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。     

A simplified method for assessing the serviceability performance of geosynthetic reinforced and pile-supported embankment

A simplified method for assessing the serviceability performance of geosynthetic reinforced and pile-supported embankment is presented, where the subsoil consolidation is introduced remaining compatible with the development of soil arching and the reinforcement sag. A piecewise function of the ground reaction curve is developed and used to quantify the arching efficiency. The link between the arching evolution and the subsoil consolidation is then established through the load-carrying equilibrium in the area between piles together with the tensile membrane theory. The reaction of the subsoil is described using the 1-D consolidation theory where the stress history is considered. A parametric study is performed to demonstrate the serviceability performance of a geosynthetic reinforced and pile-supported embankment. The serviceability design of the geosynthetic reinforced and pile-supported embankment is achieved with the proposed method which offers an approach to estimate the time consumed and the subsoil settlement required to achieve a service state.     

A simplified method for analysis of a piled embankment reinforced with geosynthetics

Piled embankments provide an economic solution to the problem of constructing embankments over soft soils. The piles and geosynthetic combination can alleviate the uneven surface settlements that sometimes occur in embankments supported by piles without reinforcement. The main focus of this paper is to present a new method for analysis of an embankment of granular fill on soft ground supported by a rectangular grid of piles and geosynthetic. This method is based on consideration of the arching effect in granular soil and similar to the method proposed by Low, B.K., Tang, S.K., Choa, V. [1994. Arching in piled embankments. Journal of Geotechnical Engineering 120 (11), 1917–1938]. The main refinements are: inclusion of a uniform surcharge load on the embankment fill, individual square caps were used, and taking into account the skin friction mechanism, which contributes to soil–geosynthetic interface resistance. Using this method, the influence of embankment height, soft ground depth, soft ground elastic modulus, and geosynthetic tensile stiffness on efficiency, stress concentration ratio, settlement ratio, tension of geosynthetic, and axial strain of geosynthetic are investigated. The results show that inclusion of a geosynthetic membrane can increase the fill load carried by piles. As a result, both the total and differential settlements of the embankment can be reduced. The new design method was verified against several current design methods. Theoretical solution showed that BS8006 [1995. Code of Practice for Strengthened/Reinforced Soils and other Fills. British Standards Institution, London, p. 162] and Guido, V.A., Kneuppel, J.D., Sweeny, M.A. [1987. Plate loading tests on geogrid-reinforced earth slabs. In: Proceedings of the Geosynthetics '87, New Orleans, USA, IFAI, pp. 216–225] methods overpredict the vertical stress acting on the geosynthetic due to that the reaction of the soft ground on the geosynthetic is not considered in their methods. It also showed that the present method is in good agreement with Low, B.K., Tang, S.K., Choa, V. [1994. Arching in piled embankments. Journal of Geotechnical Engineering 120 (11), 1917–1938] method.     

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.     

Combined effect of PVDs and reinforcement on embankments over rate-sensitive soils

A numerical study of the behavior of geosynthetic-reinforced embankments constructed on soft rate-sensitive soil with and without prefabricated vertical drains (PVDs) is described. The time-dependent stress–strain-strength characteristic of rate-sensitive soil is taken into account using an elasto-viscoplastic constitutive model. The effects of reinforcement stiffness, construction rate, soil viscosity as well as PVD spacing are examined both during and following construction. A sensitivity analysis shows the effect of construction rate and PVD spacing on the short-term and long-term stability of reinforced embankments and the mobilized reinforcement strain. For rate-sensitive soils, the critical period with respect to the stability of the embankment occurs after the end of the construction due to a delayed, creep-induced, build-up of excess pore pressure in the viscous foundation soil. PVDs substantially reduce the effect of creep-induced excess pore pressure, and hence not only allow a faster rate of consolidation but also improve the long-term stability of the reinforced embankment. Furthermore, PVDs work together with geosynthetic reinforcement to minimize the differential settlement and lateral deformation of the foundation. The combined use of the geosynthetic reinforcement and PVDs enhances embankment performance substantially more than the use of either method of soil improvement alone.     

斜坡软弱地基站场路堤加固方案现场试验研究

在斜坡软弱土地区填筑高路堤,需对地基进行加固处理来保证工程安全与控制变形,结合某新建铁路具体工点情况,进行了现场试验测试了地基侧向变形与竖向沉降。试验结果表明:打入桩和抗滑桩均能有效提高地基路堤稳定性、约束地基变形,且打入桩方案约束变形的效果略优于抗滑桩方案;抗滑桩加固时应防止桩间溜滑及桩后土体边坡失稳,必要时与排水固结或复合地基结合使用;打入桩与塑料排水板结合使用时,前者会影响后者的排水效果。     

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.     

Bemessung und Überwachung einer geokunststoffbewehrten Tragschicht auf HSP‐Pfählen

Design and monitoring of a geosynthetic reinforced base‐layer on HSP‐piles. A four‐field fly‐over and the access dam for a double‐track railway line and a bicycle path has been constructed to cross a motorway in a suburb of Rotterdam. The access dam is up to 8.5 m high and was constructed with a slope of 1:2 leading to the preferred small construction width. In spite of the underground conditions with very low shear strength a short‐term traffic initiation as well as low long‐term maintenance cost was demanded. In close collaboration between the contracting authority, the geotechnical consultant CRUX Engineering BV and the construction company Voorbij Funderingstechniek BV a flexible foundations system was developed. Advantages of the chosen system are the possibility top bridge differential settlement, adjust it to the local subsoil conditions and minimise disturbance of the adjacent recreation area. To transfer the self weight and the traffic load into sand layers with higher bearing capacity a combination of Voton HSP‐ piles (high speed piles) and an overlying load distributing geosynthetic stabilised soil layer has been constructed. This innovative combination enables high economic efficiency and an optimised load transfer in the subsoil. A geotechnical monitoring program has been installed to verify that the acting loads on the construction elements are not higher than the allowable loads.     

Geosynthetic reinforced levee test section on soft normally consolidated clays

Reinforced levee test sections have been constructed and monitored in southern Louisiana since the late 1980s. Although the results of these test sections indicated that the anticipated stresses were never realized by the geosynthetic reinforcement, each test section has provided the geotechnical industry with some very important information about the effectiveness of using geosynthetic reinforcement. The most important information acquired from these previous test sections was the fact that the foundation soils experienced significant gains in shear strength due to the more uniform consolidation of the soft foundation material during and immediately following embankment construction. The New Orleans District of the US Army Corps of Engineers (NOD) is researching the viability of a new design methodology that would adequately account for these gains in shear strength resulting from consolidation. This new methodology was derived via the results from the NOD's latest geosynthetic reinforced levee, discussed herein.     

软基上加筋防波堤的离心模型试验

通过离心模型试验，研究在有无土工织物加筋垫层条件下防波堤和软基的变形性状，得到了地基沉降、隆起及水平位移的分布规律。提出了一种量测筋材张力的新方法，对土工织物在离心试验过程中的应力状态进行了测试。研究结果表明：土工织物加筋垫层能有效地减小地基的侧向位移，有一定的加筋效果：土工织物张力的发挥水平与堤坝的沉降量密切相关，工作状态下其最大强度发挥水平约为52％。     

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.     

高填土路堤粉喷桩和土工格栅联合加固性状的数值分析

应用FLAC-3D程序对某大桥28号桥台试验区17 m高的粉喷桩与土工格栅联合加固填土路堤进行了数值分析,计算结果和实测结果的对比表明:联合加固后,路基中心沉降明显减小,但坡角处沉降影响较小,因此,可以采取优化布桩的原则,减少工程成本;联合加固可以限制侧向位移,使得路堤的整体稳定性提高,有利于快速施工,同时加固区内桩身出现应力集中现象;加固区沉降量有明显减小,但下卧层沉降量不降低反而有一定程度增大,因此建议粉喷桩宜穿透软弱层,坐落在持力层上;联合加固后下卧层顶面附加应力有向路基中心集中的趋势;使用粉喷桩处理表层软弱而下层坚实的地基土效果将更为理想。     

Effect of basal reinforcement on performance of floating geosynthetic encased stone column-supported embankment

In this paper, two centrifuge modeling tests were performed to investigate the influence of basal reinforcement on the global performance of floating geosynthetic encased stone column (GESC)-supported embankments. Based on the centrifuge tests, a 3-dimensional (3D) numerical modeling was carried out to investigate the influence of basal reinforcement on the deformation behavior of the floating GESC-supported embankment. The centrifuge and numerical modeling results showed that the basal reinforcement reduced total and uneven settlement at the embankment crest and base significantly. Moreover, the inclusion of the basal reinforcement significantly reduced the lateral displacement on top of the column, preventing outward bending of the floating GESCs below the embankment toe. However, the basal reinforcement increased the lateral displacement at the bottom of columns.     

采用减沉路堤桩处理大面积地面堆载下软土地基的设计与实践

基于20世纪70年代起瑞典、芬兰等北欧国家在软土地区的路堤下广泛采用传统路堤桩,以及20世纪80年代起中国上海等软土地区多层建筑下大量采用沉降控制复合桩基等两方面的工程经验,介绍了一种对大面积地面堆载下软土地基采用减沉路堤桩处理的新技术,该技术具有工期短、造价低和质量容易控制等显著优点。首先研究探讨了减沉路堤桩的一般概况和主要受荷与工作性状,然后提出了减沉路堤桩技术的主要设计步骤与方法,最后简要报导了采用减沉路堤桩处理的上海国际赛车场赛道地基工程实例。     

Load transfer mechanism and deformation of reinforced piled embankments

A series of twenty-eight centrifuge tests was performed on piled embankments with basal geosynthetic reinforcement to assess the influence of pile spacing, embankment height, pile cap size and geosynthetic stiffness on the load transfer mechanism and surface settlements. The measurements of the forces on the piles made it possible to assess the load transfer mechanisms, and 100% efficiency was achieved for all tests performed. The results showed that for the thicker mattress and/or closer piles, the surface settlements were smaller or negligible. Geosynthetic maximum deflections were also examined experimentally and analytically, the latter based on BS8006 (2010) and its further corrigendum in 2012. Close agreement in the predictions of the maximum reinforcement deflection was reached with BS8006 (2012) by adopting a slight modification in the ratio of diagonal and orthogonal maximum deflection (y_d/y = √2).     

斜坡基底软土桩–网复合地基变形特性离心试验研究

为探究斜坡基底软土–桩网复合地基的变形特性,以沿海铁路线某深厚软土地基断面为原型,开展比尺为1∶100室内离心模型试验。通过对试验数据的整理分析,结果表明:(1)路堤顶部两侧的竖向和横向位移、两侧沉降差在运营期都有明显增长,但竖向和横向位移主要产生于路堤施工阶段,而沉降差基本产生于运营期内;(2)加筋垫层对其下方的地基表层横向位移有明显约束。地基右侧土体的竖向和横向位移关于路堤中心不对称,表现为右侧大于左侧,地基表面呈现出明显偏向斜坡外侧的非对称"沉降盆";(3)斜坡桩和悬浮桩的桩顶竖向和横向位移明显大于平台桩,平台桩潜在破坏模式为弯拉或弯剪破坏,斜坡桩为倾斜破坏,悬浮桩为倾斜+横移破坏;(4)桩发生明显上刺和下刺现象,且淤泥质黏土表现出流体特性发生绕桩横向流动。     

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.     

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.     

高速列车荷载下桩承加筋路堤荷载传递机制数值分析

桩承加筋路堤是近来发展起来的软土地基处理技术,具有用桩数量少、节约工程成本、加快工程进度的优点。采用有限元方法研究高速列车荷载下桩承加筋路堤的应力变化规律,分析路堤各部分应力随软土的变形模量、桩弹性模量、格栅弹性模量和桩间距的变化规律。结果表明:上述4个因素对桩中应力、土中应力、桩土应力比、格栅拉应力都有一定的影响。     

路堤荷载下砼芯水泥土搅拌桩复合地基现场试验研究

针对砼芯水泥土搅拌桩复合地基处理高速公路深厚软土段的工程实例,通过地表沉降、分层沉降、深层水平位移、砼芯荷载以及桩土应力比测试,讨论了砼芯水泥土搅拌桩处理深厚软土地基的加固效果,荷载分布和传递规律。测试结果表明：砼芯水泥土搅拌桩对于路基沉降和水平位移控制效果优于水泥土搅拌桩,且横断面差异沉降较小。复合地基的主要压缩量发生在桩顶至砼芯底端一定范围的土体内,沉降发生深度由砼芯控制。砼芯水泥土搅拌桩上部出现负摩阻力,中性点位于砼芯1/3长度处。桩土应力比为水泥土搅拌的2～3倍,与刚性桩相近,桩体承担大部分路堤荷载。砼芯水泥土搅拌桩复合地基排水通畅,超静孔隙水压力消散迅速。路堤荷载下砼芯水泥土搅拌桩工作特性与载荷板试验下的测试结果有所不同。最后从沉降和承载力控制角度给出了砼芯水泥土搅拌桩复合地基的设计方法