Shear Strength of Soil COSC 323 Soils in

Shear Strength of Soil COSC 323: Soils in Construction

Shear Failure

Shear Failure

Shear Failure

Shear Strength due to Sliding Friction Normal Force N Shearing Force S Failure Plane

Shear Strength due to sliding friction Normal Force Sliding Force

Shear Strength due to sliding friction Normal Force Sliding Force

Shear Strength due to Sliding Friction Normal Stress vs. Shear Stress Normal Stress (s) Shear Stress (f) 10 k. Pa (=k. N/m 2) 6 k. Pa 20 k. Pa 12 k. Pa 30 k. Pa 18 k. Pa

Shear Stress (s) Shear Strength due to Sliding Friction Angle of internal friction (f) Normal Stress (s)

Coulomb Equation Shear strength Cohesion (Strength gained from the ionic bound between grain particles) Effective intergranular normal pressure (perpendicular to the shear plane) Angle of internal friction (Strength gained from internal friction resistance) Coefficient of friction

Shear strength of soil • Soil cannot resist tension • Soil can resist compression. • For excessive compression failure occurs in the form of shearing along the internal surface within the soil • Structural Strength of soil = f ( soil’s shear strength) • Shear Strength – Soil’s ability to resist sliding – Important for • foundation design • Lateral earth pressure calculations • Slope stability analysis

Methods of investigating shear strength • • • Unconfined compression test (for cohesive soil) Direct shear test Triaxial compression test Vane test (for soft clay) Standard penetration test (for cohesionless soil) Penetrometer test

Unconfined compression test Soil Specimen

Example • A clayey soil subjected to an unconfined compression test fails at a pressure of 2540 lb/ft 2 (i. e. , qu = 2540 lb/ft 2). What is cohesion of this clayey soil?

Direct Shear Test Normal Load Sharing Force Soil Specimen Normal Load Sharing Force

Direct Shear Test Normal Load Sharing Force Normal Stress = Normal load / the specimen’s cross-sectional area Soil Specimen Shear Stress Test with different Normal Load Shear stress = Shearing Force / the specimen’s cross- sectional area The graph can be used to determine the given soil’s shear strength for any load f c Normal Stress f: angle of internal friction c: cohesion Problem: Shear failure is forced to occur along a predetermined plane, which is not necessarily the weakest plane of the soil specimen tested.

Example • A series of direct shear tests was performed on a soil sample. Each test was carried out until the soil specimen experienced shear failure. The test data are listed next. What is soil’s cohesion and angle of internal friction? Specimen Number Normal Stress (lb/ft 2) Shearing Stress (lb/ft 2) 1 604 1522 2 926 1605 3 1248 1720

Triaxial Compression Test Wrap the specimen with rubber membrane

Triaxial Compression Test Enclose the specimen in a chamber filled with water

Triaxial Compression Test Apply a specific pressure using water s 3 s 3

Triaxial Compression Test Apply a vertical load and increase until the specimen fails s 3 s 3 Use different lateral pressure, conduct the same experiment

Triaxial Compression Test s 3 : Minor principal stress s 3 Dp : Deviator stress at failure axial = (Load at failure / cross-sectional area) s 3 s 3

Triaxial Compression Test s 3 : Minor principal stress Dp : Deviator stress at failure axial = (Load at failure / cross-sectional area) s 3 s 1 = s 3 + Dp; major principal stress How to obtain cohesion and angle of internal friction?

Triaxial Compression Test s 1= s 3+Dp s 3 s 3 s 3 s 1= s 3+Dp Before After

Triaxial Compression Test First Test Shear Stress A B Normal Stress (s 3)1 (Dp)1 (s 1)1

Triaxial Compression Test Second Test Shear Stress A B C D Normal Stress (s 3)1 (Dp)1 (s 1)1 (s 3)2 (s 1)2 (Dp)2

Triaxial Compression Test Shear Stress Strength Envelop A B C D Normal Stress (s 3)1 (Dp)1 (s 1)1 (s 3)2 (s 1)2 (Dp)2

Triaxial Compression Test Shear Stress F Strength Envelop c A B C D Normal Stress (s 3)1 (Dp)1 (s 1)1 (s 3)2 (s 1)2 (Dp)2

Example • Gien – Triaxial compression tests on three specimens of a soil sample were performed. Each test was carried out until the specimen experienced shear failure. The test data are tabulated as follows: • Required – The soil’s cohesion and angle of internal friction Specimen Number Minor Principal Stress (kips/ft 2) Deviator Stress at Failure (kips/ft 2) 1 1. 44 5. 76 2 2. 88 6. 85 3 4. 32 7. 50

Example Specimen Number Minor Principal Stress (kips/ft 2) Deviator Stress at Failure (kips/ft 2) Major Principal Stress (kips/ft 2) 1 1. 44 5. 76 7. 2 2 2. 88 6. 85 9. 73 3 4. 32 7. 50 11. 82

Example 8 6 4 2 0 2 4 6 8 10 12 14

Example 8 6 4 2 0 2 4 6 8 10 12 14

Example 8 6 4 2 0 2 4 6 8 10 12 14

Example 8 6 4 2 0 2 4 6 8 10 12 14

Example 8 2 6 4 4 2 0 2 4 6 8 10 12 14

Variations in Shear Test Procedures • Unconsolidated Undrained (UU) – Q test • Consolidated Undrained (CU) • Consolidated Drained (CD) – S test