The Direct Shear Test is one of the most widely used laboratory tests in geotechnical engineering to determine the shear strength parameters of soil, namely cohesion (c) and angle of internal friction (φ). Understanding shear strength is crucial for analyzing soil stability, designing foundations, retaining walls, slopes, and embankments. This test is simple, reliable, and provides a direct measure of the soil’s resistance to shearing forces under controlled conditions.
What Is a Direct Shear Test?
A Direct Shear Test (also known as the Shear Box Test) is a laboratory test used to measure the shear strength of soil or granular materials. In this test, a soil sample is placed in a shear box that is divided horizontally into two halves. The lower half of the box is fixed, while the upper half is allowed to move horizontally to apply shear force until the soil fails. The normal load and shear load at failure are recorded to calculate the shear strength parameters.
The test is based on Mohr–Coulomb’s failure criterion, which states: τ=c+σtanϕtau = c + sigma tan phiτ=c+σtanϕ
Where,
τ = Shear strength of soil
c = Cohesion intercept
σ = Normal stress
φ = Angle of internal friction
Purpose of Direct Shear Test
The main objectives of conducting the direct shear test are:
- To determine the shear strength parameters (c and φ) of soil.
- To study the shear stress–strain relationship of soil.
- To evaluate the soil’s stability and bearing capacity for design.
- To estimate earth pressure on retaining walls and slope stability.
Apparatus Used in Direct Shear Test
The equipment used for the direct shear test includes:
- Shear box apparatus: A square box (usually 60 mm × 60 mm × 25 mm) split horizontally into two halves.
- Loading frame: To apply normal load on the sample.
- Proving ring: To measure shear load.
- Dial gauges: To record horizontal displacement and vertical settlement.
- Weights: To apply normal stress.
- Water bath: For saturated or drained tests.
Types of Direct Shear Test
Depending on drainage conditions, there are three main types of direct shear tests:
1. Drained Test (CD Test)
In this test, drainage is allowed during both consolidation and shearing. It is suitable for cohesionless soils (sands, gravels) and simulates long-term loading conditions.
2. Consolidated Undrained Test (CU Test)
Drainage is permitted during consolidation but not during shear. It is used for cohesive soils to simulate intermediate conditions.
3. Undrained Test (UU Test)
No drainage is allowed during either consolidation or shearing. It represents short-term conditions like rapid loading in clay.
Procedure for Direct Shear Test
The standard testing procedure follows IS 2720 (Part 13): 1986 or ASTM D3080.
1. Sample Preparation
- Obtain a representative soil sample.
- Trim and fill it into the shear box (usually 60 mm × 60 mm).
- Level the surface and place a porous stone at the top and bottom for drainage (if required).
- Apply the top loading plate.
2. Application of Normal Load
- Apply a known normal load (using weights or a loading frame) to simulate overburden pressure.
- Allow consolidation under this load if a drained test is performed.
3. Application of Shear Load
- Apply horizontal shear force at a constant rate of strain (usually 0.5–1 mm/min) until failure occurs.
- Record shear load and horizontal displacement at regular intervals.
4. Observation and Failure
- Failure is identified when the shear stress reaches its peak and starts to decrease.
- Note the maximum shear load at failure.
5. Repetition
- Repeat the test for at least three different normal loads to plot the shear strength envelope.
Calculations
1. Shear Stress (τ)
τ=PAtau = frac{P}{A}τ=AP
Where,
P = Shear load at failure (N)
A = Corrected area of the sample (mm²)
The corrected area is calculated as: A=(L−Δ)×(B−Δ)A = (L – Delta) times (B – Delta)A=(L−Δ)×(B−Δ)
Where L and B are initial length and breadth, and Δ is horizontal displacement at failure.
2. Normal Stress (σ)
σ=NAsigma = frac{N}{A}σ=AN
Where, N = Normal load (N).
3. Plot and Determine c and φ
Plot shear stress (τ) on the Y-axis and normal stress (σ) on the X-axis. The straight-line envelope drawn through the points represents: τ=c+σtanϕtau = c + sigma tan phiτ=c+σtanϕ
The intercept gives c (cohesion) and the slope of the line gives φ (angle of internal friction).
Graphical Representation
The result of the test is typically presented as a shear stress vs normal stress graph. The linear relationship confirms the soil’s shear parameters, used for stability analysis and design calculations.
Advantages of Direct Shear Test
- Simple and quick to perform.
- Requires minimal equipment.
- Useful for both cohesive and cohesionless soils.
- Provides direct measurement of shear strength.
- Ideal for preliminary design and comparative studies.
Limitations of Direct Shear Test
- Failure plane is predetermined (horizontal), not necessarily the weakest natural plane.
- Stress distribution across the shear plane is non-uniform.
- Not suitable for accurate results in highly cohesive soils.
- Pore pressure cannot be measured directly.
- Limited control over drainage and boundary conditions.
Applications of Direct Shear Test
The direct shear test is widely used in geotechnical engineering for:
- Foundation design: Determining bearing capacity.
- Slope stability analysis: Estimating soil shear parameters.
- Retaining wall design: Calculating lateral earth pressure.
- Pavement subgrade evaluation: Understanding soil behavior under loads.
- Soil improvement studies: Comparing shear strength before and after stabilization.
Typical Values of Shear Parameters
| Soil Type | Cohesion (c) kN/m² | Angle of Friction (φ°) |
|---|---|---|
| Dry Sand | 0 | 30–40 |
| Wet Sand | 0 | 25–35 |
| Clay (Soft) | 15–25 | 0–10 |
| Clay (Stiff) | 25–50 | 10–20 |
| Silty Clay | 10–30 | 5–15 |
FAQs
What is the main purpose of the direct shear test? To determine the soil’s shear strength parameters (c and φ) for design and stability analysis. What is the standard size of the shear box? The most common size is 60 mm × 60 mm × 25 mm, but larger boxes are used for coarse soils. Why is the failure plane predetermined? Because the box is split horizontally, the test forces the sample to fail along that plane. Which soil is best suited for direct shear testing? Cohesionless soils like sand and gravel are most suitable. How is shear strength used in design? It helps calculate safe bearing capacity, earth pressure, and slope stability.
Conclusion
The Direct Shear Test is a fundamental laboratory test in soil mechanics, providing vital information about the shear strength of soils. Despite its limitations, it remains one of the most practical methods for quick and reliable assessment of soil behavior under shear stress. The data obtained from this test help engineers design safe and stable foundations, slopes, and retaining structures. By accurately determining cohesion and friction angle, the direct shear test ensures the long-term stability and safety of civil engineering projects.
