Research On The Effects To The Columns And Columns-piles Due To The Rebound Of Soil Caused By Excavation Of Deep Foundation Pit

Foundation pit excavation is a process of unloading for the soil under the bottom of the pit,where the soil under the bottom of the pit will rebound due to the reduce of stress level.For deep foundation pit with internal supporting systems,soil rebound caused by excavation will cause different mechanical effects on excavation supporting structures such as piles,columns,and horizontal supporting elements.In practical engineering,unloading and bouncing of soil are affected by many factors,such as the physical and mechanical properties of the soil,the change of the soil stress field,the rate of stress changing,the surrounding constraint condition,etc.Under the combined action of those complex factors,relatively accurate analysis and simulation of the unloading and rebounding of soil under the bottom of foundation pit,as well as the interaction between the rebounding of soil and the supporting structure have become hot topics for many geotechnical researchers and engineers.This paper systematically reviewed existing research results on the mechanism of soil unloading and rebounding,and its effects on internal support elements,columns and piles of ELS system.Based on Mindlin solution and Geddes solution,the theoretical calculating method of additional stress field caused by excavation unloading with consideration of existing piles are deduced,its results are compared with the numerical simulating results.In addition to the theoretical solution,the three-dimensional finite element methods combined the small strain harden soil constitutive model was issued as a reliable method to simulate the unloading and rebounding of soil under the bottom of foundation pit,which can be used for complex foundation pits.Base on a deep foundation pit project on the alluvial plains of lakes and rivers of the delta of Yangtze river,several large-scale 3D finite element models were established to study the rebounding form and the variation regularity and influence to the column piles(changing of internal force of piles,additional deformation and bearing capacity of piles in different area);influence of soil bouncing caused by unloading of deep foundation pit on inner support(vertical deflection of inner support,stability of inner support)are also studed.According to the research,the rebounding phenomenon of the soil below the excavation surface of the foundation pit is greatly affected by the rebounding modulus of the soil layers and the excavation depth of the foundation pit.Column piles are the key items to study the interaction between ground rebounding and foundation pit support system.According to the rebounding of the ground and the top loads of the pile,four stress states of the pile can be established.By Comparing the finite element numerical simulation results with the monitoring data during the construction phase of the foundation pit,it shows that the variation module is similar to the numerical results,which proves the feasibility and accuracy of the numerical calculation.After obtaining the effects of soil rebounding caused by unloading to column piles,columns and internal supporting elements of foundation pit and their changing rules,the measures to improve or mitigate the negative impact of soil rebounding to column pile,columns and internal supporting elements of foundation pits are presented.