在软土地基上进行路堤工程建设, 主要面临稳定性和变形2方面的问题, 桩承式加筋路堤不仅能够提高路堤的稳定性, 而且能够控制路基的沉降。通过一系列室内模型试验、现场原型监测和数值分析, 人们对桩承式加筋路堤中的土拱效应和加筋拉膜效应, 以及路堤荷载传递机制已形成比较一致的看法, 并在各种土拱模型理论指导下形成多种桩承式加筋路堤设计方法。但目前对桩承式加筋路堤的土拱形态和成拱条件认识不一致, 重复荷载下土拱效应演变规律缺乏定量研究, 现行设计方法对路堤沉降控制仍显不足。在综述桩承式加筋路堤已有研究成果的基础上, 重点论述桩承式加筋路堤的作用机理理论与应用研究的成果, 提出了对存在问题做进一步研究的建议。

Stability and deformation are main issues of embankment construction on soft soils. The techniques of basal reinforcement and pile support could enhance the embankment stability and control the subgrade settlement. Through laboratory model tests, prototype monitoring and numerical modeling, researches on the soil arching effect and reinforcement tensioned membrane effect as well as the load transfer mechanism have been in consistency. A variety of design methods of GRPS (Geosynthetic-reinforced and Pile-supported) embankments have come into being. However, researches on the arching shape and the conditions of arching development have not reached an agreement. Quantitative studies on the variation regularity of arching effect under repeated loading are in lack. The control of embankment settlement hasn’t received sufficient attention in current design specifications. On the basis of previous research achievements, the mechanism and application of GRPS embankments are expounded, and suggestions for further researches are put forward.

In the aim of investigating the soil arching of geotextile-encased stone columns, we established a three-dimensional finite element model of geotextile-encased granular columns composite foundation under embankment load to analyze the influences of geotextile stiffness, column spacing and embankment height. Results indicated that the ratio of geotextile modulus to soil modulus which induced the start of soil arching was 20. The modulus ratio had little effect on soil arching after reaching 67. The height of soil arching increased linearly until the height of embankment reached 5 times of net spacing and then remained constant. The required height for a complete soil arching increased with the increase of net spacing. When net spacing reached twice the pile diameter, the height of soil arch remained stable. Moreover, with the increase of embankment height, the soil arch rate decreased gradually. When the height of embankment reached 1.7 times of net spacing to form complete soil arch, the soil arching rate did not change anymore. Compared with numerical results, we found that the results calculated by the three-dimensional soil arching method proposed by Hewlett and Randolph are the closest to the numerical results, and we suggest to use this method for design.

基于单桩等效处理范围内堤身土体受力平衡, 改进了传统的HEWLETT极限状态空间土拱效应分析方法, 求得了桩体荷载分担比计算的解析表达式。研究了桩间距、桩帽大小和填料内摩擦角对桩体荷载分担比的影响。研究表明:土拱处于弹性状态时, 改进方法计算出的桩体荷载分担比小于HEWLETT极限状态计算结果;土拱处于塑性状态时, 改进方法计算结果与HEWLETT计算结果相同。设计桩承式路堤时, 若填料内摩擦角较大, 则应优先考虑增大桩帽宽度来提高桩体荷载分担比;若内摩擦角较小, 则可适当减小桩距来提高桩体荷载分担比。大内摩擦角填料有助于提高桩体荷载分担比。给出了桩体荷载分担比计算图, 便于工程应用。最后对一个工程实例作了分析。

Based on gravity balance of embankment filling above equivalent area for one pile, conventional HEWLETT's three-dimensional soil-arch analysis method is improved. An analytical solution for pile efficacy is obtained. Influence of pile spacing, pile cap dimension and angle of internal friction of embankment filling on the pile efficacy is analyzed. Pile efficacy calculated by the improved method is less than that obtained by HEWLETT's method.When the soil arch is in plastic state, both methods have the same value. Increasing pile cap dimension is more economy for embankments with large friction angle. Decreasing the pile space is more effective for embankments with small friction angle. Choosing embankment filling with large friction angle is helpful to increasing pile efficacy. Pile efficacy charts are given for engineering application. Finally, these formulas are used to analyze a practical case.

为研究地基不排水或排水不畅情况下加筋碎石桩复合地基的承载特性,配置了由钠基膨润土和甘油混合而成的新型人工软土地基,进行了4组加筋碎石桩复合地基路堤模型试验,并与排水条件下的试验结果进行对比分析。研究结果表明:加筋碎石桩复合地基沉降随筋材刚度的增大而明显减小,其桩土应力比远大于排水条件下的桩土应力比;筋材刚度较大时,桩体发生向路堤外的整体倾斜变形,而筋材刚度较小时,桩体则发生向路堤内的弯曲变形;部分加筋碎石桩复合地基的沉降明显大于全长加筋情况,且其桩土应力比小于全长加筋时的桩土应力比,但仍大于普通碎石桩的桩土应力比;不同加筋长度碎石桩桩体均在其加筋范围内发生弯曲变形,在未加筋部位发生鼓胀变形;排水条件对加筋碎石桩桩体变形和稳定性有显著影响,排水条件好,桩体弯曲变形小,反之,桩体则产生显著的弯曲变形。通过试验分析得出对于路堤荷载下的加筋碎石桩复合地基,应采用刚度较大的筋材对碎石桩体进行全长加筋,并保证地基的排水,可大幅提高路堤的稳定性。

A new type of artificial soft clay composed of sodium bentonite and glycerol was made to study the mechanical feature of composite foundation reinforced with geosynthetic-encased stone columns (GECs) in soft clay foundation under undrained or poorly drained conditions. Four groups of model tests of GECs composite foundation with varied reinforcement stiffness and length under embankment load were carried out for this purpose. The test results were compared with those in well drained condition. Results showed that: the settlement of the GECs composite foundation obviously decreases with an increase of reinforcement stiffness. The stress concentration ratio is much larger than that in well drained condition. The columns with high reinforcement stiffness are likely to tilt outward the embankment. The columns with low reinforcement stiffness, however, bend inward the embankment. For the partially encased stone columns, the settlement is significantly greater than the fully encased stone columns. The stress concentration ratio of the partially encased stone columns is smaller than that of the fully encased stone columns, but larger than that of stone columns. The GECs bend inward the embankment in the encased parts and bulge in the non-encased parts. Drainage condition significantly affects the displacement and stability of GECs. Generally, the bending deformation is smaller in the columns with well drained conditions and vice versa. It is found from the study that fully encased stone columns with high reinforcement stiffness and well drained conditions are of great importance to improve the stability of road embankment.