Shear Strength of Soil - University of Waterloo

1 1 Shear Strength of SoilDirect Shear test in sand: (a) schematic diagram of test equipment; (b) plot of test results to obtain the friction angle 2004 Brooks/Cole Publishing / Thomson Learning 2 Variation of friction angle with void ratio for Chattachoochee River sand (after Vesic, 1963) 2004 Brooks/Cole Publishing / Thomson Learning 1 1 3 = K 1 3 Soil Sampling3 Triaxial Test EquipmentSequences of stress application in triaxial test 2004 Brooks/Cole Publishing / Thomson Learning 4 Plot of deviator stress vs.

2 Axial strain- drained triaxial test 2004 Brooks/Cole Publishing / Thomson Learning Triaxial Testing Application of cell pressure (sigma 3): UNCONSOLIDATED (U) No drainage allowed e constant (fixed) if Sr =1- CONSOLIDATED (C) Drainage allowed when applying cell pressure e decreases due to sample Testing Application of deviatoric stress ( 1): Deviator stress = 1 3 Undrained (U) No drainage allowed e constant (fixed) if Sr =1- drained (D) Drainage allowed when applying cell deviator stress e decreases due to consolidation.

3 Stress applied so slow no excess pwpTriaxial Tests UU Unconsolidated Undrained UD Unconsolidated drained CU - Consolidated Undrained CD - Consolidated Drained6 Undrained Soil Strength parameters (Su, = 0)Note: Always test soil at void ratio on the fieldSkempton s Porewater Coefficients Developed B for a measure of degree of Water saturation3 =B7 Peak- and residual- Strength envelopes for clay 2004 Brooks/Cole Publishing / Thomson Learning 8 Skempton s Porewater Coefficients Developed A parameter for indication of sample OCRinterest ofpoint at stress deviatoric)(interest ofpoint at pressure porepressure pore initail)(.

4 (3131= == = =iiooiiiiwhereA What about negative values of AfSoil StrengthM-C Failure EnvelopeSample will fail at intersection with envelope 3(1) 1(1) 1(2) 1(3) 1(Failure) n 3(2)c 1(1) 1(2) 1(3) 1(Failure)Test 1 Failure Circle failure (1) failure (2)Test 2 Failure CircleNOTE: all stresses are effectiveEach test is performed at a set void ratio9 drained Soil Strength parameters (c , )Soil Strength Envelope10 Soil StrengthStress Path of Sample123 t = d' + S tan 's = ( 1' + 3')/2 Centre of Circled = c' cos 'tan = sin 'S( 1- 3)/2 = tRadius of Circledstsin = tan Unconfined compression test in progress (courtesy of Soiltest, Inc.))

5 , Lake Bluff, IL)11 Unconfined compression test: (a) soil specimen; (b) Mohr s circle for the test; (c) variation of quwith the degree of saturation 2004 Brooks/Cole Publishing / Thomson Learning Variation of friction angle with plasticity index for several clays (after Bjerrum and Simons, 1960) 2004 Brooks/Cole Publishing / Thomson Learning