第三系泥岩隧洞围岩大变形成因及应对措施

围岩大变形是软岩隧洞建设中危及隧洞施工及长期安全的重大工程灾害之一。结合第三系泥岩隧洞出现的显著围岩大变形及支护结构破坏等现象的工程现场调查,通过开展围岩监测、室内试验及数值模拟等工作,获得了第三系泥岩隧洞围岩大变形的主要成因和发生机理。研究表明:触发该隧洞围岩大变形的主要因素是低岩石强度条件下隧洞开挖卸荷引起的塑性变形以及地下水对围岩的软化作用,围岩挤压膨胀变形和不同岩层间的非一致变形共同主导了支护结构的破坏;围岩大变形的发生机理主要体现在第三系泥岩洞段横穿一条常年流水的冲沟,加之隧洞中部透水性良好的砂砾岩层,使得隧洞开挖后围岩含水率显著增加,第三系泥岩遇水泥化、软化,强度显著降低并呈现出一定的膨胀性,最终促使围岩产生显著的大变形。在此认识的基础上,提出了提高钢拱架型号、增强钢拱架之间的纵向连接、增设底拱外八字锁脚锚管、施加初期支护与二次衬砌之间的聚乙烯缓释消能层等应对措施,实施后的现场监测结果表明,所提出的控制措施有效解决了第三系泥岩洞段开挖过程中的软岩大变形难题。

Causes and Countermeasures of Large Deformation in a Tunnel with Tertiary Mudstone

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.