Geosynthetic Sheet Research Articles

Experimental study on the L-shaped anchorage capacity of the geogrid by the pullout test

The soil reinforcement by geosynthetics has been extensively applied in covers and liners of landfills. The stability of this structure is especially dependent on the effectiveness of the anchorages holding the geosynthetic sheets. The simple run-out and L-shaped anchorages are the two most commonly used approaches. For increasing the available knowledge of the anchorage system behavior, experimental studies have been conducted. This paper shows the results of the experimental analysis that are based on the results of large-scale pullout apparatus on geogrid embedded in simple run-out and L-shaped anchorage in two modes (fixed length and fixed space). The influence of different geometric parameters of the trench on the behavior of the geogrid is also examined. Based on the results, the values of pullout force were approximately 69% and 196% higher in the case of the fixed length mode and the fixed space mode, in the respective order, compared to the simple run-out anchorage. In the L-shaped anchorage, it is observed that for the initial length (L) constant, the mode is optimized with a small value of the geogrid rear heel length (B) and a greater value of the depth of the buried geogrid (D) when D+B is constant.

The Search for Internal Stability in Reinforced Soil

Professor Jonathan Wu’s career was devoted to rigorous development of internal stability in reinforced soil as a new and evolving technology. This paper presents two of Professor Wu’s students’ work on the search for internal stability by tracing his work. Engineering design of reinforced soil originated in France with Schlosser and Vidal in the late 1960s. The first designs were based on a simple limit state method. Analyzing test data, Schlosser later found that the limit method was insufficient and that internal stability involves stiffness. Wu found that stiffness was affected by reinforcement spacing. Settlement or collapse is shown in examples that violate this spacing criterion. Initial analysis and validation involved geosynthetic sheets, but these were extended to other materials and to strip reinforcement. The analysis has been validated both by laboratory data and by field observations. These findings corroborate those of international companies in the reinforced soil community, but they are often incompatible with code in North America. The studies and discussions presented in this paper are further enhanced with the most current testing and work to confirm Professor’s Wu insights and visions.

Subsidence prediction of reinforced soil layer by geosynthetic using large-scale 1g physical model

Abstract. Subsidence can result from the collapse of underground cavities. The impact of the movements on existing structures and infrastructures is generally dramatic. Geosynthetic sheets (RG) can be used to reduce their impact. This paper focuses on the use of large-scale physical modelling to study the subsidence mechanisms and to estimate the RG efficiency for cohesive or granular soil layers. The results have shown that the RG reduced the ground movement even under important overload. The deformation of the RG and the surface settlement depend both on the soil type and overload intensity. The experimental results are compared to analytical solutions proposed to design the RG for cohesive and granular soils. Stress distribution on the RG was investigated. The analytical and experimental results are rather similar, that shows the relevance of the analytical models to predict the behaviour of reinforced soil layers.

Experimental studies on inclined pullout behaviour of geosynthetic sheet Vis-À-Vis geogrid - Effect of type of anchor and sand

This paper presents the details of an experimental investigation using large-scale inclined pullout apparatus on sheet geosynthetic and geogrid embedded in run-out, I-type, and L-type anchors. The influence of the type of sand on the behaviour of the sheet and the geogrid is also investigated. The results show that in both the sheet and the geogrid, I-type anchor provides approximately 50% and L-type anchor provides 90% higher pullout force than the run-out anchor. The maximum pullout force increases by more than 20% as the inclination of pullout force increases from 0° to 30° for both the sheet and the geogrid.

Experimental study on geosynthetic-reinforced sand fill over marine clay with or without deep cement mixed soil columns under different loadings

Geosynthetics and deep cement mixed (DCM) soil columns have been widely used to improve soft soil grounds in many countries and regions. This paper presents an experimental study on a geosynthetic-reinforced sand fill over marine clay with or without DCM columns under different loadings. Two tests were conducted on the sand fill reinforced with fixed-end and free-end geosynthetics over marine clay under three-stage local loading to investigate the effects of the boundary conditions of geosynthetic reinforcement on reducing settlements. It is observed that the fixed-end geosynthetic sheet is more effective in reducing settlements than the free-end condition under identical local loading. Another test was conducted on the fixed-end geosynthetic-reinforced sand fill over the marine clay improved by DCM columns under single-stage uniform loading. The vertical stresses on the marine clay and on the DCM columns, as well as the tensile strains of the geosynthetic sheet in the overlying sand fill, were measured. The results revealed that the stress concentration ratio increases with an increase in consolidation settlements, and the maximum tensile strain of the geosynthetic sheet occurs near the edge rather than at the center of the top surface of the DCM columns.

Solution for Low Geosynthetic-Reinforced Embankments Subjected to Localized Sinkholes

Vertical earth pressure on the geosynthetic-soil interface is the key to estimate behaviors of geosynthetic sheets and surface settlement of low geosynthetic-reinforced embankments. New calculation methods of embankment backfills and vertical earth pressures on geosynthetic sheets considering static equilibrium are proposed. According to reasonable boundary conditions, a new approach for three behaviors of geosynthetics subjected to linear localized sinkholes is derived. It considers load transfer of embankments over voids, deflection of geosynthetic sheets over voids, and horizontal displacement of geosynthetic sheets in anchorage zones. Moreover, the values predicted by this study and other studies are compared with the results of the existing tests and finite element analyses. The comparisons validate the reliability and feasibility of the new improved approach.

Evaluation of tensile load model accuracy for PET strap MSE walls

There are conflicting opinions regarding whether or not mechanically stabilized earth (MSE) walls constructed with polyester (PET) straps should be classified as relatively inextensible type, and thus fall into the same category as steel reinforced soil walls, or whether they should be viewed as relatively extensible and grouped with MSE walls that are designed with geosynthetic sheet reinforcement such as geogrids and geotextiles. The differences in predicted loads using models associated with each category can be significant. The writers collected 71 measurements from eight different instrumented PET strap wall projects. The data showed that these PET strap walls behaved more like extensible geosynthetic walls constructed with geogrids and geotextiles (or at least at the stiffer end of these walls) rather than MSE walls with relatively inextensible (steel) reinforcement. The accuracy of the Coherent Gravity Method, the Simplified Method and the Simplified Stiffness Method was assessed statistically using analysis of bias values, where bias is the ratio of measured to predicted load. The Coherent Gravity Method and the Simplified Method resulted in conservative (safe) estimates of reinforcement loads under operational conditions on average. However, the Simplified Stiffness Method was the most accurate and did not result in an excessive level of conservativeness.

Soil Reinforcement with Geosynthetic for Localized Subsidence Problems: Experimental and Analytical Analysis

This study focuses on the evaluation of the load transfer mechanism and the determination of the shape of the load distribution transmitted to the geosynthetic layer when a cavity appears under granular and cohesive backfills. Trapdoor experimental technique with high-speed acquisition of digital images and continuous monitoring of load and displacements were used for this purpose. Testing different soil types, it has been demonstrated that an approximate parabolic or inverted triangular load distribution seems acceptable for granular soil layer, whereas for cohesive backfill the load distribution could be approximately represented by two concentrated forces near the edges of the cavity. In both cases, an important overload on the soil surface could change the shape of the load acting on the geosynthetic sheet. Experimental results are then approached by Terzaghi’s formulation with an appropriate shape of load distribution and a convenient value of the earth pressure coefficient. Finally, recommendations are proposed to promote a better design of such reinforcement structures.

Investigation of load transfer mechanisms in granular platforms reinforced by geosynthetics above cavities

Geosynthetic-reinforced soils constitute an interesting solution for bridging cavities. Many methods have been developed to analyze the stability of soil-geosynthetic-cavity systems, but none of them is able to take into account all the complexities of these mechanisms. Many researchers have assumed mechanisms developed in the reinforced granular platform when cavities appear, such as load transfer and expansion of materials. However, they are not fully understood because many factors can influence the design, such as the cavity opening processes, the type, and the density of the soil.In this study, a new laboratory apparatus is developed to simulate two different cavity opening procedures (trapdoor and progressive opening) for different geometric configurations. A series of tests is conducted for three granular soils with two different geosynthetic sheets. By measuring the shape of the surface soil settlement and the geosynthetic deflection, the expansion coefficient is calculated. A novel tactile pressure sensor is used to observe the load transfer during the cavity opening. The experimental data are analyzed and the influence of the experimental conditions (geometric and soil properties and the opening procedure) are also discussed. Correspondingly, elicited findings can be used to propose recommendations to improve the existing design methods.

Stability of geosynthetic reinforced embankments over stone column‐improved soft soil

The stability of embankments on soft soils is a common problem; the reinforcement ofthis type of soil by a geosynthetic sheet above stone columns is a recent technique, for anexcellent distribution of the vertical stresses on the stone columns and the soft soils. This paperfocuses on the numerical analysis of a geosynthetic‐reinforced embankment over stone column‐improved soft soil. The finite difference code Fast Lagrangian Analysis of Continua in 3 Dimensions(FLAC3D) was used to determine the optimal placement of the geosynthetic sheet. Theembankment fill, the subsoil, and the stone columns were modeled by an elasto‐plastic model witha Mohr‐Coulomb yield criterion and associated flow rule. In this study, several geometrical andmechanical parameters have been considered, such as the stiffness modulus, the placement ofgeosynthetic sheet, and the internal friction angle of the embankment fill. The factor of safetyobtained by 3D numerical method was compared with those obtained by 2D numerical methods.The results of this study show the effect of the placement and stiffness of the geosynthetic sheeton the factor of safety.

Load transfer mechanisms in geotextile-reinforced embankments overlying voids: Numerical approach and design

A numerical model was used to investigate the mechanical behaviour of granular embankments reinforced by geosynthetics in areas prone to subsidence and to overcome the shortcomings of the current design methods. The ability of the numerical model to consider the load transfer mechanisms and the deflection of the geosynthetic was established by comparison with experimental data. By testing two numerical processes, it was demonstrated that the cavity opening modes have a great influence on the shape of the load distribution transmitted to the geosynthetic sheet above the cavity and on the expansion mechanisms of the soil. An approximate conical load distribution seems well adapted when considering a progressive cavity diameter opening process, whereas an inverted load distribution seems more suitable for a gradual settlement process. In both cases, the intensity of the load transfer mechanism can be approached by the Terzaghi's formulation using an appropriate value for the ratio between the horizontal and vertical stresses. Finally, recommendations based on the experimental and numerical results are proposed to promote a better design of such structures.

METHODS FOR DESIGNING PARTIALLY INFLATED GEOTUBES

A widely used technique in constructing dikes for land reclamation is to use tubes made of sewn geosynthetic sheets. These tubes are usually filled with slurry comprising soils, such as sand, silt, and clay. The tension stress that develops in geosynthetic tubes during tube filling is the dominant factor considered in constructing safe dikes. Existing design methods are effective for sausage-shaped tube designs; however, they cannot be directly applied in flat tubes, which are commonly used in dike construction. This paper presents a procedure that can determine the relationships among tube size, pumping pressure, unit weight of the slurry, and tension stress in geosynthetic tubes. All these approaches are programmed to enable dike designers to select a suitable geosynthetic design for dike profiles.

Pengaruh Jumlah Lapisan dan Spasi Perkuatan Geosintetik terhadap Kuat Dukung dan Penurunan Tanah Lempung Lunak

One of the ways to improve the mechanical characteristics of soft soil is to provide soil reinforcement which is a geosynthetic sheet overlaid on the soft soil. Previous studies show that the installation of geosynthetic layers in soft soil proved to increase the bearing capacity and reduce the settlement. Theoretically, if more layers of reinforcement are given, the support will be stronger, while the soil of settlement will be reduced even greater. In this study, the influence of the number of layers and vertical distance between geosynthetic layers on the magnitude of bearing capacity, and the settlement in base soil will be assessed. The primary research is a load test on each clay included in model boxes of 120 cm x 120 cm x 100 cm size. Water was added to the soil to achieve the liquid limit conditions, then the soil was strengthened by inserting 60 x 60cm2 HRX200 woven geotextile layers whose tensile strength is 20 kN/m2 with various distances and number of layers among different boxes. The bottom of loading foundation on the top side of the samples is square whose side (B) is of 10 cm. The loading process were done until the soil collapse indicated by visual observation or by the condition of no increase of the load magnitude causing settlement. The results showed that 1 layer, 2 layers and 3 layers geosynthetic correspond to the bearing capacity increase of 60.57%, 213.00% and 402.64%, respectively, and experienced a reduction in soil settlement by 40%, 60% and 70%, respectively, compared to that without any reinforcement. Geosynthetic being placed at a distance of 0.4 B and 0.6 B gives the greatest bearing capacity increase, in the amount of 402.64%. Geosynthetic mounted on distance more than 0.6 B resulted in bearing capacity increase of diminishing percentages.

Model tests on methods to improve dewatering efficiency for sludge-inflated geotextile tubes

ABSTRACT: Geotextile tubes made of woven geotextile materials have been widely applied to dewater waste sludge such as digested bio-solids, sewage sludge, dredged materials, industrial solid wastes, fly ash and coal slurry. However, the dewatering process of single geotextile tubes is time consuming. Two methods to accelerate the dewatering process of slurry in geosynthetic tubes were studied using laboratory model tests. The first method was to stack one tube on top of another and use the weight of the top tube to preload the bottom one. The second method was the vacuum method proposed in this paper to use vacuum pressure to consolidate the slurry in the geosynthetic tube. A 2.0-mm-thick woven polypropylene geosynthetic sheet was selected for the geotextile tubes. The deformation, pore water pressure and tensile forces monitored during the model tests against time are presented and analysed in this paper. The properties of the soil after consolidation were also measured and compared. The model test data showed that the vacuum method resulted in a lower water content and higher deformation rate than the two-layer stacking geotextile tube method. Compared with the stacking method, the vacuum method could reduce the risk to the geotextile sheet from explosion.

Geosynthetics anchorage with wrap around: experimental and numerical studies

ABSTRACT: Geosynthetics are used as reinforcing elements in a wide variety of structures: reinforced slopes and walls; embankments on soft soils; piled embankment; reinforcement in the base layers of railroad and road constructions; reinforced foundation mattresses; bridging of sinkholes or reinforced abutments. In some cases, these reinforced structures require anchorage areas. Designing the required dimensions of this anchorage remains problematic. This paper focuses on the simple run-out and wrap around anchorages. Laboratory tests, performed with these two anchorage benches, consisted of the pull-out of three geotextiles (uniaxial or biaxial with different stiffness) anchored following various geometries in different kinds of soil. In order to confirm and to complete the experimental studies presented in these anchorage systems, a specific two-dimensional discrete-element model has been used. The soil was described by discrete elements and the geosynthetic behaviour was taken into account by the use of thin finite elements. The interface behaviour between the soil particles and the geosynthetic elements was considered at each contact point by using a Coulomb contact law. The numerical model reproduced the experimental campaign and the anchorage mechanisms reasonably. The load transfer between the geosynthetic and soil was visualised by the force and displacements distribution along the geosynthetic sheet. The numerical procedure could also be used to define the conditions of stability for reinforced slopes or walls.

Load transfer mechanisms in geotextile-reinforced embankments overlying voids: Experimental and analytical approaches

The techniques of soil reinforcement by geotextile are easy and economic solutions that limit the surface settlements of embankments prone to sinkholes. The design of such structures is based on understanding complex mechanisms, such as the tensile geosynthetic behavior under vertical loading, the frictional interaction between the soil and the reinforcement, the load transfer mechanisms and the arching effect in the soil embankment. Recently, significant progress has been made, allowing for improvement of the design methods by taking into account the frictional and sliding effects of the geosynthetic sheet in the anchorage areas and the local increase of the vertical stresses in the vicinity of the edges of the cavity. Nevertheless, the soil dilatancy, or the load transfer mechanisms in the embankment during the formation of the cavity or under static or cyclic loadings, remains unknown. Additionally, the reinforced, treated soil layer has not been specifically studied. To focus on the soil embankment behavior over a void, experimental studies were conducted as part of the FUI research project GéoInov.Sinkholes are experimentally simulated under granular embankments and treated soil layers, both reinforced by geotextiles. Each void is created by a device that allows the progressive formation of a circular cavity with an increasing diameter of 0.75, 1.25 and 2.2 m. This enables reproduction of the formation of some natural sinkholes. After the void reaches a size of 2.2 m wide, traffic loads are applied on the top surface of the embankment. Then, during both the formation of the cavity and the vehicle traffic loading, a dedicated instrumentation is used to measure the dilatancy and the soil movement, the load transfer, the deformation of the geotextile and the surface settlement. The experimental results are then analyzed and compared with existing analytical methods to improve the existing analytical methods.

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.

Analysis of geosynthetic tubes inflated by liquid and consolidated soil

When permeable geosynthetic tubes are used for dewatering of waste sludge or construction of dikes or embankments, the tubes have to be inflated using sludge or soil slurry several times. After each inflation, the soil slurry is consolidated into solid. Hence from the second inflation onwards, the geosynthetic tube is filled by both slurry and consolidated soil. In this paper, a new analytical method is proposed to provide a solution to the above specific case. Friction between geosynthetic sheet and soil, and friction between geosynthetic tube and subgrade, are considered. Parametric studies are also carried out to compare the design between geosynthetic tubes inflated using pure slurry and that using slurry and consolidated soil to study the key factors affecting the design. The study shows that tensile forces vary along the cross-section of the geosynthetic tube with the minimum value occurring at the center of the base. The effect of friction and lateral earth pressure on the geometry and tensile forces of the geosynthetic tube is insignificant when the height of the consolidated soil in the tube is small, but increases considerably with an increase in the height.

An analytical model for predicting load–deformation behavior of the FHWA GRS-IBS performance test

The Federal Highway Administration (FHWA) has recently developed a new bridge abutment system, known as Geosynthetic-Reinforced Soil-Integrated Bridge System (GRS-IBS). The term “GRS” refers to a soil mass that is reinforced internally by closely spaced (reinforcement spacing not more than 0·3 m) horizontal layers of geosynthetic sheets. The design protocol of GRS-IBS recommends a “performance test” be conducted to estimate the load–deformation behavior of a GRS-IBS system. The performance test involves a cuboidal block-faced GRS mass that is subjected to increasing axial loads. More than 100 GRS-IBS have been constructed in the US since its introduction in 2011. In light of the rapid development of the new bridge abutment system, an analytical model is developed for predicting the load–deformation behavior of the performance test up to the ultimate load-carrying capacity that is determined from the “W-equation.” This paper presents the analytical model and validation of the analytical model. Validation of the model was accomplished by comparing the analytical results with measured data of four sets of performance tests. The paper also presents a new interpretation of the W-equation, an equation that has been shown to be capable of predicting the ultimate load-carrying capacity of a GRS mass with very good accuracy. The analytical model is seen to give very good predictions of the load–deformation behavior of the performance tests up to the ultimate load-carrying capacity; hence may be used to supplement or replace the FHWA performance test that is rather laborious and time-consuming to conduct.

Centrifuge investigation of load transfer mechanisms in a granular mattress above a rigid inclusions network

Reinforcing compressible soils by rigid inclusions is a method to reduce and homogenize settlements under many types of structures. A granular mattress, set between the structure and the group of inclusions, transfers by arching effects a part of loadings to the piles embedded in rigid substrate. A geosynthetic can be added between the heads of the rigid inclusions and the granular mattress. In addition to the arching effect, a membrane effect happens caused by the stretching of the geosynthetic sheet. An experimental mobile tray device, especially designed to test this reinforcement technique in centrifuge at 20g, consists in simulating the settlement of the soft soil located between the inclusions. An initial pretension can be applied to the geosynthetic. A parametric study of the load transfer mechanisms in the mattress is conducted with three different thicknesses of granular mattress, two different rigid inclusions networks and different initial pretensions in the geosynthetic.The efficacy of the load transfer and the settlements at the surface of the granular mattress are studied and discussed. With and without geosynthetic reinforcement, load transfer mechanisms are better for thicker load transfer mattresses and for higher mesh densities. The improvement made by a geosynthetic reinforcement is clearly shown trough both load transfer and differential settlement reduction.