3D numerical study of the performance of geosynthetic-reinforced and pile-supported embankments

Geosynthetic-reinforced and pile-supported (GRPS) systems provide an economic and effective solution for embankments. The load transfer mechanisms are tridimensional ones and depend on the interaction between linked elements, such as piles, soil, and geosynthetics. This paper presents an extensive parametric study using three-dimensional numerical calculations for geosynthetic-reinforced and pile-supported embankments. The numerical analysis is conducted for both cohesive and non-cohesive embankment soils to emphasize the fill soil cohesion effect on the load and settlement efficacy of GRPS embankments. The influence of the embankment height, soft ground elastic modulus, improvement area ratio, geosynthetic tensile stiffness and fill soil properties are also investigated on the arching efficacy, GR membrane efficacy, differential settlement, geosynthetic tension, and settlement reduction performance. The numerical results indicated that the GRPS system shows a good performance for reducing the embankment settlements. The ratio of the embankment height to the pile spacing, subsoil stiffness, and fill soil properties are the most important design parameters to be considered in a GRPS design. The results also suggested that the fill soil cohesion strengthens the soil arching effect, and increases the loading efficacy. However, the soil arching mobilization is not necessarily at the peak state but could be reached at the critical state. Finally, the geosynthetic strains are not uniform along the geosynthetic, and the maximum geosynthetic strain occurs at the pile edge. The geosynthetic deformed shape is a curve that is closer to a circular shape than a parabolic one.     

Two-dimensional soil arching evolution in geosynthetic-reinforced pile-supported embankments over voids

Soil arching and tensioned membrane effects are two main load transfer mechanisms for geosynthetic-reinforced pile-supported (GRPS) embankments over soft soils or voids. Evidences show that the tensioned membrane effect interacts with the soil arching effect. To investigate the soil arching evolution under different geosynthetic reinforcement stiffness and embankment height, a series of discrete element method (DEM) simulations of GRPS embankments were carried out based on physical model tests. The results indicate that the deformation pattern in the GRPS embankments changed from a concentric ellipse arch pattern to an equal settlement pattern with the increase of the embankment height. High stiffness geosynthetic hindered the development of soil arching and required more subsoil settlement to enable the development of maximum soil arching. However, soil arching in the GRPS embankments with low stiffness reinforcement degraded after reaching maximum soil arching. Appropriate stiffness reinforcement ensured the development and stability of maximum soil arching. According to the stress states on the pile top, a concentric ellipse soil arch model is proposed in this paper to describe the soil arching behavior in the GRPS embankments over voids. The predicted heights of soil arches and load efficacies on the piles agreed well with the DEM simulations and the test results from the literature.     

移动荷载作用下桩承式加筋路堤的动力特性

为了研究桩承式加筋路堤在移动荷载作用下的特性,采用FLAC 3D软件建立了移动荷载作用下道路的三维动力流固耦合分析模型,对桩承式加筋路堤和天然路堤在移动荷载作用下的竖向变形、桩土应力比、超孔隙水压力、加速度等进行了对比分析,并研究了不同轴载对路堤竖向变形的影响。分析结果表明：移动荷载作用下,桩承式加筋路堤通过桩体土拱效应和格栅张拉膜效应的联合作用,其路面竖向变形、桩土应力比、超孔隙水压力、加速度均比天然路堤的结果明显减小;随着轴载的增加,桩承式加筋路堤路面竖向变形不断增大。     

Centrifuge tests to investigate global performance of geosynthetic-reinforced pile-supported embankments with side slopes

Three centrifuge model tests were conducted to investigate the influence of the number of geosynthetic layers and the pile clear spacing on the global performance of Geosynthetic-Reinforced Pile-Supported (GRPS) embankments with side slopes constructed on soft soil foundations. This study found that the change of the geogrid number from one to two did not significantly affect the foundation settlement, the geogrid deflection, and the vertical stress at the embankment base. For the GRPS embankment with a single geogrid layer, the geogrid strain distribution at the embankment base showed an “M” shape along the transverse direction with the maximum strain near the embankment shoulder. When two geogrid layers with sand in between were used, the upper and lower layers showed different strain distributions with the maximum strains happening near the embankment shoulder and at the center of the embankment for the upper and lower layers respectively. The strains of the upper geogrid were smaller than those of the lower geogrid. Smaller pile clear spacing reduced the geogrid deflection and the foundation settlement. Despite the change of the pile clear spacing, the progressive development of soil arching with the normalized displacement at the embankment base followed a similar trend without an obvious stress recovery stage.     

A novel 2D-3D conversion method for calculating maximum strain of geosynthetic reinforcement in pile-supported embankments

For design of a geosynthetic-reinforced pile-supported (GRPS) embankment over soft soil, the methods used to calculate strains in geosynthetic reinforcement at a vertical stress were mostly developed based on a plane-strain or two-dimensional (2-D) condition or a strip between two pile caps. These 2-D-based methods cannot accurately predict the strain of geosynthetic reinforcement under a three-dimensional (3-D) condition. In this paper, a series of numerical models were established to compare the maximum strains and vertical deflections (also called sags) of geosynthetic reinforcement under the 2-D and 3-D conditions, considering the following influence factors: soil support, cap shape and pattern, and a cushion layer between cap and reinforcement. The numerical results show that the maximum strain in the geosynthetic reinforcement decreased with an increase of the modulus of subgrade reaction. The 2-D model underestimated the maximum strain and sag in the geosynthetic reinforcement as compared with the 3-D model. The cap shape and pattern had significant influences on the maximum strains in the geosynthetic reinforcements. An empirical method involving the geometric factors of cap shape and pattern, and the soil support was developed to convert the calculated strains of geosynthetic reinforcement in piled embankments under the 2-D condition to those under the 3-D condition and verified through a comparison with the results in the literature.     

基于分段荷载传递法的桩网路堤结构荷载效应整体分析模型

为了系统分析桩网支撑路堤结构的荷载传递效应,首先基于单元分析模型和Marston埋管理论,考虑到路基荷载应力重分布时竖向应力水平和界面剪切强度发挥的影响,采用分段平衡控制方程结合边界位移协调的方法,建立了路基荷载传递分析的分段解析模型;在此基础上,考虑垫层加筋的张拉膜效应影响,结合复合地基桩土相互作用的简化分段分析,根据应力、位移连续性条件,建立路基–加筋(网)垫层–刚性桩复合地基联合作用的桩网支撑路堤结构荷载效应整体计算模型并给出求解方法,明确了桩、土间的荷载分担。通过与工程实测结果的对比,验证了该计算模型的可靠性,并算例分析了路基模型界面剪切强度发挥、筋材兜提效应和垫层材料性能等因素的影响规律,可为桩网支撑路堤结构荷载传递机制的研究提供参考。     

基于Hewlett土拱模型桩承式加筋路堤桩土应力比计算

桩土应力比是桩承式加筋路堤荷载传递以及地基沉降计算的重要参数。基于Hewlett土拱模型,单独分析拱顶或拱脚土单元,假设拱顶土单元处于极限状态(拱脚土单元处于弹塑性状态),以均匀超载模拟交通荷载,推导桩土应力比计算公式;基于抛物线模型,考虑筋-土界面摩擦以及地基反力,改进张拉膜效应分析方法,推导加筋条件下桩土应力比计算公式。最后与相关文献实测数据进行对比验证,结果表明该方法与相关文献实测结果除桩间净距为100mm存在误差外,变化规律基本一致,当桩间净距大于100mm时,误差不超过8%。     

Investigation on geogrid reinforcement and pile efficacy in geosynthetic-reinforced pile-supported track-bed

This paper presents a full-scale model study of geosynthetic-reinforced pile-supported (GRPS) track-bed to investigate the effect of geogrid reinforcement and the evolution of pile efficacy (ratio of load borne by the pile cap to the total applied load). Three testing procedures were followed: model construction, static loading and subsoil settlement (simulated by discharging of water bags surrounding the pile caps). The results indicated that partially mobilized soil arching was developed during the first two procedures. When sufficient subsoil settlement was reached, fully mobilized soil arching was established. The geogrid was proven to effectively transfer load from the water bag to the pile cap. The stress difference induced by the geogrid showed lower absolute values for the corresponding sensors above the water bag during loading and settlement procedures, due to the inverse triangular distribution of the vertical-directional geogrid tensile force above the water-bag area. The experimental results of pile efficacy were compared to the estimations of four analytical models. For the present test at partially mobilized arching state, the pile efficacy increased with the construction height increasing and decreased as the static loading increased. The partially mobilized arching also resulted in overestimations of the pile efficacy from all four analytical models. At fully mobilized arching state, the pile efficacy stayed relatively stable, being well predicted by all four analytical models.     

桩承式路堤中的静动力土拱效应

为了从更深层次理解土拱效应的工作性状,在总结桩承式路堤土拱效应中等沉面、桩体荷载分担比等问题的基础上,比较了几种桩体荷载分担比的计算方法,阐述了动荷载在桩承式路堤中的传递机理,分析了土拱效应发挥程度对动应力的影响,最后给出桩承式路堤中动应力的计算方法。研究结果表明:等沉面与土拱高度可用临界填土高度进行归一化描述,临界填土高度与桩间净距呈线性关系;桩体荷载分担比的大小与工况有关,几种计算方法有各自的适用条件;陈云敏的计算方法与实测值拟合度较高;动荷载的传递也受土拱效应的影响,随着动荷载循环次数的增加,土拱效应存在先强化后弱化的现象。     

高填土路堤荷载下混凝土芯砂石桩复合地基变形与承载力试验研究

混凝土芯砂石桩复合地基由竖向增强体混凝土芯砂石桩、水平加筋砂石垫层和地基土共同组成,是一项将砂石桩良好的排水固结特性、预制混凝土桩高强度竖向承载和加筋垫层应力扩散调整功能进行有机结合而成的新技术,适合于处理高含水率深厚淤泥质软土地基。结合实际工程应用围绕混凝土芯砂石桩复合地基的变形和承载力两大特性开展了大量的现场观测和原位测试,研究分析了高填土路堤荷载作用下混凝土芯砂石桩复合地基的表面沉降、分层沉降、深层水平位移、桩土应力分布、桩身荷载传递规律、强度和承载力特性等,进一步揭示了混凝土芯砂石桩复合地基的加固机理,对工程设计与施工具有重要指导意义。     

桩承式加筋路堤受力机理及沉降分析

作为一种经济、有效的软土地基处理方法,桩承式加筋路堤在国内外已开始使用。把单桩处理区域及上部路堤等效为圆桩体,采用弹塑性有限元法分析了瞬时加载后地基中超静孔隙水压力的分布特征及消散过程,研究了加筋格栅的受力和路堤的沉降特性等,分析了桩长、桩间距及桩托板大小对桩体荷载分担比和路堤沉降的影响。研究结果表明,打桩后桩体所受荷载向下传递,地基中的初始最大孔隙水压力出现在桩端以下土层。打穿软土层情况下,路堤的沉降量决定于浅部桩间土的压缩,而未打穿情况下,路堤的沉降量决定于桩端以下软土层的压缩。桩长是控制路堤沉降的最主要因素,其次是桩间距和桩托板尺寸。最后对一个工程实例进行了分析。     

桩承式加筋路堤的改进设计方法研究

具有深厚软土层的路堤若采用桩承加筋式复合地基,可提高地基承载力,减少路堤不均匀沉降,也可布置成疏桩,降低工程成本,在国内外得到越来越广泛的应用,但还没有可靠实用的设计计算方法,且现有的设计均忽略了桩间土的承载作用,这与工程实际有很大差别。基于三维土拱效应,改进Hewlett土拱效应算法,得到桩承式路堤的桩土荷载分担比,进而考虑加筋体影响以及桩间土承载作用,推导桩土应力比计算式,并将此式应用于路堤的设计。     

抗土洞塌陷的低填方加筋路基荷载传递机制及设计方法

低填方加筋路基对地基承载力要求较低,同时利用水平加筋法跨越尺寸较小的土洞能有效预防路堤出现突发式局部沉陷,提高路堤抗工后沉降和失稳的安全系数,正被逐步应用于岩溶土洞地区道路工程;但其作用机理复杂,现存设计方法大都偏于保守,考虑抗土洞塌陷的低填方加筋路基荷载传递机制的设计方法亟待提出。通过揭示受土洞塌陷影响的低填方加筋路基荷载传递机制,推导了考虑路基差异沉降引起土体应力偏转的竖向应力计算方法,假定塌陷区上方加筋体作用抛物线荷载,从而明晰了加筋体应力-应变状态;应对岩溶区不同形态的土洞塌陷,同时考虑设计需要满足的正常使用极限状态与承载能力极限状态,提出了抗土洞塌陷的低填方加筋路基加筋体及路堤填方高度设计方法,通过与现有设计方法的对比进行了合理性及准确性验证,可为空洞上方低填方加筋路基设计提供参考。     

基于Mindlin应力解近似计算路堤桩沉降

由于桩间土的分担荷载大部分以负摩阻力的形式传递给桩,因此近似假定路堤桩为上部荷载直接作用在中性点处的桩基,以此作为桩侧摩阻力的分布模式,用Mindlin应力解计算路堤桩地基中的附加应力与沉降,计算结果与实测结果的吻合程度较好。     

Geosynthetic-reinforced pile-supported embankments with caps in a triangular pattern over soft clay

Given the limit studies on the behavior of GRPS embankments with different numbers of geosynthetic layers and pile caps in a triangular pattern, this paper conducted a series of three-dimensional (3-D) numerical analyses. The numerical model was verified based on a well-instrumented large-scale test. A 3-D soil arch model was proposed for pile caps in a triangular pattern, in which the crown of the upper boundary was approximately 1.4 times the clear spacing of pile caps. Inclusion of geosynthetic reinforcement reduced the soil arching effect but increased the total load carried by the piles. For the case with three geosynthetic layers, the lower layer had a significant effect on load transfer than the middle and upper layers, but each layer had an almost proportional effect on mitigating the differential settlement on the top of the gravel cushion. The maximum strains in the reinforcement concentrated on the geosynthetic strips bridging over two adjacent square cap corners.     

Full-scale model tests of load transfer in geogrid-reinforced and floating pile-supported embankments

Well-designed field full-scale model tests were carried out to enhance the understanding of geogrid-reinforced and floating pile-supported (GRFPS) embankments constructed on medium compressibility soil (MCS). Two comparative test sections of GRFPS embankments with and without pile caps were built over silty clay with medium compressibility for field monitoring, over 25 months. The heavily instrumented embankments produced comprehensive high-quality data. Settlement, earth pressure, and geogrid strain measurements during embankment filling stages and the postconstruction placement stage were conducted. The influence of pile cap installation on the differential deformation and load transfer behaviour of the GRFPS embankment was evaluated. The results demonstrate the installation of pile caps can significantly improve the evolution characteristics of the stress increment ratio on the pile, facilitating a change in load sharing of the pile top from a “softening” feature to a “hardening” feature. The state of the “arching structure” heavily depends on the relative displacement. After the maximum arching is formed, although the subgrade load continuously increases, the arching enters the damage and recovery state, and the transfer of the overburden load increment is largely conducted by the tensioned membrane effect.     

Bindemittelstabilisierter Erdbau am Beispiel eines Anschlußdammes der NBS Erfurt–Leipzig/Halle

Binder stabilized earthwork on the new high‐speed railway line Erfurt–Leipzig/Halle. There is an increasing utilization of hydraulic binders to stabilize embankments, fillings and subgrades in cuts. Due to the stabilization, fill materials of lower and medium quality may be used. The higher stiffness of the embankment evens out settlement differences, thus preventing settling moulds and settlement drops especially at the transition to structures. The shear strength of the embankment can be increased in a way that a load transfer into piles without ties (e.g. geogrids) becomes possible. The application of binder stabilized earthwork is described with the example of an embankment of the high‐speed railway Erfurt–Leipzig/Halle. Due to a thick brown‐coal layer in the subsoil, the 13m high embankment is founded on driven piles. The load is being transferred to the piles by load distribution layer of binder stabilized soil built directly and without geogrid on pile heads. A shallow founded abutment and the crossing of a country road over the railway are embedded in the embankment, the core of which has been stabilized with brown‐coal filter ashes.     

附加应力法计算刚性桩复合地基路基沉降

部分刚性桩复合地基路基实际沉降超过计算沉降的重要原因之一是现有沉降计算方法存在严重缺陷。在分析桩土沉降关系和桩土作用的基础上,提出了路堤下刚性桩复合地基沉降计算新方法——附加应力法。首先根据桩土作用计算桩土附加应力,然后采用分层总和法计算复合地基沉降。经工程实例验证后,利用附加应力法研究了桩长、桩间距、扩底、桩帽等因素对路基沉降的影响,并与现行方法计算的沉降进行对比。分析表明:附加应力法可以考虑单桩竖向承载力、桩帽转移荷载能力、桩土相互作用等因素的影响,计算沉降与实测沉降接近;利用桩帽将路堤大部分荷载转移到桩顶可以有效减小路基沉降;扩底比桩长加大更经济合理;按"强桩、大间距、大桩帽"原则设计的复合地基比密桩复合地基更经济合理。     

桩承式加筋路堤土拱效应试验研究

桩承式加筋路堤中存在土拱效应,它影响着路堤的荷载传递和沉降变形性状,桩土应力比是反应土拱效应的重要参数。本文通过模型试验,研究了桩土相对位移、路堤高度、桩梁净间距、桩梁宽度及水平加筋体等因素对桩土应力比及路堤沉降的影响。结果表明:①桩土应力比随桩土相对位移的发展而变化,存在上限值和下限值;②路堤高度与桩梁净间距之比越大,桩土应力比越大;桩梁宽度与桩梁净间距之比越大,桩土应力比也越大;③使用水平加筋体能提高桩土应力比,提高的幅度与水平加筋体拉伸强度有关;④当路堤高度与桩梁净间距之比小于1.4时,无论是否使用水平加筋体,路堤顶面均会出现明显的差异沉降;当路堤高度与桩梁净间距之比大于1.6时,路堤顶面不会出现明显的差异沉降。该研究成果可为桩承式加筋路堤设计提供有益的参考。     

CFG桩复合地基桩土应力比试验分析

对CFG桩复合地基进行了现场堆载监测和载荷试验,并对两者试验结果进行了分析。结果表明：在刚性荷载和半柔性荷载条件下桩土应力比的变化规律存在着差异,半柔性荷载条件下的复合地基的桩土应力比载荷板刚性荷载下的桩土应力比小,半柔性荷载条件下的复合地基,在沉降过程中,桩和桩间土的沉降存在差异性。