EAG 345 – GEOTECHNICAL ANALYSIS (iv) Determination of shear strength parameters of soils (2) By: Dr Mohd Ashraf Mohamad Ismail

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

Unconsolidated- Undrained test (UU Test) Data analysis s3 + Dsd s3 No drainage Specimen condition during shearing sC = s3 No drainage Initial specimen condition Initial volume of the sample = A0 × H0 Volume of the sample during shearing = A × H Since the test is conducted under undrained condition, A × H = A0 × H0 A ×(H0 – DH) = A0 × H0 A ×(1 – DH/H0) = A0

Unconsolidated- Undrained test (UU Test) Step 1: Immediately after sampling Step 2: After application of hydrostatic cell pressure s’3 = s3 - Duc sC = s3 No drainage Duc = + Increase of pwp due to increase of cell pressure Duc = B Ds3 Skempton’s pore water pressure parameter, B Increase of cell pressure Note: If soil is fully saturated, then B = 1 (hence, Duc = Ds3)

Unconsolidated- Undrained test (UU Test) Step 3: During application of axial load s3 + Dsd s3 No drainage = Duc ± Dud + Increase of pwp due to increase of deviator stress Dud = ABDsd Increase of deviator stress Skempton’s pore water pressure parameter, A

Unconsolidated- Undrained test (UU Test) Combining steps 2 and 3, Duc = B Ds3 Dud = ABDsd Total pore water pressure increment at any stage, Du Du = Duc + Dud Du = B [Ds3 + ADsd] Skempton’s pore water pressure equation Du = B [Ds3 + A(Ds1 – Ds3]

Typical values for parameter B f (saturation) Degree of saturation

Typical values for parameter A

Relation between effective and total stress criteria Three identical saturated soil samples are sheared to failure in UU triaxial tests. Each sample is subjected to a different cell pressure. No water can drain at any stage. At failure the Mohr circles are found to be as shown t s s3 s1

Relation between effective and total stress criteria Three identical saturated soil samples are sheared to failure in UU triaxial tests. Each sample is subjected to a different cell pressure. No water can drain at any stage. At failure the Mohr circles are found to be as shown t s s3 s1 We find that all the total stress Mohr circles are the same size, and therefore fu = 0 and t = su = cu = constant

Relation between effective and total stress criteria Because each sample is at failure, the fundamental effective stress failure condition must also be satisfied. As all the circles have the same size there must be only one effective stress Mohr circle t s s3 s1

Relation between effective and total stress criteria The different total stress Mohr circles with a single effective stress Mohr circle indicate that the pore pressure is different for each sample. As discussed previously increasing the cell pressure without allowing drainage has the effect of increasing the pore pressure by the same amount (Du = Dsc) with no change in effective stress. The change in pore pressure during shearing is a function of the initial effective stress and the moisture content. As these are identical for the three samples an identical strength is obtained.

Significance of undrained strength parameters It is often found that a series of undrained tests from a particular site give a value of fu that is not zero (cu not constant). If this happens either the samples are not saturated, or the samples have different moisture contents If the samples are not saturated analyses based on undrained behaviour will not be correct The undrained strength cu is not a fundamental soil property. If the moisture content changes so will the undrained strength.

Unconsolidated- Undrained test (UU Test) Effect of degree of saturation on failure envelope t s or s’ S < 100% S > 100% s3a s1a s3b s1b s3c s1c

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

Some practical applications of UU analysis for clays 2. Large earth dam constructed rapidly with no change in water content of soft clay t = Undrained shear strength of clay core t Core

Some practical applications of UU analysis for clays 3. Footing placed rapidly on clay deposit t = In situ undrained shear strength Note: UU test simulates the short term condition in the field. Thus, cu can be used to analyze the short term behavior of soils

Unconfined Compression Test (UC Test) s1 = sVC + Ds s3 = 0 Confining pressure is zero in the UC test

Unconfined Compression Test (UC Test) s1 = sVC + Dsf s3 = 0 Shear stress, t Normal stress, s qu Note: Theoritically qu = cu , However in the actual case qu < cu due to premature failure of the sample

Other laboratory shear tests Direct simple shear test Torsional ring shear test Plane strain triaxial test

Other laboratory shear tests Direct simple shear test Torsional ring shear test Plane strain triaxial test

Direct simple shear test f = 80 mm Soil specimen Porous stones Spiral wire in rubber membrane Direct shear test Direct simple shear test

Other laboratory shear tests Direct simple shear test Torsional ring shear test Plane strain triaxial test

Torsional ring shear test Peak Residual t Shear displacement tf s’ f’max f’res The ring shear test is suited to the relatively rapid determination of drained residual shear strength because of the short drainage path through the thin specimen, and the capability of testing one specimen under different normal stresses to quickly obtain a shear strength envelope. The test results are primarily applicable to assess the shear strength in slopes that contain a preexisting shear surface, such as old landslides, and sheared bedding planes, joints, or faults.

Torsional ring shear test sN Preparation of ring shaped undisturbed samples is very difficult. Therefore, remoulded samples are used in most cases

Other laboratory shear tests Direct simple shear test Torsional ring shear test Plane strain triaxial test

Plane strain triaxial test s’1, e1 s’2, e2 s’3, e3 Plane strain test s’2 ≠ s’3 e2 = 0 s’1 s’2 s’3 Rigid platens Specimen

Drained and undrained conditions DRAINED condition occurs when there is no change in pore water pressure due to external loading In a drained condition, the pore water can drain out of the soil easily, causing volumetric strains in the soil UNDRAINED condition occurs when the pore water is unable to drain out of the soil In undrained condition the rate of loading is much quicker than the rate at which the pore water is able to drain out of the soil As a result, most of the external loading is taken by the pore water, resulting in an increase in the pore water pressure. The tendency of soil to change in volume is suppressed during undrained loading.

Drained and undrained conditions The existence of either a drained or an undrained condition in a soil depends on: ((i)soil types; fine-grained or coarse grained, (ii) geological formation and (iii) rate of loading) For a rate of loading associated with a normal construction activity, saturated coarse grained soils (e.g. sands and gravel) experience drained conditions and saturated fine-grained soils (e.g. silts and clays) experience undrained conditions If the rate of loading is fast enough (e.g. during an earthquake), even coarse-grained soils can experience undrained loading, often resulting in liquefaction.

Example of undrained loading (Liquefaction) When loading is rapidly applied and large enough such that it does not flow out in time before the next cycle of load is applied, the water pressure may build to an extent where they exceed the contact stresses between the grains of soil that keep them in contact with each other. These contact between grains are the means by which the weight of the buildings and overlying soil layers are transferred from the ground surface to layers of soil or rock at greater depth. This loss of soil structure causes it to lose all of its strength and it may be observed to flow like a liquid.

Example of undrained loading (Liquefaction) Uplift of sewerage during Niigata earthquake 2004 Collapse of flat house during the 1964 Niigata earthquake, Japan.

Drained and undrained conditions The shear strength of a fine-grained soil under undrained condition is called the undrained shear strength and denoted as su. su is the radius of the Mohr’s circle of total stress: The undrained shear strength depends only on the initial void ratio or the initial water content of the soil τ σ, σ’ Total stress circle

Drained and undrained conditions The undrained shear strength is not a fundamental soil parameter. Its value depends on the values of the initial confining stresses. τ An increase in initial confining stresses causes a decrease in void ratio and an increase in undrained shear strength σ, σ’

Selection of shear strength parameter – Drained or undrained ? CU with pore water pressure measurement When designing a geotechnical structure, both undrained and drained conditions must be considered to determine which one is more critical

Drained and Undrained shear strength Condition Drained Undrained Excess porewater pressure Not zero; could be positive or negative Volume change Compression Positive excess porewater pressure Expansion Negative excess porewater pressure Consolidation Yes, fine grained soil No Yes Yes, but lateral expansion must occur so that the volume change is zero Analysis Effective stress (EFA) Total stress (TSA) Design parameters Homework: Do reading from page 243 – 245 (Section 7.8) – Soil mechanics and foundations