Direct Shearbox Test

The objective of the test is to determine the effective shear strength parameters of the soil, the cohesion (c’) and the angle of internal friction (φ). These values may be used for calculating the bearing capacity of a soil and the stability of slopes. Direct Shear Test is considered one of the most common and simple tests to derive the strength of a soil and can be performed on undisturbed or remoulded samples.

The specimen is confined in the shearbox of square or circular cross-section split horizontally at mid-height, a small clearance being maintained between the two halves of the box. Porous plates are placed below and on top of the specimen if it is fully or partially saturated to allow free drainage: if the specimen is dry, solid metal plates may be used. A vertical force is applied to the specimen through a loading plate and shear stress is gradually applied on a horizontal plane by causing the two halves of the box to move relative to each other, the shear force being measured together with the corresponding shear displacement.

Required equipment

•        Shearbox apparatus for carrying out tests on soil specimens of 60 mm square and 30 mm high divided horizontally into two halves.

•        Two porous plates of corrosion-resistant material.

•        Two perforated grid plates of about the same size in plan as the porous plates, if necessary.

•        A loading cap to cover the top grid plate or porous plate.

•        A calibrated means of applying a vertical force to the loading cap such as a loading yoke.

•        A motorized loading device capable of applying horizontal shear to the vertically loaded specimen at constant rates of displacement from which a rate to suit the soil being tested can be selected.

•        A shear load measuring device (loading ring)

•        Dial gauge for measuring the relative horizontal displacement of the two halves of the shearbox.

•        Dial gauge for measuring the vertical deformation of the specimen during the test.

•        Specimen cutter.

•        Tool for removing the specimen from the cutter.

•        Levelling template for trimming the surface of the specimen in the shearbox to a known level.

•        Calibrated vernier external/internal caliper for measuring the internal dimensions and height of the cutting ring or test specimen to 0.1 mm.

•        Stop clock, readable to 1 sec.

•        Balance, readable to 0.1 g.

•        Apparatus for determining moisture content.

•        Silicone grease or petroleum jelly.

Preparation and assembly of shearbox

1. Apply a thin coating of silicon grease or petroleum jelly to the inside faces of the shearbox and to the surfaces of contact between the two halves of the box.
2. Assemble the shearbox with two halves securely clamped together, fit the baseplate and place it securely in position in the carriage.
3. Place a grid plate on the base of the box if necessary, followed by a porous plate.

Preparation of specimen

Specimen of either cohesive or non-cohesive soil may be tested in the shearbox. Preparation procedures depend on the type of soil, as indicated below. The size of the largest particle shall not exceed one tenth of the height of the specimen.

Normally three similar specimens are prepared from an undisturbed or remoulded cohesive sample, for testing under three different normal pressures. A non-cohesive sample shall be large enough to provide three separate specimens to avoid having to re-use the same material.

Undisturbed specimens are built into the shearbox by the use of the specimen cutter.

Remoulded specimens shall be compacted into an e.g. Proctor mould given sufficient compactive effort to achieve the desired density. The specimens are then built into the shearbox by the use of the specimen cutter.

Test procedure

Initial adjustments

1. Position the carriage (on its bearings) on the machine bed, and adjust the drive unit to the correct starting point of the shear test. Secure the horizontal displacement gauge in position.
2. Assemble the loading system so that the loading yoke is supported by the ball seating on top of the load cap.
3. Secure the vertical deformation gauge in position so that it can measure the vertical movement of the centre of the loading cap, ensuring that it allows enough movement in either direction.
4. Record the initial zero reading.

Consolidation

1. Apply a normal force to the specimen, to give the desired vertical stress, σ_n (in kPa), smoothly and as rapidly as possible without jolting. Start the clock at the same instant if consolidation readings are significant.
2. Except when testing dry soils, as soon as possible after applying the normal force fill the carriage with water to a level just above the top of the specimen, and maintain it at that level throughout the test.
3. Record readings of the vertical deformation gauge and elapsed time at suitable intervals to allow a graph to be drawn of vertical deformation as ordinate, against square-root of elapsed time as abscissa. A plot of vertical deformation against time to a logarithmic scale may also be made. Continue until the plotted readings indicate that primary consolidation is complete.
4. Final adjustments
5. On completion of the consolidation stage and before shearing make the following checks and adjustments:
6. Ensure that all adjacent components from the constant rate of displacement device through to the load measuring device and its point of restraint are properly in contact, but under zero horizontal load.
7. Remove the clamping screws which lock the two halves of the shearbox together.
8. Raise the upper half of the box, keeping it level, by turning the lifting screws (1/4 to 1/2 turn). The amount of clearance between the two halves should be enough to prevent them coming together during the test, but shall not permit extrusion of the soil between them. Retract the lifting screws.
9. Record the initial readings of the horizontal displacement gauge, the vertical deformation gauge and the force measuring device.

Shearing

Shear the specimen to failure:

1. Start the test and at the same instant start the timer. Record readings of the force measuring device, the horizontal displacement gauge, the vertical deformation gauge and elapsed time, at regular intervals of horizontal displacement such that at least 20 readings are taken up to the maximum load (‘peak’ shear strength).
2. Take additional readings as the maximum horizontal force is approached, so that if the ‘peak’ occurs it can be clearly defined.
3. Continue shearing and taking readings beyond the maximum force, or until the full travel of the apparatus has been reached if there is no defined peak, then stop the test.
4. Reverse the direction of travel of the carriage and return the two halves of the shearbox to their original alignment.
5. If the specimen was sheared under water, siphon off the water from around the specimen and allow to stand for about 10 min to enable free water to drain from the porous plates.
6. Remove the vertical force and loading yoke from the specimen.
7. Transfer the specimen from the shearbox to a small tray, taking care not to lose any soil. Remove any free water with a tissue.
8. Weigh the specimen on the tray to 0.1 g.
9. Dry the soil in an oven at 105^oC to 110^oC and determine its dry mass (m_d) to 0.1 g, and its final moisture content.

Calculations and plotting

From each set of data obtained during the shear test calculate the horizontal shear force, P (in N), applied to the specimen.

Calculate the Shear Stress on the surface of shear, τ (in kPa) for each set of readings from the equation:

Where

A is the initial plan area of the specimen (in mm²).

The normal stress σ_n (in kPa), applied to the specimen is given by the equation

kPa = kN/m²

From each stress-displacement graph read off the value of the maximum shear stress (the ‘peak’ strength) and the corresponding horizontal displacement and change in specimen height.

Plot each value of peak strength τ  (in kPa), as ordinates against the corresponding vertical normal stress σ_n (in kPa) applied for that test as abscissae, both to the same linear scale.

If it can be assumed that the relationship is linear, the slope of the line and its intercept with the shear strength axis can be derived from the line of best fit through the plotted points. The slope gives the angle of shearing resistance φ (in degrees), and the intercept gives the apparent cohesion c’ (in kPa), both in terms of effective stress.