Mechanism And Control Analysis Of Strata Deformation Induced By Confined Water Pumping And Recharging

The confined water in the foundation pit engineering,which just need to know how to pump it out,was supposed to be a simple question.In recent three decades,under the large scale practice of excavation engineering in soft soil area,the diversity of foundation pit engineering hazards induced by confined water becomes a major concern and the excessive settlement induced by pumping confined water is a most common phenomenon.Meanwhile,the existing mechanism of strata subsidence,which copied the theory of load induced consolidation,has a big difference with the actual situation.Therefore,it is necessary to study the soil deformation characteristics of confined aquifer during pumping and recharging and propose prevention methods of confined water in the process of the underground engineering which can provide useful guidance for practical engineering.In this paper,the soil deformation characteristics of confined aquifer during pumping and recharging is studied by use of field test,laboratory test and numerical simulation.The governance principle of confined water,which is on the premise of water level control and take the subsidence control as the core,is proposed.Finally,a comprehensive treatment method,which include waterproof curtain,pumping and recharging,is integrated and applied to design and construct the dewatering scheme of two excavation case histories.The main contributions of this thesis are described as follows:(1)The pimping-recharge test on site which focus on the first and second confined aquifer is presented for studying seepage characteristics and soil deformation characteristics of shallow confined aquifer.The results show that recharge can pretect the water level around the recharging well from drawdown,and the group well recharging play a more significant role.The bigger the injection flow is,the greater the water level raising.What is more,the recharge can control the surface subsidence efficiently,and the subsidence can be controlled with 2mm by group well recharging.(2)The laboratory test and in-site test which focus on the typical soft clay in shanghai is presented for studying the deformation characristics and regularity of soil under the influence of pumping and rerecharge.The results show that the permeability coefficient,which will reduce with the decrease of void ratio during pumping but reduce with the increase of void ratio during recharge because of the blocking or other reasons,have a non-linear relationship with void ratio.During the pumping,the soil will be compress,and the modulus of compression will increase so that the compression of soil will slow down.Compared with pumpinig,the modulus of resilience during recharge is bigger,and the surface back-elasticity is less than the compression.What is more,in in-site test,a special phenomenon called “downward rebound” appears in shallow soil layer during pumping,which reflect the specificity of the strata deformation during the consolidation induced by seepage.(3)A numerical method which considers the Biot consolidation theory is presented for analyzing the pumping and recharge induced deformation characteristics of foundation pit and soil around the pit.By using the transient analysis of the joint action of pumping,recharge and excavation of foundation pit,and the parameter analysis of different conditions of sealing structure and recharge,the characteristics of groundwater level and soil deformation will be studied.Results show that puming in confined aquifer will influence the deformation of retaining sturctures and surface subsidence greatly,and the influence range will be more than ten times as much as the excavation depth.Extend the insertion depth of the waterproof curtain can protect the confined waterlevel outside the pit from being influenced by the pumping in the pit.Moreover,recharge outside the pit can reduce the change of confined waterlevel and the soil deformation outside the pit.(4)The governance principle of confined water,which is on the premise of water level control and take the subsidence control as the core,and a comprehensive treatment method,which integrate waterproof curtain,pumping and recharging,is proposed.The proposed comprehensive method is used to design and construct the dewatering scheme of two excavation case histories,i.e.Yishan road station foundation pit of Shanghai Metro Line 9 and Hanzhong road station foundation pit of Shanghai Metro Line 13.Results reveal the practicality of the proposed method.Moreover,it is further verified by the field data.