SOIL MECHANICS Assignment #7: Shear Strength Solution.

Transcription

1 SOIL MECHANICS Assignment #7: Shear Strength Solution. PROBLEM #1: GIVEN: Direct Shear test results from a SP soil shown in Figure A (from _2012_Assignment_#8_P1.csv on the course website). Project specifications provided in the assignment handout. Figure A. Direct Shear Test Results for Problem Assignment 7 Solution Page 1 of 6

2 REQUIRED: Determine the peak and residual friction angles (φpeak and φresidual) for this soil. Determine if the soil meets the project specifications. Explain density (loose or dense) of the soil based on direct shear results. SOLUTION: Using data provided in Figure A, determine the peak and residual shear strengths for the tests at the three (3) confining (i.e. normal) stresses. This summary is provided in Table A. Table A. Direct Shear Results (from Figure A). Test Confining Stress (σ) Peak Shear Stress (τ) Residual Plot of shear stress vs. normal stress to determine φpeak and φresidual is presented in Figure B. Note, since soil is a SP, there is no cohesion (i.e. c=0). A summary of the Mohr-Coulomb Failure Criteria from the direct shear testing is presented in Table B. Table B. Direct Shear Results Summary for SP soil. State Friction Angle (φ) ( ) Cohesion (c) Peak Residual Assignment 7 Solution Page 2 of 6

3 Figure B. Shear Stress vs. Confining Stress for Direct Shear Results for Problem 1. Since φresidual = 30.8 < 32 (Required φresidual by specification), this soil CANNOT be used for the project. This soil was tested in a dense state. That can be determined by volumetric behavior of the soil during the direct shear test and from the fact that φpeak > φresidual Assignment 7 Solution Page 3 of 6

4 PROBLEM #2: GIVEN: Direct Shear test results from a SW soil plotted in Figure C (from Table 1 in the assignment handout). Project specifications provided in assignment handout. Figure C. Shear Stress vs. Confining Stress for Direct Shear Results for Problem 2. REQUIRED: Determine the peak and residual friction angles (φpeak and φresidual) for this soil. Determine if the soil meets the project specifications. Explain density (loose or dense) of the soil based on direct shear results. SOLUTION: Plot of shear stress vs. normal stress to determine φpeak and φresidual is presented in Figure C. Note, since soil is a SW, there is no cohesion (i.e. c=0). A summary of the Mohr-Coulomb Failure Criteria is presented in Table C Assignment 7 Solution Page 4 of 6

5 Table C. Direct Shear Results Summary for SW soil. State Friction Angle (φ) ( ) Cohesion (c) Peak Residual Since φresidual = 34.8 < 32 (Required φresidual by specification), this soil CAN be used for the project. This soil was tested in a dense state. That can be determined the fact that φpeak > φresidual. PROBLEM #3: GIVEN: The results of the CU triaxial testing for a CL soil shown in Figures D and E. Note effective stresses were calculated using the effective stress equation (σ = σ u) and the provided CU test data in assignment handout. Figure D. CU Test Results for Problem 3 (Total Stresses) Assignment 7 Solution Page 5 of 6

6 Figure E. CU Test Results for Problem 3 (Effective Stresses). REQUIRED: Determine the effective and total Mohr-Coulomb Failure Criteria for this soil. SOLUTION: Using Figures D and E, the Mohr-Coulomb Failure Criteria were determined for the total and effective stresses cases. Friction angles and cohesion for the total and effective stress envelopes are summarized in Table C. Table C. CU Triaxial Test Results Summary for CL soil. State Friction Angle ( ) Cohesion Effective (φ and c ) Total (φ and c) Assignment 7 Solution Page 6 of 6