CE342 Lecture Notes - Shear Strength of Soil PDF

SOIL MECHANICS CE342 - FALL 2024 DR. ASHREF ALZAWI 1 CHAPTER FIVE: SHEAR STRENGTH OF SOIL 2 Shear Strength of Soil Mohr–Coulomb Failure Criterion There are many ways of defining failure in real materials, or failure criteria. Various theories ar...

SOIL MECHANICS CE342 - FALL 2024 DR. ASHREF ALZAWI 1 CHAPTER FIVE: SHEAR STRENGTH OF SOIL 2 Shear Strength of Soil Mohr–Coulomb Failure Criterion There are many ways of defining failure in real materials, or failure criteria. Various theories are available for different engineering materials. However, no one is general for all materials. The one generally accepted and used for soil is Mohr Theory of Failure. According to Coulomb relation for shear strength: 𝜏𝑓 = 𝑐 + 𝜎 tan ∅ Where: f = shear strength c = cohesion  = normal stress on the failure plane  = angle of internal friction 3 Shear Strength of Soil Mohr–Coulomb Failure Criterion 4 Shear Strength of Soil Mohr–Coulomb Failure Criterion 5 Shear Strength of Soil Mohr–Coulomb Failure Criterion 6 Shear Strength of Soil Mohr–Coulomb Failure Criterion in terms of effective stress 7 Shear Strength of Soil Mohr–Coulomb Failure Criterion in terms of total stress 8 Shear Strength of Soil Mohr–Coulomb Failure Envelope 9 Shear Strength of Soil Determination of shear strength parameters of soils 10 Shear Strength of Soil Laboratory Shear Strength Testing 11 Shear Strength of Soil -Direct Shear Test Direct Shear Test 12 Shear Strength of Soil - Direct Shear Test Direct Shear Test Procedure A constant vertical force (normal stress) is applied through a metal platen. Shear force is applied by moving one half of the box relative to the other and increased to cause failure in the soil sample. The tests are repeated on similar specimens at various normal stresses. The normal stresses and the corresponding values of f obtained from a number of tests are plotted on a graph from which the shear strength parameters are determined. 13 Shear Strength of Soil - Direct Shear Test Stress-Strain Relationship Nature of variation of void ratio with shearing displacement Plot of shear stress and change in height of specimen against shear 14 displacement for loose and dense dry sand Shear Strength of Soil - Direct Shear Test Stress-Strain Relationship 15 Shear Strength of Soil - Direct Shear Test Stress-Strain Relationship 16 Shear Strength of Soil - Direct Shear Test Effective stress failure envelope for NC/OC clay Direct shear tests on clays must be conducted very slowly so that pore pressures generated during shearing can dissipate. This is known as a "drained" test. Time for failure is often set as 7 times the time required for primary consolidation!! 17 Shear Strength of Soil -Direct Shear Test Advantages of Direct Shear Test Inexpensive, fast, and simple, especially for granular materials. Easiness of sample preparation in case of sand. Due to the smaller thickness of the sample, rapid drainage can be achieved. Large deformations can be achieved by reversing shear direction. This is useful for determining the residual strength of a soil. Samples may be sheared along predetermined planes. This is useful when the shear strengths along fissures or an interface is required. 18 Shear Strength of Soil -Direct Shear Test Disadvantages of Direct Shear Test Failure occurs along a predetermined failure plane which may not be the weakest plane. Non-uniform of shear stresses along failure surface in the specimen. There are rather stress concentrations at the sample boundaries, which lead to highly nonuniform stress conditions within the test specimen itself. There is no means of estimating pore pressures, so effective stresses cannot be determined and only the total normal stress can be determined. It is very difficult if not impossible to control drainage, especially for fine-grained soils. Consequently, the test is not suitable for other than completely drained conditions. 19 Shear Strength of Soil -Direct Shear Test 20 Shear Strength of Soil -Direct Shear Test 21 Shear Strength of Soil -Direct Shear Test 22 Shear Strength of Soil –Triaxial Shear Test Introduction: Triaxial test was developed by Casagrande to overcome some of the serious disadvantages of direct shear test. Advantages of TST over DST: More versatile Drainage can be well controlled There is no rotation of principal stresses like the direct shear test The failure plane can occur 23 Shear Strength of Soil –Triaxial Shear Test Introduction: Since direct shear test has severe limitations on the types of stresses that can be imposed on the soil specimen, the amount of control over those applied stresses can be increased by performing a triaxial test instead. Triaxial Device: A pressurized cell in which a confined soil specimen can be loaded in a variety of ways, the most common being axial compression. 24 Shear Strength of Soil –Triaxial Shear Test Triaxial test preformed in two stages: Stage One: Consolidation Two possible drainage conditions: Unconsolidated (undrained conditions) – drainage volve is closed No volume change Excess pore pressure will be generated Consolidated (drained conditions) – drainage volve is open Volume change is allowed No excess pore pressure Stage Two: Shearing Two possible drainage conditions: Undrained – drainage volve is closed No volume change Excess pore pressure will be generated and measured Drained – drainage volve is open Volume change is allowed No excess pore pressure 25 Shear Strength of Soil –Triaxial Shear Test 26 Shear Strength of Soil –Triaxial Shear Test Types of Triaxial Shear test: 1. Consolidated – Drained (CD or S) Test: Slow Test Slow test or effective test (provides effective strength parameters) 2. Consolidated – Undrained (CU or R) Test: Regular Test Total & effective test (provides total & effective strength parameters) 3. Unconsolidated – Undrained (UU or Q) Test: Quick Test 4. Unconfined Compression (UC) Test: 27 Shear Strength of Soil –Triaxial Shear Test 1. Consolidated-Drained (CD) Triaxial Test: (Test is performed so slowly u that there is no generation of ’ excess pore water pressure) V 28 Shear Strength of Soil –Triaxial Shear Test 1. Consolidated-Drained (CD) Triaxial Test: Also called slow test Apply 3 and wait until the soil consolidates Drainage valves OPEN during consolidation as well as shearing phases Complete sample drainage is achieved prior to application of the vertical load The load is applied at such a slow strain rate that particle readjustments in the specimen do not induce any excess pore pressure. (it might take up to 2 weeks), deviatoric stress, d Since there is no excess pore pressure total stresses will equal effective stresses CD test simulates long term shear strength for cohesive soils. 29 Shear Strength of Soil –Triaxial Shear Test 1. Consolidated-Drained (CD) Triaxial Test: Effective stress failure envelope from drained tests on sand and normally consolidated clay 31 Shear Strength of Soil –Triaxial Shear Test 1. Consolidated-Drained (CD) Triaxial Test: Effective stress failure envelope from drained tests on overconsolidated clay 32  Shear Strength of Soil –Triaxial Shear Test 2. Consolidated-Undrained (CU) Triaxial Test: u ’ Wants consolidate … u V Wants dilate … u TWO different failure envelopes can be generated by CU test: 1) Undrained failure envelop: This envelope includes the effects of excess pore pressures acting on the soil. Represents the soil behavior while the soil is still adjusting to a new induced shear stress. 2) Drained failure envelop: This envelope has no excess pore pressure effects affecting the soil particles. Represents the soil behavior after all of the induced stress has been transferred to the soil particles (e.g., after consolidation has occurred), an no excess pore pressure remains in the soil due to the induced shear 33 stress. Shear Strength of Soil –Triaxial Shear Test 2. Consolidated-Undrained (CU) Triaxial Test: Also called Regular test Apply 3 and wait until the soil consolidates Drainage valves OPEN during consolidation phase but CLOSED during the shearing phase Loading does not commence until the sample ceases to drain (or consolidate). CU test simulates long term as well as short term shear strength for cohesive soils if pore water pressure is measured during the shearing phase For this Test, cT  c’ & T  ’ (cT & T for total stress, c’ & ’ for effective stress) 34 Shear Strength of Soil –Triaxial Shear Test 2. Consolidated-Undrained (CU) Triaxial Test: Total & effective stress failure envelope from CU tests on sand and normally consolidated clay 36 Shear Strength of Soil –Triaxial Shear Test 2. Consolidated-Undrained (CU) Triaxial Test: Total stress failure envelope from CU tests on overconsolidated clay 37 Shear Strength of Soil –Triaxial Shear Test 3. Unconsolidated-Undrained (UU) Triaxial Test: 𝝉𝒇 = 𝒄 = 𝒄𝒖 Where: cu = undrained shear strength  = 0 for saturated clay & silt 38 Shear Strength of Soil –Triaxial Shear Test 3. Unconsolidated-Undrained (UU) Triaxial Test: Also called quick test Critical assumptions associated with the UU test: 1. The existing effective stress of the soil sample placed in the membrane and triaxial cell is the same as its effective stress in the ground. 2. The soil is completely saturated. 3 and d are applied fast, so that the soil does not have time to settle or consolidate Drainage valves CLOSED during all phases of the test (Water is not allowed to drain) UU test simulates short term shear strength for cohesive soils For this Test,  = ’ = 0 & c = cu = Su = (1 – 3)/2 39 Shear Strength of Soil 4. Unconfined Compression Test on Saturated Clay: 40 Shear Strength of Soil 4. Unconfined Compression Test on Saturated Clay: For clay soils No confining stress → 3 = 0 Axial stress →  = 1 𝜎1 𝑞𝑢 𝜏𝑓 = = = 𝑐𝑢 2 2 Where: cu = undrained shear strength qu = unconfined compression strength 41 Shear Strength of Soil –Triaxial Shear Test 42 Shear Strength of Soil –Triaxial Shear Test 43 Shear Strength of Soil –Triaxial Shear Test 44 Shear Strength of Soil –Triaxial Shear Test 45 Shear Strength of Soil –Triaxial Shear Test 46 Shear Strength of Soil –Triaxial Shear Test 47 Shear Strength of Soil –Triaxial Shear Test 48 Shear Strength of Soil Other Methods for Estimating/Measuring Undrained Shear Strength (Su): Su/o’ Method: (Skempton (1957)) 𝑺𝒖 ′ = 𝟎. 𝟏𝟏 + 𝟎. 𝟎𝟎𝟑𝟕 𝑷𝑰 for N.C. soils 𝝈𝒐 𝑺𝒖 ′ = 𝟎. 𝟏𝟏 + 𝟎. 𝟎𝟎𝟑𝟕 𝑷𝑰 𝑶𝑪𝑹 𝟎.𝟖 for O.C. soils 𝝈𝒐 Torvane: hand-held spring device that you press into clay then spin to get a rough estimate of the undrained shear strength, Su. Pocket Penetrometer: hand-held spring device that you push into clay to get a rough estimate of the unconfined compressive strength, qu. 49 Shear Strength of Soil Other Methods for Estimating/Measuring Undrained Shear Strength (Su): Vane Shear Test (VST): Traditionally used quite heavily in Europe and Canada, the VST is becoming much more popular in the US because it can instantly provide reliable estimates of Su without having to perform any laboratory tests. Works best in N.C. clay and can have some trouble in O.C. clay. Not meant for use in granular soils! Steps for performing a Vane Shear Test: 1) Drill boring to the depth of interest 2) Vane is inserted into the boring and pushed into the soil 3) Vane is rotated a minimum of 6° until the maximum torque (Tmox) is obtained 50 Shear Strength of Soil Other Methods for Estimating/Measuring Undrained Shear Strength (Su): Vane Shear Test (VST): Steps for performing a Vane Shear Test: (cont.) 4) Vane is rotated until residual strength (Tres) is obtained 5) Compute either the undrained shear strength, Su, or the residual shear strength, Sr, as: 𝑻 Where: 𝑺𝒖 𝑽𝑺𝑻 =  = 1/2 for triangular mobilization of undrained shear strength 𝒅𝟐 𝒉 𝒅𝟑 𝝅 +𝜷  = 2/3 for uniform mobilization of undrained shear strength 𝟐 𝟒  = 3/5 for parabolic mobilization of undrained shear strength 6) 6) Correct the vane undrained shear strength for soil plasticity using the relationship by Bjerrum (1974): 𝑺𝒖 = 𝝀 𝑺𝒖 𝑽𝑺𝑻 , 𝝀 = 𝟏. 𝟕 − 𝟎. 𝟓𝟒 𝒍𝒐𝒈 𝑷𝑰 51 Shear Strength of Soil 52 Shear Strength of Soil Sensitive Soils and Quick Clays: Some soils have a peck (i.e. undisturbed) undrained strength that is MUCH greater than the residual (i.e. remolded) undrained strength. These soils are said to be sensitive. 𝑺𝒖 𝒖𝒏𝒅𝒊𝒔𝒕𝒖𝒓𝒃𝒆𝒅 𝝉𝒇 𝒖𝒏𝒅𝒊𝒔𝒕𝒖𝒓𝒃𝒆𝒅 𝑺𝒕 = = 𝑺𝒖 𝒓𝒆𝒎𝒐𝒍𝒅𝒆𝒅 𝝉𝒇 𝒓𝒆𝒎𝒐𝒍𝒅𝒆𝒅 Highly sensitive clays are called "quick clays." In the US, most normal clays have St < 4. Clays with St > 16 called as "quick." The Scandinavians call it quick clays if its St > 50. Their extra quick clays have St > 100. 53 Shear Strength of Soil Thixotropy of Clay: However, depending on the clay, only a fraction of the original undisturbed strength may be gained back over time. All clays are at least partially thixotropic! 54 Shear Strength of Soil Thixotropy of Clay: Sensitive clays tend to regain their strength over time if they become disturbed. This process is called Thixotropy. 55 Shear Strength of Soil Stress Path (p’-q’ Diagram): Many geotechnical engineers avoid drawing lots of Mohr's circles because it gets messy. Instead, they just plot the tops of the Mohr's circles. These plots are called p' and q diagrams or stress paths 56 Shear Strength of Soil Stress Path (p’-q’ plot): 57 Shear Strength of Soil 58