| Landslide disasters occur frequently in our country,which will cause a lot of casualties and economic losses.It has already become the main type of natural disasters in our country.Rainfall is the main factor that causes slope instability and induces landslide.The exit slope of Chenjiapo tunnel is taken as the research object.Through the geological exploration of the slope,the engineering geological and hydrogeological conditions of the slope are found out.According to the statistics of local rainfall on the slope,three rainfall conditions are established,which are rainstorm with different rainfall types,rainstorm with different rainfall intensity and long-term continuous rainfall.The stability of the slope under three rainfall conditions is numerically simulated by Geo Studio software,and to explore the mechanism of slope instability caused by rainfall,with a view to the research results can provide a basis for such slope protection and treatment projects.The main research results of this paper are as follows:(1)The exit slope of Chenjiapo tunnel is located in Xuanen County,Hubei Province.The geological structure of the slope is concealed fault structure with a natural slope of22?26°.The overburden of the slope is mainly gravelly soil,locally silty clay,and the underlying bedrock is siltstone.Xuanen County has a subtropical monsoon mountain humid climate with annual rainfall of 1361-1902 mm.Continuously concentrated rainfall,high intensity,and strong suddenness are the main factors inducing landslides in spring and summer.The slope is located directly above the exit of Chenjiapo tunnel.During tunnel excavation,the left side of the front edge of the slope slides downward,and it forms6.27×10~4 m~3 large landslide and 8.69×10~5 m~3 giant unstable slope under the combined action of rainfall and excavation.The landslide and unstable slope not only threaten the safe construction of the tunnel,but also directly threaten the operation safety of the expressway in the later stage.(2)When the total rainfall and the rainfall time are the same,because therainfall mainly concentrates on the first day,a large amount of rainwater infiltrates into the slope on the first day of rainfall,resulting in a rapid increase in the scope of the saturated area at the slope slide zone and the slope toe,a rapid increase in the horizontal displacement,a rapid increase in the size and distribution of the shear stress at the slope toe,a rapid development in the plastic zone and a rapid decrease in the slope stability.The slope has been damaged at the end of the first day of rainfall.Although the slope is unstable under the rainfall conditions of equal intensity,stepped,middle-peak and back-peak,its failure time is later than that of front-peak.Therefore,in terms of the timeliness of slope failure,the front-peak rainfall is the most detrimental to the slope stability.(3)When the rainfall time is the same and the rainfall is of equal intensity,with the increase of rainfall intensity,the amount of rainwater infiltrating into the slope is more.The larger the saturation zone at the slope slip zone and the slope toe,the greater the horizontal displacement of the slope.The greater the size and distribution of the shear stress at the slope toe,the faster the plastic zone develops and the more likely the slope is to be damaged.At the end of the rainfall,the minimum safety factors of the slope at 50 mm/d,70mm/d and 90 mm/d are 1.023,0.952 and 0.887 respectively,and the slope is damaged at 36h and 27 h under the rainfall intensity of 70 mm/d and 90 mm/d respectively.Therefore,the greater the rainfall intensity is,the worse the slope stability is,and the shorter the time required for slope failure is.(4)In the process of long-term continuous rainfall,with the infiltration of rainwater,the scope of saturated zone at the slope slide zone and slope toe increases gradually,the horizontal displacement increases gradually,the size and distribution range of shear stress at the foot of slope increases gradually,the distribution range of plastic zone at the slide zone,the slope toe and the surface of slope increases gradually,and the safety factors of slope decreases gradually.In the later period of rainfall,as the slope begin to drain outward,the rate of slope safety factor decrease gradually slows down and then tends to be stable.Therefore,during the long-term continuous rainfall,the slope stability continues to decrease,but the rate of decrease gradually slows down until it tends to be stable.(5)Under the condition of rainstorm,the rainstorm lasting for 3 days is enough to make the slope unstable.Therefore,the main reasons for the slope failure are the rapid increase of the sliding force,the rapid decrease of the anti sliding force and the significant decrease of the resistance slippery ability of the front edge of the slope caused by the rainstorm.Under the condition of long-term continuous rainfall,the slope may be completely unstable under the combined effect of rainfall and other adverse factors.Therefore,the main causes of slope instability are the surrounding conditions of slope,the effect of long-term continuous rainfall,the geological structure of slope itself and the influence of expressway construction and tunnel excavation.(6)Based on the comprehensive analysis of the field investigation results and the numerical simulation results,the failure tendency of the exit slope of Chenjiapo tunnel induced by rainfall is as follows:1)the slope sliding force increases continuously and the anti sliding force decreases continuously due to rainfall,and the slope stability decreases continuously;2)in the process of slope stability decreasing,local damage occurred in the soil at the front edge of the slope;3)when the failure degree of the soil at the front edge of the slope reaches a certain scale,the whole slope is unstable under the combined action of self weight and rainfall.The slope is reinforced by anti slide pile as the main support and closed slope cracks and drainage as the auxiliary measures,and the unstable slope is monitored. |
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