Study On Macroscopic Properties Of Layered Rock And Stability Of Surrounding Rock

This article takes the rich water weakly cemented sandstone of Xiangshan Tunnel as the background,and studies the stability of the Neogene rich water weakly cemented sandstone tunnel in response to the special engineering characteristics of weakly cemented sandstone and various difficulties encountered during the construction of weakly cemented sandstone formations,such as weak geological properties that are prone to collapse and frequent water and sand gushing disasters.Conducted on-site sampling and some indoor tests,combined with the results of on-site investigation and testing,and mastered the basic properties of weakly cemented sandstone;Based on the fluid solid coupling theory,the finite difference software FLAC3D is used to carry out numerical analysis of tunnel stability in many states such as natural stratum,grouting reinforcement and water stop,dewatering and grouting reinforcement,and field monitoring tests are carried out,and the following conclusions and results are obtained:（1）The Neogene weakly cemented sandstone is mainly composed of fine particles,with a water content generally ranging from 10.0%to 16.0%and a maximum of 23.1%.The mud content is about 7.2 to 15.8%,and the uniaxial compressive strength at natural water content is 2-3 MPa.After soaking in water for30 minutes,the bond breaks down and becomes completely loose.During tunnel excavation,local phenomena such as collapse,sand gushing,and base softening often occur on the face and arch of the tunnel.（2）Based on the degree of cementation,water content,and groundwater conditions of weakly cemented sandstone,this article is divided into Class Ⅰ,Ⅱ,and Ⅲ weakly cemented sandstone.Among them,Class Ⅰ roughly corresponds to general Class Ⅴ₁surrounding rock,Class Ⅱ roughly corresponds to general Class Ⅴ₂surrounding rock,and Class Ⅲ roughly corresponds to general Class Ⅵ surrounding rock.（3）In the absence of auxiliary construction methods in the natural strata,the stability analysis of the Ⅲ Neogene weakly cemented sandstone tunnel shows that the arch settlement and tunnel bottom uplift values are as high as 32.17 cm and 50.89 cm,respectively,under the 40 m natural water level.Therefore,direct excavation construction is basically not feasible without using other auxiliary methods.（4）There are significant differences in the grouting forms of surrounding rock with different sections,and the effect of full perimeter grouting and full curtain grouting is better than that of half perimeter grouting and half curtain grouting.Among them,the effect of full curtain grouting is the most significant in improving the stability of the tunnel.At the same time,the optimal thickness of the grouting ring is 5 meters,and it is recommended to use full curtain grouting within a range of 5meters for on-site construction.（5）In the case of using a full section curtain grouting with a thickness of 5meters for grouting,the decrease in water level can still have a positive effect on the stability of the surrounding rock,and precipitation work is still important.（6）After on-site surface precipitation and advanced curtain grouting to reinforce the formation,the on-site monitoring test and numerical calculation patterns are relatively similar.The on-site test data shows that the deformation of the surrounding rock does not exceed the reserved deformation amount for construction,the lining pressure is small,the structure is in good condition,and the tunnel is in a safe state.