Concept of the In-Situ Direct Shear Test:
The In-Situ Direct Shear Test is an in-situ method used to determine the shear strength parameters of soil or rock at a specific site. This test simulates the actual conditions in the field by applying direct shear stress to a prepared surface or along a natural discontinuity. It is particularly useful for evaluating the stability of slopes, foundations, retaining structures, and other geotechnical applications. The test provides values for shear strength parameters such as cohesion (c) and the angle of internal friction (φ), which are essential for design and stability analyses. The test follows standards like ASTM D3080/D3080M for soil and ASTM D5607 for rock.
Figure 1: In-Situ Direct Shear Test.
Descriptions and Procedure
Apparatus Required
- Shear box apparatus with adjustable loading mechanism
- Loading frame or reaction system
- Dial gauges or electronic displacement transducers for measuring horizontal and vertical displacements
- Hydraulic jack or screw jack for applying normal and shear loads
- Data acquisition system for recording measurements
- Excavation tools for preparing the test area
- Leveling tools to ensure flat and parallel test surfaces
Procedure
1. Site Preparation
- Selecting Test Location: Choose a test location that represents the in-situ conditions of interest.
- Excavating Test Area: Excavate to the desired depth where the shear test will be conducted.
- Preparing Test Surface: Level and smooth the test surface to ensure proper contact with the shear box.
2. Shear Box Setup
- Positioning Shear Box: Place the shear box apparatus on the prepared test surface. The shear box consists of two halves that can move relative to each other.
- Assembling Shear Box: Assemble the shear box by securing the two halves together with the test specimen in between. For natural discontinuities, ensure the discontinuity lies within the shear plane.
3. Loading Setup
- Applying Normal Load: Apply the normal load to the test specimen using a hydraulic jack or screw jack. The normal load should be applied incrementally to the desired value and maintained throughout the test.
- Measuring Initial Conditions: Record the initial vertical and horizontal positions using dial gauges or displacement transducers.
4. Shearing Procedure
- Applying Shear Load: Apply the shear load horizontally to the upper half of the shear box at a constant rate. The rate should be slow enough to ensure drained conditions, typically ranging from 0.01 to 1 mm/min.
- Recording Data: Continuously monitor and record the horizontal displacement (shear displacement) and the corresponding shear load.
- Observing Failure: Continue applying the shear load until the specimen fails, indicated by a peak in the shear load followed by a drop or plateau.
5. Post-Test Measurements
- Final Measurements: Record the final horizontal and vertical displacements after the test.
- Shear Surface Examination: Carefully dismantle the shear box and examine the shear surface for any signs of failure, such as cracks or slip surfaces.
Data Interpretation
1. Shear Stress vs. Shear Displacement Curve
- Plot the shear stress (shear load divided by the area of the shear plane) against the shear displacement to visualize the shear behavior of the specimen.
- Identify the peak shear stress, which represents the shear strength of the specimen.
2. Determining Shear Strength Parameters
- Perform multiple tests at different normal loads to determine the relationship between shear stress and normal stress.
- Plot the peak shear stress values against the corresponding normal stresses to create a failure envelope.
- • Use the failure envelope to determine the shear strength parameters: cohesion (c) and the angle of internal friction (φ). The envelope is typically linear, represented by the Mohr-Coulomb failure criterion: 𝜏=𝑐+𝜎tan𝜙 where τ is the shear stress, c is the cohesion, σ is the normal stress, and ϕ is the internal friction angle.
Summary Table
Key Points to Remember
- Purpose: Determine shear strength parameters of soil or rock in-situ.
- Apparatus: Shear box, loading frame, hydraulic jack, displacement measuring devices.
- Procedure: Prepare site, assemble shear box, apply normal load, shear load, record data, analyze results.
- Data Interpretation: Plot shear stress vs. displacement, determine shear strength parameters from failure envelope.
- Standards: ASTM D3080/D3080M (soil), ASTM D5607 (rock).
Final Summary
The In-Situ Direct Shear Test is crucial for evaluating the shear strength parameters of soil and rock in their natural state. It provides essential data for designing stable geotechnical structures such as slopes, foundations, and retaining walls. Proper execution and interpretation of the test yield reliable shear strength parameters, ensuring safe and effective design and construction practices.