循环荷载下长短组合路堤桩土拱效应及其演变规律

As a stricter settlement demand of the foundation is put forward in the construction of highway and high-speed railway, especially in deep soft ground, a more efficient foundation treatment method needs to be developed. Due to its advantages of rapid construction, easy control of settlements, and wide suitability for various geological conditions, the composite embankment with long and short piles has been gaining its popularity in the construction. The main difficulty in design is to assess the degree of support offered by the pile caps, which places restrictions on its engineering design and application. Centrifuge model tests have been becoming a powerful tool in exploring the mechanism of new types of foundation treatment, since it can simulate a full-scale prototype using a small-scale model through increasing the gravity acceleration. Therefore, the load sharing law of composite embankment with long and short piles under embankment load and cyclic loading is studied through centrifuge tests. The basic theoretical framework of three-dimensional hybrid arching analysis is established based on the vivid description of the stress concentration obtained from the finite element analysis. The length ratio coefficient and cyclic loading coefficient are separately adopted to feature the incremental generation and the progressive failure of the arches. The stress concentration ratios of the long piles and the short ones are calculated through solving system of equations including the equilibrium differential equation, boundary condition equation and relational expression of the two different soil arching effect coefficients. When the geosynthetics is taken into account, the reinforcement tension is calculated through the assumed deformed curves. And then a new equilibrium differential equation over the elevation of the pile caps is generated. The study will provide experimental and theoretical bases for the design of the composite embankment with long and short piles under cyclic loading, which will be great helpful to improve its application in the traffic engineering construction.

高等级公路和高速铁路等交通工程的建设对地基沉降控制提出了更严格的要求，尤其是在深厚软土覆盖地区，需要采取更为合理有效的地基处理方法。长短组合路堤桩是将建筑地基中的减沉桩思想应用于道路工程柔性荷载作用下的一种新型地基加固方式，具有沉降控制容易且适用于各种地质条件的优点，但其桩土荷载分担规律还不甚明了，限制了该方法的应用。本项目通过离心模型试验技术，研究典型桩长比条件下长短组合路堤桩中长桩和短桩的桩土荷载分担比，及其随循环荷载强度和次数演化规律；采用三维有限元分析方法，揭示循环荷载下长短桩组合路堤桩新型土拱效应的形态及其演化机理，建立新型土拱效应理论研究的概化模型；提出长短组合路堤桩新型土拱效应的理论框架，建立考虑长桩与短桩的桩长比以及循环荷载强度和次数的桩土荷载分担计算方法。研究成果将为长短组合路堤桩的设计提供试验和理论依据，促进其在软土地区交通工程中的推广和应用。

高等级公路和高速铁路共同面临着交通荷载的长期作用但工后沉降要求严格的需要，如何控制循环荷载作用下地基的长期沉降是保证高快速、安全运行的一个关键问题。长短组合路堤桩是将减沉桩思想应用于柔性荷载作用下的一种新型地基加固方式，具有沉降控制容易且适用于各种地质条件的优点，但其桩土荷载分担规律还不甚明了，限制了该方法的应用。. 本项目主要研究内容包括：开展长短桩组合路堤桩大型土工离心模型试验,针对目前交通工程中采用的典型布桩形式和桩基类型，分析典型桩长比的长短桩在静荷载以及有限次循环荷载作用下桩土的荷载分担规律；采用有限元分析方法建立长短桩组合路堤桩数值模型，通过与离心模型试验结果对比，揭示土拱效应随循环荷载作用次数的演变规律；基于传统等长桩的三维土拱效应分析方法，引入桩长比效应系数和循环荷载效应系数，建立循环荷载下长短组合路堤桩新型土拱效应的分析方法。通过离心模型试验、三维有限元模拟以及理论分析的综合研究得到如下主要结论：随着循环次数的增加，长桩桩土应力比先增大然后趋于稳定，而短桩桩土应力比增加幅度较小；由于循环荷载作用下长桩沉降明显小于短桩和桩间土，土拱效应增强，长桩承担更多的上覆荷载作用；但随着循环荷载次数增大，桩土应力比先增大然后逐渐趋于稳定；随着加卸载次数的增加，塑性变形累积，第一次循环完成时塑性变形较大，约占六次加卸载所产生变形的60%左右。发表（含收录）科技论文7篇，其中SCI收录1篇，EI收录3篇，1篇录用待刊；研究生培养2名，已毕业；获省级岩土力学与工程学会科学技术奖二等奖2项。. 研究成果能够用于分析和计算长短组合路堤桩的桩土荷载分担比，优化方案设计，促进其在交通工程领域尤其是软化地区的应用。因此，本项目既结合我国交通领域建设和发展的现实需求，又紧扣岩土工程地基处理研究的学科前沿，具有重要的理论意义和广阔的工程应用价值。