Study On The Deformation And Optimization Of Construction Method Of The Loess Tunnel Under The Residential Area

With the rapid development of traffic infrastructure in Northwest China,more and more tunnels cross the Loess Plateau with ravines.When the tunnel goes through the residential area inevitably,the geological environment of the tunnel,especially the underground water storage environment,has been changed by the activities of residents such as irrigation,land cultivation and forestation programme.The water will greatly weaken the strength of loess surrounding rock,making the tunnel deform or even collapse.The existing researches mainly focus on the damage to the surface buildings in the process of tunnel excavation,while the impacts of human activities on the surface of the tunnel are always ignored.Based on the large deformation phenomenon of Anxing tunnel project of Yin-Bai expressway in Gansu section,this paper makes an in-depth study and discussion on the deformation law and control technology of the loess tunnel under the residential area.The main research contents are as follows:(1)According to the range of residential activities above the tunnel,the whole tunnel is divided into residential section and non-residential section.Through the field deformation monitoring and the statistical analysis of the deformation of all monitoring sections,the results show that the deformation laws of the two sections are significantly different.The residential section has the characteristics of rapid development of deformation,long deformation time and large accumulated settlement,while the abundant water environment is an important reason for this phenomenon;(2)The mechanism of groundwater recharge in residential areas on the loess plateau are discussed.In addition to rainfall and river,human activities such as a large amount of irrigation can recharge the groundwater,which can not be ignored.However,the different land use changes of cultivated land,conversion of cropland to forest and other residential activities change the soil texture and water infiltration rate,affecting the recharge.Surface water seepage is divided into priority flow and piston flow.The mechanism of two kinds of soil water seepage is analyzed.Because of the influence of residents’ activities,the soil water in the residential area flows downward in the way of priority flow to supply the deeper groundwater;(3)Through the method of literature statistics,68 cases of collapse or large deformation of loess tunnel are collected,from which the main influencing factors of loess tunnel collapse or large deformation are summarized.Combined with the engineering environment,six factors are selected as the influencing factors of the loess tunnel under the residential area,including the surrounding rock grade,burial depth,excavation span,groundwater,rainfall and the intensity of human activity.In order to quantitatively analyze the deformation of the tunnel caused by residents’ activities,the fuzzy analytic hierarchy process is used to determine the weight of each influencing factor,and to evaluate the risk of large deformation of the tunnel in residential areas;(4)Based on the engineering practice,the numerical models of tunnel excavation in residential and non-residential areas are established,and the influence law of fluid structure coupling on tunnel deformation in residential areas is compared.The optimization of the construction methods of loess tunnel under residential areas are analyzed,mainly aiming at the excavation footage,the length of the steps and the control of the tunnel deformation by the measures of waterproof and drainage.The results show that the smaller the excavation footage and the shorter the bench length,the better the deformation control of the tunnel,but the influence is limited.Anchor pipe can control the deformation of the tunnel,but the effect is poor.