Abstract: The strength of the soil is very important in civil engineering. It is a key for calculating the subsoil bearing capacity, the slope stability and the earth pressure on the retaining wall.In all of the strength theory, only calculate the angle of internal friction ψ for the cohesionless soil. It was adopted the natural angle of repose for the coarse grained soil.The author made a lot of the large triaxial compression test and the abrupt slope test in site.Through these tests and engineering practice,it was proved that there is a newforce in the coarse grained soil.It is not the force of internal friction, it means the interlocking force. It may raise the strength of the coarse grained soil. At present,there is not this content in the national standard or in textbook. So this research work has important scientific and economic value.