Pseudo 3-D Elastic Foundation Beam Analysis Method For Deep Pit Supported By Combing Long With Short Piles

Pile-anchor and pile-strut are the most common used deep foundation pit retaining system,and the piles are often arranged in an equal length way.However,when the depth or size of foundation pit are too small and spatial effect appear markedly,the length of the piles at the corner of foundation pit can be shorter than those piles in the middle of the support section.Namely,using the supporting form of combining long and short piles is adoptable.Obviously,in comparison to equal-length piles arrangement,using the form of combining long and short piles is faster in construction speed and cheaper in engineering cost.However,it should be noticed that the study on the supporting system combining long and short piles is still at exploring stage,there is no simple,reliable,suitable engineering design method except 3-D finite element analysis.In view of the situation that the size of foundation pit is large,retaining piles take different length,the arrangement of prestressed anchor cable or inner support is complicated,the modeling and analysis of 3-D finite element method would become very difficult.Therefore,based on the traditional plane elastic foundation beam method,this paper establishes a pseudo 3-D elastic foundation analysis that can consider the excavation process and the interaction of ring beam(breast beam),the long and short piles and the anchor cable(or inner support).This method takes one of the enclosure protection planes of foundation pit as a whole,firstly,it takes one of the piles of this plane as research object,and establish the internal force and deformation compatibility equations of the pile in different conditions(excavating,setting up inner support or prestressed anchor cable),and then extends the research object from a single pile to the whole support plane in the consideration of the interaction of ring beam(breast beam)and piles,and then derives the internal force and deformation compatibility equations considering the interaction of excavation process,setting up inner support(or anchor cable)and other support units,at last,analyses the internal force and deformation of piles,ring beam(breast beam),inner support(or anchor cable)in the whole excavation process through solving the equations.The main achievements of this paper as follows:1.Under the pseudo 3-D condition,this paper established the elastic foundation beam analysis that can consider excavation process,setting up inner support(anchor cable)and the interaction of piles and ring beam(breast beam).This method is suitable for both pile-anchor and pile-strut supporting foundation pit.2.Based on the data of a pile-anchor supporting foundation pit,this paper using this method studied the influence on the internal force and deformation of the foundation pit while it has different short pile length,plie diameter ratio of long plies and short piles,ratio of long piles and short piles,length depth ratio of retaining structure plane.3.The present method is applied in two real foundation pits,the first one used pile-anchor support,and the others used pile-strut support.According to the comparison between the calculated values and actual values of displacement of the piles,the rationality of the present method in this paper is verified,and at the same time,studied the internal force and deformation regularity of long and short piles,ring beam(breast beam),prestressed anchor cable,inner support through comparing the different situation in pile-anchor support and pilestrut support.4.Corresponding FORTRAN program has been created.