| In recent years,urban underground space has rapidly developed,bringing an increasing number of underground transportation facilities such as metro stations and transit tunnels.In big cities,when the foundation pit is excavated nearby exisiting metro stations,the responses of groundwater and soil outside the foundation pit induced by dewatering would be subject to the barrier effect(i.e.,water-blocking(WB)effect and soil-blocking(SB)effect)of the metro station;however,the behaviours of a foundation pit during dewatering would be different in the cases with or without metro station.Previous studies only focused on the water-blocking effect or the soil-blocking effect,but the deformation characteristics of the foundation pit under the combined actions of these two barrier effects is still an open problem.In this thesis,the pumping tests were first carried out in the cases with and without adjacent metro stations outside the pit,in the meanwhile,monitoring the enclosure wall deflections,soil settlement outside the foundation pit,groundwater drawdowns inside and outside the pit,and the water and soil pressures.And the responses of groundwater to foundation pit dewatering considering the barrier effect of existing metro station were revealed.Then,based on a practical pumping test,a three-dimensional numerical model was established by ABAQUS(a finite element software).The model was verified by the comparison of measured and calculated results.Additionally,a multi-parameter analysis was carried out to further investigate the barrier effect of existing metro station on environmental deformation induced by dewatering under different conditions(i.e.,D = the distance between the metro station and foundation pit,H = the depth of foundation pit,B = foundation pit width and = the dewatered depth inside the pit),revealing the mechanism of the combined actions of water-blocking effect and soil-blocking effect and the characteristics of the response of adjacent metro station structures.Based on this study,the following main conclusions can be drawn.Owing to the barrier effect of the adjacent metro station,the water yield of foundation pit would be reduced,and groundwater drawdown curves are divided into several sections;moreover,groundwater drawdown appears to be greater between the station and the foundation pit,while relatively smaller behind the metro station;the enclosure wall deflection and ground surface settlement outside the pit are affected by D,H and B.D plays a leading role,followed by H and B(see points(2)-(4)).Whether water-blocking effect or soil-blocking effect is dominant is determined by the distance between the metro station and foundation pit(D);when D is small(D<20 m),the SB effect plays a dominant role,limiting the dewatered-induced surface settlement;with the increase of D,both the SB and WB effects get weakened,while the SB effect weakens more rapidly;when D is larger(D≥20 m),the WB effect plays a leading role,making the surface settlement greater;nevertheless,when D is over 40 m,the SB effect would no longer work,while the WB effect continues to weaken,making the surface settlement approach to the results without station outside.As to the depth of foundation pit(H),both the SB and WB effects becomes stronger as H increases,and the effect of H on the combined actions of SB and WB effects is subject to the variations of D.When D is small(SB plays a leading role),with the increase of H,the SB effect appears to be stronger,making the surface settlement outside the pit smaller;when D is large(WB plays a dominant role),the WB effect appears more apparent,causing the surface settlement greater;however,after H is larger than 35.5 m,the SB effect tends to be stronger rapidly,causing the ground settlement outside the pit gradually decreases(but is still greater than that without the station outside).As to the foundation pit width(B),with the increase of B,the WB effect gradually gets weakened,while the SB effect tends to be stronger,and the effect of B on the combined actions of SB and WB effects is subject to the variations of D.When D is small(SB plays a dominant role),with the increase of B,the SB effect appears more apparent,making the ground surface settlement with metro station outside the pit smaller than that without station outside;additionally,the differential settlements would gradually increase;however,when D is large(WB plays a leading role),the WB effect gradually get weakened,causing the ground surface settlement with metro station outside the pit greater than that without station outside;but the differential settlements between the two cases tends to decrease gradually.Foundation pit dewatering would induce additional bending moment in adjacent metro station structures(the maximum additional bending moments occur at the top,middle part and bottom of the metro station),deflecting the metro station into the pit.Moreover,it was found that the variations of D,H and B could affect the force and deformation of the metro station structures to different degrees.Thus,designers could appropriately enlarge the bending moment at the top middle part and bottom of the metro station considering the additional bending moment caused by the subsequent surrounding underground structure construction(such as foundation pit dewatering). |
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