Study On The Application Of Long-Short Pile Retaining Structure In Soil-Rock Composite Stratum Foundation Pit

In recent years,with the vigorous development of China’s infrastructure,the number of various types of foundation pits has increased year by year.Foundation pit engineering is developing towards the trend of ’long,deep and large’,and the requirements for foundation pit retaining structure are also gradually increasing.At present,there are some shortcomings in the design of row pile-crown beam retaining structure: First,it is designed according to equallength piles,which cannot give full play to the flexural bearing capacity of retaining structure;Second,the stability of the retaining structure is considered according to the plane strain problem,and the influence of the spatial effect of the foundation pit is less considered.The above problems are likely to cause the design of the envelope structure to be too ’conservative’,the project cost is high,and the material waste is serious.The long-short pile retaining structure can further exert the flexural bearing capacity of the pile body.Considering the spatial effect in the design can further optimize the structural design parameters,making the scheme safer and greener.Based on this,this paper first summarizes the research on spatial effect and the application cases of long-short pile retaining structure at home and abroad,analyzes the shortcomings of existing research,and makes some research on the field application and numerical simulation of long-short pile retaining structure in soil-rock composite stratum :(1)Based on the monitoring project of the pile cap foundation pit of the bridge across the Anhui-Jiangxi line and the Guixi relief line in Yingtan City,Jiangxi Province,the left and right foundation pits of the railway in the project are supported by the equal-length pile retaining structure and the long-short pile retaining structure respectively.The horizontal displacement and bending moment of the pile body of the two retaining structures are obtained by pre-embedding the inclined pipe and the concrete strain gauge.It is concluded that it is feasible to use the long-short pile retaining structure in the bias foundation pit.(2)In order to study the influence of size effect on the supporting effect of long and short pile retaining structure,the finite element software MIDAS was used to simulate the field conditions,and the rationality of the numerical model was verified by comparing the measured results.Subsequently,the plane size of the foundation pit was changed,and the influence of the size effect was analyzed by observing the principal stress,soil settlement and soil pressure changes in the soil around the pit.Finally,the deformation and stress of the longshort pile retaining structure and the equal-long pile retaining structure in the foundation pit with different plane sizes are compared.It is concluded that the size effect can optimize the deformation and stress of the long-short pile retaining structure.The small-sized square foundation pit should fully consider the effect and adopt the long-short pile retaining structure.(3)In order to study the influence of short pile embedded depth on the supporting effect of long and short pile retaining structure under spatial effect,the influence of spatial effect on the stability of soil around the pit was analyzed by observing the distribution of principal stress and soil loosening area in soil around the pit under different excavation sizes and depths.Then,by analyzing the deformation and stress distribution of the long and short pile retaining structure with the change of the short pile embedding ratio,the influence law of the short pile embedding depth on the supporting effect of the long and short pile retaining structure under the spatial effect and the reasonable embedding depth of the short pile are obtained.(4)In order to study the influence of crown beam on the deformation and stress of long and short pile retaining structure,the distribution differences of principal stress and loose area of soil around foundation pit with and without crown beam are compared firstly.Then,the distribution and maximum value of soil pressure under two working conditions are compared,and the influence of crown beam on the formation of arch effect of soil around foundation pit is analyzed.Finally,the influence of crown beam on the supporting effect is discussed by comparing the deformation and stress of long and short pile retaining structure with and without crown beam.It is concluded that the setting of crown beam can optimize the deformation and stress of long and short pile retaining structure,and the optimization effect is strongly affected by the size effect of foundation pit.The design size and reinforcement of crown beam can be appropriately reduced in small size foundation pit.