The direct shear test or box shear test is a commonly used method to evaluate the shear strength of soil. This test is particularly suitable for soils with low cohesion, also known as cohesionless soils.
The direct shear test involves placing a soil sample of a specific size and shape between two horizontal plates, with one plate being stationary and the other plate moving parallel to the soil surface. The movement of the top plate causes shearing of the soil sample along the predetermined plane, while the bottom plate remains stationary.
The shear strength of the soil is determined by measuring the force required to cause the soil sample to fail along the predetermined plane. This force is then divided by the cross-sectional area of the soil sample to obtain the shear strength of the soil.
Overall, the direct shear test or box shear test is a useful tool for evaluating the shear strength of soil, especially in cohesionless soils. By measuring the shear strength of soil, engineers and geologists can make informed decisions about the stability of structures built on or in the soil, as well as the potential for landslides or other geological hazards.
Shear Strength of Soil by Direct Shear Test
The shear strength of a soil refers to its ability to withstand and resist shearing stresses, which are forces that act parallel to its surface and cause it to deform or fail. It is considered as the maximum limit of the soil’s ability to resist such stresses. The shear strength of soil is typically expressed as a numerical value or parameter that quantifies its resistance to shearing forces, and it is an important parameter used in geotechnical engineering and soil mechanics to predict the stability and behavior of soil masses under various loading conditions.
The direct shear test is a commonly used method in geotechnical engineering to determine the cohesion and angle of internal friction of soils, which are important parameters for various engineering designs such as foundations and retaining walls. The test can be conducted under three different drainage conditions, namely unconsolidated-undrained, consolidated-undrained, and consolidated-drained conditions. Typically, cohesionless soils are tested using the consolidated-drained condition.
The effective cohesion, also known as effective stress, and the effective angle of shearing resistance are parameters that can be obtained from the direct shear test. These parameters provide valuable information for engineers when designing structures such as foundations and retaining walls. The test can be performed under different drainage conditions, including unconsolidated-undrained, consolidated-undrained, and consolidated-drained conditions, depending on the type of soil being tested.
In general, when testing cohesionless soils, the consolidated-drained condition is preferred. This condition allows for effective measurements of the cohesion and angle of internal friction of the soil. Other drainage conditions, such as unconsolidated-undrained and consolidated-undrained, may be used depending on the specific requirements of the project. However, for cohesionless soils, the consolidated-drained condition is commonly used in direct shear testing to obtain accurate and reliable results.
Apparatus Required for Direct Shear Test
To conduct a direct shear test, you will need various pieces of equipment. These include a shear box, a shear box container, a base plate with cross grooves on its top, two porous stones, two plain grid plates, two perforated grid plates, a loading pad with a steel ball, a digital weighing machine, a loading frame with a loading yoke, a proving ring, two dial gauges, weights, a tampering rod, a spatula, a rammer, and a sampler.
The shear box is used to contain the specimen during the test. The shear box container holds the shear box in place. The base plate with cross grooves on its top provides a surface for the specimen to sit on. The two porous stones are placed on either side of the specimen to allow water to drain out.
The plain and perforated grid plates are used to ensure proper drainage during the test. The loading pad with a steel ball is used to apply pressure to the specimen. The digital weighing machine is used to measure the weight of the specimen. The loading frame with a loading yoke applies the vertical load to the specimen.
The proving ring is used to measure the shear stress and shear strain during the test. Two dial gauges are used to measure the vertical and horizontal displacements of the specimen. Weights are used to provide the vertical load on the specimen. The tampering rod, spatula, rammer, and sampler are used to prepare the specimen before the test.
Fig 1: Direct Shear Test Apparatus
Test Procedure of Direct Shear Test
The direct shear test procedure involves the collection of a soil specimen, which can either be undisturbed or remolded. The sample is typically taken using a sampler and a rammer, unless the soil being tested is cohesionless. In that case, a sampler and rammer are not required.
The sampler used should have inner dimensions of 60 mm x 60 mm in plan, which are the same as the inner dimensions of the shear box. The shear box has a thickness of about 50 mm, while the sample thickness should be 25 mm.
Fig 2: Shear Box,Porous Stones, Grid Plates, Loading pad
The shear box is assembled by attaching the two halves with locking pins and placing the base plate at the bottom. Above the bottom plate, the porous stone is placed, followed by the grid plate. Plain grid plates are used for undrained conditions, while perforated grid plates are used for drained conditions. The weight of the shear box is noted at this stage.
Next, the soil specimen is placed above the grid plate. If it is an undisturbed sample, it is directly transferred to the shear box. If sandy soil is being used, it is placed in layers and each layer is tampered to achieve the required density. The weight of the shear box with the soil specimen is noted.
Above the soil specimen, the upper grid plate, porous stone, and loading pad are placed on top of each other.
Fig 3: Different Layers Positions in Shear Box
The entire box is now positioned inside a container and secured onto a loading frame. The proving ring is carefully arranged so that it makes contact with the upper half of the shear box. The loading yoke is placed onto the steel ball of the loading pad of the shear box. Two dial gauges are installed, one on the container to measure shear displacement, and the other on the loading yoke to measure vertical displacement. Next, the locking pins are removed from the shear box, and spacing screws are inserted into their designated positions in the box.
Fig 4: Applying Load on Specimen
The box used for testing the soil specimen is raised slightly in the upper half using spacing screws. The spacing is determined based on the maximum size of particles in the soil. A normal stress of 25 kN/m2 is applied to the soil specimen. Additionally, a shear load is applied at a constant rate of strain. Readings from the proving ring and dial gauges are recorded every 30 seconds. If the proving ring reaches its maximum value and suddenly drops, it indicates that the specimen has failed. The maximum value recorded at this point is considered as the failure stress. In some cases, the failure point is determined at 20% of shear strain. After failure, the box is removed and the water content of the specimen is measured. This procedure is repeated for different normal stresses ranging from 50 kN/m2 to 400 kN/m2.
Observations and Calculations for Direct Shear Test
The given instructions outline a set of calculations that should be performed using observations obtained from a test. The calculations involve determining various properties of a soil specimen and its container. Specifically, the size of the container should be determined, along with its area. The thickness of the soil specimen should also be measured, as well as its mass and volume. From these measurements, the bulk density and dry density of the soil can be calculated, as well as the void ratio.
To perform these calculations, the mass of the container and various other components should be measured, including the base plate, porous stone, and grid plate. The mass of the container and these components, as well as the soil specimen itself, should also be measured in order to obtain a total mass. These measurements can then be used to calculate the various properties of the soil specimen and its container.
Calculate the shear stress using below tabulated observations:
| S.No | Elapsed time | Horizontal dial gauge reading | Horizontal displacement | Vertical dial gauge reading | Vertical displacement | Proving ring reading | Shear force | Shear stress |
Now determine the shear stress at failure for different normal stress values:
| Test no. | Normal stress (kN/m2) | Shear Stress at Failure (kN/m2) | Shear Displacement at Failure | Initial Water Content | Final Water Content |
A graph was plotted with normal stress on the abscissa and shear stress at failure on the ordinate. The graph displayed a relationship between normal stress and shear stress, with the normal stress values plotted on the x-axis and the corresponding shear stress values at failure plotted on the y-axis. The graph’s shape and trend were represented visually, showing how shear stress at failure changed with varying levels of normal stress. The specific appearance of the graph was not described in the given context.
Fig 5: Graph plotted between Normal Stress and Shear stress
The cohesion intercept (c’) and angle of shearing resistance () in a graph provide valuable information about the behavior of a material under shear stress. By analyzing the graph, it is possible to determine the cohesion intercept (c’) and angle of shearing resistance (). These parameters are essential in calculating the shear strength (s) of the material using a specific formula. The cohesion intercept (c’) represents the intercept of the graph on the y-axis, indicating the cohesion or internal strength of the material. On the other hand, the angle of shearing resistance () denotes the slope of the graph, representing the material’s ability to resist shearing forces. By utilizing these parameters and applying the appropriate formula, the shear strength (s) of the material can be accurately determined. This information is crucial in various engineering and geotechnical applications to assess the stability and behavior of soils, rocks, and other materials under shear stress conditions. Understanding the relationship between cohesion intercept (c’), angle of shearing resistance (), and shear strength (s) is essential for predicting the performance and safety of structures and foundations, designing slopes, excavations, and tunnels, and mitigating geotechnical hazards.
Result of Direct Shear Test
Shear strength of the given soil sample is = ______________ kN/m2.