Direct Shear Tests

Introduction:
An accurate determination of soil shear strengths in the laboratory is essential to cost effective design and planning for geotechnical projects. One such laboratory method available at MEA is the Direct Shear Test. The test is commonly employed for geotechnical projects where soil shearing along a horizontal plane is anticipated. This applies to landslide remediation, slope stabilization, and pipeline analysis projects. The direct shear machine at MEA can perform tests over a large range of strain rates ranging from 0.00001 to 0.49999 in/min; these rates are suitable for both geotechnical and research projects. MEA’s direct shear machine offers a more customizable strain rate than standard direct shear apparatuses whose rates range from 0.0001 to 0.3 in/min. Figure 1 is a picture of our direct shear machine.

FIGURE 1: MEA’S DIRECT SHEAR MACHINE

Method:
The purpose of a direct shear test is to determine the shear strength of a soil; this is done by forcing the soil to shear along an induced horizontal plane of weakness at a constant rate. A minimum of three subtests are run where the sample is consolidated using three different weights and then sheared at the same constant rate. The peak shear stress and residual shear stress of each subtest are recorded and graphed. Using this data, the cohesion and friction angle of the soil can be determined. Figure 2 below shows an example test plot with five separate subtests used to complete the test. The soil in question had no cohesion and a peak friction angle of 40.5º and a residual friction angle of 37.9º.

FIGURE 2: AN EXAMPLE TEST PLOT FOR A DIRECT SHEAR TEST WITH FIVE SUBTESTS. “P” IS PEAK SHEAR STRESS AND “R” IS RESIDUAL SHEAR STRESS.

Landslides

Landsliding refers to the downward shifting of the ground on a slope. The slope can naturally exist and/or be manmade. The reason for the shifting is either there has been added force to the slope or the slope has been weakened. There are a number of ways downward force can be added to the slope. Some of the more common applications are:
  • The slope is steepened and/or heightened with fill soil
  • Slope becomes water logged and thus made heavier
  • A structure is constructed on or close to the slope
  • The water level in the waterway at the bottom of the slope dropped significantly causing downward seepage forces.
More typical causes for landsliding from slope weakening are: waterway bank erosion; man-made undercutting or excavations along the slope; removal of root reinforcement from vegetation; and weathering of the soil mantle. Landsliding can occur slowly to abruptly with little warning. More typical tell-tale signs of slope instability are ground cracking along the slope which is most commonly towards the upper portion of the slope; trees, poles, fences, etc. which are leaning downslope and ground surface bulging of heaving near the bottom of the slope. 

Rotational Landslide

Diagram of an idealized landslide showing commonly used nomenclature for its parts. Courtesy of the Utah Geological Survey.