In order to ensure the road safety

bridge was adopted in the active debris flow regions.When the debris flow broke out

the pier located in the activie debris flow regions was impacted by the debris flow.The different impact forces could destruct the pier in different degrees.The debris flow′s impact force is the main load in the debris flow regions.Now

we usually consider the debris flow as an isotropic body.In fact

the debris flow is two-phase liquid

so the impact forces of debris flow included the solid phase′s impact force and liquid phase′s impact force.The two impact forces must be calculated separately.This paper has corrected the debris flow′s impact forces′ formula.Based on the inelastic collision and dynamics theory this paper receives the formula of the solid phase′s impact force.But the liquid phase′s impact force could be calculated by the Newton′s second law

because the pier′s stiffness is greater than the liquid phase′s.The destruction of pier contains three modes: impact destruction mode

capsize mode and slide mode Set a energy-dissipating layer on the pier could not only decrease the impct force but also decrease the damage potential of the debris flow.The exemple indicates that this technique′s control effect is significant.