Study On The Mechanism Of Pile Anchor Permanent Support And Its Joint Work With Basement Exterior Wall

At present,the retaining structure of foundation pit in a large number of projects is often used as a temporary auxiliary project,which remains underground after the completion of basement construction and earthwork backfilling.These cast-in-place concrete piles and anchor structures are left in the surrounding strata of the foundation pit,which has the problems of resource waste,geological pollution and high energy consumption.It is worth thinking about how to use such a high input and large volume enclosure permanently and how to use it.This paper is based on a deep foundation pit project in Ankang City.On the premise of ensuring that the foundation pit construction meets the requirements of the specifications and the construction is carried out safely and orderly,according to the actual situation of the project,the permanent pile anchor support method is adopted.Based on the construction organization and action mechanism of the pile anchor combined support,aiming at the design of the permanent pile anchor support,the rationality and feasibility of the design of the permanent pile anchor support structure are verified by the actual monitoring,design documents and ABAQUS finite element software.Further,according to the common working mechanism of the permanent support system,ABAQUS simulation is carried out for the combined system of the pile anchor permanent support and the basement external wall in the actual project case,and the common working situation of the pile anchor permanent support and the basement external wall after the foundation pit backfilling is studied.The main research work and results are as follows(1)Combined with the action process of supporting pile,anchor cable and soil,the existing research is summarized.In the permanent design of the pile anchor supporting structure,the lateral earth pressure of the soil around the foundation pit is firstly borne by the supporting pile,the steel waist beam is fixed on the pile body,and it is rigidly connected with the anchor head and other components.After prestressed tension,the anchor cable transmits the lateral earth pressure of the surrounding environment to the rock soil layer.(2)Through the actual measurement,design calculation and ABAQUS simulation,it is concluded that the maximum horizontal displacement of the supporting pile appears in the middle of the pile body.The horizontal displacement of the supporting pile is generally largein the middle and small at both ends with the depth,showing a "medium convex type".The prestressed anchor cable has a good displacement control effect on the pile body,which can effectively limit the displacement of the pile body to the inside of the foundation pit.The maximum horizontal displacement of the design calculation is 16.59;the maximum horizontal displacement of the ABAQUS simulation is 24.32;the maximum horizontal displacement of the project site monitoring is 17.8;the difference between the three is not big and all are less than the limit value 50 required by the code,which meets the design requirements.(3)By using ABAQUS software,the model of pile anchor permanent support system and basement exterior wall working together is established.By extracting the earth pressure of basement and displacement of supporting pile under different relaxation of anchor cable,the mechanism of stress change of the system in permanent use stage is studied.It is found that the change of horizontal displacement of the supporting pile causes the change of lateral earth pressure of the backfill,which makes the stress of the outer wall of the basement change.Using the fitting formula to optimize the design of basement exterior wall can effectively reduce the thickness of basement exterior wall under the condition of ensuring crack control,which has high economic and applicable value.