Research On Design And Monitor For Supporting Structure Of A Deep Foundation Pit Project In Lanzhou

The design and construction of deep foundation pit support is an important and key problem in the capital construction projects of major cities in China.It is also a complicated and difficult problem in geomechanics.Based on the previous research results,this paper studies the supporting structure and detection of a deep foundation pit project,which is located in Lanzhou City,Gansu Province.Lanzhou City has complex and diverse landforms,inclined from southwest to northeast,and its topography is narrow and long.The landform belongs to the typical Loess Plateau.Because of the weak geology,complex soil changes and complex geological conditions,the engineering accidents caused by the instability of the pit bottom and the collapse of the pit wall are easy to occur.Therefore,the design and construction of deep foundation pit in loess site are challenging.Based on a project in Lanzhou,this paper synthesizes,improves and perfects the design and analysis methods of support structure suitable for Loess area,and designs reasonable and universal support structure.According to the basic geotechnical characteristics of the loess area where the project is located and the design experience of many practical projects,this paper designs the support of the deep foundation pit of the case project according to the current design rules,normative documents,standards,specifications and drawings of the building foundation pit,combined with the physical geography of a project in Lanzhou,the general situation of the project and the on-site investigation report.Specific designs are as follows:(1)Design of row piles and crown beams,a total of 258 supporting piles are arranged in the site,of which 16 are 800 mm diameter slope protection piles,the spacing of 800 mm diameter slope protection piles is 1.80 m,the spacing of 1000 mm diameter slope protection piles is 2.0-2.35 m,and crown beams are set at the top of piles,and stirrups are set equal spacing;for the top of crown beams in the area where the top of piles is not placed,steel bars are pre-embedded with steel mesh.In addition,the required guardrails are set at the top of the crown beam;(2)the design of hanging net and shotcrete for slope protection piles,with steel mesh hanging on the surface of row piles and high-strength concrete sealing with shotcrete thickness,and steel pipe grouting with insertion and thickness between each pile is designed.When arranging soil nails,the angle between them and the horizontal plane is set;(3)prestressing anchor.In the design of the retaining slope,two rows of pre-stressed anchors are installed between the retaining piles,three rows in the eastern part of the foundation pit and one row in the southern part of the foundation pit.The pre-stressed anchors are "one pile and one anchor".The pre-stressed anchors are mainly anchored in the pebble layer between the piles.The maximum horizontal displacement and internal force of each section of pile-anchor retaining structure of deep foundation pit in this project are obtained by calculating the deformation and internal force of each phase of retaining structure with Beijing Lizheng software and incremental method model.The overall stability of the deep foundation pit,the anti-overturning stability under seven working conditions,the calculation of embedded depth and the internal soil reaction of the embedded foundation pit are checked and calculated.The results show that the overall stability,anti-overturning stability,the calculation of embedded depth and the internal soil reaction of the embedded foundation pit meet the requirements of the code.In order to ensure the construction quality and protect the safety of foundation pit,from the beginning of construction to the end of construction,the monitor of foundation pit deformation runs through the construction process.According to the test results,the variation range of horizontal displacement at the top of foundation pit is between 4.0 mm and 9.0 mm,the variation range of vertical displacement at the top of foundation pit is between 5.0 mm and 12.0 mm,and the surrounding construction of foundation pit is carried out.(The vertical displacement of buildings ranges from 0.13 MM to 1.08 mm.All the measured data are true and accurate,and they are within the alarm range stipulated in the Code for Building Deformation Measurement,thus achieving the purpose of protecting the safety of foundation pit.