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.     

Effect of gravel subgrade on hydraulic performance of geosynthetic clay liner

The effect of a gravel subgrade on the hydraulic performance of GCLs is investigated. Laboratory test results show that the GCL specimens exhibit significant variation in thickness when compressed against gravel. The maximum and minimum thicknesses of the specimen were about 20 and 3 mm, respectively, after consolidation by an effective stress up to 138 kPa. However, the permittivity of GCLs remained very low. The permittivity of both needle-punched and adhesive-bonded geotextile-supported GCLs decreased with increasing confining stress, regardless of the type of subgrade materials. In general, larger particles led to more significant migration of bentonite. Nevertheless, there was no significant difference in the degree of bentonite migration between the two GCLs investigated.     

Influence of geosynthetic reinforcement on the load-settlement characteristics of two-layer subgrade

The possibility of realistic prediction of two-layer subgrade load-settlement characteristics is discussed. The case of improvement of the soft subgrade properties using the geosynthetic reinforcement placed at the boundary between two different subgrade layers is analysed. In the first part of the paper, a short review of the main conclusions from experimental results dealing with the influence of geosynthetic reinforcement on the load-settlement characteristics of subgrade is presented. Then, the results of using the selected analytical membrane-action model to describe the reinforcement action in soil are discussed. The model is verified on the basis of data obtained from previously published laboratory tests. Particular attention is devoted to influencing some basic initial parameters on the accuracy of obtained results. Important problems which need intensive investigations are identified.     

A simplified method for design of geosynthetic tubes

A simplified method for the design of impermeable geosynthetic tubes inflated using liquid is proposed in this paper. Adopting a computer program for an existing theoretical model, relationships between pumping pressure and geometric parameters for geosynthetic tubes can be established. A set of simplified dimensionless design equations are then derived using the Chapman–Richard curve fitting method. The validity of this simplified method was verified using other established methods and laboratory model tests. The proposed simplified method can thus be used for routine or preliminary design.     

Analysis of geosynthetic tubes filled with several liquids with different densities

A two dimensional model of a geosynthetic tube sitting on a rigid horizontal foundation and filled with several separated liquids with different densities is proposed. The material from which the tube is made is a special synthetic fabric which is inextensible, perfectly flexible, and leakproof. Such a model is useful for modeling a consolidations process in the tube filled with a slurry. The equilibrium equations of the model are formulated. Unknown values like the pressure on the top and bottom of the tube, the tension in the geosynthetic fabric, the length of the contact zone between the tube and the rigid foundation are searched with respect to the given perimeter, the volumes and densities of liquids. Such a problem is solved by the Newton’s method. The initial approximation is obtained by solving a simplified problem with one liquid with the average density. The problem is implemented in a MATLAB code for geosynthetic tubes filled with two, three, and four liquids with different densities. The tubes filled with two different liquids are studied in more detail. The graphs of the relations are compared with the graphs for the tube filled with the single liquid whose density is the average of the densities of the liquids. The comparison enables to discuss the influence of the consolidation process on the height, the contact zone, the pressures and the tension of the tube. The results of the proposed model for a tube filled with a single liquid are compared with another model.     

Numerical study on maximum reinforcement tensile forces in geosynthetic reinforced soil bridge abutments

This paper presents a numerical study of maximum reinforcement tensile forces for geosynthetic reinforced soil (GRS) bridge abutments. The backfill soil was characterized using a nonlinear elasto-plastic constitutive model that incorporates a hyperbolic stress-strain relationship with strain softening behavior and the Mohr-Coulomb failure criterion. The geogrid reinforcement was characterized using a hyperbolic load-strain-time constitutive model. The GRS bridge abutments were numerically constructed in stages, including soil compaction effects, and then loaded in stages to the service load condition (i.e., applied vertical stress?=?200?kPa) and finally to the failure condition (i.e., vertical strain?=?5%). A parametric study was conducted to investigate the effects of geogrid reinforcement, backfill soil, and abutment geometry on reinforcement tensile forces at the service load condition and failure condition. Results indicate that reinforcement vertical spacing and backfill soil friction angle have the most significant effects on magnitudes of maximum tensile forces at the service load condition. The locus of maximum tensile forces at the failure condition was found to be Y-shaped. Geogrid reinforcement parameters have little effect on the Y-shaped locus of the maximum tensile forces when no secondary reinforcement layers are included, backfill soil shear strength parameters have moderate effects, and abutment geometry parameters have significant effects.     

Experimental study on the load bearing behavior of geosynthetic reinforced soil bridge abutments with different facing conditions

This paper presents an experimental study on reduced-scale model tests of geosynthetic reinforced soil (GRS) bridge abutments with modular block facing, full-height panel facing, and geosynthetic wrapped facing to investigate the influence of facing conditions on the load bearing behavior. The GRS abutment models were constructed using sand backfill and geogrid reinforcement. Test results indicate that footing settlements and facing displacements under the same applied vertical stress generally increase from full-height panel facing abutment, to modular block facing abutment, to geosynthetic wrapped facing abutment. Measured incremental vertical and lateral soil stresses for the two GRS abutments with flexible facing are generally similar, while the GRS abutment with rigid facing has larger stresses. For the GRS abutments with flexible facing, maximum reinforcement tensile strain in each layer typically occurs under the footing for the upper reinforcement layers and near the facing connections for the lower layers. For the full-height panel facing abutment, maximum reinforcement tensile strains generally occur near the facing connections.     

Experimental study on the load bearing behavior of geosynthetic reinforced soil bridge abutments on yielding foundation

This paper presents an experimental study of the load bearing behavior of geosynthetic reinforced soil (GRS) bridge abutments constructed on yielding clay foundation. The effects of two different ground improvement methods for the yielding clay foundation, including reinforced soil foundation and stone column foundation, were evaluated. The clay foundation was prepared using kaolin and consolidated to reach desired shear strength. The 1/5-scale GRS abutment models with a height of 0.8 m were constructed using sand backfill, geogrid reinforcement, and modular block facing. For the GRS abutments on three different yielding foundations, the reinforced soil zone had relatively uniform settlement and behaved like a composite due to the higher stiffness than the foundation layers. The wall facing moved outward with significant movements near the bottom of facing, and the foundation soil in front of facing showed obvious uplifting movements. The vertical stresses transferred from the footing load within the GRS abutment and on the foundation soil are higher for stiffer foundation. The improvement of foundation soil using geosynthetic reinforced soil and stone columns could reduce the deformations of GRS abutments on yielding foundation. Results from this study provide insights on the practical applications of GRS abutments on yielding foundation.     

Bearing capacity of square footings on geosynthetic reinforced sand

The results from laboratory model tests and numerical simulations on square footings resting on sand are presented. Bearing capacity of footings on geosynthetic reinforced sand is evaluated and the effect of various reinforcement parameters like the type and tensile strength of geosynthetic material, amount of reinforcement, layout and configuration of geosynthetic layers below the footing on the bearing capacity improvement of the footings is studied through systematic model studies. A steel tank of size 900 × 900 × 600 mm is used for conducting model tests. Four types of grids, namely strong biaxial geogrid, weak biaxial geogrid, uniaxial geogrid and a geonet, each with different tensile strength, are used in the tests. Geosynthetic reinforcement is provided in the form of planar layers, varying the depth of reinforced zone below the footing, number of geosynthetic layers within the reinforced zone and the width of geosynthetic layers in different tests. Influence of all these parameters on the bearing capacity improvement of square footing and its settlement is studied by comparing with the test on unreinforced sand. Results show that the effective depth of reinforcement is twice the width of the footing and optimum spacing of geosynthetic layers is half the width of the footing. It is observed that the layout and configuration of reinforcement play a vital role in bearing capacity improvement rather than the tensile strength of the geosynthetic material. Experimental observations are supported by the findings from numerical analyses.     

Experimental evaluation of wicking geotextile-stabilized aggregate bases over subgrade under rainfall simulation and cyclic loading

Wicking geotextile has been increasingly utilized in field projects to mitigate water-related roadway problems. The previous studies showed that the wicking geotextile could provide mechanical stabilization, serve as capillary barrier, and enhance lateral drainage. The wicking geotextile differentiates itself from non-wicking geotextiles by providing capillary or wicking drainage in unsaturated conditions, whereas non-wicking geotextiles only provide gravitational drainage under saturated or near-saturated conditions. Although the previous studies have demonstrated the benefits of soil water content reduction by the wicking drainage, it is not well understood how the wicking geotextile stabilization improves overall performance of aggregate bases over subgrade under traffic or cyclic loading. This paper presents an experimental study where large-scale cyclic plate loading tests were conducted under different conditions: (1) non-stabilized base, (2) non-wicking geotextile-stabilized base, and (3) wicking geotextile-stabilized base, over soft and moderate subgrades. Rainfall simulation was carried out for each test section. After each rainfall simulation, a drainage period was designed to allow water to drain from the section. The amounts of water applied and exiting from the test section were recorded and are compared. Cyclic loading was applied after each drainage period. The test results show that the combined hydraulic and mechanical stabilization effect by the wicking geotextile reduced the permanent deformation of the aggregate base over the subgrade as compared with the non-stabilized and non-wicking geotextile-stabilized sections.     

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

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

Effect of water salinity on the water retention curve of geosynthetic clay liners

The effect of water salinity on the water retention curve of geosynthetic clay liners (GCLs), under constant volume condition is examined. The results indicate that at a constant gravimetric moisture content the total suction increases as the salinity of the wetting liquid increases. Furthermore, the difference in total suction between the GCL hydrated by saline water and distilled water is greater than the difference in the osmotic potential of the wetting water. This behaviour is possibly caused by the matric suction being affected by the expected chemically induced pore size change of the bentonite component of the GCL.     

Effect of geosynthetic component characteristics on the potential for GCL internal erosion

Experiments quantifying GCL permittivity and the ultimate water head the GCLs can sustain before the initiation of internal erosion when underlain by a 50 mm angular to subangular gravel subgrade are conducted. The influence of different geotextiles over the subgrade, water heads, hydration periods before testing, masses per unit area of bentonite within the GCL, and ionic strengths of the solution (cation exchange) are considered. Test results show that GCL with the scrim-reinforced nonwoven geotextile over the subgrade has the best hydraulic performance against internal erosion, followed by the woven geotextile coated with a 110 g/m² polypropylene film. A woven or nonwoven is the least useful for preventing internal erosion, with the corresponding threshold water head initiating internal erosion >39 m for scrim-reinforced nonwoven, 21 m for lightly coated woven, 4–5 m for woven and nonwoven alone, respectively. Cation exchange, length of hydration, and mass per unit area of bentonite do not notably affect the threshold water head for the subgrade examined. Once internal erosion occurs, there is a 3-order of magnitude increase in permittivity. The practical implications are discussed.     

Effect of geotextile and cement on the performance of sabkha subgrade

Many construction and post-construction problems have been reported in the literature when sabkha soils have been used without an understanding of their abnormal behavior, especially their inferior loading capability in their natural conditions. The strength of these soils can be further significantly decreased if the sabkha is soaked. The main objective of this study was to upgrade the load-carrying capacity of pavements constructed on sabkha soils using geotextiles, and to assess the effect of geotextile grade, base thickness, loading type (static and dynamic) and moisture condition (as-molded and soaked) on the performance of soil-fabric-aggregate (SFA) systems. In addition, the sabkha soil was treated with different dosages (5%, 7%, and 10%) of Portland cement and the performance of cement-stabilized sabkha was compared to that of the SFA system under different testing conditions. The ANOVA results indicated that the use of geotextile has a beneficial effect on sabkha soils, especially under wet conditions. Although the improvement in the load-carrying capacity of sabkha samples with high dosages of cement showed better results than the inclusion of geotextile, an economic analysis showed that the use of geotextiles would be superior. Moreover, mechanistic analysis was used to develop a prediction model for the percentage increase in the modulus of resilience.     

Ultimate bearing capacity of strip footing resting on soil bed strengthened by wraparound geosynthetic reinforcement technique

In the recent past, the wraparound geosynthetic reinforcement technique has been recommended for constructing the geosynthetic-reinforced soil foundations. This paper presents the development of an analytical expression for estimating the ultimate bearing capacity of strip footing resting on soil bed reinforced with geosynthetic reinforcement having the wraparound ends. The wraparound ends of the geosynthetic reinforcement are considered to provide the shearing resistance at the soil-geosynthetic interface as well as the passive resistance due to confinement of soil by the geosynthetic reinforcement. The values of ultimate load-bearing capacity determined by using the developed analytical expression agree well with the model footing load test values as reported in the literature.     

Chemical interaction and hydraulic performance of geosynthetic clay liners isothermally hydrated from silty sand subgrade

The effects of the silt aggregation, compaction density, and water content of the subgrade on the hydration of five different geosynthetic clay liner (GCL) products is reported based on a series of laboratory column experiments conducted over a six-year period. GCLs meeting typical specifications in terms of minimum hydraulic conductivity and swell index are hydrated to equilibrium from the same subgrade soil with sufficient cations to cause cation exchange during hydration. It is then shown that the GCL bentonite granularity and GCL structure can have a significant (~four orders of magnitude) effect on hydraulic conductivity under the same test conditions (from 8 × 10⁻¹² m/s for one GCL to 6 × 10⁻⁸ m/s for another GCL product). The effect of subgrade water content on the hydraulic performance of GCLs are not self-evident and quite dependent on the bentonite granularity, GCL structure, and permeant. Varying the subgrade water content from 5 to 16% and allowing the GCL to hydrate to equilibrium before permeation led to up to 5-fold difference in hydraulic conductivity when permeated with tap water and up to 60-fold difference when the same product is permeated with synthetic municipal solid waste leachate. When permeated with synthetic leachate, increasing stress from 70 kPa to 150 kPa led to a slight (average 37%; maximum 2.7-fold) decrease in hydraulic conductivity due to a decrease in bulk void ratio. It is shown that hydraulic conductivity is lower for GCLs with a scrim-reinforced geotextile, and/or with finer bentonite. It is shown that selecting a GCL based on the initial hydraulic conductivity and swell index in a manufacturers product sheet provides no assurance of good performance in field applications and it is recommended that designers pay more attention to selection of a GCL and preparation of the subgrade for important projects.     

Effect of ammonium on the hydraulic conductivity of geosynthetic clay liners

Hydraulic conductivity and swell index tests were conducted on a conventional geosynthetic clay liner (GCL) containing sodium-bentonite (Na-B) using 5, 50, 100, 500, and 1000 mM ammonium acetate (NH₄OAc) solutions to investigate how NH₄⁺ accumulation in leachates in bioreactor and recirculation landfills may affect GCLs. Control tests were conducted with deionized (DI) water. Swell index of the Na-B was 27.7 mL/2 g in 5 mM NH₄⁺ solution and decreased to 5.0 mL/2 g in 1000 mM NH₄⁺ solution, whereas the swell index of Na-B in DI water was 28.0 mL/2 g. Hydraulic conductivity of the Na-B GCL to 5, 50, and 100 mM NH₄⁺ was low, ranging from 1.6–5.9 × 10^?11 m/s, which is comparable to the hydraulic conductivity to DI water (2.1 × 10^?11 m/s). Hydraulic conductivities of the Na-B GCL permeated with 500 and 1000 mM NH₄⁺ solutions were much higher (e.g., 1.6–5.2 × 10^?6 m/s) due to suppression of osmotic swelling. NH₄⁺ replaced native Na⁺, K⁺, Ca²⁺, and Mg²⁺ in the exchange complex of the Na-B during permeation with all NH₄⁺ solutions, with the NH₄⁺ fraction in the exchange complex increasing from 0.24 to 0.83 as the NH₄⁺ concentration increased from 5 to 1000 mM. A Na-B GCL specimen permeated with 1000 mM NH₄⁺ solution to chemical equilibrium was subsequently permeated with DI water. Permeation with the NH₄⁺ converted the Na-B to “NH₄-bentonite” with more than 80% of the exchange complex occupied by NH₄⁺. Hydraulic conductivity of this GCL specimen decreased from 5.9 × 10^?6 m/s to 2.9 × 10^?11 m/s during permeation with DI water, indicating that “NH₄-bentonite” can swell and have low hydraulic conductivity, and that the impact of more concentrated NH₄⁺ solutions on swelling and hydraulic conductivity is reversible.     

Numerical parametric study of geosynthetic reinforced soil integrated bridge system (GRS-IBS)

A 2-D finite flement model was developed in this study to conduct a FE parametric study on the effects of some variables in the performance of geosynthetic reinforced soil integrated bridge system (GRS-IBS). The variables investigated in this study include the effect of internal friction angle of backfill material, width of reinforced soil foundation (RSF), secondary reinforcement within bearing bed, setback distance, bearing width and length of reinforcement. Other important parameters such as reinforcement stiffness and spacing were previously investgated by the authors. The performance of GRS-IBS were investgated in terms of lateral facing displacement, strain distribution along reinforcement, and location of potential failure zone. The results showed that the internal friction angle of backfill material has a significant impact on the performance of GRS-IBS. The secondary reinforcement, setback distance, and bearing width have low impact on the performance of GRS-IBS. However, it was found that the width of RSF and length of reinforcement have negligible effect on the performance of GRS-IBS. Finally, the potential failure envelope of the GRS-IBS abutment was found to be a combination of punching shear failure envelope (top) that starts under the inner edge of strip footing and extends vertically downward to intersect with Rankine active failure envelope (bottom).     

Effect of stress on water retention of needlepunched geosynthetic clay liners

Geosynthetic clay liners (GCLs) are placed at the bottom of waste disposal facilities where they hydrate from the subsoil and eventually from a hydraulic head on geomembranes (GMs) defects. Predicting hydration behavior of GCLs requires knowledge of the water-retention properties of the GCL along wetting paths. Given that GCLs could be subjected to different ranges of vertical stresses that are induced by the weight of the supported waste, the confining stress could affect water-retention properties of GCLs and should be investigated. To do so, a laboratory methodology to establish the water-retention curves (WRCs) of needlepunched GCLs under stress was undertaken. Various constant vertical stresses corresponding to different weights of the supported waste were applied to GCL specimens placed in controlled-suction oedometers. Suction values were selected so as to mimic a wetting path from the initial dry state to zero suction. Suction was controlled by using controlled suction techniques with controlled humidity imposed by a saturated saline solutions and using the osmotic technique with polyethylene glycol (PEG) solutions. Measurements were undertaken on oedometer systems as to apply confining stresses and have been complemented by standard saturated oedometer swelling tests. The data obtained confirm that increasing the stress on to the GCL results in less, albeit faster, water uptake, which could emphasize on recommendations about rapidly covering GCLs after they are placed at the bottom of a waste disposal facilities. Finally, the potential validity of the state-surface concept, which was developed in unsaturated soil mechanics, is discussed using van Guenuchten's and Fredlund and Xing's equations for water retention curves.     

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.