Research On Water Leakage Risk Analysis And Control Technology Of Metro Deep Foundation Pit In Sandy Soil Layer

The sandy soil layer has strong permeability and weak adhesion between particles.When the groundwater is abundant and the water level is high,the deep foundation pit will be deformed greatly after excavation under the action of groundwater seepage.In addition,under the influence of construction,design and other factors,it is easy to lead to safety problems in the process of deep foundation pit construction,endangering the safety of foundation pit structure and surrounding environment,resulting in the construction of deep foundation pit of subway station in sandy soil layer,which is difficult and risky.In order to analyze the deformation characteristics of foundation pit and control the risk of foundation pit,this thesis mainly takes the deep foundation pit project of Tangxia station of Guangzhou Metro Line 13 as the research background,discusses the deformation characteristics of dewatering and excavation of sandy foundation pit under the action of underground diaphragm wall+internal support by means of case investigation,field measurement and numerical simulation,and makes an in-depth analysis on the leakage of diaphragm wall of deep foundation pit in sandy soil layer,Combined with the actual situation of the site,control measures are put forward to curb the dangerous situation.The main research contents and results are as follows:（1）Through literature research,the risk cases of deep foundation pit in sandy soil layer in the past are counted.It is found that foundation pit leakage is the most common problem of sandy foundation pit.The risk factors of foundation pit leakage are identified and sorted by analytic hierarchy process,the causes of foundation pit leakage risk are discussed,and the common control measures are summarized.（2）Taking the deep foundation pit project of Tangxia station as an example,the cause of water leakage in Tangxia station is analyzed through on-site investigation and on-site monitoring.It is mainly the result of the comprehensive factors such as the characteristics of sand layer,groundwater and the quality of retaining structure,which leads to the deformation rate of diaphragm wall at the leakage part exceeding the control value.（3）Using FLAC^3D software to establish a three-dimensional numerical model for construction dynamic analysis,and combined with the field measurement and comparative analysis,the deformation law of deep foundation pit excavation under the action of precipitation and seepage is obtained.The results show that when the foundation pit is excavated to the sand layer,the risk of foundation pit excavation is the largest,the increment of surface subsidence is the largest,the uplift of the pit bottom reaches the maximum,the effect of soil internal arch on both sides of the pit foot is particularly obvious,and the change rate of horizontal displacement of the wall is large.（4）The leakage of the diaphragm wall of the foundation pit in the sandy soil layer will have a great impact on the deformation of the diaphragm wall.The horizontal deformation of the wall and soil on the leakage side increases significantly and is greater than that of the foundation pit on the non leakage side and the normal section,forming a"bulge"bending centered on the leakage part,which can dynamically reflect the deformation of the wall before and after the leakage through intensive monitoring.（5）Combined with the actual situation of the site,this thesis discusses the control effect of different measures such as back pressure soil inside the foundation pit,grouting outside the foundation pit and dewatering outside the foundation pit on the deformation of the foundation pit through numerical simulation.The results show that when the dewatering outside the foundation pit is allowed,the dewatering outside the foundation pit is the best scheme,otherwise it is better to adopt back pressure soil inside the foundation pit and grouting outside the foundation pit to block the leakage point and control the deformation of the foundation pit.This thesis contains 74 pictures,13 figures and 92 references.