| The pre-precipitation before the foundation pit excavation will cause the displacement of the surrounding soil and have a certain impact on the adjacent pile groups,thus damaging the surrounding environment and buildings(structures).Therefore,combining with theoretical analysis and numerical simulation,a series of studies are carried out on the changes of water level outside the pit,the settlement of surrounding soil and the deformation of adjacent pile caused by pre-precipitation.For the consideration of reality,in the theoretical derivation of soil settlement and vertical displacement of pile foundation caused by pre-precipitation,this paper considers the effect of seepage and blocking effect between pile groups based on whether the water level outside the pit considering the effect of water curtain.Then,based on the analysis of the field measured data,a three-dimensional fluid-structure coupling model of foundation pit dewatering and excavation is established using MIDAS/GTS NX finite element software,and the model is verified by using the field monitoring data.Finally,the pre-dewatering results before excavation are analyzed.On this basis,the change rules caused by different precipitation influencing factors and pile foundation parameters under pre-precipitation conditions are further explored.The following achievements and conclusions are obtained:(1)The calculation method of soil settlement outside the pit caused by foundation pit dewatering is systematically put forward,and the calculation method of vertical displacement of adjacent single pile and group pile is further put forward by the two-stage method.(2)From the analysis of precipitation results,it is found that when the diaphragm wall penetrates into the aquitard,the environmental response is small.The farther away from the foundation pit,the lower the water level,the smaller the surface settlement and the vertical displacement of the pile foundation.The existence of adjacent piles will also play the role of isolating piles to further limit the displacement field of surrounding soil,and the blocking effect between pile groups will also affect its own displacement field.(3)By comparing the results of different construction stages,it is found that the main influence range of surface settlement outside the pit caused by precipitation is significantly larger than that caused by excavation alone.In the coupling condition of dewatering and excavation,within the range of 2He,the main factor causing surface settlement is the unloading in the pit.The main factor causing surface settlement in the area beyond 2He is precipitation,which indicates that it is necessary to consider the compression and consolidation of soil caused by pre-precipitation before foundation pit excavation.(4)Through exploring the influence of different water curtain conditions,it is found that the water curtain increases the seepage path of groundwater,the water level difference inside and outside the pit increases,the impact of precipitation decreases,and the settlement of soil is also reduced.The vertical displacement of the front and rear piles decreases significantly with the setting and the increasing depth of water curtain,and the displacement difference between the two piles also decreases gradually,indicating that the water curtain also plays a certain role in controlling the uneven settlement between the groups of piles.(5)The soil above the original water level is not affected by the water level drop in the formation to produce compression consolidation,and the maximum value of soil settlement does not always appear on the surface due to the collaborative settlement.It is further found that part of the reason for the "grooved" settlement trend of the suspended water curtain is that with the increase of the distance,the influence depth of soil settlement extends to the whole stratum below the original water level due to the influence of groundwater recharge outside the pit,resulting in greater settlement of the soil at this point.(6)Through exploring the influence of different pre-dewatering depths,it is found that with the increase of the depth,the water level outside the foundation pit drops more and the distribution is more uneven.The greater the surface settlement and uneven settlement affected by this,and the maximum settlement value is approximately linear with the depth of drawdown.The vertical displacement of pile group will also increase with the increase of preprecipitation depth,and the displacement difference between the front and rear piles will gradually increase.However,when the pre-precipitation depth is gradually close to the aquitard,the growth rate will slow down for the low permeability of the bottom soil layer.Based on the correlation analysis,it can be seen that there is a close relationship and strong correlation between the water level change outside the pit,soil settlement and vertical displacement of pile foundation under the influence of different pre-precipitation depths in the pit,and the change trend of each other shows a positive linear relationship.(7)When the control variable method is used to study the influencing factors of pile foundation deformation,it is found that the vertical displacement of single pile caused by predewatering of foundation pit is always greater than the group pile due to the blocking effect.The increase of pile length and pile diameter in a certain range can control the vertical displacement of pile groups to a certain extent,and the blocking effect can be enhanced,but the increase of pile spacing will lead to the weakening of the blocking effect,affecting the relative displacement of the front and rear piles.In addition,according to the relative sensitivity analysis method,the sensitivity of the vertical displacement of the piles to the three influencing factors is: pile length > pile diameter > pile spacing,and except that the vertical displacement of the front pile changes in the same direction as the increase of the pile spacing,the rest decreases in the opposite direction with the increase of the parameters. |
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