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.     

A closed-form solution for column-supported embankments with geosynthetic reinforcement

Soil arching effect results from the non-uniform stiffness in a geosynthetic-reinforced and column-supported embankment system. However, most theoretical models ignore the impact of modulus difference on the calculation of load transfer. In this study, a generalized mathematical model is presented to investigate the soil arching effect, with consideration given to the modulus ratio between columns and the surrounding soil. For simplification, a cylindrical unit cell is drawn to study the deformation compatibility among embankment fills, geosynthetics, columns, and subsoils. A deformed shape function is introduced to describe the relationship between the column and the adjacent soil. The measured data gained from a full-scale test are applied to demonstrate the application of this model. In the parametric study, certain influencing factors, such as column spacing, column length, embankment height, modulus ratio, and tensile strength of geosynthetic reinforcement, are analyzed to investigate the performance of the embankment system. This demonstrates that the inclusion of a geosynthetic reinforcement or enlargement of the modulus ratio can increase the load transfer efficiency. When enhancing the embankment height or applying an additional loading, the height of the load transfer platform tends to be reduced. However, a relatively long column has little impact on the load transfer platform.     

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.     

加载条件对土拱效应影响的Trapdoor模型试验研究

土拱效应是一种土中应力的重分布现象,它是由土与土中结构物间刚度的差异而引起的。目前关于循环荷载对于土拱效应影响的研究十分有限。使用铝棒相似土代替砂土作为填料,通过自制的模型试验装置进行了土体自重、表面静荷载及循环荷载作用下的平面应变Trapdoor模型试验,利用土拱率作为土拱效应强弱的衡量指标,对比研究了不同加载条件对土拱效应的影响,并将试验所得结果与前人的研究成果进行了对比验证。结果表明,静荷载与循环荷载均会削弱已有稳定"土拱",削弱程度随荷载幅值及频率的增大而增大,随荷载作用面积的增大而减小。相同荷载水平下,循环荷载对土拱效应的削弱较静荷载更强,两者间的差异随荷载幅值的增大而减小,随荷载频率的增大而增大,且峰值荷载下的差异小于零荷载下的差异。总体而言,Evans提出的土拱率计算公式能够较好地用于土体自重及静荷载作用下的Trapdoor试验中土拱率的预测,而对于循环荷载作用下的情况,还有待对计算公式进行进一步地改进。     

桩承路堤土拱效应现场试验与分析

桩承路堤在路堤自重作用下形成竖向土拱,并通过土拱传递荷载,其浅层荷载传递机制直接影响到桩土的协调工作和地基的加固效果。为研究桩承路堤中竖向土拱效应的特性,通过设置试验段进行了现场试验,对采集到的应力与变形数据进行了分析处理。研究结果表明,刚性桩支承路堤中存在明显的竖向土拱效应,且在路堤浅层采用粗粒土填料填筑有利于路堤中连续、稳定的土拱的形成,能够起到更好的加固效果。最后提出了一些合理化建议,可供设计与施工人员参考。     

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。     

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.     

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.     

加筋形式对桩承式路堤工作性状影响的试验研究

对无加筋和采用不同加筋材料、加筋层数下桩承式路堤的工作性状进行了三维模型试验研究,侧重分析了桩土应力比、应力折减系数、填土中竖向应力分布、地基沉降等内容。结果表明加筋材料的设置有利于荷载向桩顶的转移,可有效减小沉降,但不同加筋形式下桩承式路堤的工作性状有所不同。使用单层或双层土工布时,路堤的荷载传递机理主要是填土的土拱效应和加筋材料的拉膜效应,但拉膜效应发挥相对较晚。使用双层格栅时,加筋材料与周围砂土形成半刚性平台。单层格栅的作用介于两者之间。试验结果与常规拉膜效应设计方法的对比表明,若假设荷载只由相邻桩间的加筋材料条带承担,计算的拉力将偏大,过于保守。     

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。     

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%。     

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。     

循环荷载作用下土拱效应的离散元研究

基于室内Trapdoor模型试验,采用PFC2D研究了循环荷载作用下不同路堤高度的土拱效应,从力链和位移的角度对路堤内土拱结构、填料移动的变化规律进行了宏观和微观分析。结果表明:抗扭转模型可以较好地模拟以铝棒相似土作为填料的Trapdoor试验; 在循环荷载作用下路堤内形成的土拱结构发生破坏,土拱效应得到削弱,土拱结构的破坏主要发生在初始加载阶段,并且在这个阶段高路堤底部土拱结构比低路堤受到外部荷载的影响要小; 随着加载的进行,路堤内部形成了新的稳定受力结构并基本保持不变; 在循环加载过程中低路堤加载板两侧的力链结构受到的影响和扰动比高路堤的大; 在循环荷载作用下,路堤表面发生了沉降,其中塑性位移主要发生在初始加载阶段,之后产生的几乎是弹性位移; 高路堤加载板两侧土体相较于低路堤在第一次加载时更不容易产生横向位移被挤向两端,加载板的竖向位移减少,从而减少加载板对底部土体的影响,使得路堤底部的土拱结构更不容易被影响。     

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.     

柔性桩承式加筋路堤桩土应力比分析

针对柔性桩承式加筋路堤,建立了路堤–网–桩–土相互协调共同工作的荷载传递模型,通过改进的路堤荷载传递模型和假定的柔性桩侧摩阻力分布模式分析了路堤土拱效应和桩土相互作用,根据平衡条件推导获得了新的可以考虑土拱效应、拉膜效应和桩土相互作用三者耦合条件下桩土应力比及桩土差异沉降计算公式。通过工程实例的分析计算,验证计算模型的合理性,并分析了各因素与桩土应力比的关系。结果表明：网上、下桩土应力比均随路堤填土内摩擦角的增加先增大后减小,随桩体压缩模量、路堤填土压缩模量的增加而增大,随桩间土压缩模量、桩间距的增加而减小,且网下桩土应力比大于网上桩土应力比;网上桩土应力比随土工格栅抗拉强度的增加而减小,网下桩土应力比随土工格栅抗拉强度的增加而增大,网上、下桩土应力比差随土工格栅抗拉强度、路堤填土重度和填土高度的增加而增大。桩土应力比和桩土差异沉降理论计算值与工程实例实测值对应较好。     

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.     

Early-stage anaerobic zone formation by organic eluate from wood in soil

When separated soil (SS) generated from the Great East Japan Earthquake, which contains woodchips and others (WC), is utilized as ground material, microorganisms decompose the organic carbon in WC, and oxygen is consumed in the process. This reaction results in forming an anaerobic zone and generating methane gas. To effectively utilize SS as ground material, it is necessary to prevent anaerobic zone formation to ensure safety. This study clarified the relationship between WC content and the concentration of leached organic carbon (TOC) and the relationship between TOC concentration in the eluate and the oxygen consumption rate were clarified. We obtained the relationship between WC content and the extent of anaerobic zone formation from the results,. The anaerobic zone is formed below the oxygen penetration depth (L_O2). L_O2 rapidly decreases as the WC weight ratio increases from 0 to 1 w%, and L_O2 is almost constant, i.e., around 1 to 2 m above 1 w% WC weight ratio. An increase in WC weight ratio does not significantly decrease L_O2 determined by the aerobic decomposition of the solution. From the above, contamination with WC should be limited to 1 w% or less to prevent the formation of an anaerobic zone. If SS has WC above 1 w%, it is required to put a ventilating layer such as crushed stone every few meters or the other countermeasures when SS is utilized as ground material.     

Microfoundations of project portfolio regulation in joint university-industry laboratories

Joint university-industry laboratories (joint U-I labs) consist of a new, yet little understood model of academic partnership with the firms, typically involving the establishment of a collaborative research unit to conduct research and development (R&D) projects. This type of collaboration faces an ongoing challenge in dealing with the tension between the development and deployment focus in the project portfolio. While several studies have investigated project portfolio management in a given firm, little is known about the interorganizational portfolio management and how the actors manage tensions in joint U-I labs. This study aims to investigate the microfoundations of project portfolio management in joint U-I labs. By combining the multiple-case study and at-home ethnography methods, we identified that the regulation approach explains how partners in joint U-I labs dynamically adjust and manage the project portfolio. Based on the empirical findings, we propose a new framework for project portfolio management in joint U-I labs: portfolio regulation. This framework consists of four processes: dual scouting, dual matchmaking, joint selection, and bridgemaking. These microfoundations enable actors to strategically regulate the portfolio by increasing/decreasing resources related to development or deployment. Our study explores how portfolio regulation evolves within a joint U-I lab through four stages: formation, learning, reconfiguration, and joint portfolio. The proposed framework contributes to interorganizational R&D portfolio management theory and joint U-I lab portfolio management practice.     

Experimental and numerical investigations of the behaviour of footing on geosynthetic reinforced fill slope under cyclic loading

This paper investigates the cyclic loading responses of a strip footing supported by a geosynthetic reinforced fill embankment. A series of large-scale model footing tests were conducted first to investigate the accumulation of permanent footing displacement and residual vertical soil stress over large number of load cycles. The embankment fill was a heavily compacted silty sand and the reinforcement was a flexible geogrid, so that the model test configurations were representative of actual field conditions. Both permanent displacement and residual stress accumulated asymptotically with load cycles and majority of the build-up occurred over the first few hundred cycles. The potential effect of load interruptions was part of the study. Depending on how cyclic load interruption was implemented, it may or may not induce a trailing effect on subsequent cyclic loading responses. To have more in-depth understanding, these footing tests were also investigated numerically based on a soil model that can capture the unload-reload stress-strain loop over large number of load cycles. Reasonably good agreement between experimental observations and numerical predictions was also achieved.