Abstract: Large deformation of surrounding rock is one of the major engineering disasters endangering tunnel construction and long-term safety in soft rock tunnel construction.In association with engineering field investigation of significant large deformation and support structure damage,we obtained the main causes and occurrence mechanism of large deformation in the Tertiary mudstone tunnel via deformation monitoring,indoor test and numerical simulation.Results revealed that the plastic deformation caused by tunnel excavation under the condition of low rock strength and the softening effect of groundwater on surrounding rock were major factors triggering the large deformation in the tunnel.The squeezing and expansive deformation of surrounding rock and the inconsistent deformation between different rock strata jointly dominated the failure of the support structure.The Tertiary mudstone tunnel crosses a gully with perennial water flow,coupled with the sand gravel layer with good water permeability in the middle of the tunnel,which significantly increased the water content of surrounding rock after tunnel excavation.The argillization and softening of the Tertiary mudstone led to strength reduction and expansion,which finally resulted in the large deformation of surrounding rock.On this basis,we put forward such countermeasures as improving the steel arch frame,strengthening the longitudinal connection between steel arch frames,adding foot lock anchor pipe of bottom arch,and applying polyethylene slow-release energy dissipation layer between primary support and secondary lining.Field monitoring results demonstrated that the proposed control measures effectively solved the problem of large deformation during the excavation of the Tertiary mudstone tunnel.

Key words: tunnel, the Tertiary mudstone, large deformation, cause analysis, control measures