| Based on the deep foundation pit project of weft 28 station of xi’an metro line 6,this paper analyzed the deformation characteristics of deep foundation pit of subway station in loess area and its influence on the surrounding existing underground pipeline,which could provide technical support for the design of deep foundation pit enclosure structure and the safe construction of subway station.By means of theoretical analysis,on-site monitoring and FLAC simulation,work was carried out.The main work and conclusions are as follows:(1)The main factors affecting the deformation of the retaining structure of the deep foundation pit at weft 28 station were analyzed,including the physical and mechanical properties of each soil layer,underground water level,the physical size of the foundation pit,the surrounding environment and so on.In this paper,the existing pipeline status around the deep foundation pit of weft 28 station was studied,the influence of the construction of deep foundation pit on the existing pipeline was analyzed.Then the demolition and protection scheme of the existing pipeline was put forward.(2)The design of deep foundation pit protection scheme was completed.The structural calculation and foundation pit stability checking of support scheme were carried out by using the software of LiZheng deep foundation pit.It is proved that it is reasonable to bored cast-in-place pile with steel support.(3)The design of monitoring scheme for deep foundation pit construction was completed.The measured data of pile deformation,soil deformation between piles,surface settlement,axial force of steel support and deformation of adjacent existing pipelines were analyzed.The results showed that the earth excavation,the change of soil and water pressure and the erection time of internal support all affected the stability of deep foundation pit and its surrounding environment during the construction of deep foundation pit.Besides,when the deep foundation pit was excavated,the active soil pressure of the surrounding soil increased sharply,and the erection of the steel support played a good constraint on the deformation of the pile body and the surrounding soil,and the lateral extrusion of the steel support to the soil could well limit the settlement of the surface.(4)By using FLAC software to simulate and analyze the whole process of deep foundation pit excavation and enclosure,the variation law of vertical and horizontal displacement of deep foundation pit,the law of surface deformation around foundation pit and the law of axial force of steel support were obtained.10 kV power pipes and DN800 gas pipelines had the highest risk.The main factors affecting the vertical displacement of 10 kV power pipe trench and DN800 natural gas pipe were surface subsidence,construction disturbance and pipeline material.During foundation pit construction,when the first steel support was set up,the amount of earthwork excavation was so small that the influence on pipeline settlement was very small.A large amount of earth excavation was needed before the second and third steel supports were set up,so the pipeline had significant settlement deformation.The settlement rate of pipeline decreases after the steel support was set up,which indicated that the steel support had a certain restraining effect on the settlement deformation of pipeline.The measured and simulated data of the existing pipeline deformation were basically consistent.The existing deep foundation pit excavation enclosure scheme could ensure the safety of existing pipelines. |
