Experimental And Simulation Study On The Mechanical Properties Of Red Bed Soft Rock Under The Action Of Dry-wet Cycle

Red bed geomorphology is widely distributed around the world,and southwestern China is one of the regions with the most concentrated red bed distribution.In recent years,with the promotion of the development of the western region,a large number of infrastructure projects have been built in the red bed area,and geological disasters caused by red bed soft rocks often occur,causing many safety hazards to the construction of the project.During the construction of the tunnel project,due to the influence of excavation disturbance,the rock mass originally in a relatively balanced stress state will generate concentrated stress to produce a plastic zone,resulting in a large number of cracks in the surrounding rock.Fissures provide a permeation path for water,and the reaction of red-layer soft rock with water will cause the deterioration of mechanical properties and the deterioration of engineering properties,especially in the season of heavy precipitation.The deterioration of mechanical properties will be more obvious and the deterioration rate will be faster.This will cause a large amount of deformation of the surrounding rock of the tunnel,resulting in the loss of the support structure and even collapse.Therefore,it is of great significance to study and analyze the mechanical properties of red bed soft rock under the action of dry and wet cycles.Based on this,this paper takes the Huilongsi Tunnel of Chongqing-Changshou Expressway as the background,through theoretical analysis,combined with indoor tests and numerical simulation,systematically studies the mechanical properties of red soft rock under dry and wet conditions and the surrounding rock Excavation mechanical response.The main research content and results of the paper are as follows:(1)The dry-wet cycle test of the red layer soft rock was carried out.Through mercury intrusion analysis,XRD,electron microscope scanning and other tests,the basic physical characteristics,mineral composition and porosity of the red layer soft rock under the dry-wet cycle were obtained..The results show that: under the action of dry and wet cycles,the properties of the rock samples are continuously deteriorating.The microstructure of the red layer soft rock samples changes from dense to loose and porous,and the pores increase.(2)Uniaxial and triaxial compression tests were carried out on the red layer soft rock samples under different cycles of dry and wet cycles,and the degradation effect of the dry and wet cycles on the mechanical properties of the red layer soft rock was analyzed.The results show that the dry-wet cycle effect has a significant effect on the mechanical properties of the red layer soft rock.The uniaxial compressive strength(σ),elastic modulus(E),cohesive force(C)and internal friction angle of the red layer soft rock(Φ)will continue to decrease with the increase of the number of dry and wet cycles,and the rate of decrease will gradually slow down,but the effect of dry and wet cycles on the mechanical parameters of red bed soft rock is different.(3)Based on the experimental research,the particle flow model of the red layer soft rock sample was established,and the deterioration law of the mesomechanical characteristics of the red layer soft rock under different times of dry and wet cycles was studied.The main conclusions were:(1)dry and wet The cyclic effect makes the model more prone to damage under the same conditions,and the existence of confining pressure allows the model to have a better stress state when loaded.(2)The development and distribution of fissures are formed together under confining pressure and dry-wet cycle.(3)The meso-parameters of the red layer soft rock show a decreasing trend with the increase of the number of dry and wet cycles,and the rate of decrease is also gradually slowed down,but the degree of influence of the dry-wet cycle on the meso-parameters of the red layer soft rock Also different.(4)The PFC particle flow software was used to establish the Huilongsi tunnel excavation model,which simulated the deformation law of the surrounding rock under different drying and wetting cycles.The results showed that:(1)After the drying and wetting cycle,the number of cracks in the surrounding rock Significantly increased,the distribution area is larger,and the generation time is earlier;(2)Under different dry and wet cycles,the area of ??the tunnel surrounding rock displacement is mainly concentrated in the arch top,arch bottom and side walls on both sides,and the The displacement of rock mass in the area increases faster than in other areas,the growth rate of the displacement of surrounding rock becomes faster,and the displacement becomes significantly larger.(3)With the increase of the number of dry and wet cycles,the deformation of the surrounding rock continues to increase,and the supporting structure loses its effect,which may cause geological disasters such as tunnel collapse.(4)The surrounding rock is prone to collapse without reinforcement,and the collapse is time-sensitive.With the increase of the number of dry and wet cycles,the greater the collapse height and collapse area of ??the surrounding rock of the tunnel,the change will increase linearly.