The direct shear test "in situ" is one of the "in situ" tests carried out to carry out the geotechnical reconnaissance of a terrain.

The "in situ" cutting tests obey the same principles and methodologies as the laboratory ones. Its most typical use is the determination of the shear resistance of joints or planes of weakness of rock masses.

To do this, a block of rock of the required dimensions is carved, so that the plane to be tested is located at the base of the block. The carving is done by hand, in the most careful way possible. The block is then surrounded with a metal frame, and the gap between the block and the frame is filled with (construction) mortar "Mortar (construction)"). Once hardened, the normal load is applied using hydraulic jacks. Subsequently, the tangential load is applied, also using jacks. This load usually has a certain inclination to avoid moments on the base of the block that imply non-uniform stress distributions along the failure plane. Blocks measuring 50 cm x 50 cm are common, although tests have been carried out on larger areas. As in load plate tests, it is common to resort to location in galleries.

The interpretation of the essay is direct. Displacements are measured in horizontal and vertical directions. Results are obtained on the deformability of the tested joint (normal, transverse and dilatancy moduli), as well as its shear resistance. "The reason for testing large blocks is usually the analysis of the influence of large-scale roughness on the resistance.

In the case of soft soils or rocks, it may happen that the vertical load is relatively close to the subsidence limit value. In these circumstances, the failure does not occur by sliding along the plane of the base, but by failure in the lower area, such as a footing with an inclined load, and should be interpreted as such.