ABSTRACT
In a recent field investigation of the May 2008 Wenchuan earthquake sites, we found clear evidence that some of the fatally destroyed villages are resulted from the extremely large vertical ground acceleration. Here we investigate the complete process of spalling with numerical analysis, from which we derive an intuitive physical model to explain the presently unknown mode of strong ground motion with large vertical amplitude, and to explain the observed unusual phenomena of “ground overturn” in Wenchuan earthquake. Failure process of one dimensional rock bar was firstly analyzed, it was found that when a compressive wave hits the free surface, it will be reflected as a tensile wave and the acceleration or displacement doubles at the free boundary. Rock bar may then be fractured parallel to the reflecting surface if the tensile stress generated by the reflected wave exceeds the tensile strength of the rock, a phenomenon termed as spalling. Subsequently, numerical rock slope model was established to analyze the “ground overturn” in Wenchuan. Simulation results indicated that the surface of the rock slope was broken out due to the wave reflection on the ground surface. In addition, detailed analysis, including location of the earthquake wave, the length of earthquake wave and the amplitude of earthquake wave were systematically conducted. It was found that the location of the earthquake wave directly determined the failure location of the rock slope. And with increasing of the length/amplitude of the earthquake wave, the failure degree of the rock slope increased. Such a model should provide a better understanding of the near-source earthquake hazards.
