Abstract:With the further development of hydropower engineering towards southwest area,the geological conditions of tunnel engineering are more and more complex.To solve the failures problem of surrounding rock masses (such as large deformation and collapse) with developed joint fissures,fault or other weak fractured zone during the tunnel excavation process,flood discharge tunnel at the Changheba Hydropower Station was selected as a case study example,and the supporting measures were studied to ensure the stability of surrounding rock and the construction safety.The reasons and solution for the failure of surrounding rock were concluded through comprehensive analyses of field investigation results,tunnel designed schemes,engineering geological conditions and finite element calculation.First,according to the failure of surrounding rock and actual geological conditions in the 0~30 m section of flood discharge tunnel engineering incombination with the investigation of specific construction measures of excavation and supporting,the influencing factors and modes of surrounding rock failure were studied.Then a typical finite element model of 30 m cross section for flood discharge tunnel was established.Considering the nonlinear deformation of surrounding rock,mohr-coulomb constitutive relation was used,and the stress and displacement of surrounding rock were calculated using finite element software.From the analysis of influencing factors and models of surrounding rock failure,influencing factors could be classified into three aspects:1) there were many joints in the surrounding rock of this section of flood discharge tunnel and the rock mass were cracked.This,the stability of surrounding rock was poor;2)There existed high crustal stress in the engineering area and there was some energy in the rock mass,and when the tunnel was excavated,the high crustal stress would be released so that the rock mass would be more cracked;3) the design of supporting measures was inappropriate.Ordinary bolting and shotcreting could not effectively restrain the deformation of surrounding rock and would lead to the rock failure.The physical and mechanical properties of surrounding rock were the leading factor of failures,which suggested that the failure mode of surrounding rock was rock mass structure-led failure mode.The finite element calculation results showed that,under the condition of original supporting,there was large deformation in the parts which were cut by the structural planes.Deformation of surrounding rock in fracture zone was effectively controlled after being timely supported.For the potential instability problems in the fracture zones of tunnel,advance supporting measures and spray anchoring,steel supporting,anchor wire bundle combined supporting measures were applied,which could restrain the deformation of surrounding rock and ensure the stability of surrounding rock.Moreover,the field monitoring data and application effects proved that the effectiveness of reinforcing supporting measures timely controlled the stability of surrounding rock mass.