The Influence Of Deep Excavation On The Load Sharing Characteristics Of Adjacent Rigid Long-short CFG Piled Composite Foundation

In the field of modern geotechnical engineering,dealing with weak ground and passive loading acting on piles have been among the ubiquitous upfront challenges.The long-short CFG pile composite foundation has emerged as an alternative engineering solution and has been proven over the past decades to address the soft soil problem;however,a comprehensive understanding of their behavior under passive loading still merits further studies because they must not only support vertical superstructural loads but also effectively resist passive loadings arising from various sources such as excavation and/or other man-made operations and natural phenomenon.Moreover,due to the growing engineering application and utilization of CFG composite piled raft foundation to support unprecedented demands of housing development in urban dwellings,this foundation system would inevitably appear in close proximity to supported excavations to be carried out for new underground construction activities.Accordingly,a profound insight is sought on how the soil movement induced by the stress and displacement boundary conditions of new excavation retaining structures would disturb the performance and structural integrity of nearby in-service multi-type CFG pile composite foundations.This study is directed to explore the load sharing performance of CFG pile composite foundations having long and short piles subjected to soil movement based on:(i)an indoor model test conducted on 2 × 2(2 long-2 short)pile composite foundation subjected to a movement of the front side of the model box;(ii)numerical simulation established to better comprehend the complex interaction thereof varying(1)geometrical and location parameter such as pile distance from the wall,pile number and spacing,short pile’s tip location relative to axis of wall rotation,and thickness of the interposed load transfer platform,and(2)geotechnical material parameter such as stiffness of the soil and cushion;and then(iii)theoretical analysis postulated according to the superposition method and two-stage analysis method.In the beginning,the settlement and load bearing capacity behaviors during applied working load condition was taken as initial state.Then,taking these characteristics of vertical load redistribution among long piles,short piles and soil as a reference,the influence of the impinged retaining wall movement-induced soil displacement on the vertical load carrying mechanism of existing long-short CFG pile composite foundation has been studied.Factors influencing(1)the load transfer mechanism of composite pile foundations and(2)those pertinent to the response of piles to excavation induced soil movement were systematically taken into account to decide the parametric studies conducted by performing a series of three-dimensional finite element analyses using ABAQUS software.Both the experimental and numerical models on the load transfer mechanism of rigid long-short-piles composite foundation revealed that the overall settlement of the foundation increases as the wall movement progresses under a sustained working load,with the overall foundation settlement being larger for the larger working load sustained during wall rotation.Moreover,the contribution of the soil to the load carrying capacity of composite pile foundation,which distinguishes them from the conventional pile foundation,is highly affected by the deformation and stress changes within the zone of influence of retaining wall movement.The closer the foundation is situated near to the wall,the more it is affected due to the external soil displacement.The progressive wall movement-induced soil displacement decreases the bearing capacity of the soil between piles,which comparably increases the percentage load sharing of the long and short piles,with long piles having a higher load sharing ratio than short piles.The location of the piles in the group and their proximity to the wall affect the percentage of load carried by each pile.It was also seen that increasing the stiffness of short pile increases their load sharing and carrying capacity during both active and passive loading processes.The larger the number of piles and/or the higher the area replacement ratio,the smaller the effect of passive loading on the load sharing ratio of the soil.The stress state of soil elements at different positions(considering points near the piles’head and toe)were analyzed to comprehensively understand the inherent reasons responsible for retaining wall movement-induced variations on the load transfer mechanism of composite foundations.For the soil elements located below the cushion around the upper pile shaft and facing towards the wall,the stress states on the p-q plane have changed with the trajectory of the stress paths moving from the initial state downwards toward the critical state line(K_f-line).The stress unloading for the soil element located at the periphery of the pile and situated close to the wall was larger than that appeared for the soil elements surrounded by the piles within the group at the same depth.Nevertheless,the stress path experienced by these interior stress points beneath the cushion behaved in a different way during the progression of wall movement depending on the number of piles and the proximity to the theoretical slip surface.This shows,the stress path for each monitoring point in the shallow depth sustained the process of stress unloading at various dropping amount of principal stress components in a complex manner.Moreover,the reduction in the magnitude of the stress invariants was also accompanied by the rotation of the principal stresses.However,because of the increase in pile end resistance during the process of wall movement,both the mean isotropic and deviatoric stresses increased for the soil elements situated underneath the tip of each pile.Accordingly,the stress path trajectory moved upward to the K_f-line for the soil elements below the piles’end.Therefore,the analysis of the stress state of the soil elements at different positions indicated that a new static equilibrium has been achieved through both an upward(by the increment of pile head load)and downward(with mobilized pile end resistance)load-transfer mechanisms along the piles’length,with the neutral plane moving further downward owing to the additional down-drag from the wall movement-induced relative pile-soil displacements.Lastly,an extended version of system of higher-order differential equations have been established considering the deformation mechanism under the effect of passive loading using the principle of superposition and the two-stage analysis method to analyze the load-bearing characteristics between the piles and the soil between the piles.It is hoped that the current study renders a significant indication of how the pertinent parameters employed influence the behavior of long-short CFG pile composite foundation in close-proximity to excavations.