Research On Centrifuge Modelling And Analytical Method Of Effects Of Spudcan Penetration On Adjacent Piles

Jacket platform supported by several single piles or a pile group has been universally adopted during oil development in ocean engineering,while jack-up rigs are often employed to perform oil drilling or work-overs near an existing jacket platform without drilling equipments.The jack-up needs to be positioned adjacent to the platform and at the same time penetrates large-diameter spudcan foundations into soils to provide bearing capacity.Due to limited cantilever length of the jack-up rig and deep water depth,the clearance between the spudcan foundation and the piled foundations of the platform is often less than a spudcan diameter,which will lead to additional stress and deflection on the adjacent piled foundations.Model tests combined with numerical and analytical analysis were conducted in this study to investigate the effects of spudcan penetration on the adjacent piled foundations.Conclusions are summarized as follows:(1)Soil flow mechanisms in sand and soft clay are investigated by centrifuge model tests,and results show that the general failure mechanism dominates when the soil is stiff and occurs at shallow penetration depths,while the localized flow mechanism is found in soft soils and occurs at deep penetration depths.Considering that the soil moves upwards and outwards when spudcan penetrates in soft clay,large bending stress and positive pile shaft friction will be induced in the adjacent pile.When spudcan penetrates in sand,the soil moving downwards and outwards will induce large bending stress and negative pile shaft friction in the adjacent pile.(2)The effect of spudcan penetration on the p-y relationships of laterally loaded piled foundations is analyzed by centrifuge model tests in loose sand and soft clay.Results show that reloading,loading in the opposite direction,and unloading states can be found in sequence along the pile shaft.The pile should be divided into the influenced zone and the intact zone when influenced by spudcan penetration and the lateral pile head load simultaneously,and the depth of the influenced zone is related to the penetration depth.(3)An auto soil displacement calculation method during spudcan penetration in the free field is developed based on the finite element remeshing technique.It is suggested that an uniform triangular mesh with 0.05 times the spudcan diameter(D)in grid size,and 0.03 D in remeshing interval be proper to give accurate soil displacement prediction.The Couple Eulerian-Lagrangian(CEL)finite element method is also adopted to aid soil displacement calculation and results show that a fine mesh of 0.05 D near the spudcan penetration area and a simulation speed of 0.5m/s are suggested in the CEL model to give accurate calculation results.(4)A method to predict the effects of spudcan penetration on the adjacent laterally-loaded single piles is proposed by combination of soil displacement prediction and the nonlinear foundation beam model with the p-y relationships considering the effects of spudcan penetration.The method is characterized by an influencing factor describing the effects of spudcan penetration on the p-y relationship,and variation of the factor with spudcan penetration depth is suggested based on centrifuge model test results.It is indicated that the p-y relationship involving the influencing factor gives closer prediction of lateral pile response,and the penetration-induced pile response will be underestimated if the influencing factor is not considered.(5)A method to predict axial pile response is proposed by combining large deformation finite element calculation with the nonlinear beam-column model,and was certified by comparison with centrifuge model test results.(6)A group effect analyzing method is proposed by combination of the nonlinear beam model and the Elastic analysis method to evaluate the effects of spudcan penetration on the lateral pile group effect of adjacent laterally-loaded pile groups.And the proposed method is certified by the centrifuge model test and numerical simulation results.(7)Spudcan penetration on a fixed-head pile group is analyzed by the proposed group effect analyzing method,and a parametric study is conducted to investigate the effects of pile spacing and the relative position between the spudcan and the pile group.Results show that the pile head load redistributes during spudcan penetration for the fixed-headed pile group.The Y multiplier,indicating the pile group effect,firstly increases during spudcan penetration and then decreases at deep penetration depths.However,the Y multiplier variations with penetration depth will not change with varying pile spacing and relative position between the spudcan and the pile group.The study not only provides reference to understanding of the influencing mechanism of spudcan penetration,but also proposes relevant analyzing methods for quantifying its effects on the adjacent piled foundation.