Influence of pore water chemistry on GCL self-healing with hydration from silica sand

The self-healing of a GCL with a circular hole is examined in experiments where the GCL, overlain by geomembrane, is hydrated from a silica sand subgrade (SSS) having three different pore water chemistries. Factors considered included: hole size, subgrade initial moisture content w_fdn, GCL mass per unit area, and overburden stress (20–100 kPa). GCL self-healing is better for w_fdn = 16% than for w_fdn = 10%, which is better than for 5%, when the SSS pore water has negligible cations (ionic strength, I < 0.1 mM). However, only the 14.3 mm-diameter hole fully self-healed and only when w_fdn = 16%. In contrast, when the GCL is hydrated from SSS with pore water having an ionic strength, I, of 20 and 30 mM, the self-healing for w_fdn = 5% is better than for w_fdn = 10%, which is better than for w_fdn = 16%, although none of the holes self-healed. When a ~0.5 m hydraulic head was applied above the GCL under σ_v = 20–100 kPa, a 38.1 mm-diameter hole self-healed with water having I < 0.1 mM, a 25.4 mm-diameter hole self-healed with pore water with I = 20 mM and 30 mM, but none self-healed with simulated synthetic landfill leachate (SSL). Post-hydration hydraulic conductivity (k) tests with SSL suggest that a hole up to 14.3 mm-diameter would not pose a significant adverse impact on the k compared to an intact GCL; however, this is not the case for the larger holes tested.     

Experimental investigation of the effect of airgaps in preventing desiccation of bentonite in geosynthetic clay liners exposed to high temperatures

This paper investigates whether the introduction of an airgap above a composite liner made of a geomembrane (GMB) and a Geosynthetic Clay Liner (GCL) can decrease thermal loads on the GCL, reduce the risk of bentonite desiccation and/or help maintain its low hydraulic conductivity. A composite liner, subject to 20?kPa overburden load, over a well graded sand was subjected to a thermal gradient. In addition, to the reference base case in which no airgap was present, two designs included air gaps through the placement of a 10?mm and 20?mm-thick geocomposites (GC) on top of the GCL-GMB, respectively.Temperatures on top of the GCLs were found to be significantly reduced by the presence of air gaps, relative to the reference base case. All three designs resulted in GCL desiccation cracks at the end of the tests, due to the relatively high temperature gradients and low water retention of the subsoil, even in the presence of air gaps. However, X-Ray imaging revealed that crack patterns in bentonite samples from designs with air gaps were finer and narrower. Subsequent rehydration (and permeation tests) with distilled water indicated that significant self-healing of bentonite was in evidence in all three cases. However, while in the absence of an air gap the saturated hydraulic conductivity was found to be 2.8 times its pre-heating value, no significant increase was recorded for other two cases. X-Ray imaging of rehydrated samples confirmed that more effective healing had occurred in samples with an air gap.     

Hydraulic conductivity behaviour of soil blended with geofiber inclusions

Efficiency of fiber reinforcements to ensure the sealing efficiency of the landfill cap soil barriers so as to isolate the waste from the environment was demonstrated in the present study. Evaluation of hydraulic conductivity of soil barrier materials with different types of fibers, fiber dosage and fiber lengths are very important to ensure the sealing efficiency of the fiber reinforced soil barriers. An attempt was made to evaluate the hydraulic conductivity of the soil barrier material at a known effective stress using a flexible wall permeameter. Soil samples of 100 mm diameter and 100 mm height were prepared and tested in the present study. In all the cases, the hydraulic conductivity test phase was started after the completion of initialisation, saturation and isotropic consolidation phases of the soil samples. In the present study, seventeen (17) hydraulic conductivity tests were conducted on two different soil types for studying the influence of fiber content, fiber length and fiber type on the hydraulic conductivity of fiber reinforced soil. The fiber content, f used were 0.25%, 0.50% and 0.75% and the fiber lengths, l were 30 mm, 60 mm and 90 mm. Two types of fibers namely polyester (PET) fibers and polypropylene tape (PP-T) fibers were used for hydraulic conductivity tests. The repeatability of test results was also demonstrated. As the fiber content and fiber length were increased, initially there was a marginal decrease in hydraulic conductivity of the soil and thereafter marginally increased. Short fibers and low fiber contents were found to have greater influence in reducing the hydraulic conductivity of the soil and the variation was found to depend on the soil type also. Even with long fibers, the hydraulic conductivity of selected barrier material remained within the permissible limit required for a barrier material. The hydraulic conductivity of PP-T fiber reinforced soil is more, compared to hydraulic conductivity of PET fiber reinforced soil at all the fiber contents varied in the present study. The use of Scanning Electron Microscopy (SEM) is also attempted for the interpretation of the results.     

Saturated hydraulic conductivity of compacted bentonite–sand mixtures before and after gas migration in artificial seawater

To understand the self-healing property of an engineered barrier for radioactive waste disposal, the hydraulic conductivity of compacted bentonite–sand mixtures saturated with artificial seawater (SW) before and after gas migration was examined. Na- and Ca-bentonites were mixed with fine sand at a ratio of 70% bentonite in dry weight. Two aspects were considered during the experiment: the hydraulic conductivity of the specimen that was resaturated after gas migration and the distribution of water content immediately after gas migration to study gas migration pathways. The gas migrated through the entire cross-section of the specimen, and gas breakthrough occurred in the equilibrium swelling pressure range approximately. Subsequently, the gas flow rate reached a sufficient large value when the gas pressure was approximately twice the equilibrium axial pressure (the sum of swelling and confining pressures), which excluded the back pressure. Although the gas migration pathway was not visible when the specimen was observed immediately after gas migration, the water content distribution showed that several parts of the specimen with lower water content were connected in the direction of gas migration. After resaturation, the change in permeability was within a limited range—two to three times larger than that before gas migration for each type of bentonite in SW. This slight change suggests that gas migration creates a pore structure that cannot be sealed via crystalline swelling of montmorillonite in SW, even if highly compacted bentonite is used under a constant-volume condition.     

Influence of bio-clogging on permeability characteristics of soil

It is necessary to enhance the barrier performance of cutoff walls in order to improve the contamination control level, especially for reconstruction or expansion of existing landfill sites. This paper presents a comprehensive laboratory investigation on the synergistic effects of microorganisms and fibers on the hydraulic conductivity of silty sand to evaluate the applicability to the field condition as an alterative barrier material. Inside the soil, the added carbon fibers not only provided good biocompatibility, but also formed spatial three-dimensional network between soil particles to improve the bacterial adhesion that eventually caused 2–3 orders of magnitude decrease in soil permeability. The resistance of the biofilm to extreme conditions was tested by permeation with solutions of different salinity and pH values, and by subjecting specimens to various hydraulic gradients and soil conditions. Despite the microbial growth inhibition occurred at these conditions, however, biofilm can largely remain intact and continue to reduce k, which due to the gradual adaptation of microorganisms to the extreme environment and the gradual recovery of their activity. Results of these tests demonstrate that biofilm treatment may be a feasible technology for creating waste containment barriers in soil.     

Effects of lime treatment on the microstructure and hydraulic conductivity of Héricourt clay

This study aims at evidencing the effects of lime treatment on the microstructure and hydraulic conductivityof a compacted expansive clay, with emphasis put on the effect of lime hydration and modification.For this purpose, evolutions of hydraulic conductivity were investigated for both lime-treatedand untreated soil specimens over 7 d after full saturation of the specimens and their microstructureswere observed at the end. Note that for the treated specimen, dry clay powder was mixed with quicklimeprior to compaction in order to study the effect of lime hydration. It is observed that lime hydration andmodification did not affect the intra-aggregate pores but increased the inter-aggregates pores size. Thisincrease gave rise to an increase of hydraulic conductivity. More precisely, the hydraulic conductivity oflime-treated specimen increased progressively during the first 3 d of modification phase and stabilisedduring the next 4 d which correspond to a short period prior to the stabilisation phase. The microstructureobservation showed that stabilisation reactions took place after 7 d. Under the effect of stabilisation,a decreasing hydraulic conductivity can be expected in longer time due to the formation ofcementitious compounds.
2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.     

Interface transmissivity of conventional and multicomponent GCLs for three permeants

A laboratory investigation of the interface transmissivity is reported for five different geosynthetic clay liners (GCLs) and a range of different geomembranes (GMBs) for a range of stresses from 10 to 150?kPa. The GCLs were prehydrated under normal stress before permeation. The GCLs examined comprised three multicomponent (a smooth coated, a smooth laminated, and textured coated) and two conventional (one with granular and one with powdered sodium bentonite) GCLs. The effect of a 4?mm circular defect in the coating of a multicomponent GCL directly below the 10?mm diameter hole in the GMB is investigated. The effect of GMB stiffness and texture is examined. Additionally, the effect of hydration and permeation of smooth coated GCL with highly saline solution and synthetic landfill leachate (SL3) is presented. It is shown that the 2-week interface transmissivity (θ_2-week) can be one to two orders of magnitude higher than steady-state interface transmissivity (θ _steady-state) at low stresses (10?kPa–50?kPa), whereas at high stresses (150?kPa) the variation is substantially less. For a smooth coated GCL hydrated and permeated with reverse osmosis (RO) water, GMB stiffness and texture has a limited effect on interface transmissivity when the coating is placed in contact with GMB at normal stresses of 10?kPa–150?kPa, whereas coating indentations result in much high interface transmissivity when placed in contact with GMB. GCL prehydration and permeation with highly saline solutions leads to higher interface transmissivity compared to RO water. With a 4.0?mm defect in the coating, the interface transmissivity between the coating and woven geotextile is higher than that between the coating and GMB for the stress levels and GCL examined.     

Hydro-mechanical behavior of a lateritic fiber-soil composite as a waste containment liner

In waste disposal landfill projects, the hydraulic conductivity of the barriers is a major consideration. The use of fibers mixed with backfill may improve the overall performance of the barriers. Fiber-soil composites show a more resistant and ductile behavior than the soil alone. The presence of fibers may reduce cracking problems related to shrinkage or traction in liners or covers. In this study, laboratory tests were performed to evaluate the use of fiber-soil composites as a containing barrier. Hydraulic conductivity and diametral-compression tests were carried out on PET fiber reinforced and unreinforced compacted soil specimens. The tests were conducted under confinement conditions similar to those found in the field. Diametral-compression tests were used to induce cracks in the specimens. Hydraulic conductivity was measured at different stages during the diametral loading. In the tests performed under low confinement pressure (10?kPa), the crack openings led to a significant increase in hydraulic conductivity. The results showed that the addition of fibers increases the tensile strength of the soil-fiber mass and delays the opening of cracks. Moreover, in the tests under high confinement pressure (100?kPa), a decrease in hydraulic conductivity occurred at all stages of the diametral load application.     

竖向应力作用下GCL的膨胀特性和渗透性能

近年来,土工织物膨润土垫(GCL)被越来越多地应用到各种防渗工程之中,它的防渗有效性也成为了设计人员和研究人员所关注的焦点.GCL的防渗有效性包括渗透性能、吸附能力和内部剪切强度三个方面.通过水化膨胀试验和渗透试验研究了GCL在竖向应力作用下的膨胀特性和渗透性能,并分析了正应力和加压水化顺序的影响.试验结果表明:(1)随着竖向应力的增大,GCL的膨胀量不断减小,而GCL的渗透系数则出现先减小后略有增大的规律;(2)水化加压顺序对GCL的膨胀量和渗透系数均有影响;(3)在实际工程应用中,GCL铺设完成后在堆载之前最好完全水化,这样能够大大提高GCL的防渗有效性.     

Hydraulic conductivity of bentonite-polymer composite geosynthetic clay liners permeated with bauxite liquor

The high ionic strength of the porewater in red mud (bauxite liquor from digestion) can suppress swelling of montmorillonite, resulting in geosynthetic clay liners (GCLs) that are too permeable to be effective as liners in red mud disposal facilities. Bentonite-polymer composite GCLs (BPC GCLs) have been developed as more resilient lining materials, and some BPC GCLs have been shown to have very low hydraulic conductivity to bauxite liquors that have extreme ionic strength and pH. In this study, a nationwide investigation was conducted in China to evaluate the characteristics of bauxite liquor in Chinese impoundments, and to evaluate the suitability of GCLs containing granular sodium bentonite or BPCs for containment. Hydraulic conductivity tests were conducted on six BPC GCLs with two characteristic Chinese bauxite liquors that are hyperalkaline (pH > 12) and had ionic strengths of 76.9 mM and 620.3 mM. The BPC GCLs had hydraulic conductivity ranging from 10^?8-10^?12 m/s, which is higher than the hydraulic conductivity of BPC GCLs to deionized water (10^?12-10^?13 m/s), but lower than the hydraulic conductivity of conventional GCLs with granular sodium bentonite GCLs to the same liquors (10^?7-10^?8 m/s). The hydraulic conductivity of the BPC GCLs depends on the chemical properties of the leachate, the polymer loading, and the type of polymer. Microstructural analysis by scanning electron microscopy (SEM) suggests that the hydraulic conductivity of BPC GCLs is controlled by pore-blocking by polymer hydrogel, which is affected by the bauxite liquor.     

The effect of thickness reduction on the hydraulic transmissivity of geonet drains using rigid and non-rigid flow boundaries

In this paper, relationships between in-plane flow capacity reduction and thickness reduction are presented in tri-planar and bi-planar geonets for rigid and non-rigid flow boundaries. Using these equations, the long-term flow capacity of geonets can be determined using creep test results. To validate these relationships, geonet thickness was measured under different conditions and the theoretical values of the transmissivity reduction ratios were calculated by substituting the results in the equations. Transmissivity tests were then performed under the same conditions to obtain experimental values of the reduction ratios. A comparison showed that the theoretical and experimental values of the transmissivity reduction ratios were in agreement, and the relationships provide a useful tool to predict the drainage capacity of both tri-planar and bi-planar geonets influenced by loading pressure. However, special precautions must be taken when applying the equations to investigate the hydraulic capacity of other types of geosynthetic drains as well as when the geonet is covered by geotextile material acting as a filter between the geonet and adjacent soil, is overlain by geosynthetic clay liner material where the swelling potential of the bentonite in the geonet exists, is placed in inclined positions or is subjected to complex combinations of load.     

Temperature and concentration dependence of ammonium migration in bentonite-clay mixtures: A case study in Hanoi,Vietnam

Groundwater in southern Hanoi, Vietnam has been recently detected to possess high concentration of ammonium ion (NH₄⁺). Otherwise, one of the abundant sources of NH₄⁺ comes from municipal solid waste landfills. Bentonite-clay mixtures (BCMs) widely utilized as landfill bottom barriers in various countries, but limited in Vietnam should perform well to isolate NH₄⁺ from groundwater. This study is to evaluate combined effects of temperature and initial ammonium concentration on adsorption, diffusion, and permeability through mixtures of indigenous clay with 0 %, 5 %, 10 %, 15 % bentonite. The results indicated more effective NH₄⁺ adsorption capacity for low initial concentration than high initial concentration in all temperatures (20, 35, and 50 °C). The temperature dependency showed an increase in adsorption coefficient from 20 °C to 35 °C and a decrease in the range of 35 °C and 50 °C. Whereas diffusion coefficient and hydraulic conductivity for all cases keep increasing gradually in both temperature ranges. The reasonable mass of bentonite content of 15 % should be added into local clay for landfill bottom liners in such conditions of elevated temperature at 50 °C and interaction of ammonium solution 1000 mg/L. The micro-structures via SEM images of these materials provided the proofs of both improvement of hydraulic barrier properties for indigenous clay owing to bentonite presence and NH₄⁺ effects on their micro-structures.