This is the earliest and simplest device to determine soil strength parameters.
As seen in Figure 1, it consists of upper and lower shear boxes, and a soil specimen is placed inside the box. Vertical normal force Fv and hence the normal stress σ (= Fv/specimen area) is applied and kept constant. In most devices, the upper box is fixed, and the lower box is movable on low-friction rollers at the base.

Also, special care is taken to minimize friction at contacting surfaces between the upper and the lower shear boxes such as with low-friction Teflon push bolts. The lower box is pulled or pushed to apply shear force T, and hence the shear stress τ (= T/specimen area) is induced along the middle plane of the specimen.

FIGURE 1 – Direct shear test setup.

In this device, shear failure surface is forced to develop in a near-horizontal direction.
Measurements during the test are constant σ, and changes in τ, vertical deformation δv, and horizontal shear deformation δh. The change in δv measurement is directly proportional to the volume change of the specimen ΔV (= Δδv ∙ specimen area) since the cross-sectional area of the specimen remains the same. Thus, under a given normal stress σ, τ versus Δδh and ΔV versus Δδh are plotted as seen in Figure 2.
Figure 2(a) defines the peak shear strength and the residual shear strength. The former is generally used as the shear strength of the soil τf. The latter is the strength after a large deformation, and it may be used to evaluate the stability of earth structures when large deformation is allowed beyond its peak strength.

FIGURE 2 – Direct shear test result.

Soil may contract or dilate during shearing, as seen in Figure 2(b), mostly depending on its initial density. It is interesting to notice that soil is a very unique material, which increases its volume upon application of shear stress (dilatancy), particularly for dense sands and heavily overconsolidated clays. It is because densely packed grains or particles have to move or roll over neighboring grains to change their relative positions during shearing, as seen in Figure 3.

FIGURE 3 – Dilatancy model.

Accordingly, shear stress–deformation relations and their volume change characteristics during shear are largely influenced by initial density of specimens.
Figure 4 shows these for dense, medium dense, and loose soils. As seen in the figure, the shear stress–deformation curves emerge to the residual shear strength at a large shear deformation. The void ratios also emerge to a certain value at a large shear deformation. When soil assemblage is sheared at large deformation, certain zones within the specimen (shear zone) are subjected to large shear deformation.
Along these shear zones, where shear failure is taking place, particles are oriented to a preferred direction, which changes from their original loose or dense configurations, and a steady-state flow (failure) mechanism is created. This is the reason why all strengths emerge to the residual strength and all void ratios become a certain value at large shear deformation, regardless of their original denseness. In Figure 4(b), initial dense soils undergo initial contraction and then dilation.

FIGURE 4 – Shear stress–deformation and void ratio for loose to dense soils.

On the other hand, loose soils contract all the way till failure. For a specimen in between dense and loose, there is a specimen of which the void ratio remains nearly the same during the shear. That void ratio is called critical void ratio and this specimen does not contract or dilate during shear.
For a given soil with a similar density, several direct shear tests are conducted under different normal stresses. Peak shear strength values τf are measured for each test. Then σ and τf relations are plotted as in Figure 5 A linear relation is obtained through the data points and the intersection on the τf axis gives the cohesion component c; the slope of the line makes the internal friction angle φ.
For different soils and different densities, lines are different, so different c and φ values are obtained.

FIGURE 5 – Determination of j and c from direct shear tests.

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