This work presents the experimental results of the strength of sands and clay soils in the following conditions: plane shear, triaxial stress state, with passive and active loading. The obtained experimental results are compared with existing theories of strength and the reasons for their non-conformity are identified. Experimental data on the determination of the position of shear surfaces with active and passive resistance of soils is analysed. A new concept of the theory of soil strength is considered, which allows to take into account the fundamental parameters of the strength of...