Research On Bearing Mechanism And Settlement Of Single Dx Pile And Dx Pile Group

DX pile is a new kind of pile with varied cross-section recently developed. The bearing bells are constructed by the rotary extruding-expanding rig on the pile shaft in-situ. The rotary extruding-expanding construction method extremely expands the application of DX pile. Engineering practice shows that the DX pile has several remarkable advantages, such as high bearing capacity, small settlement and fast construction. The existence of bearing bell makes the DX pile become a new type of pile with bells, pile shaft and pile tip resist the load together, which makes the bearing mechanism rather complicated.In this dissertation, a systemic research on the bearing mechanism and settlement characteristics are conducted by model tests, in-situ test, numerical simulations and theoretical analysis. In addition, based on the Geddes solution, through theoretical deduction obtained the settlement calculation method for DX pile.The following conclusions can be made in this research:1) Compared with the conventional straight pile with same pile diameter and pile length, the ultimate bearing capacity of DX pile is obviously increased, usually can double the bearing capacity. At the meanwhile, the DX pile can provide high bearing capacity with very small settlement, under the same load, the settlement of DX pile is only half of the settlement of straight pile.2) At the beginning of loading, bearing bells begin to carry large amount of load, which means DX pile can provide high resistance under small settlement. As the increase of the load, the capacities of bells in total load keep constant of increase slightly. Generally speaking, the capacities of bells in total load roughly between30%-40%. The Q-s curve of DX pile shows slow failure pattern, usually the lower bell still have bearing potentials. DX pile has better bearing safty.3) Through the model test of DX pile group, in sand, as the increase of pile spacing, the bearing capacity and the settlement of4-pile and6pile group show no obvious changes; while in clay, the capacities increase slightly.4) In pile group, while the pile spacing is small, the soil between piles and the foundation piles become a whole, the shaft resistance increases and the bell resistance decreases; while the pile spacing increases, the overall effect weakened, each foundation pile settled separately. Through the study, the critical pile spacing is about 2.5D, which is2.5times of diameter of bells.5) As the increase of the number of piles in pile group, the bearing capacities of foundation piles decrease which means its capacity cannot play sufficiently. It can be seen that a simple increase in the number of piles to improve the bearing capacity and control settlement cannot play a good role.6) Based on Geddes solution, introduced the layer wise summation method to calculate the settlement of DX pile. According to the calculated values and the measured value, the relative error between those two values is in the reasonable range, and can provide reference for further research.