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.     

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。     

桩承式路堤土拱数值模拟中路堤土本构模型的选择

数值模拟方法已成为研究桩承式路堤中土拱最重要的手段,其关键在于路堤填土要采用合理的本构模型。建立桩承式路堤平面土拱分析的弹塑性有限元模型,考虑摩尔-库伦模型（MC）、硬化土模型（HS）和小应变硬化土模型（HSS）3种不同的路堤土本构模型,用有限元方法模拟不同路堤土本构模型下桩承式路堤中的土拱形态和土拱效应。计算结果表明：3种不同路堤土本构模型下平面土拱的形态都是半个椭圆。路堤土采用HS和HSS模型,获得的土拱形态、效应和桩帽-土差异沉降相同。较之HS和HSS模型,路堤土采用MC模型时计算得到的桩帽-土差异沉降较小,桩帽荷载分担比略大。当路堤高度较小时,采用MC模型获得的土拱远小于HS和HSS模型下的计算结果。土拱效应的数值模拟中路堤土可采用简单的MC模型,但土拱形态的数值模拟中路堤土宜采用HS模型。     

Effect of geogrid reinforcement on the load transfer in pile-supported embankment under cyclic loading

This paper investigated effects of geogrid reinforcement on the load transfer in pile-supported embankment under cyclic loading using self-moving trapdoor tests. In the self-moving trapdoor test setup, the trapdoor between two stationary portions (which were equivalent to the piles) was supported by compression springs to simulate the subsoil. Quartz sand and a biaxial geogrid were used as the test fill and reinforcement material, respectively. Tests results show that soil arching above the geogrid reinforcement and load transfer to the stationary portions (caused by the soil arching and tensioned membrane effect) experienced a process of “relatively enhancing - relatively degrading” with an increase in the number of cycles and maintained similar degrees within each complete cycle of cyclic loading. Moreover, the inclusion of geogrid reinforcement reduced the mobilization of soil arching, but increased the degree of load transfer to the stationary portions. In addition, cyclic loading with a higher frequency tended to mobilize more soil arching and induce a higher degree of load transfer to stationary portions. Also observed was that a higher frequency cyclic loading tended to decelerate the degradation of load transfer to stationary portions and caused less surface settlement, which indicating increased load-carrying capacity of pile-supported embankment.     

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.     

Large-scale tests of pile-supported earth platform with and without geogrid

In recent years, concrete piles, such as cast-in-place piles and precast concrete piles, have been increasingly used to support superstructures and embankments when they are constructed on soft soils. On the top of pile head elevation, a certain thick granular cushion including geosynthetic reinforcement is usually installed to transfer more external load onto the piles through soil arching effect and membrane effect. This technique involving the use of rigid piles, gravel cushion and geosynthetics is usually referred to as geosynthetic-reinforced and pile-supported earth platform. This paper presents two well-instrumented large-scale tests of pile-supported earth platform with and without geogrid reinforcement. The performance of the pile-supported platform with geogrid and its load transfer behavior were investigated and compared with those for the test without geogrid. The validation of the EBGEO (2010) calculation was performed based on the test results. The test results indicate that under lower applied load, the loads carried by the piles in the test with geogrid were close to those in the test without goegrid, while with an increase in external load the loads carried by piles in the test with geogrid increased faster than those in the test without geogrid. The negative skin friction for the test with geogrid was smaller than that for the test without geogrid. Based on the contours of earth pressures on foundation base the maximum earth pressures were distributed along the edge of central cap in the test with geogrid. The minimum earth pressures were on midway subsoil between two caps in both tests. Based on the test results, the efficacy for the test with geogrid was 2.5% greater than that for the test without geogrid at the end of loading. The efficacies predicted by the EBGEO (2010) calculation agreed well with the measured efficacies.     

中低压缩性土地区桩承式加筋路堤现场试验研究

将桩承式加筋路堤技术应用于中低压缩性土地区高速铁路桥台和涵洞之间填方路基的处理,通过逐渐改变CFG桩桩长形成刚度均匀变化的地基加固区,严格控制线路纵向差异沉降。通过现场试验对桥台、涵顶和路基中心地基沉降进行了长期观测,同时对桩承式加筋路堤桩间土沉降、孔隙水压力、格栅上下表面土压力和格栅变形进行了长期监测分析。研究结果表明:桩承式加筋路堤可有效减小中低压缩性土地基沉降,总沉降小且很快趋于稳定;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帽传递,格栅下桩土应力比明显高于格栅上,张拉膜效应明显,格栅上桩土应力比接近1.0,土拱效应较弱;格栅在路肩处发挥的作用强于线路中心处。     

桩承式路堤土拱效应颗粒流分析

土拱效应是桩承式路堤中荷载传递机制中的关键因素。参照前人室内模型试验,采用颗粒流软件PFC2D建立离散元(DEM)数值模型,对桩承式路堤中的接触力分布、主应力偏转、竖向位移和侧向位移等进行深入分析。模拟中,路堤填料和桩间土采用Disk单元模拟,桩和模型箱采用Wall单元模拟,路堤DEM模型采用分层压实法生成;路堤填料细观参数通过建立数值双轴试验进行标定,桩间土细观参数通过建立数值压缩试验进行标定。模拟结果表明:桩承式路堤中土拱由多个不同圆心的半球形拱共同组成,拱的高度约为5(s-a)/6;在该高度内路堤中的主应力发生明显偏转,竖向位移量和侧向位移量较大。     

桥梁逆作承台–群桩复合基础应用与现场实测研究

为进一步研究逆作复合基础在桥梁工程中的适用性,开展了桥梁逆作承台–群桩复合基础的实桥应用研究,针对有水和无水的施工环境,分别提出了桩与承台间预留孔采用带剪力连接键的钢护筒和收口网模板的构造设计与连接施工工艺,并通过12组接头承载性能试验验证了工艺方案的可靠性。通过现场实测,揭示了桥梁逆作承台–群桩复合基础受力机理,测试结果表明:采用逆作方案,先施工桩基和预留桩孔的承台,待盖梁施工完成后再封桩,使桩与承台形成整体共同参与受力,能够使封桩前承台底土承担上部荷载,有效发挥浅层地基土的承载能力;封桩后,基础沉降增量和承台底土压力增量均明显减缓,继续增加的上部荷载主要由桩基承担,能够有效控制封桩后桥梁沉降。     

筒桩桩承式加筋路堤现场试验研究

为了进一步明确筒桩桩承式加筋路堤的工作机制,在广州绕城高速公路九江—小塘段进行现场试验。试验结果证明,筒桩单桩竖向承载力大,均以刺入方式破坏,并且筒桩单桩复合地基承载力大,沉降小。筒桩桩承式加筋路堤荷载传递机制主要受"土拱效应"和"拉膜效应"控制,桩土应力比随路堤荷载以及桩顶与桩间土之间沉降差的变化而变化。路堤荷载下筒桩复合地基,总沉降小,桩帽上和桩间土上的土体存在沉降差,沉降差的发展可以反映土拱效应的发挥程度。另外,路堤荷载在地基土中产生的超孔隙水压力很小,且随深度迅速减小,地下6.0m处超孔压已接近0。路堤侧向变形小且随深度迅速减小,最大侧向变形发生在地下3.0～4.5m处。     

桩承式加筋路堤土拱及格栅受力模型试验研究

介绍了桩承式加筋路堤足尺模型实验装置,该实验装置利用PVC材料水袋模拟桩间软土,从而在一定程度上能够控制桩土差异沉降。路堤填筑过程中测试了路堤内部土压力以及格栅拉力,并且重点分析了桩帽和桩间不同位置处土压力以及格栅拉力随填筑高度的变化规律。实验结果表明,路堤在填筑过程中发生了明显的土拱效应,路堤填筑完成后桩土应力比约为8.46,土拱高度约为1.125倍桩间净距;单向土工格栅能够进一步将桩间上方土压力传递到桩顶上方;随着路堤填筑高度的增加,格栅拉力增长并不大,路堤横向滑移引起的格栅拉力可以忽略不计。     

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.     

刚性承台下垫层-桩-土相互作用的近似解析法

基于桩基沉降分析中已被广泛接受的剪切位移法,考虑刚性承台下的垫层-桩-土相互作用,提出了带垫层桩体复合地基的一种近似解析方法,分析时假定垫层与承台接触紧密,根据变形协调和荷载平衡条件,可推导出了桩土分担比和基础沉降的显式解答。作者所求得的桩土荷载分担比与边界元解答具有较好的一致性,而在反映基础沉降特性的无量纲刚度上,两者还存在着一定的误差。该方法可以进一步推广用于非均质地基中的群桩沉降分析。     

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。     

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.     

层状地基中单桩-桩帽-土共同作用等效剪切位移法

基于单桩-桩帽-土共同作用的特征,推导了桩和桩帽下土体的荷载传递矩阵,提出了层状地基中带帽单桩的等效剪切位移法.当桩与桩帽下土体以及桩帽下土体与桩帽外侧土体交界面处的位移协调时,分别建立桩与桩帽下土体的平衡微分方程;考虑交界面处的相对滑动时,桩与桩帽下土体交界面处的侧摩阻力较大导致非线性,主要表现在桩与桩帽下土体间的相互滑动;当帽桩较小时,桩帽下土体与桩帽外侧土体交界面处的侧摩阻力引起的桩帽外侧土体自由位移场比较小.与有限元法、已有理论分析方法以及模型试验的对比表明桩帽的加入可明显增大带帽桩的整体刚度,并且一定尺寸的桩帽下桩体长径比的增大不会无限增大带帽单桩的整体刚度,桩帽始终贡献一定的荷载分担比例.     

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。     

黄土塬地区大直径长桩承载性状与优化设计研究

 黄土塬地区桩基问题研究匮乏,依托陇东首栋超高层建筑,在试验桩身上布置混凝土应变计、钢筋应力计,承台底板下和桩端布置土压力盒,对原地基土、单桩基础和单桩承台基础分别进行现场原位载荷试验;利用ANSYS有限元软件对全短桩基、全长桩基及长短桩组合桩基在竖向荷载作用下的筏板沉降变形、地基土应力场与沉降变化进行分析。结果发现：(1) 黄土塬场地地基土夹层交互分布、湿陷性不连续,存在由非湿陷性黄土变成湿陷性黄土的可能,桩周土层对桩基内力传递与分布影响显著,桩身出现多个中性点,湿陷性土层下限深度确定更加复杂;(2) 各级荷载作用下,桩基Q-S曲线呈缓变型发展,表现为典型的摩擦型桩,桩身内力发挥具有异步性;试验加载至8 000 kN时,桩顶最大沉降为8.15 mm,单桩和单桩承台端阻力分别仅占桩顶荷载的4.8%和2.1%;(3) 单桩承台基础中承台底部实测反力呈倒“盆”形分布、边缘应力较大,桩–土–承台体系的承载性能优于单桩基础;桩基础设计时,可结合经验以承载力和最大允许变形量进行控制,提高桩身线刚度抵抗自身压缩变形,减小桩基上部沉降;(4) 长短桩组合桩基础充分利用与发挥了长桩控制沉降的作用与地基土浅层承载的能力,减少了长桩数量,节省了桩基造价,值得进一步深入研究。     

A simplified model to analyze the reinforced piled embankments

It is an economic way to use the piled embankment for the construction of embankment over soft soil. The combination of piles and reinforcement can effectively reduce the differential settlement at the surface of embankment. The paper presents a simplified model for analysis of an embankment of granular fill on soft ground supported by reinforcement and piles. This model is based on consideration of the arching effect in granular material proposed by Hewlett & Randolph. The vertical equilibrium of the unit body at the center of pile caps immediately below the reinforcement is established. The refinements of the model are that the failure mechanisms of the arch both at the crown and at the pile cap were considered, three-dimensional situation was taken into account for reinforced piled embankment, calculation of the vertical stress carried by the subsoil due to arching effect and reinforcement for multi-layered soil was proposed. Using the simplified model, the influence of embankment height, one-dimensional compression modulus of subsoil, tensile stiffness of reinforcement on stress reduction ratio (SRR) and tensile force of reinforcement is investigated. It is found that the model can be used to assess the relative contribution of the reinforcement and subsoil. The results show that subsoil gives a major contribution to overall vertical equilibrium, while the reinforcement gives obvious contribution at relatively large settlement. The inclusion of the reinforcement can reduce the vertical stress acting on the subsoil. The simplified model is then evaluated by three case studies. The results of this model show good consistence with these cases.     

Mehrlagig mit Geogittern bewehrte Erdkörper über pfahlartigen Gründungselementen – Numerische Simulation des Verformungsverhaltens unter statischer und zyklischer Einwirkung

Numerical simulation of the deformation behaviour of multi‐layered geogrid‐reinforced embankments on pile foundations under static and cyclic loading. Embankments for traffic constructions above soft soil are often founded on piles and geogrids are inserted at the bottom of the embankment. In the framework of present design procedures the cyclic (dynamic) traffic loads are considered in a very simplified manner. They are replaced by a static load with a magnification factor. The established model perception for static loading is a redistribution of stress due to arches in the embankment and tensile stress in the geogrids. However it has to be expected that the load bearing and deformation behaviour of such soil structures will change during the life time of the structure (millions of cycles). The cycles cause an accumulation of deformations and changes of stresses in the soil. This may cause a large destruction of the arches and may lead to unexpected settlements. Numerical strategies and constitutive models for the investigation of the behaviour of soils under high‐cyclic loading using finite element method were recently developed. This paper presents the results of such calculations of multi‐layered geogrid‐reinforced embankments on soft soil for the 2D case. The results show that, depending on the position of the geogrids in the embankment, their contribution is unequally to the bearing behaviour and that the stress arches will actually be destroyed under cyclic loading.