Abstract:After the completion of excavation of the 6th layer of the underground powerhouse,the longitudinal cracks appeared in the concrete upper surface of the rock-anchored beam. Monitoring data indicate that the width of the cracks in lower part between the concrete and the rock mass is larger than the upper one,which is an unusual phenomenon. Detailed cause analysis of these cracks was made based on the in-situ geological environment,monitoring data,constructional procedure and the numerical simulation. The analysis results indicate that the distance between the top of the bus gallery and the rock-anchored beam is only 5.2 m,and rock mass is cut by 3 weak interlayers. Due to the construction of the main machine hall,the ground stress was readjusted,and the obviously concentrated stress phenomenon appeared at the rock mass,which resulted in the non-uniform deformation of the rock mass. Numerical calculating results indicate that as the excavation goes on,the rock mass between the interlayer JC2–2 and the lower one has the trend to be squeezed out of the palisades,while this deformable body lies in the lower part of the rock-anchored beam,the widths of the cracks in lower part are larger than ones in the upper part,which explains the unusual phenomenon of the monitoring results. Simulation results also indicate that this value in cross section CZ0+87 is larger than that in cross section CZ0+127,which agrees well with the monitoring data. Finally,forecasting is made on the displacement development trend of cracks in the rock- anchored beam;and several advices are given.