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

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

3. 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

4. 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

5. 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

6. 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

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

8. 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

9. 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

10. Vertical Displacement Vs. Horizontal DisplacementDH
Soil Mechanics Lab CE 350

11. ِ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