Study On Settlement Regulation Caused By Horizontal Precipitation And Pilot Tunnel Excavation Construction With PBA Method

The underground excavation stations with PBA method in Beijing often encounter groundwater in the excavation area,and it need to carry out precipitation construction.The horizontal precipitation is applied in the PBA construction station precipitation project.However,the interaction between precipitation and undercut construction will undoubtedly aggravate the settlement and improve the difficulty of underground excavation construction,there is a lack of research on the settlement effect caused by the joint action of PBA construction station and horizontal precipitation at home and abroad.A station of Beijing Metro Line 17(referred to as M station)is constructed by PBA method.It is a typical groundwater-rich clay and silt interbedded stratum.The horizontal precipitation technology has achieved good application in this project.Taking the construction stage of the cross-channel and main body guide tunnel of Beijing Metro M station as an example,the FLAC3 D numerical simulation analysis and field measurement results analysis are carried out to systematically study the ground settlement and dome settlement law at different construction stages,and verify the accuracy of numerical simulation.Finally,effective settlement control measures are proposed for different construction stage settlement characteristics.The main research conclusions are as follows:(1)By simplifying the process and carrying out numerical simulation analysis,it is shown that the influence of the upper horizontal channel excavation on the surface settlement is greater than that of the lower horizontal channel excavation;the influence of the lower confined water layer precipitation on the surface and vault settlement is greater than that of the upper layer.The influence of layer precipitation is analyzed.The analysis of the measured data of different sections shows that the maximum part of the arch and surface settlement is located at the intersection of the upper transverse channel and the upper main body tunnel.The settlement of the same section is larger than the surface settlement and the section away from the transverse channel.The settlement curve is symmetrical with the axis of the station main body,and the axis of the transverse channel and the axis of symmetry of the adjacent section are obviously shifted toward the vertical direction.(2)Numerical simulation and measured results show that the ratio of surface and vault settlement caused by precipitation,cross-channel excavation and main tunnel excavation is basically the same,which are 10%,30% and 60% respectively.It can be seen that the excavation of the main body guide hole is the most important process for causing settlement.During this construction period,measures to control the settlement should be emphasized,and it can be inferred that the settlement weight caused by precipitation will become smaller after the completion of the station as a whole,proving the horizontal well.Precipitation technology can effectively control settlement.(3)The influencing factors and control measures of PBA station settlement mainly consider the following six aspects: formation conditions,groundwater conditions,group hole effect,excavation footage,construction process and speed,and support strength.For the project,the following measures for controlling settlement are proposed: the upper and lower horizontal passages and the lower main body guide hole are replaced by deep hole grouting to reinforce the stratum;the cross channel and the main body guide hole are connected by advanced small duct grouting and deep hole injection.The combination of pulp and the method of locking the anchor bolt on the four sides increases the range and strength of grouting;during the excavation of the main guide hole,the thickness of the initial branch is appropriately increased and the footage of the excavation is reduced.