Experiment # 7 Direct Shear ASTM D3080/98 Soil Mechanics Lab CE 350

Content Definition of friction force Shear Strength Parameters Test Procedure Data and calculations Determination of Shear Strength Parameters Applications Soil Mechanics Lab CE 350

F = Shear Force Required to Cause Slip Frictional Strength W N F Tan Ø = = Friction F = Shear Force Required to Cause Slip  F = N Tan Ø Ø Sliding block-friction analogy. Stress equation. Expand the concept to explain that friction is composed of both surface roughness and particle interlock effects. Stress equation shows that frictional shear strength increases as the pressure between the particles increases. Soil Mechanics Lab CE 350

Friction + Cohesion τ =  Tan Ø + c @ Failure τ = Shear Stress W C F N F = C + N Tan Ø Ø C Glue For Soil: Ø = Angle of Internal Friction C = Cohesion Sliding block with glue is the analogy for combined friction/cohesion forces between soil particles. Stress equation for the strength of any soil includes a contribution due to friction and a contribution due to cohesion. This is a basic concept of soil mechanics. τ =  Tan Ø + c @ Failure τ = Shear Stress  = Normal Stress Soil Mechanics Lab CE 350

Apparatus Vertical dial gage Horizontal dial gage Load Ring Shear Box 1:10 10P + 4.5kg F Load Ring P Shear Box Soil Mechanics Lab CE 350

Data τ= Shear Force / Ac  = (10P + 4.5)*9.81/Ac Ac = Ao(1-Δh/Lo) Hz. dial reading (div.) Hz. disp. mm Vl. dial reading Vl. disp. mm Corrected area Load ring reading Shear force (N) Shear stress (N/cm2) Normal stress (N/cm2) R1 R1*C1 R2 R2*C2 R3 R3*C3  = (10P + 4.5)*9.81/Ac Ac = Ao(1-Δh/Lo) τ= Shear Force / Ac Soil Mechanics Lab CE 350

Direct Shear Test Data Dense Sand Loose Sand Shear Stress Δh @ Failure Soil Mechanics Lab CE 350

Shear Stress Vs. Displacement Peak Stress τ 3 τ 2 Shear stress, τ τ 1 N3 = 12 kg N2 = 8 kg N1 = 4 kg Horizontal displacement, DH Soil Mechanics Lab CE 350

Shear Stress Vs. Normal Stress Failure Envelope (s2,τ2) (s3, τ 3)  Shear Stress, τ (s1, τ 1) C Normal Stress s Soil Mechanics Lab CE 350

Vertical Displacement Vs. Horizontal Displacement DH Soil Mechanics Lab CE 350

ِApplications Prediction of soil behavior Estimation of bearing capacity Estimation of bearing capacity of pils Estimation of lateral earth pressure Soil Mechanics Lab CE 350