Effect Of Cross Wall Layout On Foundation Deformation Caused By Dewatering Before Soil Excavation

In recent years,China's urbanization process has been accelerating,and a large number of foundation pit projects adjacent to existing underground structures appear in the urban environment.The environmental deformation caused by foundation pit construction needs strictly control.At present,most of the studies in the field of foundation pit only focus on the environmental impact of foundation pit excavation,and put forward various support forms(internal support,pile anchor,etc.)to control the deformation caused by foundation pit excavation.However,in the whole process of foundation pit construction,only excavation caused deformation?Has a large number of engineering cases show that the dewatering process can cause significant before the excavation deformation of retaining structure and ground settlement,which means that the traditional considering excavation of foundation pit deformation research may underestimate the environmental effects of construction of foundation pit,more importantly,has yet to have a study and put forward to effectively control the deformation caused by dewatering before excavation.In order to reduce the deformation of foundation pit caused by dewatering before excavation,a new method is proposed in this paper.The cross wall and the diaphragm wall sink into the soil synchronously and play the role of supporting the diaphragm wall in the whole section in the process of dewatering before excavation.In the process of excavation,the cross wall can be removed continuously along with the progress of excavation,or the cross wall can be retained as the inner wall of underground space according to the requirements of foundation pit design.Based on engineering examples,this study,through indoor model tests and numerical simulation,revealed the control effect of setting a cross wall in the pit on the deformation of the foundation pit caused by dewatering before excavation,and further explored the influence of the layout of the cross wall on the deformation of the foundation pit.Specifically,in this study,based on the measured data of engineering,the author firstly carried out three sets of fine dewatering tests before excavation,and preliminarily explored the influence rule of the cross wall on the lateral movement of the retaining structure and the ground settlement outside the pit during the dewatering before excavation.Then,based on the measured data and the existing model test results,more than 100 three-dimensional finite element fluid-solid coupling numerical models with different cross wall interval,different cross widths and different cross heights were established respectively.Furthermore,by using the results of these numerical models,the influence of different spacing,width and height of the cross wall on the deformation of the foundation pit caused by dewatering before excavation was studied,and the mechanism of different layout of the cross wall on the deformation of the foundation pit and soil was discussed.The following conclusions are drawn from the study of indoor model tests and numerical model calculation in this paper:(1)A fully penetrating cross wall(that is,the cross wall wall is in contact with the ground connecting walls on both sides of the foundation pit,which can be used for subway and other narrow strip foundation pits)is set in the foundation pit,which can effectively control the maximum lateral movement of the retaining structure caused by dewatering before foundation pit excavation and the maximum ground settlement outside the pit.Within a certain range,due to the space effect of the foundation pit,the lateral movement of the retaining structure and the subsidence of the ground outside the pit caused by the dewatering before excavation are gradually reduced with the decrease of the spacing between the cross walls in the pit.The study shows that the ratio of the distance between the cross wall and the width of the foundation pit(d/B)has a critical value on the influence of the lateral movement of the retaining structure and the surface settlement outside the pit.The critical value is related to the dewatering depth,and the critical value is d/B=1.5ln H_d.When d/B<1.5ln H_d,the lateral movement of the enclosure structure and the ground settlement outside the pit decrease rapidly with the decrease of the spacing between the cross walls.When d/B>1.5ln H_d,the change of the spacing of the cross wall had little effect on the lateral movement of the enclosure structure and the ground settlement outside the pit.Therefore,if the foundation pit deformation control requirements are extremely high(such as adjacent to the existing subway tunnel),the foundation pit deformation can be effectively controlled by setting the cross wall in the pit and meeting d/B<1.5ln H_d.Of course,the interval between the crosss should not be too small,should be fully combined with the foundation pit design and construction conditions,engineering budget and other practical issues to choose the appropriate spacing to set the cross wall.(2)The installation of non-penetrating cross wall in the foundation pit(that is,the cross wall wall is only in contact with the ground wall on one side of the foundation pit,which is applicable to the case that only one side of the foundation pit has a protected building or underground structure)has a significant impact on the lateral movement of the retaining structure at the position of the cross wall and the ground settlement outside the pit at that position.The lateral displacement at the cross wall and the ground subsidence outside the pit decrease with the increase of the width of the cross wall.The results show that the control effect of non-through cross wall on foundation pit deformation is different for projects with different requirements of dewatering depth(H_d).If the H_d is larger(for example,greater than20 m,or the dewatering depth reaches the position of confined aquifer),the deformation limitation efficiency of non-penetrating cross wall is lower,and a wider cross wall is needed to reflect better control effect.In this case,it is recommended that the ratio of the width of the cross wall to the width of the foundation pit b/B is greater than 0.75.For H_d smaller projects(e.g.,less than 10 m,or dewatering depth not confined aquifer position),not through the cross wall deformation control efficiency is relatively high,short cross can achieve a large deformation control effect,at this point,for the general deformation control,suggested that cross wall width and excavation width ratio b/B is between 0.25 to 0.5.(3)Reasonable design of the height of the cross can reduce the economic cost and control the deformation of the foundation pit at the same time.Study shows that cross wall height above the the dewatering depth(h_c1)on the cross location to the lateral wall and the pit ground settlement distribution has a great influence,with the increase of h_c1,cross location even lateral wall and the pit ground settlement significantly decreases,and so when the pit building deformation requirement is high,increase hc1 can effectively control deformation of foundation pit.On the contrary,the change of the cross height(h_c2)below the dewatering depth has a very limited control effect on the maximum lateral movement of the diaphragm wall at the position of the cross wall and the ground settlement outside the pit,but it has a certain limitation effect on the lateral movement of the diaphragm wall at the local position below the dewatering depth.In the concrete engineering practice,it is suggested that it is not necessary to set transverse cross wall full depth range partition,generally(that is,the protected objects only in shallow buried position),cross bottom can be set up in dewatering depth(H_d),and the top of the cross can be set up in 1/3H_d-2/3H_d location(dewatering depth,the greater the cross wall wall top surface should be more close to 1/3 H_d location,and the smaller the depth of dewatering,top of the cross location can be more close to 2/3 H_d),because,within the scope of this deformation control setting cross wall can play a better role.However,due to the limited cross wall for the deformation limits)(mainly in the cross distribution scope,if cross wall bottom only when rainfall depth location,dewatering depth below the ground even still there will be a certain lateral wall,as a result,when dewatering depth below have higher requirements for deformation control of deep underground structure(e.g.,close to the foundation pit of metro tunnel),can consider to stretch the cross wall below dewatering depth,but it doesn't need to stretch a lot,according to the calculated results found that When the bottom surface of the cross exceeds the dewatering depth,only 1/3 times of the distance between the dewatering depth and the ground connecting wall can control the deformation of the diaphragm wall under the dewatering depth.