Hydraulic Conductivity Requirement Research Articles

Evaluation of red soil-bentonite mixtures for compacted clay liners

Compacted clay liners are an integral part of the waste landfills, which are provided to contain the leachate within the landfills and protect the surrounding environment. Generally, locally available natural soils are used for the construction of compacted clay liners if they satisfy the design criteria. However, not all soils in their natural state satisfy all the design criteria for the liner materials. Thus, there is a definite need to modify the locally available natural soils by blending with bentonite to meet the required design criteria for the liners. In view of this, the present study evaluates the suitability of an Indian red soil enhanced with bentonite as a liner material. To achieve this, a series of experiments were carried out using locally available red soil and bentonite. First, the suitability of the red soil was evaluated as a liner material. The experimental results showed that the red soil met all the selection criteria stipulated by the Environmental Protection Agencies (EPAs) for the liners except the hydraulic conductivity criterion. Therefore, the red soil was mixed with bentonite contents of 10%, 20% and 30%, and the red soil-bentonite mixtures were evaluated for their suitability for liners in their compacted state. Further, as the liners in the arid and semi-arid regions are subjected to moisture variations due to seasonal moisture fluctuations and other factors, the red soil-bentonite mixtures were subjected to wet-dry cycles, and their suitability was evaluated after wet-dry cycles. The experimental results revealed that all the red soil-bentonite mixtures met the stipulated EPA criteria for the liners in the as-compacted state. However, the red soil-bentonite mixtures with 20% and 30% bentonite contents only satisfied the hydraulic conductivity requirement even after wet-dry cycles. The experimental findings were supplemented with the microstructural insights captured through digital camera images, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) studies.

Evaluation of potential shaft backfill formulations for use in Canada's deep geological repository

The Canadian Nuclear Waste Management Organization (NWMO) is evaluating Sedimentary and Crystalline geosphere media as potential hosts for a Deep Geological Repository (DGR) for permanent isolation of used nuclear fuel and fuel wastes.Bentonite-aggregate mixtures containing from 50 to 90% by dry mass bentonite were tested. The aggregates used were relevant to the NWMO's DGR geological options (granitic sand (GS) and sand-sized crushed limestone (CRL)). Of importance is that the moisture conditioning fluids used are representative of those that will be encountered at the locations considered by NWMO for their aDGR. These materials, if moisture-conditioned with local groundwater will mean that the porewater of the backfill will be close to chemical equilibrium with its surroundings at the time of placement. The effectiveness of Standard and Modified compaction efforts in densifying these mixtures were evaluated, providing guidance regarding what materials and compaction options are likely to achieve the swelling pressure (Ps) and hydraulic conductivity (k) performance goals (>100 kPa and < 10−10 m/s, respectively) set for shaft backfill once it comes to density equilibrium and chemical balance with the surrounding rock mass.When Standard Compaction Effort was used, aggregate type did not affect densification, while aggregate content and compaction composition fluid did. Dry density increased with increasing aggregate content, while effective montmorillonite dry density (EMDD) showed a small increase as bentonite content increased from 50 to 90%.When Modified Compaction Effort was used, aggregate type and composition of the compaction fluid had little effect on the densification achieved. Dry density achieved steadily decreased with increasing bentonite content and EMDD achieved was little affected by bentonite contents >60% by mass.Hydraulic conductivity values measured were consistent with previously reported data for bentonite and bentonite-aggregate mixtures. The hydraulic conductivity of mixtures prepared using crushed limestone were slightly lower than determined for granitic sand.The swelling pressure and hydraulic conductivity requirements set for the SBF were met for all materials examined (50–90% bentonite blended with crushed limestone or granitic sand, using either standard or modified compaction effort at low salinity. Where highly saline conditions existed, the Ps and k requirements for the shaft backfill were not be met for materials compacted using standard compaction effort.

The Application of Porous Material in the Capillary Barrier System for Residual Soil Slope Stabilization

A drainage system that is used to flow excess rainfall infiltration is an excellent alternative to enhance lateral diversion capacity at the interface of Grade VI and Grade V soil slope layers thus delaying breakthrough occurrences. This study aims to explore the performance of porous transport layer on slope behavior via numerical simulation by GeoStudio SEEP/W. In this study, a 2-dimensional non-homogenous residual soil configuration model, which is composed of Grade VI and Grade V residual soil was constructed. Laboratory tests were conducted on the soil samples collected from the Faculty of Engineering, UTM JB to determine the soil properties as input parameters in numerical modeling. The soil slope model is designed as an infinite residual soil slope according to Jabatan Kerja Raya (JKR) design specification on the cut slope and fill slope with 27 ° inclination respectively. High rainfall intensities were chosen to represent rainfall infiltration in Johor. The slope model was modeled with a porous transport layer at the Grade VI and Grade V layer interfaces. The porous transport layer is introduced as the transport layer and its effectiveness is expected to be similar to the optimum compacted soil mix in the previous study. In addition, porous concrete as a transport layer is introduced and it must meet the adequate porosity and hydraulic conductivity requirements. The suction distribution pattern with depth and time was subsequently observed to identify the volume of infiltrated water and retention time in the transport layer. The porous transport layer is also expected to be efficient in diverting infiltrating water and decreasing the interface's suction distribution between Grade VI and Grade V soil layers. HIGHLIGHTS The water built up within the residual soil slope is one of the common factors of rainfall-induced residual soil slope failure thus porous transport layer introduced The detailed laboratory testing and numerical modelling conducted in order to identify the performance of porous transport layer within residual soil slope The application of porous transport layer sandwiched between residual soil is effective to decrease the water built up and matric suction thus decrease the risk of slope failure GRAPHICAL ABSTRACT

Recent developments in design criteria for granular and geotextile filters

Granular and geotextile filters are commonly provided in several hydrological infrastructures to limit soil erosion and allow unimpeded water seepage. The success of a filter depends on forming a bridging structure, which is governed by the grain size distribution of soil and the constriction size distribution of filter. Currently, the retention requirement is satisfied considering representative grain and opening size, whereas the hydraulic conductivity requirement is satisfied considering empirical factors for avoiding excessive clogging. In this paper, the design criteria for granular and geotextile filters are reviewed, and improved design criteria are presented. A probabilistic retention criterion is developed, considering the grain size and constriction size as random variables. The influence of filter thickness is incorporated into the criterion by considering the number of constrictions in a filtration path. A hydraulic conductivity criterion is developed theoretically based on governing flow equations and the expected partial clogging of geotextiles. The limit states for the developed criteria are evaluated based on the wide range of experimental data. The developed design criteria are applicable to granular and nonwoven geotextiles, which offers an improvement in design compared to the existing criteria in practice.

Mechanical and Chemical Behaviour of Water Treatment Sludge and Soft Soil Mixtures for Liner Production

Background: Clay-based and geosynthetic liners are generally used as hydraulic barriers in solid waste disposal facilities, mining tailing ponds, and soil-based wastewater treatment technologies, avoiding the leaching of hazardous compounds into subsoil and groundwater. Water treatment sludge (WTS) is a water treatment plant (WTP) residue which due to hydraulic properties seems to decrease the permeability in WTS:soil mixtures and may become an alternative material to produce sustainable waste-based liners. Objectives: This research aims to characterize and analyse physical, chemical and mechanical parameters of a WTS, a soft soil and four mixtures WTS:soil following 05:95%, 10:90%, 15:85%, 20:80% ratios, Thereby, evaluating the best ratio for producing waste-based liners for civil engineering applications. Methods: The geotechnical characterization was performed for particle size distribution, specific surface, specific gravity, Atterberg limits, and Normal Proctor compaction; chemical composition due to oxides analyses through X-ray fluorescence (XRF), mineralogical description by X-ray diffraction (XRD) and scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) coupled for imaging; and mechanical behavior performing - –oedometric consolidation, consolidated undrained (CU) triaxial, and falling head permeability. Tests were conducted for all mixtures, the soil, and WTS, with pointed-out exceptions. Results and Discussion: The results showed that the fine-grained WTS filled the soil voids but rearranged soil particles, thus, compacted dry unit weight decreased with WTS addition, probably due to its chemical composition with high amounts of aluminium and silica. The compressibility of the compacted mixtures did not differ significantly compared to the soil, while the shear strength analysis demonstrated a reduction in cohesion and an increase in the effective internal friction angle proportional to WTS addition. Hydraulic conductivity increased with WTS until 10% of residue introduction, decreased for 15%, and continued to decrease for 20%, reaching optimum permeability at 15%. Conclusion: The incorporation of WTS can improve or just not interfere with soil’s properties to be used as liner material in solid wastes storage facilities, mining ponds and soil-based wastewater treatment technologies. Furthermore, 15% of WTS (15:85% mixture) incorporation provided the best results meeting the hydraulic conductivity requirement for liner materials, i.e., equal, or lower than 10-9 m/s. The reuse of WTS for this purpose would allow producing a new added-value material in the scope of circular economy.

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.

Construction of Plastic Concrete Seepage Cut-Off Wall for Polavaram Earth Cum Rockfill Dam

Abstract: Polavaram Project is being executed on River Godavari near Ramayyapeta village of Polavaram Mandal, West Godavari District, Andhra Pradesh. This multipurpose major project envisages construction of 2454m long Earth-cum-Rock fill (ECRF) dam across river Godavari near Polavaram with a maximum height of 50.32m. The proposed ECRF dam will be founded on a bed of sand. The depth of sand bed over the underlying rock strata varies from 30m to 90m at different location. To restrict the flow of seepage water through the sand bed, a plastic concrete diaphragm wall/Cut-Off wall is proposed with its top end embedded in the clay core of the ECRF Dam and the bottom end embedded into underlying hard rock. The wall was constructed of plastic concrete using panel construction method. Plastic concrete was selected to provide a seepage Cut-Off wall that has sufficient strength to withstand both static and seismic stresses beneath the new embankment, and yet is flexible enough to undergo seismic deformations, without cracking, with the surrounding soils. This paper illustrates the construction of the plastic concrete Cut-Off wall for the Polavaram ECRF dam, including the field and laboratory testing performed to confirm design wall stiffness, strength, and hydraulic conductivity requirements. The trial laboratory and field testing programs to determine plastic concrete mix design, and the QA/QC testing conducted during construction are presented. Keywords: ECRF, Plastic concrete, Cut-Off wall, Hydraulic grab, Trench cutter, Koden, Tremie

Hydraulic conductivity requirement of granular and geotextile filter for internally stable soils

A filter media satisfying the hydraulic conductivity requirements allows unimpeded seepage without generation of surplus pressure head and decrease of flow rate. This paper proposes design criteria for the hydraulic conductivity requirements of a filter based on governing flow equations. The results have shown that the hydraulic conductivity requirements of pressure head and flow rate are satisfied with a single condition of hydraulic conductivity of filter greater than or equal to the hydraulic conductivity of soil times the hydraulic gradient in soil. The proposed model is developed for saturated conditions and is also applicable for partially saturated conditions. The developed model is validated based on the experimental evaluations of sandy soil with three granular filters and two needle punched non-woven geotextile filters. The developed design criterion applies to internally stable soils with granular and geotextiles filters and offers an improvement in the standards and current design guidelines for protective filters.

Hydraulic conductivity and strength of foamed cement-stabilized marine clay

In traditional soft ground improvement of cement stabilized columns, strength was paid more attention than hydraulic conductivity, but the latter can improve the settlement uniformity and long term safety. This study introduced the pore-making technology referring the foaming concrete/mortar in an attempt to improve the hydraulic conductivity of the stabilized marine clays. The flexible wall permeameter and unconfined compressive strength (UCS) tests were conducted to depict the macro behaviour, and then the mercury intrusion porosimetry (MIP) tests and scanning electron microscopy (SEM) tests were performed to clarify the micro-structures. The hydraulic conductivity of foamed cement-stabilized marine clay at a given cement ratio (20%–50%) and density (900–1400 kg/m3) was found to be approximately 100–1000 times of that of (cement stabilized) clays without foaming, whereas the strength decreased only by 2–4 times. Microstructure investigation indicated that the hydraulic conductivity of foamed cement-stabilized marine clays was mainly dominated by the macro pore volume (>1 μm). Furthermore, a greater cement ratio and more active metakaolin additives enable developing an innovative material for columns that satisfy both the strength and hydraulic conductivity requirement.

A Critical Evaluation of the Suction and Swelling Characteristics of Sand–Bentonite–Coal Ash Mixes

In recent times, compacted bentonite–sand/coal ash mixtures have been suggested as backfilling materials for waste disposal systems. These soil mixes containing mineral montmorillonite from the varying percentage of bentonite may exhibit swell/shrink behaviour with fluctuating moisture conditions. This paper presents the results of matric suction using pressure plate apparatus and swelling characteristics in terms of swelling pressure, free swell and swell potential on compacted sand–bentonite–coal ash mixtures. Bentonite content was varied as 5, 10, 15, 20, 30, 40% in case of sand–bentonite mixes; where as it was kept as 5, 10, 15, 20% for coal ash–bentonite mixes. Coal ash percentage was varied from 10, 20, 30, 50% for sand–bentonite–coal ash mixes. Data was fitted using two models—Van Genuchten, Fredlund and Xing to obtain the soil water characteristic curve (SWCC) and to estimate the SWCC variables for different soil mixes. Test results depict that soils with high bentonite content exhibited a prominent swelling behaviour, while addition of coal ash had a stabilizing effect on the mix by reducing the swelling characteristics. The air entry value (AEV) and residual suction (Ψr) values decreased as the effective grain size (D10) of soil increased. A distinction in the values of the AEV and Ψr was observed using the Van Genuchten, Fredlund and Xing Models. Also, keeping in mind the statutory requirement of low hydraulic conductivity for use as a liner material, hydraulic conductivity of soil mixes was ascertained in both saturated and unsaturated state. Addition of 10% bentonite to sand and coal ash both is recommended for use as liner and cover materials in case of engineered landfills considering the hydraulic conductivity requirement and swelling characteristics.

Deformation and Compression Behaviour of a Cement\u2013Bentonite Slurry for Groundwater Control Applications

Cement bentonite (CB) barriers are self-supporting, low permeability, structures used to retard groundwater flow and as such strength and hydraulic conductivity parameters are often stipulated when developing the mixtures. This paper reports an investigation into the deformation and compression behaviour of a CB containing ground granulated blastfurnace slag using the unconfined compressive strength apparatus, triaxial (undrained, unconsolidated) and oedometer. Samples were also exposed to drying and rewetting to investigate possible response to changes in environmental conditions. Cracking was observed prior to peak stress suggesting that the hydraulic conductivity of a barrier may be adversely affected before the shear strength is reached in undrained conditions. The compression response of CB indicates the presence of a threshold stress; once exceeded the magnitude of settlements are significantly greater than those encountered below this threshold. If a barrier experiences localised changes in loading conditions then there is the potential for damage from induced differential settlements; thus it is recommended that the threshold stress should also be considered at the design stage in addition to strength and hydraulic conductivity requirements. The response of the material exposed to drying-rewetting was unexpected and requires further investigation to determine how a barrier will respond to changing environmental conditions.

Sorption behaviour of heavy metals in sand-bentonite-coal ash mixes for use as a liner material in landfills

The heavy metals are among the potential groups present in the landfill leachate in addition to various organic and inorganic contaminants and their content often exceeds the permissible limits. These heavy metals in leachate are an environmental threat unless contained or adsorbed by an impermeable liner. The barrier material/liner for landfills is generally fabricated by mixing bentonite in locally available soil to meet out the hydraulic conductivity requirement in absence of clay in the vicinity. The sorption characteristics have an important bearing to evaluate the adsorption behaviour and migration of contaminants in order to restrict the mobility of contaminants within the liner to prevent contamination of geo-environment. This study includes the sorption behaviour of landfill liner fabricated by mixing sand, bentonite and coal ash in varying proportions using Cd, Zn, Ni and Pb as the target metals including potting of adsorption isotherms. Double distilled water was used as the permeating fluid in the sorption study. With a change in percentage of sand/bentonite/coal ash in the mix, the constituent material dominating the adsorption properties differed leading to an altogether different adsorption behaviour of the soil mix. The addition of bentonite was found to be beneficial to both sand and coal ash in improving the sorption behaviour. Langmuir Isotherm was found to be best fit to the experimental data. The adsorption capacity of the formulated soil mixes with sand–bentonite and coal ash–bentonite continued to increase up to the limiting percentages considered in this study i.e. 40 and 20% for sand–bentonite and coal ash–bentonite mixes, respectively, although coal ash in itself does not prove to be a good adsorbent.

The Compressibility and Swell of Mixtures for Sand-Clay Liners

Sand-clay liners utilize expansive clay to act as a filler to occupy the voids in the sand and thus reduce the hydraulic conductivity of the mixture. The hydraulic conductivity and transfer of water and other substances through sand-clay mixtures are of prime concern in the design of liners and hydraulic barriers. Many successful research studies have been undertaken to achieve appropriate mixtures that satisfy hydraulic conductivity requirements. This study investigates compressibility and swelling properties of mixtures to ensure that they were acceptable for light structures, roads, and slabs on grade. A range of sand-expansive clay mixtures were investigated for swell and compression properties. The swelling and compressibility indices were found to increase with increasing clay content. The use of highly expansive material can result in large volume changes due to swell and shrinkage. The inclusion of less expansive soil material as partial replacement of bentonite by one-third to two-thirds is found to reduce the compressibility by 60% to 70% for 10% and 15% clay content, respectively. The swelling pressure and swell percent were also found significantly reduced. Adding less expansive natural clay to bentonite can produce liners that are still sufficiently impervious and at the same time less problematic.

Improvement of attenuation functions of a clayey sandstone for landfill leachate containment by bentonite addition

Enhanced sand–clay mixtures have been prepared by using a sandstone arkosic material and have been evaluated for consideration as landfill liners. A lab-scale test was carried out under controlled conditions with different amended natural sandstones whereby leachate was passed through the compacted mixtures. The compacted samples consisted of siliceous sand (quartz–feldspar sand separated from the arkose sandstone) and clay (purified clay from arkose sandstone and two commercial bentonites) materials that were mixed in different proportions. The separation of mineral materials from a common and abundant natural source, for soil protection purposes, is proposed as an economic and environmentally efficient practice. The liner qualities were compared for their mineralogical, physicochemical and major ions transport and adsorption properties. Although all samples fulfilled hydraulic conductivity requirements, the addition of bentonite to arkose sandstone was determined to be an effective strategy to decrease the permeability of the soil and to improve the pollutants retention. The clay materials from arkose sandstone also contributed to pollutant retention by a significant improvement of the cation exchange capacity of the bulk material. However, the mixtures prepared with clay materials from the arkose, exhibited a slight increase of hydraulic conductivity. This effect has to be further evaluated.

Field applicability of self-recovering sustainable liner as landfill final cover

Preventing the penetration of rainwater into a landfill site is the main purpose of the final cover in landfill sites. Conventional designs of landfill covers use geotextiles, such as geomembrane and geosynthetic clay liners, and clay liners to lower the permeability of the final cover of landfill sites. However, differential settlement and climatic effects in landfill sites instigate crack development or structural damage inside the final cover. This study therefore investigates the field applicability of a self-recovering sustainable liner (SRSL) as an alternative to the landfill final cover. The SRSL utilizes the precipitation reaction of two chemical materials to form precipitates that fill the pores and thereby lower the overall permeability of the liner. To examine the field applicability of the SRSL system, uniaxial compression tests and laboratory hydraulic conductivity tests were performed under various climatic effects such as wet/dry and freeze/thaw processes. Furthermore, field-scale hydraulic conductivity tests were performed with intentionally induced cracks to demonstrate the self-recovery performance for practical applications. Extensive laboratory and field test results confirmed the capability of the SRSL final cover system to fulfill the strength and hydraulic conductivity requirements, even in harsh field conditions.

Sand–Attapulgite Clay Mixtures as a Landfill Liner

This paper investigates the potential use of sand–attapulgite (palygorskite) mixtures as a landfill liner. The sand and attapulgite clay used in this study were brought from Wahiba (eastern Oman) and Al-Shuwamiyah (southern Oman), respectively. Initially the basic properties of the sand and clay were determined. Then the attapulgite clay was added to the sand at 5, 10, 20 and 30% by dry weight of the sand. The sand–attapulgite clay mixtures were subjected to mineralogical, chemical, microfabric and geotechnical analyses. The X-ray diffraction (XRD) qualitative analysis showed that attapulgite is the major clay mineral. The chemical compounds, exchangeable cations and cation exchange capacity (CEC) for the␣samples were determined. The CEC for the sand–clay mixtures is low but increases with the increase in clay content. The scanning electron microscope (SEM) examination showed that the addition of clay developed coating between and around the sand grains which results in filling the voids and reducing the hydraulic conductivity of the sand–clay mixtures. The hydraulic conductivity values for the pure clay and sand + 30% clay mixture prepared at 2% above optimum water content are slightly higher than hydraulic conductivity requirements for landfill liners but can be acceptable. The geotechnical study which included grain size distribution, Atterberg limits, specific gravity, compaction, hydraulic conductivity and shear strength tests showed that the sand+30% clay mixture prepared at 2% above optimum water content can be considered to satisfy the requirements for landfill liners. For all sand–clay mixtures no swelling was recorded and the addition of clay to the sand improved the shear strength.

Optimal Design of a Compacted Soil Liner Containing Sorptive Amendments

The design of a compacted soil liner that includes sorptive amendments is presented and evaluated as a combinatorial optimization problem. An objective function based on the materials costs, opportunity costs, and construction costs of the liner was used to evaluate the effect of incorporating four sorptive materials: benzyltriethylammnonium-bentonite, hexadecyltrimethylammonium-bentonite, shale, and granular activated carbon (GAC) into a compacted clay liner in order to mitigate transport of organic solutes through the liner. The results from this study indicate that the inclusion of sorptive amendments as a component in compacted soil liners can effectively retard the transport of organic contaminants through the liner without violating regulatory hydraulic conductivity requirements. In all cases when aqueous transport was considered as a constraint in the objective function formulation, the resulting liner always contained some amount of sorptive amendment. In general, shale and GAC were selected for use in the sorptive liner design for all organic solutes tested. The modeling framework presented in this study is general and could be used to evaluate other types of sorptive materials or additional constraints, and thus represents a flexible new tool for the design of compacted soil liners.

Evaluation of a compacted bentonite/sand seal for underground waste repository isolation

This study investigates the performance of an optimum compacted bentonite/sand mixture seal for the isolation of underground waste repositories. Engineering geological tests such as compaction, flow, swelling, mechanical and shear strength tests have been conducted to select an optimum mixture and to recommend a stable bentonite/sand seal length-to-radius ratio (L/a) as far as the factor of safety (F) is concerned. The results of the compaction permeameter tests led to a recommendation to select an optimum compacted bentonite/sand mixture possessing a bentonite content of about 20% to satisfy the minimum regulatory hydraulic conductivity requirement. Engineering geological analysis of the seal/rock mechanical interaction with regard to reduce the possibility of seal slip led to a recommendation to utilize a seal L/a of at least 25.

Feasibility of using fly ash admixtures in landfill bottom liners or vertical barriers at contaminated sites

Results from a comprehensive experimental program conducted to examine the feasibility of constructing landfill liners or vertical barriers to pollutant migration at contaminated sites using fly ash based materials are described. The materials evaluated are fly ash, lime‐fly ash, poly vinyl alcohol (PVA) polymer‐lime‐fly ash and bentonite‐fly ash. The focus of this paper is to comparatively evaluate the engineering properties of fly ash based materials in relation to potential use at landfills and/or at contaminated sites. Experimental results indicate that Alberta fly ash itself can not be compacted to achieve a hydraulic conductivity of less than 1×10‐7 cms‐1, the “benchmark”; requirement for low permeable barriers, and therefore is not suitable as a construction material for landfill bottom liners or vertical barriers. Fly ash amended with up to 20% lime satisfied the hydraulic conductivity requirement of less than 1×10‐7cms‐1. However, lime‐fly ash becomes brittle with age. The resultant inflexibility will produce cracks when subjected to differential loading and settlement conditions at landfill sites. Fly ash mixed with Ca‐bentonite (55% to 45%), or Na‐bentonite (90% to 10%), and compacted at wet‐of‐optimum moisture content exhibited hydraulic conductivity values less than 1 x 10‐7 cms‐1. Therefore, fly ash‐bentonite (or FAB) mixtures in a compacted state can be used as a landfill bottom liner material. However, utilization of FAB in the slurry form, which is required for constructing vertical barriers, may not be preferable due to segregation. Addition of 4 to 10% PVA polymer to lime‐fly ash satisfied the hydraulic conductivity requirement of less than 1×10‐7cms‐1. The PVA‐lime‐fly ash (or LFP) samples are highly ductile and flexible. Hardened LFP samples exhibit compressive, tensile and flexural strengths adequate to cause the material to remain in elastic equilibrium under impact loads of landfills, and lateral loads due to soil pressures. Hardened LFP slurry is highly likely to retain its structural integrity under differential settlement conditions better than any other fly ash based material.

Water movement through an experimental soil liner

A field-scale soil liner was constructed to test whether compacted soil barriers in cover and liner systems could be built to meet the U.S. EPA saturated hydraulic conductivity requirement (⩽ 1 × 10 −7 cm s −1). The 8 × 15 × 0.9 m liner was constructed in 15 cm compacted lifts using a 20,037 kg pad-foot compactor and standard engineering practices. Water infiltration into the liner has been monitored for one year. Monitoring will continue until water break through at the base of the liner occurs. Estimated saturated hydraulic conductivities were 2.5 × 10 −9, 4.0 × 10 −8, and 5.0 × 10 −8 cm s −1 based on measurements of water infiltration into the liner by large- and small-ring infiltrometers and a water balance analysis, respectively. Also investigated in this research was the variability of the liner's hydraulic properties and estimates of the transit times for water and tracers. Small variances exhibited by small-ring flux data suggested that the liner was homogeneous with respect to infiltration fluxes. The predictions of water and tracer breakthrough at the base of the liner ranged from 2.4–12.6 y, depending on the method of calculation and assumptions made. The liner appeared to be saturated to a depth between 18 and 33 cm at the end of the first year of monitoring. Transit time calculations cannot be verified yet, since breakthrough has not occurred. The work conducted so far indicates that compacted soil barriers can be constructed to meet the saturated hydraulic conductivity requirement established by the U.S. EPA.