Slope Stability Concepts Slope Stability Concepts to be

Slope Stability Concepts

Slope Stability Concepts to be considered: • Modes of Failure • Limit Equilibrium Method/FEM/Probabilistic Method • Definitions for Factor of Safety • Total vs. Effective Stress Analyses • Choice of Shear Strength Parameters CE 5326: Analysis & Design of Earth Structures

Failure Modes • Slides • Rotational Slides (curved/circular failure surface; moderately fast to extremely slow movement; when the failure surface consists of several parallel failure surfaces, failure is called a slump) • Translational Slides (planar failure surface; often controlled by structural features) • Compound (curved/planar combination) CE 5326: Analysis & Design of Earth Structures

Failure Modes • Falls • vertical movement > horizontal movement; extremely fast • Flows • moving mass broken up; has lost its internal structure; mass resembles a viscous fluid; extremely fast movement • Creep • Extremely slow movement CE 5326: Analysis & Design of Earth Structures

Rotational Slides CE 5326: Analysis & Design of Earth Structures

Rotational Landslides (Slumping) CE 5326: Analysis & Design of Earth Structures

Translational Slides Where movement occurs predominantly along more or less planar or gently undulatory surfaces. Movement is frequently, structurally controlled by discontinuities and variations in shear strength between layers of bedded deposits, or by the contact between firm bedrock and overlying detritus. CE 5326: Analysis & Design of Earth Structures

Translational Slides CE 5326: Analysis & Design of Earth Structures

Rock Falls/Topples CE 5326: Analysis & Design of Earth Structures

Creep o Slow mass movement of soil down rather steep slopes primarily under the influence of gravity, but aided by saturation with water, etc. CE 5326: Analysis & Design of Earth Structures

Limit Equilibrium Method • Postulated failure mechanism; assume a failure surface • Failure mass moves as a rigid body • Determine the shear resistance needed to maintain failure mass in “limiting equilibrium” • If this shear resistance is smaller than the shear strength of the soil, then the slope is safe; i. e. has a F. O. S. larger than 1. 0. CE 5326: Analysis & Design of Earth Structures

Limit Equilibrium Method W 2 (ext. load) o x 2 idealized slope shear resistance = cu CG W 1 radius, r x 1 assumed failure surface length = L CE 5326: Analysis & Design of Earth Structures Mcausing = W 1 x 1 + W 2 x 2 Mresisting = ( ) (L) (r)

Definition of F. O. S. CE 5326: Analysis & Design of Earth Structures

Effective Stress Analysis • • • When excess pore water pressures do not exist Or when excess pore water pressures can be predicted with reasonable accuracy When excess pore water pressures are zero, the pore water pressures can be determined based on equilibrium conditions • Static water • Seepage analysis CE 5326: Analysis & Design of Earth Structures

Types of Triaxial Compression Tests o Unconsolidated Undrained (UU-Test); Also called “Undrained” Test o Consolidated Undrained Test (CU- Test) o Consolidated Drained (CD-Test); Also called “Drained Test” CE 5326: Analysis & Design of Earth Structures

Consolidated Drained Test (CDTest) CE 5326: Analysis & Design of Earth Structures

Effective Stress Analysis Long-term Stability; Use CD-Strength Parameters CE 5326: Analysis & Design of Earth Structures

Total Stress Analysis • When excess pore water pressures exist, thus making estimation or prediction of pore water pressure difficult • Usually used in “short-term analysis” • Immediately after construction/excavation • Rapid loading situations e. g. rapid draw down CE 5326: Analysis & Design of Earth Structures

Unconsolidated Undrained Test (UU-Test) CE 5326: Analysis & Design of Earth Structures

Application of UU-Strength Parameters Short-term Stability; Use UU-Strength Parameters CE 5326: Analysis & Design of Earth Structures

Consolidated Undrained Test (CU- Test) CE 5326: Analysis & Design of Earth Structures

Application of CU-Strength Parameters CE 5326: Analysis & Design of Earth Structures

Pore Pressure Parameters • When soil masses are loaded under undrained conditions excess pore water pressures develop • Generally, excess pore water pressures are not known, and therefore slope stability analysis is conducted using total stress analysis rather than effective stress analysis • However, an alternative to this approach is to predict pore pressure parameters, A and B CE 5326: Analysis & Design of Earth Structures

Pore Pressure Parameters • Pore Pressure Parameters relate excess pore pressure, u to changes in stress, • When S =0%, B=0 and S =100%, B 1. 0 CE 5326: Analysis & Design of Earth Structures

Pore Pressure Parameter, B CE 5326: Analysis & Design of Earth Structures

Pore Pressure Parameters • Excess pore pressure, u due to changes in stress, 3 as well as ( 1 - 3 ) CE 5326: Analysis & Design of Earth Structures

Pore Pressure Parameter, Af CE 5326: Analysis & Design of Earth Structures