Geotechnical Engineering Instruction on SOIL SLOPE STABILITY INVESTIGATION & EVALUATION Approved: ___________________________________ Date: 10/10/14 Christopher C. Benda, P.E. Geotechnical Engineering Manager VTrans GEI 14-01 October 2014 Table of Contents 1.0 INTRODUCTION ................................................................................................................................................. 1 1.1 TARGET AUDIENCE .......................................................................................................................... 1 1.2 PURPOSE ........................................................................................................................................... 1 1.3 PROJECT DEFINITION, TYPE AND SCOPE ....................................................................................... 1 1.4 DOCUMENT OVERVIEW ................................................................................................................... 2 1.5 DOCUMENT LIMITATIONS ............................................................................................................... 2 2.0 PROJECT DETERMINATION, ASSESSMENT AND MANAGEMENT ...................................................... 3 2.1 REQUESTS FOR GEOTECHNICAL ASSISTANCE ................................................................................ 3 2.2 INITIAL SITE ASSESSMENT REPORT ................................................................................................. 3 2.3 PROJECT DOCUMENTATION ............................................................................................................. 5 2.4 PROJECT MANAGEMENT .................................................................................................................. 5 3.0 GEOTECHNICAL INVESTIGATION ............................................................................................................... 6 3.1 INITIAL SCOPING ACTIVITIES .......................................................................................................... 6 3.2 RESOURCE IDENTIFICATION (SURVEY, HYDRAULICS, ENVIRONMENTAL, ROW) ....................... 6 3.3 SITE RECONNAISSANCE .................................................................................................................... 7 3.4 DRILLING & SUBSURFACE INVESTIGATION .................................................................................... 7 3.4.1 Field Work Order ....................................................................................................................................... 8 3.4.2 Boring Locations and Depths .................................................................................................................... 8 3.4.3 Groundwater Determination .................................................................................................................... 11 3.4.4 Field Sampling & Testing ........................................................................................................................ 11 3.4.5 Geotechnical Instrumentation ................................................................................................................. 13 3.4.5.1 Inclinometers ..................................................................................................................................................... 13 3.4.5.2 Monitoring Wells ............................................................................................................................................... 14 4.0 LABORATORY TESTING ................................................................................................................................ 15 4.1 MOISTURE CONTENT, PARTICLE SIZE ANALYSIS AND SOIL CLASSIFICATION .......................... 16 4.2 ATTERBURG LIMITS AND SOIL BEHAVIOR .................................................................................... 16 4.2.1 Plastic Limit .............................................................................................................................................. 16 4.2.2 Liquid Limit .............................................................................................................................................. 16 4.2.3 Plasticity Index ......................................................................................................................................... 16 4.2.4 Liquidity Index ......................................................................................................................................... 17 4.3 SOIL SHEAR STRENGTH .................................................................................................................. 17 4.3.1 Strength Tests ........................................................................................................................................... 18 4.3.1.1 Unconfined Compression Test ......................................................................................................................... 18 4.3.1.2 Triaxial Compression Test ............................................................................................................................... 18 4.3.1.2.1 Unconsolidated-Undrained (UU) ............................................................................................................. 18 4.3.1.2.2 Consolidated-Undrained (CU) ................................................................................................................. 19 4.3.1.2.3 Consolidated-Drained (CD)...................................................................................................................... 19 4.3.1.3 Direct Shear ....................................................................................................................................................... 19 4.3.2 Consolidation Test .................................................................................................................................... 19 4.3 EXTERNAL TESTING SERVICES ...................................................................................................... 20 5.0 ANALYSIS AND DESIGN PROCEDURES ..................................................................................................... 20 5.1 GENERAL APPROACH ...................................................................................................................... 20 5.2 DEVELOPMENT OF SUBSURFACE SOIL PROFILE ........................................................................... 21 5.3 DETERMINATION OF SOIL AND ROCK STRENGTH PARAMETERS FOR DESIGN .......................... 22 5.3.1 Cohesionless Soils, c = 0 .......................................................................................................................... 22 5.3.2 Cohesive Soils, φ = 0................................................................................................................................ 24 5.3.3 φ-c Soils, φ > 0 and c > 0 .......................................................................................................................... 34 5.3.4 Rock Fill Soil Strength Parameters ......................................................................................................... 34 5.3.5 “Back-Calculation” or “Back-Analysis” Method ................................................................................... 34 5.4 GROUNDWATER EFFECTS ............................................................................................................... 35 VTrans GEI 14-01 ii October 2014 5.4.1 Hydraulic Conductivity ............................................................................................................................ 36 5.4.2 Effect of Groundwater and Excess Pore Pressures ................................................................................ 36 5.5 DRAINAGE CONDITIONS AND TOTAL VS. EFFECTIVE STRESS ANALYSIS .................................... 37 5.5.1 Drained vs. Undrained Loading Conditions ............................................................................................ 37 5.5.2 Total vs. Effective Stress Analysis ........................................................................................................... 38 5.5.2.1 Total Stress Analysis ......................................................................................................................................... 39 5.5.2.2 Effective Stress Analysis ................................................................................................................................... 39 5.6 SLOPE STABILITY MODELING ....................................................................................................... 40 5.6.1 Factor of Safety for Slope Stability Analyses .......................................................................................... 41 5.6.2 Software Programs ................................................................................................................................... 41 5.6.3 Acceptable Methods for Slope Stability Analyses .................................................................................... 42 5.6.3.1 Computer Analysis ............................................................................................................................................ 42 5.6.3.2 Simplified Design Charts .................................................................................................................................. 42 5.6.4 Search for Critical Failure Surfaces ......................................................................................................