Study On The Influence Of Deep Excavation Dewatering On Surface Settlement In Loess

With the economic growth and urbanization in northwest China,the development of high-rise buildings and underground rail transit is increasing day by day,which makes the number of deep foundation pit in loess area increase continuously.In order to ensure the construction of deep foundation pit in dry environment,the dewatering work destroys the balance of the groundwater seepage field around the dewatering well,forms the regional funnel-shaped curved surface,and makes the hydraulic gradient increase.When the water surface tends to be stable,the surrounding surface will consolidate and settle in the vertical direction,which will endanger the safety of the surrounding buildings and underground pipelines.Therefore,it is particularly important to accurately predict the surface settlement and building tilt caused by drainage.In this paper,based on elastic semi-infinite body theory,shear displacement method and seepage mechanics,theoretical formula derivation and numerical simulation research are carried out,and measured values of a foundation pit case in loess area are used for verification.The main work and achievements are summarized as follows:(1)The stress changes of the soil layer before and after precipitation were analyzed to dredge the additional stress that increased linearly with the decrease of water level in the area,which was equivalent to the uniform physical load.Simplifying the infiltration curve,the calculating formula for calculating the additional stress at any point in the saturated area is derived quantitatively from the two-dimensional plane and three-dimensional space,and the calculated value can show the attenuation of the additional stress on the soil layer under the infiltration curve.(2)Considering the influence factors of soil settlement,such as the constraint of the enclosure structure on soil settlement,the horizontal component of seepage force and the selection of soil modulus,the calculation method of surface settlement caused by precipitation was modified.The empirical coefficient of land subsidence caused by precipitation in 10 metro stations in the loess region was obtained by collecting the standard algorithm values and the measured values.Combined with the actual case,the modified value,the standard algorithm value with the introduction of the empirical coefficient,and the monitoring value were compared and analyzed.The average absolute error between the corrected settlement value and the monitoring value was only 14.8%,which was more consistent.(3)Based on the actual monitoring data of a loess foundation pit case,the variation rules of accumulated water level,ground and building settlement in the process of foundation pit dewatering were obtained.Diagramical method and formula method are used to obtain the influence radius R of case precipitation.Combined with the relationship between R and the depth of water stop curtain embedded in the foundation pit bottom,the traditional formula for calculating the influence radius of foundation pit precipitation is improved,which can provide reference for similar foundation pit cases in the loess area when studying foundation pit precipitation.(4)The ABAQUS two-dimensional finite element model of foundation pit dewatering was established based on the transient analysis method,and compared the simulated settlement value with the monitored value to verify the accuracy of the model.The variation rules of pore water pressure,seepage velocity,pressure and shear force on the interface between the retaining structure and soil,as well as stress-strain of soil were studied and analyzed under different precipitation depths.