Experimental study on the permeability and self-healing capacity of geosynthetic clay liners in heavy metal solutions

Geosynthetic clay liners (GCLs), which have a very low permeability to water and a considerably high self-healing capacity, are widely used in liner systems of landfills. In this study, a series of experimental tests were carried out under complex conditions on typical commercial GCLs from China. In particular, the effects of pH values and lead ions (Pb²⁺) were tested in addition to other factors. The swelling properties of natural bentonite encapsulated between geotextile components in the GCLs were tested first. The swelling capacity was reduced rapidly at pH values < 3 and concentrations of Pb²⁺ >40 mM. Permeability tests on GCLs with different concentrations of lead ions were then performed by using the self-developed multi-link flexible wall permeameter, and data showed that increases in lead ion concentrations greatly improved the permeability. Finally, self-healing capacity tests were conducted on needle-punched GCLs under different levels of damage. Results showed that the GCLs have a good self-healing capacity with small diameter damage holes (2 mm, close to three times the original aperture), but with a damage aperture larger than 15% of the sample area, the self-healing capacity could not prevent leakage; hence, in certain situations it will be necessary to repair the damage to meet the anti-seepage requirement.     

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.     

Limited reinforced space in segmental retaining walls

Current design of geosynthetic reinforced segmental retaining walls considers an a priori limitless length for reinforcement installation. Such length is typically 0.5–0.7 times the height of the wall. However, often there are constraints on such space; e.g., bedrock formation located at a small distance behind the facing. The objective of this note is to introduce a procedure for assessing the required long-term strength of the reinforcement while considering its limited length. Predictions by a conventional slope stability analysis were first checked against a continuum-mechanics based numerical analysis. Upon obtaining good agreement, a design chart was developed. The chart enables the determination of the reduction in the lateral earth pressure coefficient due to the constrained space. The revised earth pressure coefficient can be used with current analytical methods to account for the limited space. The results appear to be valid for conventional walls retaining a limited volume of soil. Comparison with limited experimental results for unreinforced backfill shows reasonably good agreement.     

钠基膨润土防水毯在陡河青龙河防洪排涝综合整治工程中的应用

通过新型环保防渗材料——钠基膨润土防水毯(GCL-NP)在陡河青龙河防洪排涝综合整治工程应用案例,介绍了膨润土防水毯的工程设计和施工要点,实践证明,所用的钠基膨润土防水毯防渗效果优异,达到了设计要求,保证了工程质量.     

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.     

Evaluation of back-calculated elastic moduli of unreinforced and geocell-reinforced unbound granular material from full-scale field tests

This paper presents a field-scale experimental track over a poor subgrade with an unreinforced section and a geocell-reinforced section subjected to in-situ performance tests. Plate load tests and Benkelman beam tests were carried out distributed in several unreinforced and reinforced layers. The objective was to: (1) examine the variability of the elastic modulus of unbound granular material (UGM) due the influence of its thickness and the presence of poor subgrade in its base, (2) evaluate the modulus improvement factor (MIF) generated by the geocell reinforcement in the UGM and (3) verify the most appropriate condition to apply the MIF to transport infrastructure design. The results showed that there is a significant influence of the thickness of the UGM layer on its elastic modulus when the layer is supported directly over a soft subgrade. The MIF values obtained in field suggest that its determination is mostly related to the UGM maximum elastic modulus rather than its decreased values (by virtue of poor subgrade or reduced thicknesses), and that the analytical formulation presented for MIF calculation has good predictive capability to be applied to pavement design.     

Investigation on geogrid reinforcement and pile efficacy in geosynthetic-reinforced pile-supported track-bed

This paper presents a full-scale model study of geosynthetic-reinforced pile-supported (GRPS) track-bed to investigate the effect of geogrid reinforcement and the evolution of pile efficacy (ratio of load borne by the pile cap to the total applied load). Three testing procedures were followed: model construction, static loading and subsoil settlement (simulated by discharging of water bags surrounding the pile caps). The results indicated that partially mobilized soil arching was developed during the first two procedures. When sufficient subsoil settlement was reached, fully mobilized soil arching was established. The geogrid was proven to effectively transfer load from the water bag to the pile cap. The stress difference induced by the geogrid showed lower absolute values for the corresponding sensors above the water bag during loading and settlement procedures, due to the inverse triangular distribution of the vertical-directional geogrid tensile force above the water-bag area. The experimental results of pile efficacy were compared to the estimations of four analytical models. For the present test at partially mobilized arching state, the pile efficacy increased with the construction height increasing and decreased as the static loading increased. The partially mobilized arching also resulted in overestimations of the pile efficacy from all four analytical models. At fully mobilized arching state, the pile efficacy stayed relatively stable, being well predicted by all four analytical models.     

Experimental and theoretical studies on the ultimate bearing capacity of geogrid-reinforced sand

Geosynthetic reinforced soil (GRS) structures have gained popularity in replacing concrete rigid piles as abutments to support medium or small-spanned bridge superstructures in recent years. This study conducted 13 model tests to investigate the ultimate bearing capacity of the GRS mass when sand was used as backfill soil. The GRS mass was constructed and loaded to failure under a plane strain condition. Test results were compared with two analytical solutions available in literature. This study also proposed an analytical model for predicting the ultimate bearing capacity of the GRS mass based on the Mohr-Coulomb failure criterion. The failure surface of the GRS mass was described by the Rankine failure surface. The effects of compaction and reinforcement tension were equivalent to increased confining pressures to account for the reinforcing effects of the geosynthetic reinforcement. The proposed model was verified by the results of the model tests conducted in this study and reported in literature. Results indicated that the proposed model was more capable of predicting the ultimate bearing capacity of the GRS mass than the other two analytical solutions available in literature. The proposed model can be used to predict the ultimate bearing capacity of GRS structures when sand was used as backfill material. In addition, a parametric study was conducted to investigate the effects of friction angle of backfill soil, reinforcement spacing, reinforcement strength, and reinforcement stiffness on the ultimate bearing capacity of the GRS mass calculated with and without compaction effects. Results showed that the ultimate bearing capacity of the GRS mass was significantly affected by the friction angle of backfill soil, reinforcement spacing and strength. Compaction effects resulted in an increase in the ultimate bearing capacity of the GRS mass.     

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.     

Stress-controlled direct shear testing of geosynthetic clay liners II: Assessment of shear behavior

This paper is the second of a two-paper set on stress-controlled direct shear testing of geosynthetic clay liners (GCLs). Design of the apparatus, preliminary experiments, and shear deformation mechanisms in heat-treated and non-heat treated needle-punched (NP) GCLs were discussed in Part I. The objective of Part II (this paper) was to evaluate the effects of physical factors (i.e., peel strength and initial normal stress, σ_ni), environmental factors (i.e., temperature and hydration solution), and creep on the internal shear behavior of NP GCLs. In addition, failure conditions of GCLs in the stress-controlled direct shear tests were compared to displacement-controlled direct shear tests to verify results. An increase in internal shear strength developed from increased GCL peel strength or increased normal stress. Elevated temperatures were observed to decrease internal shear strength for both non-heat treated and heat-treated NP GCLs. Specimens hydrated with a calcium-rich synthetic mining solution experienced increased internal shear strength due to cation exchange in the bentonite, whereas specimens hydrated with a highly alkaline synthetic mining solution experienced decreased internal shear strength. Creep tests revealed an increase in time-to-failure with decrease in applied shear stress. Finally, stress states at failure from stress-controlled and displacement-controlled shear tests corresponded to a unique failure envelope, which validates the efficacy of using stress-controlled direct shear tests to assess internal shear behavior and shear strength of NP GCLs.