Hydrodynamic assessment of bentonite granule size and granule swelling on hydraulic conductivity of geosynthetic clay liners

Flow in an idealized geosynthetic clay liner (GCL) containing bentonite comprised of equisized and equispaced square granules was simulated using a hydrodynamic model to quantitatively evaluate the premise that the hydraulic conductivity of GCLs diminishes as the bentonite granules hydrate and swell into adjacent intergranular pores, creating smaller and tortuous intergranular flow paths. Predictions with the model indicate that hydraulic conductivity decreases as granules swell and intergranular pores become smaller, and that greater granule swelling during hydration is required to achieve low hydraulic conductivity when the bentonite is comprised of larger granules, or the bentonite density is lower (lower bentonite mass per unit area). Predictions made with the model indicate that intergranular pores become extremely small (<1 μm) as the hydraulic conductivity approaches 10⁻¹¹ m/s. These outcomes are consistent with experimental data showing that GCLs are more permeable when hydrated and permeated with solutions that suppress swelling of the bentonite granules, and that the hydraulic conductivity of GCLs with bentonite having smaller intergranular pores (e.g., GCLs with smaller bentonite granules, more broadly graded particles, or higher bentonite density) is less sensitive to solutions that suppress swelling.     

Investigation of initial hydration and rehydration of geosynthetic clay liners from sandy subgrades via X-ray computed tomography images

The hydraulic conductivity of geosynthetic clay liners (GCLs) widely used as barrier systems considerably depends on their hydration status after the initial hydration of virgin GCLs and the rehydration of desiccated GCLs. Free hydration tests were performed on virgin and desiccated GCLs over sandy subgrades to compare their hydration level. In addition, high-resolution micro-X-ray computed tomography (CT) images of both GCLs and sandy subgrades with different gravimetric water content (i.e. 15%, 20%, and 25%) after the initial hydration were analyzed for better insights. The results show significant influences of subgrade water content on moisture content and thickness of virgin GCLs. Water loss of sandy subgrades and the time interval necessary for reaching a steady state of desiccated GCLs during rehydration was greater and longer than virgin GCLs during initial hydration. X-ray CT images verified a dense distribution of bentonite particles, macropores, and minor desiccation cracks that existed in poorly-hydrated GCLs over unsaturated sand. On the other hand, the completely saturated sandy subgrade facilitated the hydration of GCLs, leaving a lot of macropores in the sand. The relationship between water distribution and the frequency of macropore generation observed in the upper contact zone of sandy subgrades was also indicated via these X-ray CT images.     

Two-dimensional analysis of stacked geosynthetic tubes on deformable foundations

Stacked geosynthetic tubes resting on a deformable foundation such as soil are analyzed. The tubes contain a slurry which applies hydrostatic pressure. The material of the tubes is assumed to act like an inextensible membrane and to have negligible weight, and the foundation is assumed to exert a normal upward pressure which is proportional to the downward deflection. Friction is neglected between tubes and at the foundation interface. Two configurations are considered: (a) one tube on top of another and (b) one tube straddling two tubes underneath it. For the latter formation, the case of external fluid acting on one side is analyzed, to simulate an application as a dike, and rigid blocks are utilized to prevent sliding of the tubes. Equilibrium shapes of the tubes are obtained numerically from a closed-form integral formulation, and the tension in each tube is computed.     

Numerical study of the impact of climate conditions on stability of geocomposite and geogrid reinforced soil walls

Geosynthetic-reinforced soil (GRS) walls using marginal soils can operate under unsaturated conditions depending on climate conditions and drainage inside the reinforced zone. Geocomposite reinforcements have been suggested to act as internal drainage layers, but their hydraulic behavior can also be strongly affected by climate conditions. Numerical analyses were conducted to observe the impact of four distinct tropical climate conditions (arid, semi-arid, humid subtropical and humid tropical) on suction profiles and stability of reinforced soil walls constructed using geogrid and geocomposite reinforcements. The climate simulation involved the incorporation of a soil-atmosphere interaction on water balance and on the unsaturated transient infiltration. Results indicate the GRS walls can operate under relatively high suction levels under arid climates in which cumulative evaporation overcomes infiltration. Any climate that has rainy seasons with consecutive rainfalls with intensities close to the infiltration capacity of soil and/or monthly cumulative precipitation higher than 200 mm/day led to critical conditions in terms of soil water saturation and stability. Under unsaturated conditions of soil, the drainage effectiveness of geocomposites is significantly reduced and adverse capillary break effects become critical.     

Water retention curves of a geosynthetic clay liner under non-uniform temperature-stress paths

This paper presents a novel suction-controlled chamber that permits the determination of the full water retention curves of geosynthetic clay liners (GCLs) under non-uniform temperature-stress paths. It investigates field conditions encountered in brine ponds (low confining stress settings) and heap leach pads (high confining stress settings) during construction and operation stages. Consequently, the analysis of the moisture dynamics in a GCL was defined under the wetting path (construction) and drying path (operation). High vertical stresses were found to facilitate a more rapid water uptake as capillarity is established faster than at low, confined stresses. In general, the drying curves increase the water desorption over the suction range investigated due to the low water viscosity caused by high temperatures. The wetting of the GCL at 20 °C and drying at 70 °C under either low, confined stress (2 kPa) or high confining stress (130 kPa) shows a reduction in the volumetric water contents. Furthermore, on the drying path, the coupled effect of elevated temperature and high confining stress accelerates water desorption leading possibly to potential desiccation.     

Numerical modeling of floating geosynthetic-encased stone column–supported embankments with basal reinforcement

The performance of the floating geosynthetic-encased stone column–(GESC)-supported embankments with basal reinforcement was examined using a 3-dimensional (3D) hydro-mechanical coupling finite element model. Comprehensive parametric analyses were performed on the governing factors such as consistency of substratum soil, tensile stiffness of basal reinforcement and encasement, and embankment height. The results indicated that a higher embankment load is transferred to the surrounding soil when a GESC was constructed on a weaker substratum. This causes larger increases in the settlement and lateral displacement of the GESC on the weaker substratum. The tensile strain of the basal reinforcement and hoop strain in the encasement also increases. In addition, high tensile stiffness in basal reinforcement and encasement is necessary to ensure feasible settlement reduction in a floating GESC-supported embankment with basal reinforcement.     

Laboratory investigation of GCL hydration from Lateritic subsoils

A laboratory investigation on the hydration behavior of GCLs from lateritic soils was conducted under isothermal and thermal conditions (tropical climate), varying subsoil moisture contents, GCLs bentonite particle size and mineralogy. GCL hydration levels from lateritic subsoils under isothermal conditions (55%) were similar to literature findings. A slight decrease in water content of some GCLs after long periods of contact with the lateritic soils indicates that equilibrium can demand long time in these soils. GCL with granular bentonites were less efficient to hydrate from lateritic subsoils. GCLs with activated-calcium bentonites maintained hydration levels in long-term. Nonwoven geotextile facing down favored capillary effects. Thermal cycles significantly influenced GCLs hydration from subsoils. Capillary connections developed during hydration under isothermal conditions due to suction gradient reductions. Post-hydration tests under isothermal conditions showed more alterations in GCLs swelling and cation exchange properties than thermal cycles test. An increase in the saturated hydraulic conductivity of GCLs was observed in both lateritic soils, mainly for isothermal condition, although continued attending hydraulic conductivity requirements for barrier applications.     

Model test of clogging effects on composite foundation of geosynthetic-encased steel slag column

Experiments were conducted to study the performance of geosynthetic encased steel slag column with a diameter of 150 mm and a length of 900 mm in a soft clay foundation. The effect of clogging was simulated by mixing the slag with 10% and 20% fines. The measured bearing capacity of the column treated foundation is notably increased to about 10 times than that of the untreated foundation, and is seldom affected by the intrusion of fines. The vertical stress within the soil at column tip attenuates to 85% and 60% of the stress close to the ground surface for the column with no fines and 20% fines, respectively. For the cases with fines content of 10% and 20%, the maximum excess pore pressure is in average 5% and 10% greater than the case without fines, respectively, and the dissipation rate of excess pore pressure is in average 18% and 24% slower than the case without fines. The column treated foundation prevents the water ponding on the surface as that occurs for the untreated foundation. The undrained shear strength of the soil close to the column increases by 18% at the depth of 100 mm, and 6% at the depth of half column, regardless of the fines.     

A large-scale shaking table model test for acceleration and deformation response of geosynthetic encased stone column composite ground

Effective mitigation of seismic-induced ground hazards requires an improved understanding of ground response in terms of earthquake wave propagation and ground deformation. Here, this paper examines the effects of geosynthetic-encased stone columns (ESCs) and ordinary stone columns (SCs) on the acceleration amplitude and frequency content responses of sand profiles, and the deformation of the ground using a large-scale shaking table model test. The model was excited by 15 shaking events including El Centro motion, Wenchuan Qingping motion and Kobe motion with peaks ranging from 0.1 to 0.9 g. The results indicate that the ESCs more significantly amplify surface accelerations compared to the SCs in the frequencies ranging from 10 to 17 Hz and from 2.5 to 9 Hz. The horizontal peak acceleration values in the ESCs composite ground are approximately twice those of the SCs composite ground. The acceleration response of the ground is influenced by the applied acceleration peak and frequency content, reinforced type, and structure. After the seismic excitation, the ESCs composite ground develops much narrower surface cracks distributed in a larger area compared to the SCs.     

Geosynthetic-encased stone columns in soft clay: A numerical study

This paper presents the findings of a series of numerical studies on the contribution of geosynthetic encasement in enhancing the performance of stone columns in very soft clay deposits. In this study, the imposed loading is from a fill embankment, and the stone columns act like reinforcements. Observed settlement of a trial embankment built on very soft clay strengthened with stone columns indicated that the stone columns alone were not adequately effective in reducing settlement because the very softy clay could not provide adequate confining stress to the stones. An alternative system utilizing geosynthetic encasement was examined numerically. As the primary issue is the development of settlement with time after the completion of stone column installation, a fully coupled analysis was performed. To reduce the computational effort, a unit cell idealization was adopted. This study showed that the use of geosynthetic encasement has the potential of significantly enhancing the effectiveness of stone columns in very soft clay and the simplified analysis presented in earlier work is valid. Furthermore, the predicted performance was found to be insensitive to assumed stiffness parameters of the compacted stone. However, it was found to be dependent on the locked-in stress in the geosynthetic encasement induced during installation.