Strength

Strength

1. Frictonal

   • effective friction angle=inversetan(u)
     • This is the like the sliding block from general physics 
     • phi'=effective friction angle
   • Factors effecting Frictional Strength:
     • Mineralogy
       • Clays have low effective friction angles
     • Particle Shape
       • angular shapes have high effective friction angles
       • Round shapes have low effective friction angles
     • Void Ratio
       • Low void ratio creates a high effective friction angles
     • Gradation
       • Well-Graded creates high effective friction angles
       • Poorly-Graded creates low effective friction angles
     • Organic Materials
       • Creates low effective friction angles

2. cohesive

   • Some soils have shear strength when effective stress is zero or at least seems to be
   • c'=effective cohesion
     • c'=o in sands and gravels when no cementing a gens or clay are present
       • c'=0 in clays that are NC
   • Two types of Cohesive Strength
     • True
       • Truly bonding
         • Cementation-> CaCO3, Fe2O3
         • Electrostatic + electromagnetic attractions
         • Primary valence bonding -> adhesion
     • Apparent
       • appears to be caused by bounding between soil particles
         • it is actually fictional strength

3. Mohr-Coulomb Failure Criterion

   • Effective Stress Analysis
     • Shear strength of a soil is developed only by the solids of the soils
       • sigma'
       • c'=shear intercept
       • phi'=Slope of the shear-sigma'
     • s=c'+simga'*tan(phi')
     • no ue; drained
   • Total Stress Analysis
     • ue; undrained
     • s=ct+simga*tan(phi)
   • FACTOR OF SAFETY
     • F=s/shear stress

4. Drained & Undrained

   • Undrained
     • How does undrained develop?
       • Hydraulic conductivity(k)=low; rapid loading
         • Sand: very rapid loading
         • Clay: short term
     • Water can not flow out/into the soil -> the soil will develop excess water pressure
   • Drained
     • No excess pore water pressure, ue=0, uh=u
     • Develop in two ways
       • Hydraulic conductivity(k)=high; rate of loading=small
         • Sand
       • long time
         • Clay
   • Note: Both of these conditions are with saturated soils
   • 3 Cases
     • Case 1: Shear Strength Under the Drained Condition
       • no ue
       • use effective stress Mohr-Coulomb Failure
     • Case 2: Shear Strength under the undrain condition with positive excess pore water pressures
       • most common
       • + pore water pressure=increase mean normal stress
         • Examples: loading & construction
       • factor of safety increases with t
         • lowest F occurs immediately after construction
           • shear strength is low, full shear stresses
     • Case 3: Shear Strength under the Undrained condition with negative excess pore water pressure
       • - pore water pressure=decrease in normal stress
         • Ex: Excavation
       • ue gradually dissipates -> loss shear strength
       • factor of safety decreases with time
         • Therefore, effective stress analysis must be used

5. Shear Strength Evaluation

   • Direct Shear Method
     • Vertical P
     • Simgaz=p/A