Abstract：The impact pressure of rock-fall is an important parameter for shield structure design. While not considering the elastoplastic properties of impacted object，the value of impact pressure becomes extremely high and can¢t be used in project design. However，the actual rock-soil materials are all elastoplastic bodies and will produce great plastic deformation under the rock-fall impact. For this purpose，the initial yield pressure should be studied and serve as a basis for judging whether the materials begin to yield or not. Based on the Hertz contact mechanics，under the assumption that the soil obeys the Mohr-Coulomb yield criterion，the initial yield pressure and the corresponding impact velocity of soil are studied. The result shows that the impact velocity of initial yield is very small for the soil，and the cohesion has an obvious influence on it while the internal friction angle does relatively small.
