Shear Strength

Shear strength is the ability of soil to resist forces that cause sliding along its internal planes, and it’s a critical factor in geotechnical engineering. One of the most common and straightforward methods to measure this property is the Direct Shear Test.Huge mountains and their towering slopes stand firm against the forces of nature, thanks to an internal strength known as shear strength. This property of soil allows it to resist forces that cause sliding along its internal layers. In our previous article, we explored the concept of shear strength as defined by the Mohr-Coulomb theory. In this discussion, we will examine a widely recognized method for assessing shear strength: the direct shear test.

Shear strength refers to the capacity of soil to withstand sliding forces acting along its internal surfaces. Based on the principles of Mohr-Coulomb theory, shear strength (τ’) is defined as:

τ’ = c + σ’ tan φ

Where:

• τ’ = Shear stress at failure
• σ’ = Effective normal stress
• c = Effective cohesion between particles
• φ = Effective angle of shearing resistance

The direct shear test is one of the most straightforward and widely used methods to determine the shear strength of soil. It involves shearing a soil sample along a horizontal plane to measure its resistance to sliding.

1. Shear Box: The soil specimen is placed inside a brass or gunmetal shear box, typically 60x60x50 mm in size. The box has grid plates—plain for undrained tests and perforated for drained tests.
2. Porous Stones: For drained tests, porous stones are placed at the top and bottom of the sample to allow water to escape.
3. Normal Load: A pressure pad applies a normal load to the sample.
4. Shear Mechanism: The lower half of the box is fixed, while the upper half is pushed forward at a constant rate using a geared jack.

1. Apply Normal Stress: A normal load is applied to the sample (e.g., 25 kN/m2).
2. Shear the Sample: A shear load is applied at a constant strain rate.
3. Monitor Readings: Shear displacement is tracked using a dial gauge, and shear force is measured using a proving ring.
4. Failure Criteria: The test continues until the sample fails (proving ring readings decrease after a peak) or reaches 20% longitudinal displacement.

The direct shear test can be performed under three conditions:

1. UU (unconsolidated-undrained): No consolidation or drainage during shearing.
2. CU (Consolidated-Undrained): Consolidation allowed, but no drainage during shearing.
3. CD (Consolidated-Drained): Both consolidation and drainage are allowed during shearing.

Analyzing Results

1. Shear Stress and Strain:
   • Shear stress (τ) = shear load / cross-sectional area of the sample.
   • Shear strain = shear displacement (ΔH) / initial length (L).
2. Stress-Strain Curve:
   • In dense sands, shear stress initially peaks at low strains before decreasing to an ultimate stress.
   • For loose sands, shear stress increases gradually and stabilizes at ultimate stress.
3. Failure Envelope:
   • Shear stress vs. normal stress for multiple tests.
   • The slope of the failure envelope gives the angle of shearing resistance (φ), and the intercept gives cohesion (c).

1. Uneven stress distribution: stresses are higher at the edges, leading to gradual failure.
2. Decreasing Shear Area: The shear area diminishes during the test, complicating accurate stress calculations.
3. Pore water pressure, which affects effective stress, cannot be measured.
4. Fixed Failure Plane: The horizontal failure plane may not represent the weakest plane in real-world conditions.

Beyond the Direct Shear Test: The Triaxial Test

To address these limitations, the Triaxial Test was developed. It provides a more comprehensive understanding of soil behavior by allowing control over stress conditions and measuring pore water pressure.

• The direct shear test is a simple and effective method to determine soil shear strength.
• It helps engineers understand how soil resists sliding forces, crucial for designing stable structures.
• Notwithstanding its limitations, the test continues to be an essential component in the field of geotechnical engineering.

By mastering the direct shear test, engineers can ensure the safety and stability of structures built on soil.