Shear Strength of Soil Student Handout
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GEO-MECHANICS(CE2204)Shear Strength of SoilsLecture Week No 4Mdm Nur Syazwani Noor RodiLINTON UNIVERSITY COLLEGESCHOOL OF CIVIL ENGINEERING
NOTATIONS - Total Stress [kN/m2; kPa] - Effective Stress [kN/m2; kPa]N - Normal Stress [kN/m2; kPa]v - Vertical Total Stress [kN/m2; kPa]h - Horizontal Total Stress [kN/m2; kPa]1 - Principle Stress [kN/m2; kPa]3 - Minor Stress [kN/m2; kPa]u - Pore Water Pressure [kN/m2; kPa] - Shear Strength [kN/m2; kPa] - Angle of Friction  - Change inC - Cohesion of Soils [kN/m2; kPa]
SHEAR FAILURE IN SOILS
WHAT IS SHEAR STRENGTH?The shear strength of a soil is its resistance to shearing stresses.It is a measure of the soil resistance to deformation by continuous displacement of its individual soil particlesShear strength in soils depends primarily on interactions between particlesShear failure occurs when the stresses between the particles are such that they slide over each other
SHEAR STRENGTH IN SOILSAt failure, shear stress along the failure surface () reaches the shear strength (f).
SHEAR STRENGTH PARAMETERSSoil derives its shear strength from two sources:Cohesion (C), is a measurement of the forces that cement between particles of soils (stress independent component) - Cementation between sand grains- Electrostatic attraction between clay particles Internal Friction angle (), is the measure of the frictional resistance between particles of soils (stress dependent component)
MOHR-COULOMB FAILURE CRITERIONThis theory states that a material failure is due to the critical combination of normal stress and shear stress
The relationship between normal stress and shear is given as
MOHR-COULOMB FAILURE CRITERION
MOHR CIRCLE 3 1General State of Stress
MOHR CIRCLEAs loading progresses, Mohr circle becomes larger
.. and finally failure occurs when Mohr circle touches the envelope
General State of Stress
TYPES OF SHEAR TESTLaboratory Tests:Shear BoxTriaxial Compression
In Situ Tests:Standard PenetrationShear Vane
UNDRAINED TESTSNO drainage of pore watersimulates short term condition (e.g. end of construction)excess pore water pressure, u is often finite
DRAINED TESTDrainage ALLOW for pore watersimulates long term condition (e.g. many years after construction)excess pore water pressure, u = 0; however u is not necessarily = 0
TYPES OF SHEAR TEST(DIRECT SHEAR BOX)
TYPES OF SHEAR TEST(DIRECT SHEAR BOX)
max (kN/m2)N (kN/m2)TYPES OF SHEAR TEST(DIRECT SHEAR BOX)
Example 1A drained shear box test was carried out on a sandy clay and yielded for the following results:Area of shear plane = 60mm x 60mmDetermine the apparent cohesion and angle of friction for the soil
Normal Load (N)108202295390484576Shear Load at failure (N)172227266323374425
TestMaximum Shear Stress (kN/m2)maxNormal Load, P (kg)Normal Stress, N (kN/m2)17036.7 100213073.4 2003220128.4 350
TYPES OF SHEAR TEST (TRIAXIAL TEST)
TYPES OF SHEAR TEST (TRIAXIAL TEST)The test is designed to mimic actual field or in situ conditions of the soil.Triaxial tests are run by:saturating the soilapplying the confining stress (3)applying the vertical stress (known as deviator stress) until failure3 main types of triaxial tests:Unconsolidated - UndrainedConsolidated DrainedConsolidated Undrained
UNDRAINED TEST (Unconsolidated-Undrained) Fast - Undrained - Short term Cu & u for saturated soils (S=1), NO Volume Change
1 uf = 0Stage CDrained FailureDRAINED TEST (Consolidated-Drained) Extremely slow Drained Long term Cd & d for saturated soils (S=1), NO Volume Change
CONSOLIDATED UNDRAINED TEST Intermediate Drained Long & Short term C & ( Cd & d ) ; Cu & u
Example 1A drained triaxial compression test carried out on three specimens of the same soil yielded the following results:
Draw the shear strength envelop and determine the shear strength parameters, C & , assuming that the pore water pressure remain constant during the axial loading stage.
Test No.123Cell pressure (kPa)100200300Deviator stress at failure (kPa)210438644
Example 2Three consolidation undrained triaxial tests were carried out on 38mm diameter samples of the same clay. The applied axial force at failure of the samples were found to be as follows:-Determine the shear strength parameters of the clay in term of total stress.
Test No.123Cell pressure (kN/m2)2575120Applied axial force at failure (kN)0.0860.1200.149
Example 3The following results were obtained from undrained triaxial tests on specimens of a saturated normally consolidated clay.Determine the shear strength parameters of the clay in term of total and effective stress.
Test No.123Cell Pressure (kN/m2)100200300Ultimate Deviator Stress (kN/m2)137210283Ultimate Pore Pressure (kN/m2)2886147
Example 4The following results were obtained from undrained triaxial tests on specimens of an overconsolidated clay.Determine the shear strength parameters of the clay in term of total and effective stress.
Test No.123Cell Pressure (kN/m2)100250400Deviator Stress at failure (kN/m2)340410474Deviator Pore Pressure (kN/m2)-4264177
Example 5Referring to Example 2, if the shear strength parameters of the clay in term of effective stress were C = 10 kN/m2 and = 30, determine the pore water pressure in each sample at failure.
Example 6Consolidated undrained triaxial tested were carried out on 3 samples of the same clay soil and the following results were obtained at the point of failure:-
Determine the 6 unknown value (?) in the table by Calculation and Graphical method
Sample No.Cell Pressure (kN/m2)Deviator Stress at failure (kN/m2)Pore Water Pressure (kN/m2)Cu
TYPES OF SHEAR TEST(SHEAR VANE TEST)
TYPES OF SHEAR TEST(SHEAR VANE TEST)Suitable for determining the in-situ undrained shear strength of unfissured saturated clays and siltsThe vane consists of four rectangular blades in a cruciform at the end of a steel rodShear strength is measure by pushing the vane into the soil and rotated by applying a torque at the surface end of the rodThe vane is first rotated at 6-12 per minute to determine the undisturbed shear strength and then the remoulded strength is measured by rotating the vane rapidly
Example 1A shear vane used to test a soft clay had a diameter of 75mm and a length of 150mm. The average torques recorded after slow and then rapid rotations were 64 and 26 Nm respectively. Determine the undrained strength of the clay.