In the tunnel project,when the buried depth is shallow and the soil is poor(such as loess),the risk of subsurface excavation construction is large,which is not conducive to the safe construction of the tunnel.If the site has open excavation construction conditions,open excavation construction can be selected.In the railway engineering,the excavation depth and width of such open-cut tunnel foundation pit are generally large,and the excavation length is long.Due to the different surrounding environment,excavation width and depth of different mileage,there are often a variety of supporting methods of foundation pit engineering.In this paper,the loess deep foundation pit project of Zhongchuan Tunnel(open cut)in Lan Zhou-Zhang Ye three-four line railway is taken as the research object,and the support method of foundation pit engineering,the deformation and stability of foundation pit caused by excavation construction are analyzed and studied.The surrounding environment of the loess tunnel foundation pit project is relatively complex,and the excavation width and depth are different at different mileages.Some sections of the foundation pit have pit-in-pit section forms.Through analysis and comparative study,soil nailing wall support,pile anchor support and pile-support support are designed.Through the deformation monitoring and numerical simulation method of foundation pit excavation,the deformation law and stability of foundation pit under different influencing factors are studied.The following conclusions were obtained :(1)The surrounding environment of the loess foundation pit is complex.There are many traffic roads,farmland and woodland.Rainfall and irrigation water will penetrate into the soil around the foundation pit,which will adversely affect the stability of the foundation pit.According to the surrounding environment and engineering geological conditions of each foundation pit,as well as the width and depth of foundation pit excavation,through analysis and research,DK64+360~DK64+600(pit-in-pit foundation pit)outer pit support methods are soil nailing wall support(left)and pile anchor support(right),and the inner pit adopts the support method of cast-in-place pile and inner support.The DK64+600~DK65+740 section is supported by soil nailing wall.(2)The vertical displacement of the surface around the soil nailing wall mainly occurs in the excavation stage,and tends to be stable after the excavation.The maximum value of the deep horizontal displacement of the side wall of the foundation pit appears in the middle and lower part of the soil nailing wall.The deformation near the top and bottom of the foundation pit is small,and the deformation is ‘bow’.Comparing the deformation results of soil nailing wall foundation pit with different excavation depths,it is found that the increase of excavation depth will increase the deformation of soil nailing wall foundation pit.When the excavation depth is large,the growth rate of the vertical displacement of the surface around the foundation pit and the deep horizontal displacement deformation of the side wall of the foundation pit will be accelerated.The position of the maximum horizontal displacement deformation of the deep horizontal displacement of the side wall of the foundation pit of the soil nailing wall will move down with the increase of the excavation depth of the foundation pit.The ratio of the maximum deformation position to the excavation depth of the foundation pit is in a small range,which appears in the design excavation depth of 56.9 % ~ 67.8 %.(3)The vertical displacement of the surface around the pile-anchor support increases with the distance from the edge of the foundation pit,and the settlement gradually increases.The maximum value is 4.8 mm at 5 m from the edge of the foundation pit,and then gradually decreases.The surface settlement around the foundation pit is ‘spoon-shaped’.The horizontal displacement of the pile body is presented as the deformation inside the foundation pit,and the horizontal displacement deformation at 0.5 m below the pile top is the largest,which is 5.8 mm.(4)The correctness of the model is verified by comparing the numerical simulation analysis results of the outer pit deformation established in this paper with the actual monitoring results,and the deformation of the inner pit excavation is further predicted.The results show that before and after the excavation of the inner pit,the influence range of the deformation of the soil nailing wall part and the pile-anchor support outer pit increases by 10.5 % and 8.7 %;the horizontal displacement of the side wall of the soil nailing wall foundation pit increased by 14.5 %,and the horizontal displacement of the pile body of the pile anchor foundation pit increased by 21.8 %.The bottom heave of the outer pit increases by 11.0 %,and the bottom heave values of the two support methods are not much different.(5)The numerical simulation analysis of the foundation pit with different excavation distances(b)and different excavation depths(h)of the inner and outer pits is carried out.It is concluded that when the excavation depth of the inner pit is the same(h),the excavation distance of the inner and outer pits is different(b).The horizontal displacement deformation trend of the side wall of the two supporting methods is basically the same,the horizontal displacement is different,and the maximum deformation position does not change.When the excavation distance(b)of the inner and outer pits is the same,the deformation trend of the horizontal displacement of the side wall of the foundation pit supported by the soil nailing wall does not change under different excavation depths(h).The deeper the excavation depth of the inner pit,the deeper the maximum horizontal displacement of the side wall of the foundation pit appears.The change of the excavation depth of the inner pit of the pile anchor support only affects the horizontal displacement,and does not affect the position of the maximum value,and the maximum value appears near the pile top.The smaller the ratio of the excavation depth of the inner pit(h)to the excavation distance of the inner and outer pits(b)is,the smaller the influence of the excavation of the inner pit on the deformation of the outer pit is.When h/b is less than 1,the speed at which the influence of the inner pit depth on the deformation of the outer pit gradually increases.When h/b is greater than 1,the growth rate of the influence of the inner pit excavation depth on the deformation of the outer pit gradually increases. |