Analysis Of Influencing Factors On Surface Settlement Of The Foundation Pit Construction In Qingyang Station Of Hefei Subway

With the development of cities,a large number of foundation pits have appeared,surface settlement and deformation would be caused by excavation of the foundation pit.The safety of surface settlement and pipelines would be affected when surface settlement reach to a certain degree,therefore it is necessary to strictly control the surface settlement caused by excavation of foundation pit.This paper,based on the data of surface settlement monitoring of the foundation pit at Qingyang station of Hefei Metro Line2,establishing models through MIDAS/GTS finite element software,analyzes influence factors of surface settlement of foundation pit of Qingyang station.Studying influences of buried width of underground diaphragm wall,support numbers and support stiffness on surface settlement,the paper summarizes some rules to provide experiences for similar projects in the future.The followings are rules:(1)With the increase of depth of foundation pit excavation,the settlement of ground surface increases gradually and tends to be stable.(2)The maximum settlement value appears neither in a near place,nor in a far place away from the enclosure pile,but in a little farther place away from the enclosure pile.The reasons are as follows: the support in the foundation pit has enough rigidity,and they have a strong friction with soil,hindering settlement of the soil that near the enclosure piles.But in the place that has a certain distance away from foundation pit the surrounding soil has no greater friction with the retaining pile,and settlement of soil will become larger.So the maximum settlement value appears in a little far away from the retaining pile.(3)The diaphragm wall buried width has an effect on the ground settlement around foundation pit,the larger the diaphragm wall buried width,the greater the inhibition of surface subsidence.When diaphragm wall buried width is 0.6m,the surface settlement is larger and when diaphragm wall buried width is 0.8m,the surface settlement reduced accordingly.But the diaphragm wall buried width approaches to 1m,the surface settlement is not suppressed more,which shows that diaphragm wall buried width has an optimum buried width.Before the construction,if we calculate the optimum inbuilt width of diaphragm wall buried width more accurately,it can save a lot of cost of the project.So in the design stage,according to the surrounding environment of the foundation,we design the optimum inbuilt width which has a reasonable width.Calculating the optimum buried width accurately before construction will save a great deal of project cost.Thus,it is necessary to calculate the optimum diaphragm wall buried width according to the surrounding environment of foundation pit in design stage.(4)The number of support has an effect on the ground settlement around the foundation pit.In the condition that other factors remain unchanged,the more the supports,the more obviously in inhibition of the surface settlement.When the number of supports are 3,the surface settlement is large,and when the number of the supports are 4,the surface settlement reduces accordingly.However,when the number of supports approach to 5,the ground settlement is not significantly suppressed,which indicates that there is an optimal value for the number of supports.Calculating the optimal supports number accurately before construction will save a great deal of project cost.(5)The support stiffness has an effect on the ground settlement around the foundation pit.In the condition of other factors remain unchanged,the more the support stiffness,the more obviously in inhibition of the surface settlement.When the inside and outside diameter difference of the support is 10 mm,the surface settlement is large and when the inside and outside diameter difference of the support is 14 mm,the surface settlement reduces accordingly.However,when the difference between internal and external diameters increases to 18 mm,the ground settlement is not significantly restrained,which indicates that there is an optimal supporting stiffness in support stiffness.Calculating the optimal support stiffness accurately before construction will not only save a great deal of project cost,but accelerate construction progress.(6)Hefei Metro Line 2 Qingyang station foundation pit locates in the center of the city,thus special attentions should be paid on safety of foundation pit during excavation.With the development of the economy,resources are scarce and environment is destroyed to a certain extent in Hefei,therefore,it is necessary to take into account the relationship between safety and benefits of economy and environment.For the purposes of this paper,the diaphragm wall buried width,support number and support stiffness,all of them has an optimum value.When the design value is higher than the optimal value,the construction organization costs will be greatly increased,and resources will be wasted seriously,and when the design value is lower than the optimal value,the foundation excavation will exist security risks.On account of that,if design organizations calculate the optimal value accurately,project constructions and resource utilization will achieve to the optimal state.