Study On Soil Nail Wall Retaining Structure Of Metro Deep Foundation Pit Based On Soil And Rock Combination Stratum

In order to alleviate the traffic pressure,Jinhua government started the construction of jinyidong urban rail transit.Compared with ordinary civil foundation pits,soil nail wall support has not been applied under special geological conditions in Jinhua area.There are still many uncertainties in its applicability in subway foundation pits.This article analyzes theoretically and scientifically.The engineering behavior of the soil-nailed wall retaining structure of the deep foundation pit of the subway has been studied in several aspects including actual measurement and numerical simulation.The main research results are as follows:1、In terms of foundation pit deformation: the ground settlement is generally triangular and groove-shaped.The settlement value is basically between 0 and 0.57‰H(H is the excavation depth of foundation pit,the same below),the average value is 0.3‰H,about 4mm,and the maximum value does not exceed 11 mm.The main influence range is within the range of(0～1.8)H.The horizontal displacement of deep soil gradually increases with the increase of the excavation depth.The maximum value of the horizontal displacement is about 1.05‰H,and the maximum value is basically no more than 12 mm.The average value of the maximum horizontal displacement depth of deep soil is about 0.43 H,and the maximum value of the horizontal displacement is between 0.5‰H and0.7‰H.2、The axial force of the soil nails is distributed in the shape of large middle and small ends along the length of the nail.The axial force of soil nails along the depth direction has a large difference between the upper and lower sides of the rock surface.As a whole,the central soil nails are much larger than the upper soil nails and slightly larger than the lower ones.The soil-nailed wall surface layer forces are distributed in a parabola as a whole.When the excavation is completed,the maximum surface soil pressure increases by about 63.6%,which is about 0.28 times the surface pressure of the static soil pressure.The location of the maximum value is more consistent with the maximum point of the horizontal displacement of the soil mass,which occurs approximately between(0.29～0.36)H.Obtain the friction coefficients a and b in each soil layer,and the most dangerous slip surface of the soil-nailed wall has obvious inflection points at the soil-rock interface.3、Compared with the pile-anchor support,it is found that the soil-nail wall support in the deep subway foundation pit where the rock surface is relatively high,the surrounding environment risk is low,and the construction scope allows,it can better exert the self-stability ability of the rock formation and to meet the stability requirements of foundation pit excavation.Compared with pile-anchor support,the deformation of the soil around the foundation pit and the force of the supporting structure have increased.The surface settlement increased by about 26.39%,and the horizontal displacement of the deep soil increased by about 32.13%,but the maximum value was less than 15 mm.4、For foundation pits with excavation depth of less than 15 m and shallow burial depth and better soil conditions,the number of soil nails is generally not less than two,the length of soil nails is preferably not less than 6m,and the soil nails are best set in the middle of the foundation pit.For foundation pits with excavation depth of more than 20 m and deep buried rock surface(more than 0.5 times the excavation depth)with poor soil conditions,the number of soil nails should generally not be less than3,and the length of soil nails should preferably not be less than 8m.5、In the soil-rock combination stratum,the overall stability of the foundation pit supported by the soil nail wall decreases linearly with the decrease of the rock surface height,and the soil deformation around the foundation pit increases linearly.Different from deformation,the growth rate of soil nail axial force first increases and then decreases with the decrease of rock surface height.When the proportion of the rock layer in the excavation area decreases,the axial force of soil nails increases rapidly,and the maximum value of the axial force of soil nails increases by about 5.33 k N for every one meter of rock face drop.When the rock face is lower than the basement,the drop in height of the rock face has little effect on the axial force of soil nails and basically remains stable.