SOIL MECHANICS Assignment #8: Shear Strength Solution.

Transcription

1 SOIL MECHANICS Assignment #8: Shear Strength Solution. PROBLEM #1: GIVEN: A regional residential building contractor is planning on building a custom 3,700 ft² home on Cape Cod. The subsurface investigation for the house site shows that a poorly graded sand deposit exists from the existing ground surface to a depth of 15 ft. Density testing on the sand insitu showed yielded an average moist unit weight of 112 pcf at an average moisture content of 12%. REQUIRED: Determine the shear strength parameters for use in a preliminary shallow foundation design. SOLUTION: 112 pcf d pcf 1 w SP. Use Figure 7 (NAVFAC DM 7.01, 1986) Figure 7. NAVFAC DM 7.01, Assignment 8 Solution Page 1 of 6

2 Using Figure 7 of NAVFAC DM7.01: 26 ' 33. PROBLEM #2: GIVEN: Soil from a local contractor s soil pit is proposed for use as backfill behind a Mechanically Stabilized Earth (MSE) Wall for a local project. The project specifications require that the backfill material have a minimum residual soil friction angle of 32 at the required compaction (i.e. 95% Modified Proctor). The results from the direct shear testing performed at the minimum required compaction by a local testing firm are provided in a CSV file (14.330_2013_Assignment_8_P2.csv). REQUIRED: Determine the peak and residual friction angles ( peak and residual ) 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 (plotted from the provided data), 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 ( ) Shear Stress ( ) Peak Residual Plot of shear stress vs. normal stress to determine peak and residual is presented in Figure B. Note that the data points at = 14.4 would cause the best fit line to have a negative cohesion (which is impossible). Therefore, do not use them to calculate the Mohr-Coulomb Failure Criteria. Assume no cohesion (i.e. c=0) since no soil classification is provided. A summary of the Mohr-Coulomb Failure Criteria from the direct shear testing is presented in Table B Assignment 8 Solution Page 2 of 6

3 Figure A. Direct Shear Results for Problem # Assignment 8 Solution Page 3 of 6

4 Figure B. Shear Stress vs. Confining Stress for Direct Shear Results for Problem 2. Table B. Direct Shear Results Summary for Problem #2. State Friction Angle ( ) ( ) Cohesion (c) Peak Residual Since residual = 35.5 > 32 (Required residual by specification), this soil CAN 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 peak > residual Assignment 8 Solution Page 4 of 6

5 PROBLEM #3: GIVEN: During the subsurface exploration at a new roadway embankment, soil samples were collected from a CL soil layer. The results of the CD triaxial testing for this soil are presented in Table 1. Table 1. CD Triaxial Testing Results for CL soil layer samples. Test Minor Principal Stress at Failure ( ' 3f ) Major Principal Stress at Failure ( ' 1f ) REQUIRED: Determine the effective and total Mohr-Coulomb Failure Criteria for this soil. SOLUTION: The effective Mohr circles are presented in Figure C. Using Figure C, the Mohr- Coulomb Failure Criteria were determined for the effective stresses case. Friction angles and cohesion for the effective stress envelope are summarized in Table C. Table C. CU Triaxial Test Results Summary for CL soil. State Friction Angle ( ) Cohesion Effective ( and c ) Note that since no determination of the soils over consolidation was provided, fit the best fit line in Figure C to the tangent of all the Mohr circles Assignment 8 Solution Page 5 of 6

6 Figure C. CD Test Results for Problem 3 (Effective Stresses) Assignment 8 Solution Page 6 of 6