Research On The Application Of "End-suspended Pile" In Soil-rock Combination Foundation Pit

With the development of China’s economic society and urban construction in the past 20 years,in the face of expensive urban land and tight construction land,coupled with the restrictions on high-rise building volume ratios,engineering construction units are trying to increase the use of underground space.Therefore,the development and utilization of underground space for various purposes,the innovation of foundation pit support forms,and support design theories and methods have achieved unprecedented development.In the practice of foundation pit engineering,the stratum distribution,the complexity of the surrounding environment,and the difference in safety level will affect the choice of foundation pit support types.In recent years,a large number of places have appeared in Qingdao,Jinan,Dalian,Urumqi,Guangzhou,Shenzhen,and Chongqing soil-rock combination foundation pit.The upper part of this type of foundation pit is a weak soil layer,and the lower part is a high-strength rock layer.For such foundation pit designers,the "the end-suspended pile" support is generally used to reduce the drilling strength of the lower hard rock layer.However,as a new type of support,China has not yet formed a mature calculation theory and a relatively complete standard system for it.so this article will take a soil-rock combination foundation pit in Laizhou,Shandong as the research background,a numerical model of the 3 pits of the foundation pit was established to study the deformation of the supporting structure during the excavation process.The values of several supporting parameters and the overall stability safety factor of the foundation pit were evaluated.In addition,by introducing the unified strength theory,the earth pressure acting on the "the end-suspended pile"supporting structure was calculated,and the following conclusions were reached:(1)With the increase of the excavation depth of the foundation pit,the position of the maximum positive bending moment of the pile gradually moved down,and finally stabilized in the middle of the foundation pit.It reaches the maximum when digging to the rock-inserting surface,and the excavation of the lower rock layer will cause the bending moment of the pile to decrease to a certain extent.(2)During the excavation of the foundation pit,the maximum horizontal displacement of the hanging pile pile always appears at the top of the pile,and the displacement value reaches the maximum when the bottom of the pit is dug.It is said that under the premise of reasonable design of various support parameters,the excavation of the lower high-strength rock layer will not have much impact on the horizontal displacement of the pile body.The surface settlement outside the pit also mainly occurs during the excavation of the upper soil layer.As the distance from the pit edge increases,the settlement value increases first and then decreases.The settlement curve is roughly V-shaped.The area where the settlement occurs is mainly concentrated in the overload range.The maximum settlement occurs at the center of the overload range.(3)At the position where the prestressed anchor is installed,the bending moment of the pile body and the displacement curve of the deep soil will turn.When the anchor’s axial force develops stably,it will be found that the axial force value in the free section exceeds the original prestress value.It shows that the design of various parameters of the anchor rod is reasonable,and it can be closely combined with the surrounding soil to exert the anchoring effect.(4)From the simulation results of changing the support parameters,it can be seen that compared to the effect of the change in the pile spacing on the deformation of the supporting structure,the effect of the change in the pile diameter is more obvious,so the designer can start with the pile diameter to support the support.The force of the protective structure is adjusted.Reserving the width of the rock shoulder and the depth of the rock socket are two important design parameters of the supporting structure of the end-suspended pile.The rock shoulder width and the depth of the rock socket are too small.A safety accident may occur due to excessive deformation.If the value is too large,the construction cost will be increased and waste will be caused.Therefore,it is necessary to comprehensively determine the value of support parameters from multiple perspectives such as economics and deformation.(5)The introduction of the unified strength theory and the Hoek-Brown strength criterion not only accurate the earth pressure value on the supporting structure of the end-suspended piles,but also after the recalculated rock and soil parameters are substituted into the model,both in the support effect and safety The coefficients have been improved,indicating that the two theories can provide a theoretical basis for the optimization of the support parameters of the foundation pit.