Study On The Deformation Mechanism Of Surrounding Rock And Mechanical Behavior Of Supporting Structures For Layered Soft Rock Tunnel Under High Geo-stress

The regional geological environment in midwest of China are characterized by complex geological structures,hydrogeological conditions and changeable stratigraphic lithology.With the continuous development of China's transportation from the second geomorphic ladder(1000-2000 m above ground level)to the first geomorphic ladder(3000-5600 m above ground level),a large number of deep-buried and super-long tunnels in soft-weak surrounding rock with characteristics of large buried depth,high geo-stress,high geological disaster risk and rich water problem will be built or designed in the next ten years.Hence,the analysis method of surrounding rock stability and control technique of supporting structure safety of deep buried soft rock tunnel in this complex geological environment will become more and more important.The deformation and failure mechanism of soft surrounding rock under high geostress,the bearing mechanism of supporting structures and its mechanical behavior in construction period and long-term service state need to be further studied.Supported by the national key R&D Project of regional integrated transport infrastructure security technology of China,this paper adopts data investigation,theoretical analysis,laboratory tests and numerical methods to intensively study the deformation mechanism of surrounding rock and mechanical behavior of supporting structures in deep buried soft rock tunnels under high geo-stress.The main works and research results are as follows:(1)To study the mechanical properties,failure mechanism and water-softening characteristics of soft rock,uniaxial and conventional triaxial compression tests were systematically conducted on phyllite,sandstone,mudstone and slate under different confining pressures and water contents.Based on the energy storage and release theories,the energy damage evolution process of soft rock was studied.The stress thresholds when soft rock enters energy hardening and energy softening stages were figured out.Thus,a new rock energy brittleness evaluation index was established.(2)The asymmetric deformation failure characteristics and asymmetric mechanical behavior of supporting structures in layered soft rock tunnels under high geo-stress were investigated.Based on the deformation data of 93 layered soft rock tunnels under high geostress,the relationships between roof settlement,horizontal convergence,largest deformation and geo-stress,rock compressive strength and overburden depth were systemically analysed.Based on the power exponent variation law between the largest deformation and the strengthstress ratio,a new classification criterion for large deformations is proposed for layered soft rock tunnels under high geo-stress.(3)Based on the Sichuan-Tibetan expressway,five typical tunnels of cross fault zone tunnel,shallow buried soft rock tunnel,shallow buried bias tunnel,deep buried hard rock tunnel and deep buried soft rock tunnel were selected and field tests were conducted to obtain the surrounding rock pressure and internal forces on the secondary lining.By comparing the test results,the evolution laws of surrounding rock pressure,stress of steel arches,axial force and bending moment of secondary linings during construction period were revealed.(4)Discrete element method was adopted to study the influence of lateral pressure coefficient,bedding angle,bedding thickness and shear stress field on asymmetric mechanical behavior of supporting structures.Meanwhile,a double primary support method was proposed to restrain the large deformation of tunnels in layered soft rock under high geo-stress.The bearing mechanism and damage evolution law of double primary support was studied using a damage evolution rheological constitutive model.(5)Long-term health monitoring of surrounding rock pressure and internal forces on the secondary lining was carried out based on the Zhegu mountain tunnel.At the same time,using theoretical analysis and numerical simulation methods,the long-term structural safety performance of soft rock tunnel under high geo-stress and rheological effect was studied.