A direct shear test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil[1][2] or rock[2] material, or of discontinuities in soil or rock masses.
The test is, however, standard practice to establish the shear strength properties of discontinuities in rock.
A confining stress is applied vertically to the specimen, and the upper ring is pulled laterally until the sample fails, or through a specified strain.
The load applied and the strain induced is recorded at frequent intervals to determine a stress–strain curve for each confining stress.
These advantages have to be weighed against the difficulty of measuring pore-water pressure when testing in undrained conditions, and possible spuriously high results from forcing the failure plane to occur in a specific location.