The triaxial shear test is a laboratory procedure used to determine the mechanical properties of soil. In this test, a cylindrical soil sample is subjected to controlled pressure from all sides while being sheared vertically. This helps assess the soil's strength and behavior under different stress conditions. The results from the triaxial shear test provide valuable data for civil engineering projects, such as the design of foundations and slopes. By understanding how soil reacts to stress, engineers can make informed decisions to ensure stability and safety in construction.