Study On The Influence Of Excavation On Deformation Of Adjacent High-speed Railway Subgrade In Soft Soil Area And The Protection Distance

With the continuous development of high-speed railway(HSR)system and urban underground space,the study of the influence of underground space development projects including foundation pit excavation on the adjacent HSR subgrade has become a critical issue.HSR operates at an extremely speed,which demands a high smoothness of the track and a strict deformation control of the subgrade.The soil around the excavation near HSR will experience deformation,which may further induce undue deformation of the HSR subgrade,causing serious hazards to the running trains.The southeastern coastal area of China where soft soil is widely distributed has a dense HSR network.Therefore,the HSR subgrade in this area can be easily and severely affected by the adjacent excavation in particular.Considering that there are few study results about the regular pattern of such influence,it's necessary and urgent to find out the regular pattern and work out a prediction method of the influence of excavation on the adjacent HSR subgrade in the soft soil area.Focusing on a foundation pit engineering project which is adjacent to HSR subgrade in soft soil area,the study has built an analysis model for the influence of the excavation on the deformation of HSR subgrade by using the three dimensional elasto-plastic numerical analysis method,from which a regular pattern on such influence in soft soil area is obtained.Thus a prediction method for such influence and a calculating method for protection distance are proposed.Protective measures and their effects are also studied.Major research results are as follows:(1)A numerical simulation of the influence of excavation on the deformation of HSR subgrade is carried out.The regular pattern of lateral displacement of the retaining structure and the ground settlement around the foundation pit are analyzed and thus the characteristics of the horizontal displacement and settlement of the HSR subgrade both in longitudinal and transverse directions are obtained.(2)A basic analysis model based on the characteristics of the soil layers in soft soil area is established and the regular pattern of the deformation of HSR subgrade induced by adjacent excavation is worked out.The influences of the depth of excavation-H,the thickness of soft soil above the excavation face-D,the stiffness of support system-?,the length-width ratio of the foundation pit-L/B,the width of the subgrade-w,the height of the subgrade-h and the distance between the foundation pit and the subgrade-d on the subgrade deformation are quantified respectively,and the deformation sensitivity to each factor is also studied.(3)Based on the analysis and fitting of the simulation results,a simplified calculation formula for the maximum horizontal displacement and the maximum settlement of HSR subgrade is worked out,which comprehensively involves three factors including the depth of excavation,the stiffness of the support system and the distance between the foundation pit and the subgrade.A calculating method for the protection distance between the foundation pit and the subgrade is proposed and a curve of the relation between the protection distance and the depth of excavation under the conditions of the typical maximum allowable deformation of a HSR subgrade and the typical stiffness of the support system is obtained.Therefore,a prediction curve of horizontal displacement and settlement of HSR subgrade is drawn,and a prediction process for HSR subgrade deformation in soft soil area is proposed.(4)The protective effects of such measures as the thickening of the retaining wall,the setting of the isolation piles and the reinforcement of subgrade foundation on HSR subgrade are studied from the perspectives of the sources of influence,the transmission path and the protected object.