Three-dimensional physical modeling of load transfer in basal reinforced embankments under differential settlement

This study developed a large-scale laboratory apparatus to evaluate the load transfer behavior of basal reinforced embankment fill because of soil arching and geogrid tensile force. A 3D trapdoor-like test system performed five scaled model tests using analogical soil. The instrumentation system involved multiple earth pressure cells, dial gauges, multipoint settlement gauges, and geogrid strainmeters. Comprehensive measurements were performed to investigate the evolution of soil stress and displacement at specific fill elevations with variations in the area replacement ratio and geogrid stiffness. The critical height of the soil arching was determined to be ～1.1–1.94 times the clear pile spacing based on the soil stress and displacement profiles. The distribution of the geogrid tensile strain between and above the adjacent caps demonstrated that the maximum geogrid strains occur on top of the caps, and the tensioned geogrid effect on the load transfer efficiency was evaluated. The cap size and center-to-center pile spacing affect the pile efficacy more significantly than the stiffness of the geogrid. The measured critical heights of arching, stress concentration ratios, and geogrid strain matched those calculated by several well-recognized analytical methods. This experimental program facilitates the development of arching models that account for differential settlement impact.     

CFG桩网复合地基加固物流地坪深厚软土地基的三维数值模拟分析

结合湖州某物流园采用CFG桩网复合地基处理大面积深厚软土的工程实例,运用Midas GTS有限元软件建立桩网人工复合地基三维固结模型,计算模拟不同垫层厚度、土工格栅层数、桩长等设计参数对沉降变形的影响以及土工格栅在桩网固结模型中的受力特征。计算结果表明:地基整体沉降变形随填土垫层厚度的增加而增大,随土工格栅层数的增加而减小,随桩长增加而减小;土工格栅最大拉力随着土工格栅层数增加而减小,在桩帽顶部角点处产生部分应力集中,且在桩帽之间挠度明显增加。为此,控制垫层厚度、土工格栅层数、桩长等措施能有效减小地基沉降变形,为今后软基沉降控制等相关工程提供借鉴。     

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.     

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.     

CFG桩复合地基土工格栅抗拉强度数值分析

对土工格栅在CFG桩复合地基中的工作机理进行了数值模拟,研究了在不同的桩间距、垫层模量下,格栅抗拉强度EA对路基的沉降、坡脚水平位移和桩承担荷载的影响,研究表明,格栅抗拉强度EA对桩与土沉降影响不大,通过改变格栅抗拉强度EA来影响桩土沉降差远没有通过改变垫层模量来影响桩土沉降差效果好,提高格栅抗拉强度可以减小路基坡脚水平位移。     

不同车速下高速公路X形桩网复合地基动力特性研究

基于大比例模型试验系统,开展高速公路车辆荷载作用下X形桩网复合地基动力模型试验研究;测试了不同车速作用下桩网复合路基的动力响应,包括路基内部动应力分布、动应力扩散、格栅动应变、格栅累积应变、桩土差异沉降等的变化规律。结果表明,高速和低速状态下,动应力均会在格栅处产生波动,并且高频车速对车辆附加荷载贡献较小;当车速达到高速状态时,格栅的动应变变化较小,路肩处的垫层应力传递系数要小于路基中心处;格栅动应变比与格栅应变增量比存在线性关系,并且格栅的累积变形主要是在低速状态下产生的;车速越快,X形桩身动应力幅值越大,其桩身轴力小于圆形桩。     

交通荷载作用下桩承式加筋路堤性能分析

为了研究交通荷载作用下桩承式加筋路堤的性能,采用FLAC3D软件建立桩承式加筋路堤的三维动力流固耦合分析模型,对比分析桩间距、路堤高度、格栅模量、桩体模量等对路面沉降及路基中孔隙水压力的影响。结果表明:随着格栅模量、桩体模量的增加或桩间距、路堤高度的逐渐减小,桩承式加筋路堤路面的工后沉降逐渐增大;随着桩间距、路堤高度、格栅模量的增加或桩体模量的逐渐减小,路基中累积的超孔隙水压力最大值逐渐减小。     

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。     

Reinforcement load in geosynthetic-reinforced pile-supported model embankments

The stress conditions of geosynthetic reinforcements (GRs) are crucial in achieving the accurate serviceability design of geosynthetic-reinforced pile-supported (GRPS) embankments. However, the sensitivity of load distribution to the settlement process has been reported in geosynthetic-reinforced embankment overlying cavities. In this study, a three-dimensional model embankment was used to perform experiments and evaluate the load acting on the GR. A flexible pressure-mapping sensor was introduced to investigate the pressure distribution for two types of supporting conditions: partitioned displacement by multiple movable trapdoors and even trapdoor settlement underneath different subsoil materials. The results showed that the load on the GR was concentrated on the strip areas between adjacent pile heads along with the settlement. The measured load on the GR strip area was related to the settlement process and finally exhibited a U-shaped distribution after detachment from the support underneath. The soil arch height in the subgrade continuously increased with the settlement; meanwhile, the pile head load increased rapidly at first and then decreased slightly or remained stable depending on the foundation support stiffness. For both types of settlement behaviours, soil arching exhibited stress history-related characteristics that influence the load transfer in GRPS embankments.     

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。     

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.     

路桥过渡段桩承式加筋路堤现场试验研究

桩承式加筋路堤与路堤填土加筋技术联合应用于黄土地区路桥过渡段,减小路桥过渡段差异沉降和桥头跳车现象。通过 现场试验 对桩承式加筋路堤中心轴和路肩对应位置处格栅上、下表面桩顶和桩间土土压力、桩间格栅变形以及加筋路堤各断面格栅上、下表面土压力和格栅变形进行监测分析,研究结果表明：桩承式加筋 路堤通过土拱效应和张拉膜效应将路堤荷载向桩顶转移,从而可有效减小桩间土荷载;桩承式加筋路堤中心轴处路堤荷载转移主要以土拱效应为主,以张拉膜效应为辅,而路肩处格栅张拉膜效应较显著,路堤荷载传递由土拱效应和张拉膜效应共同完成,格栅在路肩处发挥作用效果大于路堤中心轴处;路堤加筋技术在桥台附近减载作用明显,随着距桥台距离的增加,减载作用逐渐减弱。     

超大面积深厚软土桩-网复合地基承载性状分析

通过有限元Plaxis软件,对厦深高速铁路潮汕车站的超大面积深厚软土桩-网复合地基承载性状进行全断面数值模拟,系统地分析了其沉降变形、土压力变化、桩体受力、土工格栅拉力、超孔隙水压力变化等情况。结果表明:土层的沉降随填筑高度的变化具有一定的间歇性,沉降主要集中于加固区下面的下卧层中;土压力的发展变化呈现出明显的阶梯状,并贯穿于整个施工阶段。在竖向,管桩桩体轴力的峰值出现在淤泥质黏土层顶部,而没有出现在桩体顶部;在横向,沿路基中心向外,桩体剪力及弯矩依次逐渐增大。土工格栅的最大拉力出现在桩帽边缘处,桩间的拉力较小,在管桩顶部加设桩帽有助于均匀格栅中的拉力,避免局部应力集中;桩端土层以下超孔隙水压力随填筑加载而增长的幅度较大,桩端土层以上则较小,桩体有效地将上部荷载传递到深部较好土层,减轻了浅部软弱土层的负担,从而达到控制沉降的目的。     

滨海超深软土刚性桩复合地基工作性能研究

滨海地区深厚软土地基沉降控制是工程建设的关键.本文以珠海航空新城某近海市政道路工程为依托,对超深软土刚性桩复合地基工作性能进行了研究,研究结果表明:(1)采用等效桩墙简化的平面应变模型可以有效模拟刚性桩复合地基的受力及变形;(2)持力层沉降占地基总沉降比例超80％,当边桩桩间距较大时,最大沉降及最大桩-土差异沉降均发生...     

桩网复合地基的模型试验加固效果对比研究

为了探究不同形式的桩网复合地基加固软土的效果,设计了3组复合地基进行室内模型试验.第1组为PVC管材加固软土地基,第2组为桩承式复合地基,第3组为PVC管材与砂桩复合地基.在模型箱钢板内侧壁涂上黄油,并覆以塑料薄膜来减小边界效应；在PVC管材和土工编织网(格栅)上粘贴应变片,来测试PVC桩体应力及加筋拉力；在桩间土、砂...     

桩承地基处理大面积软基的现场测试及分析

为了解桩承地基的工作机理,对深圳机场停机坪大面积桩承地基处理区域的桩顶沉降、桩间土沉降以及孔隙水压力进行现场测试,并对测试结果进行详细的分析和比较。结果表明:桩承地基能够有效地减小地基总沉降和桩土沉降差,桩土沉降差在19～25 mm;路堤填土中存在明显的土拱效应,土拱高度集中在0.83～1.04倍桩净间距,平均为0.94的桩净间距;桩土沉降差与土拱效应发挥程度密切相关,桩土沉降差是产生土拱效应的前提条件。     

多层加筋垫层刚性桩网复合地基的承载特性

为研究多层加筋垫层刚性桩网复合地基的承载特性,将设置有多层土工格栅的加筋垫层视为大挠度薄板进行分析,运用层合板理论,模拟多层土工格栅与碎石垫层之间的相互作用,建立加筋垫层抗弯刚度矩阵的计算方法。考虑刚性桩网复合地基的三维应力和位移边界条件,根据静力平衡条件,建立加筋垫层应力函数和挠度微分控制方程,并利用伽辽金方法进行求解。在此基础上,利用Winkler地基梁理论和大挠度薄板理论对桩土应力比和格栅拉力进行计算。最后,运用实际工程对计算方法进行验证,并综合分析格栅总层数、铺设间隔和位置等因素对桩土应力比及格栅拉力的影响。研究结果表明:理论计算结果与实测结果较为吻合;随着格栅总层数的增大,桩土应力比增大而格栅拉力降低,铺设2～3层格栅效率最高;随着铺设格栅间隔和底层格栅距桩帽距离的增大,桩土应力比降低,而格栅拉力增大。     

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。     

Effect of geogrid reinforcement on soil - structure – pipe interaction in terms of bearing capacity,settlement and stress distribution

In this study, the contribution of single and multiple layers of geogrids to bearing capacity and stress behavior was determined by laboratory experiments. The effects of parameters such as the depth of the first geogrid, the vertical spacing between the geogrid layers and the number of geogrid layers on the bearing capacity and settlement behavior of soil and stress distribution on geogrid and pipe by using strain gauges have been investigated. The results of experiments were given in dimensionless form of bearing capacity ratio (BCR), settlement reduction factor (SRF) and stress capacity ratio (SCR). As a result of experiments, the contribution of the geogrid on the soil-structure-pipe interaction has been observed together with the stress distribution on the geogrid contributed to the efficient use of the appropriate geogrid capacity.     

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%。