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.     

土工织物散体桩桩体大三轴试验研究

对密实状态的土工织物散体桩桩体进行大三轴试验,研究其在不同围压和不同聚丙烯土工编织布筋材强度下的应力–应变特性,并据此提出桩体强度及模量理论计算公式。研究结果表明:土工织物散体桩桩体在三轴压缩下呈剪胀破坏,其剪切带上主要是筋材的横向筋丝产生断裂,且桩体剪切角与碎石的莫尔–库仑理论破裂角比较接近;在加载初期,土工织物散体桩应力–应变曲线下凹,而后近似线性增长至峰值应力,随后应力随应变减小并趋于稳定,表现为应变软化特性;同一筋材强度、不同围压的土工织物散体桩桩体强度所对应的轴向应变值比较接近;土工织物散体桩桩体的似黏聚力随筋材强度呈较好的线性增长关系,其较碎石的似黏聚力大很多,而筋材对桩体碎石的内摩擦角影响不大;建立了土工织物散体桩桩体强度及模量的理论计算公式,并采用试验值对理论公式进行修正,经修正后的结果与试验值吻合很好。     

Interfacial properties of geocell-reinforced granular soils

To provide an accurate response of Geocells under pull-out conditions such as what happened in retained backfills, interfacial characteristics of Geocell-backfill are required. A series of direct shear tests was carried out to investigate influence of soil physical properties on interfacial properties of Geocell-reinforced granular soils. Variable parameters encompass poorly graded coarse-grained soils with different medium particles sizes (3, 6 and 12?mm), different normal stresses (100, 200 and 300?kPa) and different relative densities (50 and 70%). To compare the developed strength of the shear plane, in unreinforced and Geocell-reinforced statuses, shear characteristics mobilized at the shear plane including friction angle, dilation angle and apparent cohesion have been evaluated. The results indicated improvement of the interface's shear strength characteristics due to the presence of Geocell. The shear strength in the Geocell-soil interface was increased by increasing the medium grain size and relative density of the soil. From the obtained results, for coarse aggregates (cell aspect ratio-ratio of Geocell's cells diameter (b) to the medium grains size (D₅₀)- smaller than 8.5), Geocell reinforcement was two times, at least, more successful than compaction effort, in improving shear characteristics of the unreinforced medium dense fill materials. It has been recommended using Geocells in environments with low normal stress and coarse aggregates. Furthermore, the results clarify that Geocell with cell aspect ratio equal to 4, has the best performance in improvement of interface's shear strength.     

Geosynthetic-encased stone columns: Numerical evaluation

Stone columns (or granular piles) are increasingly being used for ground improvement, particularly for flexible structures such as road embankments, oil storage tanks, etc. When the stone columns are installed in extremely soft soils, the lateral confinement offered by the surrounding soil may not be adequate to form the stone column. Consequently, the stone columns installed in such soils will not be able to develop the required load-bearing capacity. In such soils, the required lateral confinement can be induced by encasing the stone columns with a suitable geosynthetic. The encasement, besides increasing the strength and stiffness of the stone column, prevents the lateral squeezing of stones when the column is installed even in extremely soft soils, thus enabling quicker and more economical installation. This paper investigates the qualitative and quantitative improvement in load capacity of the stone column by encasement through a comprehensive parametric study using the finite element analysis. It is found from the analyses that the encased stone columns have much higher load carrying capacities and undergo lesser compressions and lesser lateral bulging as compared to conventional stone columns. The results have shown that the lateral confining stresses developed in the stone columns are higher with encasement. The encasement at the top portion of the stone column up to twice the diameter of the column is found to be adequate in improving its load carrying capacity. As the stiffness of the encasement increases, the lateral stresses transferred to the surrounding soil are found to decrease. This phenomenon makes the load capacity of encased columns less dependent on the strength of the surrounding soil as compared to the ordinary stone columns.     

Shear strength and failure mechanism of needle-punched geosynthetic clay liner

The internal shear strength of a geosynthetic clay liner (GCL) within composite liner systems is crucial for the stability of landfills and should be carefully considered in the design. To explore the shear strength and failure mechanism of the extensively used needle-punched GCL, a series of displacement-controlled direct shear tests with five normal stress levels (250–1000 kPa) and eight displacement rates (1–200 mm/min) were conducted. The shear stress to horizontal displacement relationships exhibit well-defined peak shear strengths and significant post-peak strength reductions. The monitoring results of the thickness change indicate that the degree of volumetric contraction is related to the reorientation of fibers and dissipation of pore water pressure. Furthermore, the peak and residual shear strengths both depend on the displacement rate because of the rate-dependent tensile stiffness of needle-punched fibers and shear strength of the soil/geosynthetic interface. Through additional tests and lateral comparison, it was discovered that the shear behavior of sodium bentonite, degree of hydration, and pore water pressures all affect the shear mechanisms of the NP GCL. In particular, the failure mode transfers from fiber pullout to fiber rupture with the increase in water content as the hydrated bentonite particles facilitate the stretching of needle-punched fibers.     

Three-dimensional numerical analysis of individual geotextile-encased sand columns with surrounding loose sand

Use of geotextile-encased sand columns (GESC) to improve weak soils is an emerging technology that has great promise for field applications. This paper contains the results of a numerical study with the goal of quantifying the benefits of geotextile encasement under different conditions. A three-dimensional finite difference method implemented in FLAC3D 5.01 was used to evaluate the performance of a vertically loaded individual GESC installed in loose sand. The numerical model was first verified using the results of experimental tests performed on 150-mm diameter GESC installed in loose sand. The influence of various parameters was investigated in this study, including GESC diameter and length, soil thickness, geotextile encasement length, geotextile stiffness, and friction angle and dilation angle of the infill material. The results of the numerical model showed that vertically loaded GESC of smaller diameter experienced less settlement and lateral expansion than those of larger diameter. The geotextile material with higher stiffness had a substantial influence on the performance of GESC. The maximum effective geotextile encasement length depended on the load on the column head or the compressibility of the column.     

Centrifuge modeling of geosynthetic-encased stone column-supported embankment over soft clay

Geosynthetic-encased stone column (GESC) has been proven as an effective alternative to reinforcing soft soils. In this paper, a series of centrifuge model tests were conducted to investigate the performance of GESC-supported embankment over soft clay by varying the stiffness of encasement material. The enhancement in the performance of stone columns encased with geosynthetic materials was quantified by comparing the test with ordinary stone columns (OSCs) under identical test conditions. The test results reveal that by encasing stone columns with geosynthetic material, a significant reduction in the ground settlement, relatively faster dissipation of excess pore pressure and enhanced stress concentration ratio was noticed. Moreover, with the increase in the encasement stiffness from 450 kN/m to 3300 kN/m, the stress concentration ratio increased from 4 to 6.5, which signifies the importance of encasement stiffness. In addition, a relatively lower value of soil arching ratio observed for GESCs compared to OSCs indicate the formation of a relatively strong soil arch in the GESC-supported embankment. Interestingly, under embankment loading, GESCs fail by bending while OSCs fail by bulging. The stress reduction method can be used to calculate the settlement of GESC-supported embankment with larger stress reduction factor than that in the OSC-supported embankment. Finally, the limitation of the construction of the embankment at 1 g was addressed.     

Shaking table tests on geosynthetic encased columns in soft clay

This paper presents the results of an experimental research on the behavior of geosynthetic encased stone columns and ordinary stone columns embedded in soft clay under dynamic base shaking. For this purpose, a novel laminar box is designed and developed to run a total of eight sets of 1-G shaking table tests on four different model soil profiles: Soft clay bed, ordinary stone column installed clay bed, and clay beds with geosynthetic encased columns with two different reinforcement stiffnesses. The geosynthetic encased columns are heavily instrumented with strain rosettes to quantify the reinforcement strains developing under the action of dynamic loads. The responses of the columns are studied through the deformation modes of the encased columns and the magnitude and distribution of reinforcement strains under dynamic loading. The response of the granular inclusion enhanced soft subsoil and embankment soil and the identification of the dynamic soil properties of the entire soil body are also discussed in this article. Finally, to determine the effect of dynamic loading on the vertical load carrying capacity, stress-controlled column load tests are undertaken both on seismically loaded and undisturbed columns.     

Numerical evaluation of a granular column reinforced by geosynthetics using encasement and laminated disks

Structures built on soft strata may experience substantial settlement, large lateral deformation of the soft layer and global or local instability. Granular columns reinforced by geosynthetic materials reduce settlement and increase the bearing capacity of the composite ground. Reinforcement is more common in the form of geosynthetic encasement, but laminated disks can also be used. This paper compares these two forms of reinforcement by means of unit cell finite element analyses. Numerical results were initially validated using field and experimental data, and parametric studies were subsequently performed. The parametric studies varied the geosynthetic interval and the geosynthetic tensile stiffness of the laminated disks as well as the length of the reinforced column. The analyses showed that in both modes; encasement and laminated disks; the geosynthetic increases the vertical stress mobilized on the reinforced column and reduces settlement on soft soil. It was also observed that in order to achieve the same performance as with encased column, the optimum interval between laminated disks is dependent on the stiffness of the geosynthetics and the column reinforced length.     

Performance of geosynthetic-encased stone column-improved soft clay under vertical cyclic loading

This paper presents the results of a laboratory investigation into the performance of geosynthetic-encased stone column-improved (GESC-improved) soft clay under vertical cyclic loading. A reduced-scale model is adopted to perform a series of tests considering the principal parameters, such as the cyclic loading characteristics, including the loading frequency and amplitude, and the encasement length. The results indicate that, among other things, the overall benefit of the geosynthetic encasement of stone columns installed in soft clay is greater under cyclic loading than under static loading, and that the cyclic effect tends to lead to a stress concentration ratio that is smaller than that under static loading. The effectiveness of this encasement in improving the performance of GESCs becomes greater when subjected to cyclic loading with a lower loading frequency and/or a smaller amplitude. The settlement and pore pressure variations with the encasement length, together with the exhumed GESCs taken after the tests, suggest that full encasement is necessary to maximize the performance of GESCs under cyclic loading.     

不同填料土工织物散体桩桩体单轴压缩试验

选用碎石、圆砾和砂3种填料,以及5种不同强度的聚丙烯土工编织布套筒,制备成15组尺寸为?300 mm×600mm,填料压实度?=0.9的土工织物散体桩,对桩体进行单轴压缩试验,以研究不同填料土工织物散体桩在轴向荷载作用下的强度特性。研究结果表明:不同填料桩体在单轴压缩下具有不同的破坏模式,碎石填料局部刺破编织布套筒形成较大破口,圆砾填料致套筒横向筋丝断裂、纵向筋丝分离,而砂填料致套筒横向筋丝断裂较均匀且无明显破口。桩体强度与筋材和填料强度均呈正相关关系,3种填料桩体轴向应力–应变曲线在加载初期因填料受到初始压密而略有上凹,而后近似线性增长至桩体强度,峰值强度后呈现应变软化现象;综合本文试验数据及前期所做的单轴、三轴压缩试验数据,修正了桩体强度理论计算公式,得到的桩体强度修正值与试验值吻合较好。     

Stress-controlled direct shear testing of geosynthetic clay liners I: Apparatus development

The use of geosynthetic clay liners (GCLs) in waste containment applications can induce long-term normal and shear stresses as well as expose GCLs to elevated temperatures and non-standard hydration solutions. Considering the importance of GCL internal shear strength to the design and integrity of waste containment barrier systems, innovative laboratory testing methods are needed to assess shear behavior of GCLs. There were two main objectives of this study: (i) develop a stress-controlled direct shear apparatus capable of testing GCLs exposed to elevated temperatures and hydrated in non-standard solutions; and (ii) assess internal shear behavior of GCLs under varying experimental conditions (e.g., stress, temperature, solution). These two objectives were partitioned into a two-paper set, whereby Part I (this paper) focuses on the shear box design and Part II focuses on an assessment of shear behavior. The direct shear apparatus includes a reaction frame to mitigate specimen rotation that develops from an internal moment within needle-punched reinforced GCLs. Rapid-loading shear tests were conducted to assess functionality of the apparatus and document baseline shear behavior for a heat-treated and a non-heat treated needle-punched GCL with comparable peel strength. These two GCLs failed at comparable applied shear stress; however, the heat-treated GCL yielded lower shear deformation and failure occurred via rupture of reinforcement fiber anchors, whereas the non-heat treated GCL yielded larger shear deformation and failure via pullout of reinforcement fibers.     

Cyclic shear behavior of GMB/GCL composite liner

The composite liner system consisting of geomembrane (GMB) and geosynthetic clay liner (GCL) has been widely used in landfills. Although there have been a lot of studies on the monotonic shear behavior of GMB/GCL composite liner, the dynamic test data are still very limited and consequently, the dynamic shear mechanism is not clear. A series of displacement-controlled cyclic shear tests were conducted to study the shear behavior of GMB/GCL composite liner, including the shear stress versus horizontal displacement relationships, backbone curves, and shear strengths. Hysteretic loops in the shape of parallelogram were obtained and equivalent linear analyses revealed that the secant shear stiffness decreased and the damping ratio increased with the rise in loading cycles. According to the test results, it is generally acceptable to predict the dynamic peak strength of a GMB/GCL composite liner with its static strength envelope. Furthermore, the dynamic softening mechanism and rate-dependent shear stiffnesses were well described by the proposed equations, which also facilitate the accurate modeling of the cyclic shear behavior.     

A numerical modelling technique for geosynthetics validated on a cavity model test

Numerical modelling approaches can aid in designing geotechnical constructions involving geosynthetics. However, the reliability of numerical results depends on how the model is developed, the constitutive model, and the set of parameters used. By comparing the numerical results with experiment, the present work verifies a numerical modelling technique developed to model multilayered geosynthetic lining systems for landfills. The numerical modelling technique involves strain softening at interfaces and allows the axial stiffness of the geosynthetics to evolve as a function of strain. This work focuses on a two-dimensional finite-difference model, which is used to simulate three types of experimental tests: conventional uniaxial tensile tests, direct shear tests, and a large-scale test that was used to assess the overall mechanical behaviour of a reinforced geosynthetic system that spanned over a cavity. This reinforced geosynthetic system consisted of a 50 kN/m polyvinyl alcohol geogrid reinforcement embedded in a layer of sand, a geosynthetic clay liner, a high-density polyethylene geomembrane, and a non-woven needle-punched geotextile. The uniaxial tensile tests, direct shear tests, and the large-scale test were numerically modelled and the numerical results were compared with experimental results. The results of the numerical modelling technique presented very closely match the results of the three experimental tests, which indicates that the numerical model correctly predicted the measured data.     

On the shear failure mode of granular column embedded unit cells subjected to static and cyclic shear loads

Since the initial conception of geosynthetic encased columns (GECs), exhaustion of column capacity due to vertical loads in bulging and punching failure modes were readily recognized. This lead to a vast majority of the available research on GECs to be about the behavior of columns under the action of vertical loads. Recently, two other likely and perhaps more dominant failure modes for granular columns namely, shear and bending failure modes, were identified. The purpose of this paper is to study the behavior of unit cells containing ordinary stone columns (OSCs) and GECs under static and cyclic lateral loads where shear failure of the column is imminent. 1-g physical tests are conducted with a novel apparatus, designated as Unit Cell Shear Device (UCSD), to model the behavior of the unit cells located close to the toe of an embankment where OSCs and GECs experience significant lateral loading. Overall failure envelope and strength parameters for GECs with varying reinforcement stiffnesses are quantified under static and cyclic lateral loading conditions. The distribution and magnitude of reinforcement strains in horizontal (hoop) and vertical direction of the columns are also considered.     

A numerical analysis of a fully penetrated encased granular column

This paper studies the performance of an individual encased granular column that is embedded in soft soil using a numerical analysis. The numerical analysis is verified by experimental tests that are performed in the laboratory, using a model encased sand column that is embedded in a soft clay deposit. In addition to bearing stress-settlement response, detailed characterizations of the encased column, in terms of the distribution of lateral earth and sleeve-induced pressure along the column length, are determined. The numerically analyzed results are compared with those for the model tests and analytical results. Parametric studies over the encasement stiffness, the diameter of the granular column and the loading area are conducted to determine the influence of encasement on the column. The sleeve-induced confining pressure and the bearing stress of the encased sand columns, calculated using the cavity expansion theory and the simplified approach that assumes a constant volume for the granular column, are compared with the numerical results to justify the use of these two methods. The numerical results show that the stiffness of the encasement significantly affects the bulging length of an encased granular column. An increase in the column diameter or the loading area produces a significant reduction in the sleeve-induced confining pressure, which leads to a reduction in the bearing stress improvement of an encased granular column, but the total load supported by the loading plate has an almost linear relationship with the loading plate diameter/column diameter ratio.     

Centrifuge model tests on the deformation behavior of geosynthetic-encased stone column supported embankment under undrained condition

A series of centrifuge model tests were carried out to investigate the performance of geosynthetic-encased stone columns (GESCs) supported embankment under undrained condition. The influence of stiffness of encasement, basal reinforcement and embankment loading on the deformation behavior of GESCs were also assessed. The centrifuge test results reveal that under undrained condition, compared to ordinary stone column (OSC) supported embankment, the settlement of column has reduced by 50% and 34% when columns were encased with high and low stiffness geogrids respectively. Moreover, under identical embankment loading condition, the stress concentration ratio has increased significantly upon inclusion of basal reinforcement in the GESCs supported embankment. In case of OSCs supported embankment, columns experiences bulging in the top portion, inward bending in the central portion and a noticeable shear at the bottom portion. However, when columns were encased with geogrid layer, bulging in the top portion was significantly reduced but the inward bending of columns were noticed. With the inclusion of basal reinforcement, bending curvature of columns increases thereby inducing higher settlement in columns and relatively lesser settlement in surrounding soil. The differential settlement between the encased column and the surrounding soil under embankment loading has been considerably reduced with the inclusion of basal reinforcement.     

竖向土工加筋体对碎石桩承载变形影响的模型试验研究

在碎石桩桩顶一定深度内包裹竖向土工加筋体形成筋箍碎石桩,能有效提高碎石桩的承载能力,控制复合地基沉降量。采用分级加载方式,设计并完成了两组较大比例室内模型试验,对比分析了筋箍碎石桩和传统碎石桩的承载变形特性,进而探讨了筋箍碎石桩的加筋机理和鼓胀变形模式,重点分析了竖向土工加筋体的应力应变特征。分析结果表明：竖向土工加筋体能有效约束碎石桩的侧向鼓胀,在微小侧向变形内提供足够的径向约束应力;筋箍碎石桩的最大鼓胀变形多发生于加筋体以下区域,其破坏模式与筋体材料、桩体、桩周土体及其相互作用和协调变形密切相关;筋箍碎石桩的桩顶和桩底桩土应力比均明显大于传统碎石桩,上部土工加筋体在提高桩体刚度的同时,可有效地将上部荷载传递至桩底较好土层。