Effect of reinforcement force distribution on stability of embankments

The effect of reinforcement force distribution on the stability of reinforced embankments is studied. The stability of reinforced embankments is analyzed using the extended generalized method of slices by incorporating the effect of reinforcement. The proposed method is capable of handling features such as a tension crack in the embankment, a varying soil strength profile in the foundation soil and a general slip surface. The method allows the tensile force distribution along the reinforcement to be varied. The stability analysis of reinforced embankments is solved using a spreadsheet optimization tool. The versatility of the proposed method is demonstrated through several cases of reinforced embankments. The results obtained from the proposed method are in good agreement with those obtained from other analytical or numerical methods. The assumed force distribution along the reinforcement appears not to affect the embankment stability in undrained condition. In drained condition, it has some effect on the location of the critical slip surface, but small effect on the factor of safety.     

Experimental study of embankments with different reinforcement materials and spacing between layers

Worldwide, waste tires are being discarded in landfills at a huge environmental cost, therefore, their use as a three-dimensional reinforcement material is a wise solution to reduce their environmental impact, and fire risk in the case of shredded tires. In this research a series of experimental model tests of embankments reinforced with Geocell and tires were conducted to compare the performance of these types of reinforcement. The models tested had different Geocell embedment depths, number of Geocell layers, vertical spacing between Geocell layers and density or soil stiffness. Testing consisted of applying pressure at the crest of the embankment and monitoring the pressure distribution, as well as the vertical and horizontal deformations inside of the embankment. The results suggested that when compared with unreinforced embankments, reinforced embankments effectively improve the bearing capacity, thereby, reducing vertical and lateral displacements. This study also showed that an optimal embedment depth and spacing between Geocell reinforcement layers can further improve the slope performance. Comparisons between Geocell reinforced embankments and waste tire reinforced embankments, showed that waste tire reinforcement has a superior performance over the Geocell-reinforced embankments. This difference in performance between the two types of reinforcement is more apparent if the embankment backfill has lower stiffness. i.e. lower density.     

Required unfactored strength of geosynthetics in reinforced 3D slopes

The design of reinforced earth structures uses idealized two-dimensional (2D) geometry – classifying as a plane-strain analysis. This 2D idealization greatly simplifies design by ignoring stabilizing effects posed by three-dimensional (3D) characteristics. While the outcome of this 2D idealization is conservative in terms of required reinforcement strength, ignoring 3D end effects in back-calculations of experimental and field data may overestimate the contribution of the reinforcement to stability thus possibly leading to unconservative learned lessons related to design. The objective of this study is to explore 3D effects on the required strength of reinforcement in geosynthetic-reinforced earth structures (GRESs) using a modified 3D limit equilibrium (LE) slope stability analysis. To determine the stability of GRESs, a rotational, 3D failure mechanism, derived from variational LE analysis, is applied using a log-spiral surface generalized to 3D conditions. In order to determine the long-term strength of geosynthetics required to ensure sufficient internal stability, the moment equilibrium approach is applied and its respective equations solved. In order to conveniently assess the end effects on the required total strength of reinforcement and the volume of failing mass considering the feasible length of potential failure, a series of design charts are presented. These charts can also be useful in forensic studies when back-calculating the in-situ mobilized strength of the geosynthetic for 3D failures. The impact of seismicity and the assumed function of forces distributed amongst the reinforcement layers were investigated to highlight their importance. To keep this study focused on 3D end effects, this study is limited to a simple 3D GRES problem; however, extending the present framework to deal with complex homogenous problems is straightforward.     

A reinforcing method for steep clay slopes using a non-woven geotextile

The effect of non-woven geotextile reinforcement on the stability and deformation of two clay test embankments is examined based on their performance for about 3 years for the first embankment and about years for the other. Horizontal planar sheets of a non-woven geotextile are expected to work in three ways: for compaction control; for drainage; for tensile reinforcement. The degree of stability of the steep slopes of the test embankments decreased during heavy rainfall. It is found that the use of non-woven geotextile reinforcement may effectively improve embankment performance. Only the stability analysis in terms of effective stresses can explain the performance of the test embankments. The horizontal creep deformation of the embankments during 2–3 years, which is partly attributed to the creep deformation of the non-woven geotextile, was found to be small. The results of both laboratory bearing capacity tests of a strip footing on a model sand ground reinforced with the non-woven geotextile and plane strain compression tests on sand specimens reinforced with the non-woven geotextile show that the non-woven geotextile gives tensile reinforcement to soils.     

A comprehensive method for analyzing the effect of geotextile layers on embankment stability

Commercial software is used widely in slope stability analyses of reinforced embankments. Almost all of these programs consider the tensile strength of geotextiles and soil–geotextile interface friction. However, currently available commercial software generally does not consider the drainage function of nonwoven geotextile reinforcement. In this paper, a reinforced channel embankment reinforced by a nonwoven geotextile is analyzed using two methods. The first method only considers the tensile strength and soil–geotextile interface friction. The second method also considers the drainage function. In both cases, the reinforced embankment is modeled in rapid drawdown condition since this is one of the most important conditions with regard to stability of channel embankments. It is shown that for this type of application, modeling a nonwoven geotextile reinforced embankment using commercial software which neglects the drainage function of the geotextile may be unrealistic.     

Analysis of back-to-back mechanically stabilized earth walls

Back-to-back Mechanically Stabilized Earth (MSE) walls are commonly used for embankments approaching bridges. However, available design guidelines for this wall system are limited. The distance between two opposing walls is a key parameter used for determining the analysis methods in FHWA Guidelines. Two extreme cases are identified: (1) reinforcements from both sides meet in the middle or overlap, and (2) the walls are far apart, independent of each other. However, existing design methodologies do not provide a clear and justified answer how the required tensile strength of reinforcement and the external stability change with respect to the distance of the back-to-back walls. The focus of this paper is to investigate the effect of the wall width to height ratio on internal and external stability of MSE walls under static conditions. Finite difference method incorporated in the FLAC software and limit equilibrium method (i.e., the Bishop simplified method) in the ReSSA software were used for this analysis. Parametric studies were carried out by varying two important parameters, i.e., the wall width to height ratio and the quality of backfill material, to investigate their effects on the critical failure surface, the required tensile strength of reinforcement, and the lateral earth pressure behind the reinforced zone. The effect of the connection of reinforcements in the middle, when back-to-back walls are close, was also investigated.     

3D effects of turning corner on stability of geosynthetic-reinforced soil structures

Current design procedures of Geosynthetic-Reinforced Soil Structures (GRSS's) are for walls/slopes with long straight alignments. When two GRSS segments intersect, an abrupt change in the alignment forms a turning corner. Experience indicate potential instability problems occurring at corners. The purpose of this study is to explore the effects of turning corner on the stability of reinforced slopes. Three-dimensional (3D) slope stability analysis, based on limit equilibrium, resulted in the maximum tensile force of reinforcement. Parametric studies required numerous computations considering various geometrical parameters and material properties. The computed results produced efficient practical format of stability charts. For long-term stability of reinforced slopes with turning corner, the influences of pore water pressure and seismic loading are also considered. Turning corner can improve the stability of reinforced slopes by virtue of inclusion of end effects. However, localized increase of pore water pressure or directional seismic amplification may decrease locally thus stability requiring strength of reinforcement larger than in two-dimensional (2D) plane-strain. While using 2D analysis for non-localized conditions may require stronger reinforcement, it also requires shorter reinforcement than in 3D analysis; i.e., 2D analysis may be unconservative in terms of reinforcement length.     

Comparison of predicted and observed behaviour of two test embankments

The results of finite element analyses of two test embankments, one geotextile reinforced the other a blank control section, are compared with field observations and the results of a simple limit equilibrium analysis. The results of both types of analyses indicate good agreement between predicted and observed failure heights. The geotextile reinforcement was found to significantly increase embankment stability for this case. The finite element analysis provided useful information concerning the development of plastic failure within the soil and the deformations which occurred as tension developed in the reinforcement. It is suggested that this information, which is not provided by simple limit equilibrium analysis, may be essential to the safe design of reinforced embankments.     

Long-term reinforcement strains for column supported embankments with viscous reinforcement by FEM

The time-dependent performance of deep-mixing-method column supported embankments reinforced by viscous reinforcement is investigated for different long-term (at 99% degree of consolidation) reinforcement strains using fully coupled three-dimensional finite element method. The influence of long-term reinforcement strains on long-term net embankment height, maximum crest settlement, maximum differential settlement at the crest and horizontal toe movement of embankments numerically constructed over two soft foundations is explored with the consideration of viscosity of two reinforcement products. Based on a series of numerical simulations, an approach to controlling the deformations of column supported embankments to modest levels while maximizing their long-term service heights is proposed. Also, a correlation between long-term reinforcement strain and end-of-consolidation reinforcement strain is suggested for the studied cases.     

土工格栅加筋边坡优化设计研究

基于加筋材料的拉拔试验结果和极限平衡理论,针对具体边坡工程进行了不同加筋方案的计算与分析,对比了计算模型和设计方法的适用性,给出了满足边坡稳定条件的最佳设计方案。计算结果表明:采用改进瑞典法或荷兰法的计算结果相近且较原瑞典法有明显的提高,更能体现加筋效果;地震效应和地下水对加筋结构有较大影响;水利法应用于稳定地基上加筋边坡目的性强,能获得满足稳定性条件的合理布筋量;当地下水位较高时,筋材宜通铺。双层加筋效果较单层加筋有明显提高,但并非后者的简单叠加。单层加筋时,铺设位置对于边坡稳定性的影响有限,若铺设于坡身更能减少布筋量,降低造价。对比分析还表明,无论采用何种加筋方式,加筋前后的最危险滑弧位置均会发生改变,后者会向边坡中心和地基深处发展,对于提高其稳定性有明显作用。     

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.     

轮胎与格室加筋路堤性能及承载力研究

为研究废旧轮胎与土工格室加筋路堤边坡的性能,分别对废旧轮胎、土工格室加筋路堤边坡开展了室内模型试验,并考虑了填料两种不同相对密度的影响。试验结果表明:相对素土路堤而言,废旧轮胎和土工格室加筋路堤均能有效地提高承载力,增强其稳定性,减小不均匀沉降。加筋后均有效地增大了附加应力的扩散角,使得附加应力分布更为均匀,并且素土路堤与加筋路堤中轴线上附加应力差值随路堤深度增大而减小。中轴线以外的质点侧向位移随路堤深度的增加,呈现出先增大后减小的趋势,几种路堤中,废旧轮胎加筋路堤侧向位移最小。加筋效果随相对密度增大而减小,在低相对密度条件下,加筋后承载力能达到素土路堤2倍以上,而在高相对密度下却不足2倍。最后根据土工格室加筋地基承载力计算方法及对废旧轮胎加筋机理分析,提出了关于废旧轮胎加筋地基承载力计算方法。     

Corner reinforced slopes: Required strength and length of reinforcement based on internal stability

This paper develops an analysis procedure for turning corner in Geosynthetic-Reinforced Soil Structures (GRSS's). The procedure includes the calculations of the required strength and length of the reinforcement for internal stability. The calculations are based on the variational limit equilibrium analysis of three-dimensional (3D) stability of slopes. Seismic effects are also considered using the pseudo-static method. Results are presented in a condensed form of design charts, providing a simple tool to determine the required tensile strength and embedment length of the reinforcement. Two examples are given to demonstrate the use of the design charts. Compared with the conventional design based on plane-strain analysis, the presented design procedure yields longer reinforcement for the 3D internal stability of the corners. Generally, 3D design requires longer reinforcement than 2D as the seismic acceleration increases. The trend of obtained result is in good agreement with performance observations related to corners reported in commentary of AASHTO.     

Required strength of geosynthetics for reinforced 3D slopes in cohesive backfills with tensile strength cut-off

In extant studies, most of the stability analyses of geosynthetic-reinforced slopes focused on two-dimensional conditions using the Mohr-Coulomb (M-C) failure criterion to describe the strength of backfills. However, in reality, all failures of slopes indicate a somewhat three-dimensional (3D) feature, and the M-C criterion is observed to overestimate the tensile strength of cohesive soils. To partially remedy this shortcoming, the concept of tensile strength cut-off is adopted to include the actual tensile strength of backfills in the yield envelope, and a kinematic approach is presented to evaluate the required strength of geosynthetics for 3D reinforced slopes in cohesive backfills. A 3D rotational mechanism of collapse that is associated with the strength envelope with tension cut-off is developed. The amount of required reinforcement is evaluated and listed as a dimensionless coefficient. The results indicate that the inclusion of the 3D effect and soil cohesion can lead to substantial savings in terms of the reinforcement to be made. In addition, a higher amount of reinforcement is required when the effect of tension cut-off is considered; this effect is more distinct for backfill with a higher amount of cohesion.     

Experimental studies of the geosynthetic anchorage – Effect of geometric parameters and efficiency of anchorages

The soil reinforcement by geosynthetic is widely used in civil engineering structures: embankments on compressible soil, slope on stable foundations, embankments on cavities and retaining structures. The stability of these structures specially depends on the efficiency of the anchors holding the geosynthetic sheets. Simple run-out and wrap around anchorages are two most commonly used approaches. In order to improve the available knowledge of the anchorage system behaviour, experimental studies were carried out. This paper focuses on a three-dimensional physical modelling of the geosynthetics behaviour for two types of anchors (simple run-out and wrap around). The pull-out tests were performed with an anchorage bench under laboratory controlled conditions with three types of geosynthetic (two geotextiles and one geogrid) and in the presence of two types of soil (gravel and sand).The results show that there is an optimum length for the upper part of the geosynthetic for the wrap around anchorage.     

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.     

Consolidation of soft clay foundation improved by geosynthetic-reinforced granular columns: Numerical evaluation

Soft clays are problematic soils as they present high compressibility and low shear strength. There are several methods for improving in situ conditions of soft clays. Based on the geotechnical problem's geometry and characteristics, the in situ conditions may require reinforcement to restrain instability and construction settlements. Granular columns reinforced by geosynthetic material are widely used to reduce settlements of embankments on soft clays. They also accelerate the consolidation rate by reducing the drainage path's length and increasing the foundation soil's bearing capacity. In this study, the performance of encased and layered granular columns in soft clay is investigated and discussed. The numerical results show the significance of geosynthetic stiffness and the column length on the embankment settlements. Furthermore, the results show that granular columns may play an important role in dissipating the excess pore water pressures and accelerating the consolidation settlements of embankments on soft clays.     

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％。     

Mechanical behavior of embankments overlying on loose subgrade stabilized by deep mixed columns

Deep mixed column(DMC) is known as one of the effective methods for stabilizing the natural earth beneath road or railway embankments to control stability and settlements under traffic loads.The load distribution mechanism of embankment overlying on loose subgrades stabilized with DMCs considerably depends on the columns' mechanical and geometrical specifications.The present study uses the laboratory investigation to understand the behavior of embankments lying on loose sandy subgrade in three different conditions:(1) subgrade without reinforcement.(2) subgrade reinforced with DMCs in a triangular pattern and horizontal plan,and(3) subgrade reinforced with DMCs in a square pattern and horizontal plan.For this purpose,by adopting the scale factor of 1:10.a reference embankment with20 cm height,250 cm length,and 93%maximum dry density achieved in standard Proctor compaction test was constructed over a 70 cm thick loose sandy bed with the relative density of 50%in a loading chamber,and its load-displacement behavior was evaluated until the failure occurred.In the next two tests.DMCs(with 10 cm diameter.40 cm length,and 25 cm center-to-center spacing) were placed in groups in two different patterns(square and triangular) in the same sandy bed beneath the embankment and.consequently,the embankments were constructed over the reinforced subgrades and gradually loaded until the failure happened.In all the three tests,the load-displacement behaviors of the embankment and the selected DMCs were instrumented for monitoring purpose.The obtained results implied 64%increase in failure load and 40%decrease in embankment crest settlement when using the square pattern of DMCs compared with those of the reference embankment,while these values were 63%and 12%,respectively,for DMCs in triangular pattern.This confirmed generally better performance of DMCs with a triangular pattern.     

桩承式加筋路堤三维动力流固耦合分析

为了研究交通荷载作用下桩承式加筋路堤的动力特性,采用FLAC 3D软件建立了路堤的三维动力流固耦合分析模型,对无筋无桩、有筋无桩、无筋有桩、有筋有桩4种情况的路堤在动荷载作用下的竖向位移、水平位移、桩土应力比、超孔隙水压力、加速度等进行了计算分析,对比研究了4种情况下各自的特点,揭示了桩承式加筋路堤的作用机制.数值分析...