Study On The Design Method Of Foundation Pit Supported By Pile And Anchor Considering The Recovery Period

Pile anchor supporting structure is a kind of support system which has been applied and developed since 1980 s.Because of its advantages in deformation control and convenient construction,it has become the most commonly used and preferred mainstream support type in foundation pit engineering.But the traditional anchor cable is permanently held underground after construction,these abandoned anchor cables not only caused great waste,but also caused serious underground pollution,indiscriminately occupying the space outside the red line,huge obstacles to the development and construction of underground space in the later period.With the continuous development of urban underground space development and utilization and the "four sections and one environmental protection" awareness deeply rooted in people's minds,the recoverable anchor cable has developed rapidly in recent years,however,the relevant design theories have not been updated with the development of new technologies.At present,most of the pile-anchor design adopts the classic ultimate earth pressure theory for earth pressure calculation,but in the actual engineering,the displacement change of the support structure often can't make the soil reach the limit state.And studies have shown that,the recovery of anchor cable will reduce the safety reserve of foundation pit support system in varying degrees,and it will cause the deformation of the foundation pit to increase.At present,there is no relevant norms and regulations as technical support in China,if the blind recycling will cause safety problems in the foundation pit.Therefore,it is urgent to explore a practical design method that considers the process of anchor cable recovery.The purpose of this paper is to study the design method of the whole process of pile-anchor supporting foundation pit considering the recovery period.Study on the calculation model of non limit earth pressure,foundation pit deformation and overall stability in the process of anchor cable recovery,etc.Based on the foundation pit project of Diming Lake City in Jinan City,the influence of different recovery schemes on the deformation and overall stability of the foundation pit is studied.Finally,the reasonable recycling suggestions are put forward.(1)The traditional theory of ultimate earth pressure is based on the limit equilibrium state of soil.In practical engineering,the change of support structure displacement can not make the soil reach the limit state.In this paper,a simple calculation formula of non limit earth pressure is introduced in consideration of displacement of supporting structure.The formula considers the influence of displacement,excavation depth and embedment depth of foundation pit supporting structure.And it is applied to the actual engineering case to calculate the earth pressure on the supporting pile and guide the next construction.(2)In view of the favorable role of the backfill soil between the pile and the structural wall in resisting the deformation of the side wall of the foundation pit,the influence of the factors such as the backfill relaxation correction coefficient and the backfill width on the deformation of the retaining pile is studied.The results show that:(1)When there is no backfill to directly recover the anchor cable,the displacement of the supporting pile is too large,which does not meet the requirements of engineering deformation control;(2)Generally,the relaxation correction coefficient ? can be 0.5~ 1.0.With the increase of ?,the horizontal displacement of supporting pile tends to decrease;(3)With the increase of backfill width b,the horizontal displacement of the supporting pile shows a decreasing trend,which starts to decrease rapidly,and then decreases slowly.(3)According to the established non limit earth pressure calculation model,the earth pressure behind the pile is calculated by the displacement of the supporting pile.At the same time,the soil spring is used to represent the backfill between the pile and the structural wall,The theoretical model of pile anchor support structure considering anchor cable recovery is established,and the foundation pit excavation period is combined with the anchor cable recovery period,and the pile anchor support design method considering anchor cable recovery is proposed.(4)For the anchor cable recovery construction scheme,relying on the foundation pit project of Greenland Minghu city in Jinan City,the deformation of supporting structure,overall stability of foundation pit and recovery period are analyzed.The optimal scheme of anchor cable recovery is as follows: the anchor cable shall be recovered while backfilling,two at a time.That is to say,first construct the base plate and side wall,backfill the soil to the position of the fourth anchor cable,then recover the fourth and third anchor cables,then backfill to the position of the second anchor cable,then recover the second and first anchor cables,and finally backfill to the level with the surface.