Capillary Barrier Performance Research Articles

Construction and Test of an Instrumented 2D Channel with Rainfall and Insolation Control

This paper presents the construction and testing of a large instrumented 2D channel for the simulation of the performance of compacted barriers under controlled conditions of insolation and rainfall. Details of the main apparatus devices and capabilities and the results of a long-term test performed on a capillary barrier (CB) are presented. The performed test aimed to simulate the behavior of the CB over 1 year in typical semi-arid conditions. The channel behavior was considered very promising. Boundary conditions in terms of rainfall and insolation were adequately properly imposed and suction values, moisture content, runoff and bottom drainage were measured throughout the test. As for the CB performance, the upper clayey layer of soil presented undesirable shrinkage cracks that impacted the CB performance, mainly at the end of the period of evaluation. The obtained results point to the need to use of thicker layers than used in the experiments in order to preserve the integrity of the clayey soil near the interface with the bottom coarse soil layer.

Cover systems with synthetic water‐repellent soils

AbstractA cover system is a crucial component of engineered landfills, to minimize water percolation into the underlying waste. Capillary barriers are an alternative cover system, which has been widely used in the arid and semiarid regions as no cohesive, low‐permeability materials are used. However, the performance of capillary barriers in tropical climate has been unsatisfactory (breakthrough observed). In recent years, synthetic water‐repellent granular materials have drawn increasing attention due to their distinctive hydraulic behavior (inhibited water infiltration and high water entry pressure), suggesting they may also be used to improve the performance of cover systems. In this study, flume tests were conducted with inclined model slopes under artificial rainfall. By monitoring the surface runoff, lateral diversion, and basal percolation and conducting water balance analysis, the performance of monolithic cover, conventional capillary barrier, and water‐repellent cover systems were evaluated. The study revealed that (a) the barrier effect and diversion capacity were significantly strengthened by induced water repellency, providing a promising solution to extend the application of capillary barrier covers; and (b) cover systems can be formed using one raw material to decrease the construction cost, by using synthetic water‐repellent soil as the underlying layer.

다중 덮개시스템의 모세관 방벽 효과 평가

Capillary barriers, consisting of relatively fine-over-coarse materials, have been suggested as an alternative to traditional compacted soil covers. So, We were analysed to capillary barrier effect according to five cases of multi-layer cover systems. Water balance simulation was conducted with unsaturated flow model HELP to assess unsaturated hydraulic parameters such as hydraulic conductivity, climate affecting the performance of capillary barriers. Simulation were conducted for 5 Cases in the Ulsan area. Result of simulation indicated that three cases was formed unsaturated condition and capillary barrier effect.

Flow Processes in the Dry Regime: The Effect on Capillary Barrier Performance

Engineered capillary barriers typically consist of two layers of granular materials designed so that the contrast in material hydraulic properties and sloping interface retain infiltrating water in the upper layer. We conducted two benchtop capillary barrier experiments, followed by interpretation and numerical modeling. The hydraulic parameters for two coarse materials were measured using standard methods, and we found that the materials had similar hydraulic properties despite being morphologically different (round vs. angular). The round sand provided a better functioning capillary barrier than the angular sand, but neither experiment could be characterized as a perfectly working capillary barrier. In both cases, >93% of the infiltrating water was successfully diverted from the lower layer; however, infiltration into the underlying layer was observed in both systems. Based on this work, we believe that noncontinuum processes such as vapor diffusion and film flow contribute to the observed phenomena and are important aspects to consider with respect to capillary barrier design as well as dry vadose zone processes in general. Using a theoretical film flow equation that incorporates the surface geometry of the porous material, we found that infiltration into the coarse underlying sand layer appeared to be dominated by water film flow. The NUFT (Nonisothermal Unsaturated–Saturated Flow and Transport) model was used for qualitative comparison simulations. We were able to reproduce the barrier breach observed in the experiments using targeted parameter adjustment, by which pseudo‐film flow was successfully simulated.

Effects of infiltration and evaporation on geosynthetic capillary barrier performance

This study includes an experimental investigation of the transient movement of water in unsaturated soil layers underlain by a geocomposite drainage layer (GDL) during cycles of infiltration and evaporation. The distribution in volumetric water content with depth in a soil column having a height of 1350 mm underlain by a GDL was measured during transient infiltration. The capillary break effect was observed to affect the soil up to a height of 500 mm above the GDL, with an increase in volumetric water content up to 20% above that expected for the case of infiltration under a unit hydraulic gradient. Due to the long duration of this test (2000 h), a shorter 150 mm high soil column was also evaluated to investigate the soil–GDL hydraulic interaction during cycles of infiltration and evaporation. The capillary break was observed to have re-established itself after infiltration was stopped and the soil near the interface dried. The suction and volumetric water content measured in the soil at breakthrough were consistent after multiple cycles of wetting and drying. The conditions in the soil after each breakthrough event corresponded to the point on the drying-path water retention curve of the nonwoven geotextile where it transitioned from residual to saturated conditions.

Analytical and Numerical Modeling Assessment of Capillary Barrier Performance Degradation due to Contaminant-Induced Surface Tension Reduction

Analytical and numerical models of capillary barrier performance commonly use hydraulic characteristics measured using pure water. However, the potential exists for an infiltrating solution to have a surface tension lower than that of pure water due to the presence of surface-active contaminants (surfactants). A lower surface tension solution may impact capillary barrier performance due to the dependence of capillarity on surface tension. An existing analytical solution for capillary diversion length (L) was modified to include the effect of surface tension reduction on steady-state capillary barrier performance during uniform and constant infiltration. The L for a surfactant-contaminated system was found to be less than for a pure water system and equal to L for a pure water system multiplied by the relative surface tension. Numerical modeling using HYDRUS-2D also showed that diversion was less in the surfactant-contaminated system and that the difference in the performance of the two systems was due to ...

Visualization experiment to investigate capillary barrier performance in the context of a Yucca Mountain emplacement drift

The use of capillary barriers as engineered backfill systems to divert water away from radioactive waste potentially stored in a Yucca Mountain emplacement drift is investigated. We designed and conducted a flow visualization experiment to investigate capillary barrier performance in this context. A two-dimensional, thin slab, test system replicated the physical emplacement drift to one-quarter scale (1.4-m diameter) and included the simulated drift wall, waste canister, pedestal, capillary barrier backfill, and host-rock fracture system. Water was supplied at the top of the simulated drift and allowed to discharge by way of wicks located along the left wall of the cell (simulated fractures) or by a gravity drain at the bottom of the right side (simulated impermeable rock with floor drain). Photographs captured the migration of water and a blue dye tracer within the system, analytical balances measured the mass balance of water, while tensiometers measured the capillary pressure at numerous locations. Of particular concern to this test was the drainage of the capillary barrier, which terminates against the drift wall. We found that while the simulated fractures (left side) and drain (right side) each influenced the performance of the capillary barrier at early time, they had little differential affect at later times. Also of concern was the small disparity in capillary properties between the fine and coarse layer (limited by the need of a fine-grained material that would not filter into the coarse layer under dry conditions). While the capillary barrier was able to divert the majority of flow toward the edges of the system and away from the simulated waste canister, the barrier did not preclude flow in the coarse layer, which was noted to be visually wet next to the waste canister on day 92 and was continuing to take on water at termination on day 112.

Capillary Barriers: Design Variables and Water Balance

Water balance simulations were conducted with the unsaturated flow model UNSAT-H to assess how layer thicknesses, unsaturated hydraulic properties, and climate affect the performance of capillary barriers. Simulations were conducted for four locations in semiarid and arid climates. Hydraulic properties of four finer-grained and two coarser-grained soils were selected to study how saturated and unsaturated hydraulic properties affect the water balance. Results of the simulations indicate that thickness and hydraulic properties of the surface layer significantly affect the water balance of capillary barriers. As expected, increasing the thickness or reducing the saturated hydraulic conductivity of the finer-grained surface layer reduces percolation. Unsaturated hydraulic properties of the coarser layer also affect the water balance, including the storage capacity of the surface layer as well as the onset and amount of percolation from the cover. Thickness of the coarser layer has a much smaller impact on the water balance. Climate also affects the water balance. Greater soil water storage capacity is required at sites where the season with more frequent and less intense precipitation does not coincide with the season having highest evapotranspiration.

Capillary barrier performance in heterogeneous porous media

The effects of heterogeneities on the performance of capillary barriers is investigated by numerically simulating three systems comprised of a fine soil layer overlying a coarse gravel layer with (1) homogeneous, (2) layered heterogeneous, and (3) random heterogeneous property fields. The amount of lateral diversion above the coarse layer under steady state infiltration conditions is compared among the simulations. Results indicate that the performance of capillary barriers can be significantly influenced by the spatial variability of hydraulic properties. In the layered heterogeneous systems, realizations with highly stratified regions within the fine layer performed best and resulted in localized capillary barriers that delayed breakthrough into the coarse layer. In contrast, realizations of the random heterogeneous system performed worst because of channeled flow that produced numerous localized regions of breakthrough into the coarse layer. Results of the homogeneous model were comparable to the mean results of the layered heterogeneous realizations, but homogeneous results underpredicted the frequency and amount of breakthrough for all realizations of the random heterogeneous system. These results indicate that homogeneous models can be used to estimate the average behavior of layered heterogeneous systems with reasonable accuracy. In addition, engineered capillary barriers may be improved through emplacement and packing methods that induce highly stratified features within the fine layer of a capillary barrier system.

On the performance of capillary barriers as landfill cover

Abstract. Landfills and waste heaps require an engineered surface cover upon closure. The capping system can vary from a simple soil cover to multiple layers of earth and geosynthetic materials. Conventional design features a compacted soil layer, which suffers from drying out and cracking, as well as root and animal intrusion. Capillary barriers consisting of inclined fine-over-coarse soil layers are investigated as an alternative cover system. Under unsaturated conditions, the textural contrast delays vertical drainage by capillary forces. The moisture that builds up above the contact will flow downdip along the interface of the layers. Theoretical studies of capillary barriers have identified the hydraulic properties of the layers, the inclination angle, the length of the field and the infiltration rate as the fundamental characteristics of the system. However, it is unclear how these findings can lead to design criteria for capillary barriers. To assess the uncertainty involved in such approaches, experiments have been carried out in a 8 m long flume and on large scale test sites (40 m x 15 m). In addition, the ability of a numerical model to represent the relevant flow processes in capillary barriers has been examined.

Generalization of Ross' tilted capillary barrier diversion formula for different two‐phase characteristic curves

Capillary barriers, consisting of tilted fine over coarse layers under unsaturated conditions, have been suggested as a means to divert water infiltration away from sensitive underground regions. Ross [1990] has developed a relationship for the steady‐state diversion length of tilted capillary barriers assuming the quasi‐linear set of two‐phase characteristic curves. Due to its simplicity, this equation has been used to evaluate potential tilted capillary barrier performance even in cases where the layer characteristic curves are different than the quasi‐linear equation set. In the present study, Ross' capillary barrier diversion relationship is extended to other two‐phase characteristic curves. This generalization allows for the use of any set of two‐phase curves in evaluating tilted capillary barrier performance. This generalized Ross' equation compares well with detailed numerical simulations.

Capillary Barriers and Subtitle D Covers: Estimating Equivalency

Accumulating data on traditional compacted soil-surface covers are demonstrating that they are likely to degrade and have reduced effectiveness as long-term barriers; therefore, suitable alternatives are being examined. One possible alternative that is receiving increased attention is capillary barriers. The U.S. Environmental Protection Agency (USEPA) allows for alternatives to be used, but requires that they achieve infiltration and erosion protection equivalent to that of designs contained in design guidance documents. A method of comparing a capillary barrier to a design that features a compacted soil layer that meets the minimum requirements for a solid-waste landfill cover (so-called Subtitle D) under identical, transient conditions is introduced in the present paper, allowing equivalency to be demonstrated. The approach uses daily climatic data rather than monthly or yearly averages, which can provide misleading results. The concept of adding a “transport layer” at the fine/coarse interface of the capillary barrier to laterally drain water and reduce the moisture content is also presented. Numerical modeling results for a variety of climates show that the capillary barriers may be equivalent (or better) compared to a Subtitle D cover at many locations. The inclusion of a transport layer may significantly improve capillary barrier performance.

The effect of constant anisotropy on capillary barrier performance : J. C. Stormont, Water Resources Research, 31(3), 1995, pp 783–785

The effect of constant anisotropy on capillary barrier performance : J. C. Stormont, Water Resources Research, 31(3), 1995, pp 783–785

Geotextiles to Mitigate Frost Effects in Soils: A Critical Review

The use of geotextiles to mitigate frost effects in soils has been studied, but few techniques have been developed. Guidelines developed for the placement of granular capillary barriers are presented to serve as preliminary guidelines for geotextile capillary barriers. Laboratory research shows that pore size distribution, wettability, and, for some geotextiles, thickness influence capillary barrier performance in a given soil. Geotextiles that easily wet do not reduce frost heave and may even exacerbate it. On the basis of the literature reviewed, guidance for selection of geotextile capillary barriers in field trials is given. If geotextiles function as capillary barriers during freezing and reinforce or separate and filter the subgrade at the base course interface during thaw, then the potential exists for their use in a combination of functions to reduce frost-related damage in geotechnical structures. It was found that properly designed geotextiles have the potential to reduce frost heave by functioning as capillary barriers, they can be filters for capillary barriers, and they can provide reinforcement or separation or filtration (or all of these) of the subgrade soil to reduce thaw-related damage.

Geotextiles to Mitigate Frost Effects in Soils: A Critical Review

The use of geotextiles to mitigate frost effects in soils has been studied, but few techniques have been developed. Guidelines developed for the placement of granular capillary barriers are presented to serve as preliminary guidelines for geotextile capillary barriers. Laboratory research shows that pore size distribution, wettability, and, for some geotextiles, thickness influence capillary barrier performance in a given soil. Geotextiles that easily wet do not reduce frost heave and may even exacerbate it. On the basis of the literature reviewed, guidance for selection of geotextile capillary barriers in field trials is given. If geotextiles function as capillary barriers during freezing and reinforce or separate and filter the subgrade at the base course interface during thaw, then the potential exists for their use in a combination of functions to reduce frost-related damage in geotechnical structures. It was found that properly designed geotextiles have the potential to reduce frost heave by functioning as capillary barriers, they can be filters for capillary barriers, and they can provide reinforcement or separation or filtration (or all of these) of the subgrade soil to reduce thaw-related damage.