The technique enables the engineer to select specimens of rock from exposed faces or bore holes, observe direction of fault lines, then set and test the fault in the shear box.

The apparatus consists of a diagonally split box assembly. The upper half incorporates a vertical ram for compressive loading, and the lower half, two horizontal rams for reversible shearing action. The force applied by the horizontal ram is aligned with the centre of the box and the discontinuity in the specimen. Pressure is applied to the rams by means of hand-operated hydraulic pumps.