The Direct Shear Test is a laboratory procedure used to determine the shear strength of soil materials. By subjecting a soil specimen to a controlled normal stress and measuring the resistance along a horizontal shear plane, engineers derive the fundamental parameters of cohesion ($c$) and the angle of internal friction ($\phi$).
Test Objectives
- Determine peak and residual shear strength parameters
- Analyze the drained strength behavior of sandy and silty soils
- Provide input for safety factor calculations in slope design
Design Application
- Supports the design of retaining walls and gravity structures
- Essential for slope stability and landslide risk evaluation
- Used in bearing capacity equations for shallow foundations
Structural Safety
- Prevents sudden shear failure in earth embankments
- Ensures stability across varying soil stratification
- Provides high-reliability data for geotechnical modeling
Shear Metrics
- Mohr-Coulomb Failure Envelope determination
- Relationship between Normal Stress ($\sigma$) and Shear Stress ($\tau$)
- $\tau = c + \sigma \tan \phi$
The test is conducted in a metal "shear box" split horizontally into two halves. A soil sample is placed inside, and a vertical (normal) load is applied. While this load is maintained, a horizontal force is applied to one half of the box to induce shear displacement.
By repeating the test at three or more different normal loads, our engineers can plot the failure envelope. The intercept on the shear stress axis gives the cohesion, while the slope of the line indicates the friction angle. This data is the "DNA" of soil stability, allowing us to predict how your foundation will react under lateral and vertical pressure.
Our Laboratory Procedure
- 1 Placement of specimen in the digital shear box assembly
- 2 Application of specific normal stress for consolidation
- 3 Measurement of shearing force at a constant strain rate
- 4 Generation of shear stress vs. normal stress reports