Rockfall modeling in jointed rock for underground openings with consideration of seismic and thermal effect

A rockfall model for underground excavation in the jointed rock is developed using the three-dimensional discontinuum code, 3DEC. The factors considered in this model include the stochastic attribute of the joint network, the joint mechanical properties, the seismic loading, and the thermal loading. The Yucca Mountain site data were compiled and analyzed to feed into the numerical model. The 3DEC rockfall model incorporates the development of joint patterns generated from multiple sampling from a synthetic rock mass volume that contains a realistic joint population based on field mapping data. Site-specific ground motions and waste generated heat load are included in the model.; Variation of joint pattern combined with ground motions using the Latin hypercube technique is considered in seismic analysis in order to obtain a representative block size distribution during seismic shaking. Three levels of mean exceedance probability for seismic ground motions were used to determine the sensitivity of rockfall to seismic events. The seismic results show that higher rockfall volumes and frequency are predicted with higher level of ground motions. The results also indicate that rockfall in the jointed medium is primarily controlled by geologic structure.; Thermal-mechanical analyses were conducted using both a base case set of thermal properties and a sensitivity case considering the values for thermal conductivity and specific heat one standard deviation smaller than the mean. There was no rockfall predicted at any time for the thermal only scenario (i.e., no seismic loading) for all cases analyzed. The impact of thermal loading is found to stabilize the rock mass. Rock mass expansion on heating induces tangential compression around the excavations and provide increased normal stresses to the predominant vertical joint sets, thus increasing their shearing resistance. Thus, the most conservative thermal state, from a rockfall standpoint, is actually when the rock is at or near ambient temperature.; The results of the sensitivity study on the joint mechanical properties show that the variability of joint mechanical properties has minimal impact on rockfall except the joint dilation angle. The sensitivity analyses show that only half of the rockfall volume is predicted with dilation angle increased to 11° comparing with the base case assuming no dilation. The impact on drift stability due to the effect of rock joint degradation is assessed based on a conservative estimate of the reduction of joint cohesion and friction angle. The prediction shows that joint strength degradation has a minor impact on underground stability.; The 3DEC rockfall model is validated against laboratory testing, field observations, alternative numerical model, and a dynamic tunnel experiment. It is shown that the 3DEC rockfall model is capable of predicting block formation and correctly modeling the mechanical behavior of the jointed rock under dynamic loading.