Engineering Geological Characteristics Of Silty Mudstone In Jiangdihe Formation Of Diversion Project In Central Yunnan And Analysis Of Its Influence On Surrounding Rock Stability At Tunnel Entrance

Mudstones such as mudstone,shale,silty mudstone,etc.developed in the red layer in central Yunnan have weak water permeability and strong hydrophilicity,and are easily softened（or swelled）by water,and easily disintegrated（or contracted）by water loss.,Low-strength engineering geological characteristics,the construction of tunnels in the red rock soft rock in central Yunnan may cause large deformation of the surrounding rock and even engineering geological problems such as roof collapse,collapse,and side wall destruction.In this paper,the representative rock strata of the Cretaceous Upper Cretaceous Jiangdihe Formation（K₂j¹）in the middle Yunnan red bed soft rock is selected as the research object.First,through field investigation and data collection,the geological environment characteristics of the middle Yunnan red bed soft rock formation are analyzed;Laboratory tests,including physical property test,physical mechanical property test,rheological property test and hydraulic property test,study the engineering geological characteristics of silty mudstone of Jiangdihe Formation;finally,the entrance section of Fengtun tunnel is selected as the research object The stability of the surrounding rock was analyzed in two aspects,analysis and numerical calculation analysis.The stability of the surrounding rock before and after softening by water rock was analyzed through finite element numerical simulation.A reasonable excavation method was proposed for the surrounding rock of the tunnel after softening And supporting measures,providing reference for engineering design and construction.The main research results are as follows:（1）The mineral components of silty mudstone in the first section（K₂j¹）of the red bed Jiangdihe Formation in central Yunnan were analyzed by microscopic identification and X-ray powder diffraction test.Clay minerals in silty mudstone account for 42.1%,of which the relative contents of illite,illite mixed layer and montmorillonite are 57%,13%and 8%,respectively.These three clay minerals are relatively hydrophilic,which can lead to large swelling and poor disintegration resistance.（2）Through the uniaxial compression test,the instantaneous mechanical properties of silty mudstone（K₂j¹）in the three states of drying,natural and saturated water are studied.It can be seen that its compressive strength and elastic modulus decrease with increasing water content,and Poisson’s ratio It increases with the increase of water content,indicating that the mechanical properties of the rock decrease with the increase of water content;the mechanical properties of silty mudstone（K₂j¹）under different confining pressures are carried out by conventional triaxial compression test（σ₁>σ₂=σ₃）The study shows that as the confining pressure increases,the compressive strength and elastic modulus of the rock sample gradually increase,and the mechanical properties of the rock increase as the confining pressure increases.（3）The long-term strength of silty mudstone（K₂j¹）obtained by uniaxial compression rheological test is 62%of the short-term strength,and the long-term c value is 2.5233MPa and theφvalue is 35.37°through the shear rheological test.The rheological constitutive relationship is consistent with the Burgers model.（4）Through the study of hydraulic properties test（disintegration resistance test,free expansion rate test,expansion force test,softening test）,it is k Nown that silty mudstone（K₂j¹）has poor disintegration resistance,strong swellability,and relatively high softening coefficient Small,indicating that its mechanical properties are greatly reduced after encountering water.（5）From the qualitative analysis and numerical calculation analysis,the stability of the surrounding rock at the entrance of Fengtun tunnel is analyzed.In the numerical calculation,the stability of the surrounding rock of the entrance section of Fengtun tunnel before and after softening by water-rock interaction was analyzed through finite element numerical simulation and comparison.The supporting forces have increased significantly,indicating that the effect of water and rock has deteriorated the stability of the surrounding rock;in view of the deformation of the surrounding rock in the numerical calculation,a new excavation and supporting scheme is proposed,and the numerical calculation method is adopted to adopt The stability of the surrounding rock under the new construction scheme is analyzed.The results show that the new scheme has a better control of the stability of the surrounding rock and provides a reference for the construction of the tunnel on site.