桩承式加筋路堤土拱形态与土拱效应演变规律

Basal reinforced piled embankment has been applied more extensively in highway and railway roadbed engineering in soft soil area because of its reliable performance and investment saving. The intrinsic theory is soil arching effect developed in embankment fill as the result of differential settlement over pile and around soil. The arching effect, combining with the membrane effect of basal reinforcement, causes the piles to support the most of the embankment loads. The total settlement and the uneven settlement of roadbed would be controlled. But at present, the cognition about arching shape and the conditions of arching development is not identical, the load transferring mechanism has not yet been verified sufficiently, there are few studies on influence of pile-soil-reinforcing inclusion interaction on varying law of arching effect. Scaled model test, centrifugal model test and mathematical modeling is to be used in this project to verify the arching shape and explore the conditions of full arching development. The arching effect and load transferring of reinforced piled embankment are considered as a dynamic process under the multifactor actions, the influences of the fill properties, soft soil foundation, basal reinforcement, and pile settlement on arching effects will be analyzed. Through the researches of this project, the varying law of arching effect of piled reinforced embankment will be revealed, the load transfering model and the arch model including the arch shape and full arching conditions will be founded. Adherence to the principle controlling roadbed settlement, the analytical methods of the pile efficacy and roadbed settlement will be provided. The output of this research will establish the scientific theory for the engineering application of basal reinforced piled embankment.

因性能可靠和节省投资，桩承式加筋路堤在松软土地区公路和铁路路基工程中得到了越来越多的应用。其内在机理是基于桩土差异沉降形成路堤土拱效应，土拱效应与加筋拉膜效应相结合，使桩承担大部分路堤荷载，从而控制路基总沉降和不均匀沉降。但目前对土拱形态和成拱条件认识不一致，荷载传递机制实证不足，缺乏重复荷载下桩-筋材-土共同作用对土拱效应演变规律影响的定量研究。本项目将采用缩尺模型试验、离心模型试验和数值模拟方法，试验验证土拱形态，研究全拱条件，把桩承式加筋路堤的土拱效应和荷载传递视为多因素作用下的动态过程，分析填土性质、地基条件、加筋作用、桩顶沉降等对土拱效应和路基沉降的影响。通过本项目的研究，将揭示桩承式加筋路堤土拱效应的演变规律，建立包括土拱形态、全拱条件在内的土拱模型和荷载传递模型，提出以沉降控制为原则的桩体荷载分担比和路基沉降分析方法。研究成果将为桩承式加筋路堤的工程应用奠定科学理论基础。

在松软土地区公路和铁路路基工程中，为满足河流、物流等的横向穿越，需设置众多桥（隧）结构物。由于结构物多采用桩基础，桥隧与临接路基之间往往会产生较大的差异沉降（错台），使行车出现颠簸（俗称桥头跳车）现象。这不仅会增加后期维护费用，而且容易造成交通事故。桩承式加筋路堤是解决桥头跳车问题的先进技术之一，在道路和铁路工程中得到了越来越多的应用。. 本项目针对该技术在设计和应用中存在的一系列科学问题展开研究，这些问题涉及填土的土拱形态及成拱条件、水平加筋材料的张力膜效应以及综合考虑填土性质、桩基端承条件和重复荷载等主要影响因素，核心问题是土拱效应、张力膜效应和桩体荷载分担比的变化规律。. 通过本项目历经四年的研究，项目组取得的主要研究成果包括：.（1）研究结果表明，在二维平面内桩承式加筋路堤中的竖向应力土拱近似呈三角形形态，并据此提出了同轴三角形土拱模型。该模型能够描述土拱形成、土拱简化形态以及桩土荷载分担的变化，推导出了桩土荷载分担的具体计算公式。.（2）研制了桩承式加筋路堤中加筋材料张力膜效应模拟试验系统，进行了在不同条件下物理模拟试验。发现加筋材料张力膜具有明显的三维变形特征，在正方形布桩情形下，张力膜的空间变形形态可采用空间抛物面与抛物柱面的组合来模拟。筋材拉力最大值出现在桩帽角点，其次为桩帽边缘，其余区域拉应力较小且呈平台分布。.（3）土拱高度大于1.0倍，但小于1.5倍的桩帽净间距。进一步证明在一般情形下，路堤高度大于1.5倍的桩帽静间距可满足应力全拱条件。.（4）桩承式加筋路堤的土拱效应、张力膜效应和桩体荷载分担比均受一系列因素影响：(a)路堤填土的粘聚力，与假定粘聚力为 0 kPa的填土相比，填土的黏聚力会增强土拱效应，使桩体荷载分担比增大；(b)桩体端承条件，当采用悬浮桩时，随着上覆荷载的增加，桩体向下刺入，同时桩承担的荷载减小，土拱效应减弱；(c)重复荷载，在重复荷载下，土拱效应减弱，使一部分原本由桩承担的荷载转而由桩间土承担。. 关于桩承式加筋路堤的变形规律、整体破坏机制、桩体刚度的影响等，离心模型试验已完成，进一步的分析正在进行之中，相信能够得到预期的科研成果。. 上述已经取得的和即将完成的研究成果，将为桩承式加筋路堤的设计和应用奠定重要的理论基础，对该项技术进一步推广应用和比较彻底地解决桥头跳车问题，具有重要的实用价值。