Research On Precipitation Optimization And Law Of Confined Water In Deep Foundation Pit In Water-rich Pebble Formation

With the continuous increase in the excavation depth of foundation pit engineering,the impact of deep confined water at the bottom of the pit on the excavation of deep foundation pits gradually surfaced to engineers.After excavation of the founcdation pit,the distance between the bottom of the foundation pit and the top plate of the confined aquifer is reduced,and accordingly,the pressure in the upper part of the confined aquifer is also recduced.When the foundation pit is excavated to a certain depth,the pressure on the upper part of the confined aquifer may be less than that of the confined water in the aquifer,resulting in instability of the bottom of the foundation pit and sudden inrush,which seriously endangers the safety of the foundation pit.However,due to the strong regional engineering of deep funcdation pits,there is still no unified well-point precipitation design standard.Traditional design methods often result in a high project cost,a large amount of water in the foundation pit,and a large impact on the surrounding environment,problem.This paper through the theoretical analysis,numerical calculation,orthogonal test,objective function method and other means to optimize the design of the precipitation project by relying on the confined water dewatering project of the deep foundation pit in the typical water-rich sandy gravel stratum of Fuzhou Rail Transit Line 2.The main research contents and results are summarized as follows:(1)By studying the hydrogeological data of the deep pit of the Jinyu Station of the No.2 line of the Fuzhou Rail Transit Line,the special scheme for the construction of the precipitation,and the on-site investigation,it was found that the deep pit project has the following features:it is located in a soft and rich water sand and gravel stratum,and the excavation range of the foundation pit There are a large number of weak silt and silty soil layers with poor soil quality.Below the excavation surface of foundation pit,there is a confined aquifer with strong permeability and large water content,which poses a greater threat to safe excavation of foundation pit;(2)Through on-site pumping tests to verify whether the estimated single well water volume,the initial precipitation scheme is reasonable and whether the hydraulic connections within and outside the confined aquifer are significant,Field tests show that the pumping volume of a single well is about 192m3/d,which is 48m3/d less than the estimated single well water inflow,and the precipitation scheme meets the design requireiments for depth reduction and there is a certain hydraulic connection between the pressure confined aquitfer inside and Ooutsitde the pit;(3)A three-dimensional numerical model was established using MIDAS/GTS to simulate the pumping test.The callculation results showed that the time history curves of-the numerical pumping test and the on-site pumping test were basically the same.It can be verified that applying MIDAS/GTS to the deep foundation pit dewatering project,and qualitatively analyzing the influence of the pumping process on the structure itself and the surrounding environment is reasonable and feasible(4)Using the objective function method,the single well water inflow was used as the design variable,and the optimization of mathematical model was established with the aim of minimizing the depth of water level drop.Matalb was used to solve the optimization model.The calculation results show that the single well water inflow after optimization is about 150m3/d,which is about 40m3/d before optimization,and the pressure head at each control point meets the design requirements for depth reduction.(5)Using MIDAS/GTS,a three-dimensional numerical model was established for the well site dewatering plan of the pressure-bearing aquifer in Jinyu Station before and after optimization,and a virtual simulation calculation was performed.The calculation results show that the overall pressure head in the pit increases by about 1.75m,which can effectively prevent the settlement of the stratum and buildings(structures)outside the pit caused by the excessive precipitation of the pit.(6)For the water-rich sand and gravel stratum in Jinyu Station,the maximum settlement on the ground surface outside the pit,the maximum horizontal displacement of the earth-linked wall,and the maximum uplift at the bottom of the pit are used as evaluation indicators.Comprehensive consideration to connect wall inserted into the confined aquifer depth(H),filter tube inserted into the confined aquifer depth(H),and even wall thickness(d),reinforce the bottom thickness(w),excavation depth five factors(1),the numerical calculation combined with orthogonal test method is studied considering factors on the sensitivity of the influencing factors;(7)Based on the orthogonal test,the maximum settlements on the ground surface outside the pit,the maximum horizontal displacement of the earth-linked wall,and the maximum uplift at the bottom of the pit were used as evaluation indicators to find the optimal combination of operating conditions with minimal impact on the index.In the future,it can provide a certain guiding role for the construction of similar projects.