neutral, u Effective, s’ + Step 1: At the end of consolidation sVC shC s’VC = sVC s’hC = shC Drainage Step 2: During axial stress increase sVC + Ds shC s’V = sVC + Ds = s’1 s’h = shC = s’3 Drainage Step 3: At failure sVC + Dsf shC s’Vf = sVC + Dsf = s’1f s’hf = shC = s’3f Drainage

s1 = sVC + Ds s3 = shC Deviator stress (q or Dsd) = s1 – s3

Volume change of sample during consolidation Volume change of the sample Expansion Compression Time

Stress-strain relationship during shearing Deviator stress, Dsd Axial strain Dense sand or OC clay (Dsd)f Loose sand or NC Clay (Dsd)f Volume change of the sample Expansion Compression Axial strain Dense sand or OC clay Loose sand or NC clay

CD tests f s or s’ s3 Shear stress, t s3c s1c s3b s1b (Dsd)fb s3a s1a
How to determine strength parameters c and f Deviator stress, Dsd Axial strain (Dsd)fc Confining stress = s3c s1 = s3 + (Dsd)f s3 (Dsd)fb Confining stress = s3b (Dsd)fa Confining stress = s3a f Mohr – Coulomb failure envelope Shear stress, t s or s’ s3c s1c s3b s1b (Dsd)fb s3a s1a (Dsd)fa

Since u = 0 in CD tests, s = s’
Strength parameters c and f obtained from CD tests Since u = 0 in CD tests, s = s’ Therefore, c = c’ and f = f’ cd and fd are used to denote them

CD tests fd s or s’ Failure envelopes For sand and NC Clay, cd = 0
Shear stress, t s or s’ fd Mohr – Coulomb failure envelope s3a s1a (Dsd)fa Therefore, one CD test would be sufficient to determine fd of sand or NC clay

t CD tests f c s or s’ Failure envelopes For OC Clay, cd ≠ 0 OC NC s3
(Dsd)f c sc OC NC

Some practical applications of CD analysis for clays
1. Embankment constructed very slowly, in layers over a soft clay deposit Soft clay t t = in situ drained shear strength

t = drained shear strength of clay core t Core 2. Earth dam with steady state seepage End of construction – construction pore pressure usually reaches their maximum values when the embankment reaches maximum height Steady-state seepage – after the reservoir has been filled for a long time, pore pressures are determined by steady state seepage conditions and may be estimated from a flow net where gravitational flow conditions govern.

3. Excavation or natural slope in clay t t = In situ drained shear strength Note: CD test simulates the long term condition in the field. Thus, cd and fd should be used to evaluate the long term behavior of soils

Consolidated- Undrained test (CU Test)
Total, s = Neutral, u Effective, s’ + Step 1: At the end of consolidation sVC shC s’VC = sVC s’hC = shC Drainage Step 2: During axial stress increase sVC + Ds shC s’V = sVC + Ds ± Du = s’1 s’h = shC ± Du = s’3 No drainage ±Du Step 3: At failure sVC + Dsf shC s’Vf = sVC + Dsf ± Duf = s’1f s’hf = shC ± Duf = s’3f No drainage ±Duf

Volume change of sample during consolidation Volume change of the sample Expansion Compression Time

Stress-strain relationship during shearing Deviator stress, Dsd Axial strain Dense sand or OC clay (Dsd)f Loose sand or NC Clay (Dsd)f Du + - Axial strain Loose sand /NC Clay Dense sand or OC clay

CU tests fcu s or s’ s3 Shear stress, t s3b s1b s3a s1a (Dsd)fa
How to determine strength parameters c and f Deviator stress, Dsd Axial strain (Dsd)fb Confining stress = s3b s1 = s3 + (Dsd)f s3 Total stresses at failure (Dsd)fa Confining stress = s3a Shear stress, t s or s’ fcu Mohr – Coulomb failure envelope in terms of total stresses ccu s3b s1b s3a s1a (Dsd)fa

CU tests f’ fcu s or s’ s’3 = s3 - uf Shear stress, t s3b s1b (Dsd)fa
How to determine strength parameters c and f s’1 = s3 + (Dsd)f - uf s’3 = s3 - uf uf Mohr – Coulomb failure envelope in terms of effective stresses f’ C’ Effective stresses at failure Shear stress, t s or s’ Mohr – Coulomb failure envelope in terms of total stresses fcu s3b s1b (Dsd)fa ufb ufa ccu s’3b s’1b s3a s1a s’3a s’1a (Dsd)fa

Shear strength parameters in terms of total stresses are ccu and fcu
CU tests Strength parameters c and f obtained from CD tests Shear strength parameters in terms of total stresses are ccu and fcu Shear strength parameters in terms of effective stresses are c’ and f’ c’ = cd and f’ = fd

CU tests f’ fcu s or s’ Failure envelopes
For sand and NC Clay, ccu and c’ = 0 Shear stress, t s or s’ fcu Mohr – Coulomb failure envelope in terms of total stresses s3a s1a (Dsd)fa f’ Mohr – Coulomb failure envelope in terms of effective stresses Therefore, one CU test would be sufficient to determine fcu and f’(= fd) of sand or NC clay

Some practical applications of CU analysis for clays
1. Embankment constructed rapidly over a soft clay deposit Soft clay t t = in situ undrained shear strength

2. Rapid drawdown behind an earth dam Core t = Undrained shear strength of clay core Rapid draw down – the upstream slope stability may be critical for the rapid draw down condition where the water in the reservoir can drop drastically.

3. Rapid construction of an embankment on a natural slope t = In situ undrained shear strength t Note: Total stress parameters from CU test (ccu and fcu) can be used for stability problems where, Soil have become fully consolidated and are at equilibrium with the existing stress state; Then for some reason additional stresses are applied quickly with no drainage occurring

EAG 345 – GEOTECHNICAL ANALYSIS

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EAG 345 – GEOTECHNICAL ANALYSIS

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