Experimental Study On Strength Softening Of Slate After Water Absorption Due To Structure Effects For Muzhailing Deep-buried Tunnel

Muzhailing Tunnel has a large buried depth and the rock mass is broken.The surrounding rock deformation is significantly affected by the rock mass structure effect.Under the coupling effect of high ground stress and groundwater,the creep properties of the surrounding rock of the tunnel are significant,which has a serious impact on the excavation of the tunnel and construction safety.Due to the lack of experimental equipment and experimental methods that can simulate the actual environmental conditions of the project,the strength softening after water absorption due to structure effect of slate and its water absorption softening mechanism have not been fully recognized,which leads to the lack of reliable basis of tunnel support design and control methods.So it is difficult to achieve effective control of the surrounding rock of the Muzhailing tunnel.The paper relies on the National Natural Science Foundation of China(51874311,Rheological characteristics of soft rock under temperature coupled pressure and its control measures),and large deformation control and design project of Weiwu Expressway Muzhailing super long tunnel,using water absorption test device,true triaxial test system,and five-axis creep system of State Key Laboratory for Geomechanics and Deep Underground Engineering,adopting rock mechanics,rheological mechanics,damage mechanics,from the aspects of site geological conditions,indoor experiments,theoretical analysis,numerical simulations to carry out the research work.The research is divided into experimental study on strength softening of slate after water absorption due to structure effects,numerical simulation on strength softening of slate after water absorption due to structure effects,and long-term time effect of creep experiment on strength softening of slate after water absorption due to structure effects,to obtain mechanical properties of slate affected by water content and structure effects.The main research progresses are as follows:(1)The mineral content and microstructure of salte are one of the important reasons leading to the large deformation of the Muzhailing Tunnel.The quartz content is low but the clay mineral content is high.It is easy to absorb water and cause large deformation damage.(2)The slate water absorption is divided into three stages.The slow water absorption stage is the main water absorption stage.The inversion analysis determines that the porosity and initial water content are the key parameters of slate for water absorption.(3)The uniaxial compression failure mode of slate is affected by water content and structure effect.Under conditions of the same bedding degree,the uniaxial compressive strength decrease with the increase of water content;Under conditions of the same water condition,the uniaxial compressive strength decrease before and increase after with the increase of bedding degree,showing a "U" type distribution.(4)The single weak surface theory is used to explain the variation of uniaxial compressive strength of slate under the influence of structure effect in saturated state,and the range of bedding degree which is prone to shear failure is determined.(5)Through the microstructural and theoretical analysis,it is considered that the water absorption softening mechanism of slate is that water enters the slate interior along the weak surface of the layer,causing the coupling effect of physical,chemical and stress in the vicinity of the layer,which causes the slate to produce micro-crack along the bedding plane.The internal cohesion is reduced,and the macroscopic performance is reduced in strength and the softening effect is obvious.(6)The discrete element numerical simulation results show that the crack propagation of the bedding degrees of 0°,30°and 90°is mainly controlled by tensile stress.The crack propagation of the bedding degrees of 450 and 600 is mainly controlled by shear stress;The water content mainly influence crack propagation of bedding degrees of 0°,30° and 90°.The lower the water content,the more obvious the model crack is controlled by the tensile stress;the damage pattern of the numerical model is in a good correspondence with the indoor experimental results.(7)The creep parameters of slate are obviously affected by water content and structure effect,and the overall creep is small.The creep deformation of slate includes transient creep and steady creep,and the accelerated creep phenomenon is not obvious.Creep equation is established based on moisture content and structure effects.(8)The energy released during the initial loading and breaking moments of the slate accounted for the majority of the total energy during the creep experiment.Under conditions of the same water,the initial energy release increases with the increase of the bedding degree;Under conditions of the same water,the energy release at the time of failure decreases first and then increases with the increase of the bedding degree;the initial energy,the cumulative energy at the time of destruction,the initial frequency value,and the main frequency value at the time of destruction have a tendency to decrease with the increase of water content.This paper mainly achieved the following innovations:(1)The effects of water content and structural effects on the mechanical properties of slate were analyzed by multi-experiment and multi-scale measurement.The softening mechanism after water absorption of slate was revealed.(2)The numerical simulation of the strength softening after water absorption due to structure effect of slate was carried out by using discrete element numerical simulation method.The effects of water content and structure effect on the crack propagation law and failure morphology of slate were analyzed.(3)Through the self-developed five-axis creep experimental system,the creep experiment on strength softening of slate after water absorption due to structure effects was carried out.The creep equation considering the water content and structural effect was established.The variation characteristics of acoustic emission energy and the variation of dominant frequency characteristics during creep experiments influenced by water content and structure effects were mastered.