U型槽结构下振冲碎石桩联合抗拔桩数值分析及优化设计

Numerical analysis and optimization design of vibroflotation gravel pile combined with uplift pile under U-shaped groove

[目的]研究沿海下穿道路工程U型槽结构下振冲碎石桩联合抗拔桩的加固效果及其优化设计.[方法]采用有限元软件ABAQUS进行建模,分析了振冲碎石桩的打入对砂土地基抗液化能力的改良效果,讨论了U型槽、抗拔桩、碎石桩各设计参数对U型槽抗浮效果的影响.针对以上结果基于NSGA2遗传算法,开展抗液化能力、抗浮效果、工程造价三目标的优化设计.[结果]振冲碎石桩可以大大增强砂土地基的抗液化能力,未使用振冲碎石桩的砂土均发生液化,而采用振冲碎石桩处理后的砂土地基液化率仅为4.7%;U型槽底板厚度、抗拔桩的桩径、桩长和振冲碎石桩的桩径、桩长均对U型槽底板上浮位移有较大影响;采用NSGA2遗传算法得出的最优方案在满足安全储备的情况下,造价比原始方案节省了10.0%,且砂土地基液化率降低了45.7%.[结论]基于NSGA2遗传算法提出的以砂土液化率、底板上浮位移和工程造价为目标的优化设计方案可为今后同类工程提供设计指导.

[Objective]Due to insufficient urban road space,U-shaped grooves in road cuts are widely used because of their small deformation,high stiffness,and the ability to prevent road sinking and cracking.The design of U-shaped grooves in sandy strata under high water levels requires consideration of sand liquefaction and the anti-floating stability.As a continuation of previous research,here we further study the reinforcement effect and optimization design of the combination of vibroflotation gravel piles and uplift piles under the U-shaped groove of coastal underpass road engineering,providing design reference for similar projects.[Methods]The finite element software ABAQUS was used to model the combined uplift pile of vibroflotation gravel pile(VGP)under U-shaped groove in a coastal road project.The shear stress method was used to study the improvement effect of VGP on the anti liquefaction ability of sand,and the impact of VGP combined with uplift pile on the anti floating performance of U-shaped groove was explored.On this basis,NSGA2 genetic algorithm was adopted to optimize the design of U-shaped groove bottom plate thickness,length and diameter for uplift piles and VGPs with three objectives:sand liquefaction rate,uplift displacement,and engineering cost.[Results]The finite element software ABAQUS was used for modeling,and the improvement effect of VGP on the liquefaction resistance of sandy soil ground was analyzed.The duration of liquefaction in the sandy soil ground treated with VGP had been reduced from 30 seconds to 5 seconds when the diameter of the VGP was 0.8 m.And liquefaction rate had changed from 100%to only 4.7%.Meanwhile,increasing the diameter of VGPs can not only shorten the duration of liquefaction,but also reduce the number of units involved in soil liquefaction.When the diameter of the VGP increased from 0.8 m to 1.0 m,liquefaction occurred in 0-3 seconds and only 2.6%of soil underwent liquefaction.Subsequently,the influence of design parameters of U-shaped groove,VGP and uplift pile on the anti-floating effect of U-shaped groove was discussed.The thickness of the U-shaped groove bottom plate,the diameter and length of VGP and the uplift pile all had a significant impact on the upward displacement of the bottom plate,while the influence of the elastic modulus was relatively small.Increasing the pile diameter of the VGP can better enhance the anti-floating effect of the U-shaped groove than the pile length.When the diameter and length of the uplift pile was 0.7 m and 13.5 m,it could meet both safety and economic requirements.Finally,optimization design of VGP combined with uplift pile on the U-shaped groove of a coastal underpass road project was carried out based on NSGA2 genetic algorithm with three objectives(sand liquefaction rate,uplift displacement,and engineering cost).The original design scheme was:U-shaped groove bottom plate thickness d u=1.5 m,uplift pile length l b=15.0 m,pile diameter d b=0.8 m,and VGP diameter d s=0.8.The optimal scheme obtained was as follows:U-shaped groove bottom plate thickness d u=1.25 m,uplift pile length l b=13.50 m,pile diameter d b=0.9 m,and VGP diameter d s=1.00.Compared with the original design scheme,the cost was saved by 10.0%and the liquefaction rate of sandy soil ground was reduced by 45.7%.[Conclusions]VGP can significantly improve the anti-liquefaction effect of sandy soil ground.Increasing the diameter of VGP can enhance the anti-liquefaction ability of composite ground,shorten the liquefaction time of sandy soil ground,and reduce the proportion of soil liquefaction.The thickness of the U-shaped groove bottom plate,the diameter and length of VGP and the uplift pile all has a significant impact on the upward displacement of the bottom plate,while the influence of the elastic modulus is relatively small.The optimized design scheme of U-shaped groove bottom plate,VGP,and uplift pile based on NSGA2 genetic algorithm with the goals of sand liquefaction rate,bottom plate uplift displacement,and engineering cost can provide design guidance for similar projects in the future.