Study On Bearing Behavior Of Piles For Excavation Beneath Existing Buildings

Excavation beneath existing buildings, inevitably leads to changes in pile bearing behavior of the original pile foundation. Considering the field conditions, the bearing behavior of piles was not easy to be obtained by field tests after excavation, so it is particularly necessary to evaluate the bearing behavior of piles using theoretical analysis and finite element simulation. On the basis of single pile model, the investigation accounts for the effects of excavation width, excavation depth, soil parameters on the variation trends of pile shaft resistance, pile base resistance and pile bearing stiffness were analyzed under the condition of excavation. Finally, a three-dimensional numerical model was established for the simulation of bearing behavior of piles after excavation. The main work and conclusions are summarized as follows:(1) The plane model of single pile was established combined with case background, Mindlin’s stress solution and a simplified method were adopted to considering the changes of vertical effective stress around pile caused by excavation, the formula of vertical effective stress around pile shaft and pile base were obtained(2) The formula of ultimate shaft resistance per unit area was derived under the condition of excavation. Subsequently, the ultimate shaft resistance before and after excavation were obtained using of Mindlin’s stress solution and a simplified method, respectively. The relationship among loss ratio of the ultimate shaft resistance, excavation width and depth, friction angle of soil were also analyzed. The results show that the loss ratio increases with the increase of excavation width and depth, but decreases with the increasing friction angle of soil. The loss ratio of center pile is larger compared to corner pile, the loss ratio increases with excavation carried on, and decreases along the pile shaft from top to bottom, the maximum one turn out to be the loss ratio of entire length of pile shaft.(3) The formula of ultimate base resistance per unit area was derived under the condition of excavation. Subsequently, the ultimate base resistance before and after excavation were obtained using of Mindlin’s stress solution and a simplified method, respectively. The relationship among loss ratio of the ultimate base resistance, excavation width and depth, parameters of pile base were also analyzed. The results show that the loss ratio increases with the increase of excavation width and depth, but decreases with better quality of the rock mass and larger embedded depth. The loss ratio of center pile is larger compared to corner pile, the loss ratio increases with excavation carried on. The difference of the loss ratio for different excavation width and depth is more obvious for poorer quality of the rock mass and smaller embedded depth.(4) On the assumption of the hyperbola-type load transfer function along the pile shaft and below the pile base, the stiffness curves of pile shaft and pile base were obtained before and after excavation. thus load-settlement curves for pile-head due to excavation were obtained based on iterative solution for load transfer method. The variation of reduction factors for bearing capacity with settlement control value, and the variation of reduction factors for initial rigidity and ultimate resistance with excavation depth and unloading ratio were also analyzed. The results show that bearing stiffness for pile-head was decreased after excavation, the reduction factors for bearing capacity increases with the increase of settlement control value. Both the initial rigidity and the ultimate resistance decreased with increasing excavation depth and unloading ratio, both of the two reduction factors increases along the pile shaft from top to bottom. For a certain soil, the reduction factors for initial rigidity is smaller compared to that of ultimate resistance. For a given unloading ratio, both of the two reduction factors were smaller for the condition of weaker soil.(5) Based on the finite-element parameters derived from the results of static load tests and numerical simulation, the three-dimensional finite element framework takes into account the interaction of piled raft foundation, ground and excavation, and is able to investigate the post-excavation performance of the existing pile groups. The investigation accounts for the effects of end-bearing soil stiffness and excavation depth on the variation trends of pile-head rigidity, pile force, pile shaft resistance, ratio of pile base resistance and soil rebound. The results show that the maximum axial force and shaft resistance, the smallest pile loading stiffness occurred on center pile after excavation. When the stiffness of the end-bearing soil was smaller, the linear load-settlement curve for pile-head turned to be more bending. The shaft resistance increased firstly and then decreased along the pile shaft from top to bottom, and excavation depth was a key factor that positively influences the mobilization degree of the shaft resistance. The ratio of pile base resistance increases with loading carried on and excavation depth, and increasing the stiffness of the end-bearing soil was crucial to enhancing the critical ratio of pile base resistance. Piles have a certain resilience, the largest resilience occurred on center pile, and the largest soil rebound occurred at central area of pit.