Buffering Effect of Overlying Sand Layer Technology for Dealing with Rockfall Disaster in Tunnels and a Case Study

An overlying sand layer combined with a concrete arch operates as a buffering structure for dealing with rockfall disasters in tunnel construction, especially through karst and weak strata. However, the buffering effect and construction parameters of overlying sand technology always depend on experience. To successfully deal with rockfall disasters encountered when a tunnel passes through a top karst cave or collapse cavity, it is essential to study the buffering effect of overlying sand technology, to give a quantitative estimation of the rockfall impact force, and to optimize the construction parameter. First, a sand layer overlying a concrete arch supporting scheme in tunnel is introduced. For the optimization of the sand layer construction parameters, a series of discrete element-based simulations are employed to analyze the influence of the sand layer thickness, friction coefficient between sand particles, and sand layer porosity on the buffering performance. The results indicate that the thickness of the overlying sand layer is the dominant factor affecting the buffering effect of supporting structure. Thus, overlying sand thickness is selected as the key factor to be optimized. A collapse cavity in the Yuelongmen tunnel, Chengdu–Lanzhou Railway Project is then taken as a case of parameter optimization. Finally, construction parameters of overlying sand layer technology applied in other tunnels are discussed, which further validates the numerical simulation process and parameter optimization. This study verifies that overlying sand layer technology is an effective preventative treatment for limiting the impact of rockfall in tunnels with overhead cavities. Moreover, the results provide a fundamental reference and guidance for dealing with rockfall disasters in tunnel construction.