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Effects of vegetation type on water infiltration in a three-layer cover system using recycled concrete

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Abstract

To promote environmental sustainability, recycled construction concrete is suggested for civil infrastructure works. The aim of this study was to investigate the effects of vegetation type on water infiltration under extreme rainfall conditions in a proposed three-layer landfill cover system containing recycled concrete. Three soil columns, namely bare, covered with shrub (Schefflera arboricola), and covered with grass (Cynodon dactylon), were subjected to ponding tests. Each column was compacted with a bottom layer of silty soil, an intermediate layer of coarse recycled concrete aggregate, and an upper layer of fine recycled concrete aggregate. Water breakthrough occurred only in the bare cover system after 48 h of ponding, equivalent to a rainfall return period of greater than 1000 years in Hong Kong. Under the vegetated covers, suction maintained in the bottom silty soil layer was higher than under the bare cover by 49–52 kPa and hence no percolation was observed after 48 h of ponding. Comparing the two vegetated cover systems, suction maintained under the shrub cover was 2–12 kPa higher (2%–8% lower volumetric water content) than that under the grassed cover in the layers of recycled concrete. This implies that shrub cover can be more effective than grass cover in reducing water infiltration in humid climates.

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... In addition, the enormous amount of construction waste has been generated with the tremendous growth of construction activities (Ng et al. 2019a). Recently, the utilization of this construction waste in various civil engineering applications has gained importance for the sustainable waste management system approach (Ng et al. 2019b). ...
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Plant evapotranspiration (ET) is considered to be a hydrological effect that would induce soil suction and hence influence the stability of geotechnical infrastructure. However, other hydrological effect, such as the change of soil water retention curve (SWRC) induced by roots, is generally ignored. This study aims to investigate and compare the effects of root-induced changes in SWRC with the effects of ET on suction responses in clayey sand. Two series of laboratory tests together with 21 numerical transient seepage analyses were conducted. A tree species, Schefflera heptaphylla, which is commonly used for ecological restoration in many subtropical regions, was selected for investigation. In order to consider any effects of tree variability on induced suction, six tree individuals with similar age were tested with and without the supply of light. It is revealed that under dark condition when ET was minimal, vegetated soil could induce higher suction than bare soil by 100% after subjecting to a wetting event with a return period of 100 years. This may be explained by the increases in the air-entry value and the size of hysteresis loop induced by roots. Water balance calculation from the numerical analyses shows that even under the supply of light, the amount of ET was only 1.7% of the total volume of water infiltrated. This means that during the wetting event, the contribution of root-water uptake to induced suction in vegetated soil was relatively little, as compared with the effects of root-induced change in SWRC.
Article
A capillary barrier as a cover system is a two-layer system of distinct hydraulic properties to prevent water infiltration into the underlying soil by utilizing unsaturated soil mechanics principles. This paper illustrates the application of the capillary barrier system on a slope that experienced shallow slip failures to prevent future rainfall-induced slope failures. In this study, the capillary barrier system was designed as a cover system for residual soil slopes with a steep slope angle under heavy rainfall conditions of the tropics. The capillary barrier system was constructed using fine sand as the fine-grained layer and granite chips as the coarse-grained layer. Both layers were contained in a cellular confinement system. The slope was instrumented with tensiometers and piezometers. The tensiometers were installed at different depths from about 0.5 m to 2.0 m below the slope surface. An adjacent original slope without the capillary barrier system was also instrumented using tensiometers to investigate the performance and effectiveness of the capillary barrier system in reducing rainwater infiltration and maintaining negative pore-water pressure in the slope. The detailed installation of a matric suction measurement device is discussed comprehensively in this paper. The measurement results showed that the capillary barrier system was effective in maintaining the negative pore-water pressures during rainfalls, particularly on the crest of the slope. DOI: 10.1061/(ASCE)GT.1943-5606.0000600. (C) 2012 American Society of Civil Engineers.
Article
Water infiltration rate and hydraulic conductivity in vegetated soil are two vital hydrological parameters for agriculturists to determine availability of soil moisture for assessing crop growths and yields, and also for engineers to carry out stability calculations of vegetated slopes. However, any effects of roots on these two parameters are not well-understood. This study aims to quantify the effects of a grass species, Cynodon dactylon, and a tree species, Schefflera heptaphylla, on infiltration rate and hydraulic conductivity in relation to their root characteristics and suction responses. The two selected species are commonly used for ecological restoration and rehabilitation in many parts of Asia and U.S. A series of in-situ double-ring infiltration tests was conducted during a wet summer, while the responses of soil suction were mointored by tensiometers. When compared to bare soil, the vegetated soil has lower infiltration rate and hydraulic conductivity, due to the clogging of soil pore by plant roots. This results in at least 50% higher suction retained in the vegetated soil. It is revealed that the effects of root-water uptake by the selected species on suction were insignificant due to the small evapotranspiration (< 0.2 mm) when the tests were conducted under the wet climate. There appears to have no significant difference (less than 10%) of infiltration rates, hydraulic conductivity and suction retained between the grass-covered and the tree-covered soil. However, it is identified that the grass and tree species having deeper root depth and greater root area retained higher suction. This article is protected by copyright. All rights reserved.
Article
Grass is recognized as being beneficial in reducing rainfall infiltration in some kinds of surface cover systems such as landfill cover, because rainwater discharges as surface runoff due to reduced water permeability caused by evapotranspiration-induced soil suction as well as foliage interception. However, the distributions of grass-induced suction in various compacted soils during rainfall are rarely reported. Moreover, it is not straightforward to determine an optimum soil dry density for minimizing rainfall infiltration and at the same time encouraging plant growth. This is because there are conflicting requirements for vegetated cover systems, i.e., compacted soil should not be too dense as to impede root growth, while on the other hand to minimize infiltration. This study thus aims to investigate, quantify, and compare grass-induced suction distributions in silty sand compacted at different densities when subjected to artificial rainfall in the laboratory. A grass species, Cynodon dactylon, which is common in many parts of Asia, was selected for testing. Compacted soil with and without a growing grass patch was tested at three relative compactions (RCs) of 70%, 80%, and 95%, in six test boxes. Test results reveal that at an RC of 95%, suction ( 40 kPa) retained in vegetated soil after rainfall is 100% higher than that ( 20 kPa) in bare soil. Among the vegetated soil compacted at the three RCs, suction retained was the highest at an RC of 95% ( 40 kPa), whereas suction decreased to 0 kPa at an RC of 70% after rainfall. As the average depth of grass roots decreased by 36% due to an increase in RC from 70% to 95%, the depth of influence of suction for vegetated soil at an RC of 95% reduced to less than half of root depth, which was the shallowest among the three compacted soil specimens.
Article
Water characteristic curves (WCCs) of recycled materials are not well researched. In this study, the WCCs of three specimens from recycled concrete aggregate, three specimens from reclaimed asphalt pavement, and one specimen from spent copper slag are studied. The objective of this study is to obtain WCCs of the recycled materials by using the Tempe cell pressure extractor for standard soil-water characteristic curve determination. Properties of drying WCC of the recycled materials were determined by using a best fit WCC equation. The results showed air-entry value of the recycled materials with high gravel content that occurred at low matric suction below 1.0 kPa and a steep WCC. The results from the WCC and saturated coefficient of permeability tests were incorporated into a statistical model to indirectly predict the permeability function of the recycled materials. The permeability functions of the recycled materials are also presented.
Article
A capillary barrier is a two-layer cover system having distinct hydraulic properties to minimize water infiltration into the underlying soil by utilizing unsaturated soil mechanics principles. In this study, a capillary barrier system was designed as a cover system for a residual soil slope to maintain stability of the slope by minimizing infiltration during heavy rainfalls in the tropics. The capillary barrier system (CBS) was constructed using fine sand as the fine-grained layer and recycled crushed concrete aggregates as the coarse-grained layer. The coarse-grained layer is commonly constructed using gravels or granite chips. However, due to scarcity of aggregates and in consideration of environmental sustainability, recycled crushed concrete aggregates were used as the coarse-grained layer in this project. The suitability of recycled crushed concrete aggregates as a material within the coarse-grained layer of a CBS is subject to the hydraulic property requirement. For comparison, another CBS was constructed using fine sand as the fine-grained layer and a geosynthetic (Secudrain) as the coarse-grained layer. The performance of each constructed CBS on the residual soil slope was monitored using tensiometers installed at different depths — from 0.6 to 1.8 m below the slope surface — and a rainfall gauge mounted on the slope. An adjacent original slope without the CBS was also instrumented using tensiometers and piezometers to investigate the performance and effectiveness of the CBS in reducing rainwater infiltration and maintaining negative pore-water pressures in the slope. Real-time monitoring systems were developed to examine pore-water pressure, rainfall, and groundwater level in the slopes over a 1 year period. Characteristics of pore-water pressure distributions in the residual soil slope under a CBS with recycled crushed concrete aggregates and in the original slope during typical rainfalls are highlighted and compared. The measurement results show that the CBS was effective in minimizing rainwater infiltration and therefore, maintaining stability of the slope.
Article
Evapotranspiration (ET) covers have gained considerable interest as an alternative to conventional covers for the final closure of municipal solid waste (MSW) landfills, but often produce higher rates of percolation in regions that receive more than 32 cm year À1 of precipitation. The goal of this project is to design ET covers for MSW landfills in northwestern Ohio (long-term annual rate of precipitation of 83 cm year À1) that produce rates of percolation < 32 cm year À1 , the rate considered acceptable by the Ohio Environmental Protection Agency (OEPA), and promote habitat restoration. To attain this goal, an adequate soil water-storage capacity was provided using dredged sediment amended with organic material. Two plant mixtures were tested to evaluate the performance of ET covers immediately following construction (immature plants seeded onto the soil) and in the future (mature plants transplanted from a restored tall-grass prairie that is more than 10 years old). ET covers were constructed in drainage lysimeters (1.52-m diameter, 1.52-m depth) and watered at a rate of 91.12 to 95:72 cm year À1 , which included simulated 100-year rain events (11.7 cm over 24 h) in July and October. During the 1-year monitoring period, the ET covers using the mature plant mixture produced considerably less percolation (0.12 to 11:44 cm year À1) than the covers with the immature plant mixture (6.71 to 24:16 cm year À1). Thus far, all ET covers have produced rates of percolation less than the maximum standard by the OEPA, and they will continue to be monitored.
Article
Evapotranspiration from a grass-covered ground is known to induce suction by soil evaporation and grass transpiration. However, grass-induced suction in the ground when it is subjected to wetting and drying are not yet well understood. In this study, a laboratory test program was conducted to investigate the magnitude and distribution of suction induced by Bermuda grass growing in silty sand. In total, four test boxes compacted with silty sand were prepared, three of which covered with Bermuda grass while one test box was left bare as control. All the four test boxes were subjected to wetting and drying in a plant room with temperature and humidity controlled. Under identical atmospheric conditions and initial soil density and water content, peak suction induced within the root zone in grassed soil was 1.5 times higher than that in bare soil after 20 days of drying. A vertical suction influence zone was identified to be up to four times the root depth while the lateral suction influence zone was one diameter of ring collar away from the centre of the plot. Upon wetting, suction retained at depth right below the root zone in grassed soil was found to be 40% higher than that in bare soil. For three grass replicates that were germinated under identical atmospheric conditions, they produced different shoot lengths and induced different magnitudes of suction. No direct correlation between grass shoot length and grass-induced suction could be found.
Conference Paper
Substantial economic and population growth generate a large amount of solid waste in the world. It is common to dispose solid waste by dumping them in sanitary landfills. The most important requirement of a landfill is that it does not pollute or degrade its environment. A method to reduce leachate from a landfill is to reduce rainwater infiltration into a landfill. The use of a cover system for the landfill such as a capillary barrier system (CBS) can reduce rainwater infiltration. A CBS is a man-made two-layer cover system designed as an unsaturated system, harnessing the distinctly different hydraulic properties between a fine-grained layer (sand) and a coarse-grained layer (gravel) of soils. Previous research works indicated that a single layer CBS could be used as slope stabilization purposes against rainfall-induced slope failure in Singapore. In this study, a double layer CBS is proposed to minimize the infiltration of rainwater into the landfill. The recycled materials were used in the double layer CBS to reduce the cost associated with the construction of a cover system in the field and to maintain environmental sustainability. One dimensional seepage analyses were per-formed to determine the optimum thickness of fine-grained and coarse-grained layers within a double layer CBS and to determine the suitable material combination within the double layer CBS. One dimensional infiltration tests were carried out in the laboratory to verify the results of the numerical analyses.The results from the numerical analyses and infiltration tests indicated that double layer CBS performed better than single layer CBS in minimizing rainwater infiltration.
Article
An arrangement of unsaturated fine-grained soil overlying unsaturated coarse-grained soil along a sloping contact can, under appropriate circumstances, divert infiltrating water away from the coarser material. Such an arrangement is called a capillary barrier. The water diverted by a capillary barrier flows downdip above the contact. The volume of water moving laterally increases in the downdip direction as additional infiltration is diverted by the barrier. Sufficiently far downdip, the laterally moving water wets the contact to the point that an amount of water equal to the infiltration flows downward through the coarse soil. The lateral flow at such a point represents the diversion capacity of the capillary barrier because this flow will not increase farther downdip. If the width (measured in the direction of dip) of the system is large enough that total infiltration exceeds the diversion capacity, the downdip portion of the barrier will not be effective. The diversion capacity can be calculated exactly in the quasi-linear approximation where the relationship between relative permeability krel and pressure potential psi takes the form krel = ealphaPsi. This calculation shows that an upper bound on the width of capillary barriers is Ks tan phi/qalpha, where Ks is the saturated hydraulic conductivity of the fine soil, phi is the dip angle of the contact, and q is the infiltration rate.
Article
Remediation of landfill wastes, mine land wastes, and oil shale residues normally includes a cover over the waste. Nearly all currently used landfill covers (caps) employ barrier-type systems. The authors discuss innovative vegetative landfill covers that use no barriers. They consist of a layer of soil covered by native grasses to control infiltrating precipitation as follows: (1) the soil stores infiltrating water; and (2) natural evapotranspiration empties the soil water reservoir. The vegetative cover concept has been extensively verified in the field. Because they are natural, they should perform better than conventional covers over decades or centuries and they are less expensive to build and maintain. The concept has been proven and it should be widely used. The authors propose that the name evapotranspiration cover should be associated with correctly designed and constructed vegetative covers.
Article
Evapotranspiration (ET) covers have gained interest as an alternative to conventional covers for the closure of municipal solid waste (MSW) landfills because they are less costly to construct and are expected to have a longer service life. Whereas ET covers have gained acceptance in arid and semi-arid regions (defined by a precipitation (P) to potential evapotranspiration (PET) ratio less than 0.75) by meeting performance standards (e.g. rate of percolation), it remains unclear whether they are suitable for humid regions (P:PET greater than 0.75). The goal of this project is to extend their application to northwest Ohio (P:PET equals 1.29) by designing covers that produce a rate of percolation less than 32cmyr(-1), the maximum acceptable rate by the Ohio Environmental Protection Agency (OEPA). Test ET covers were constructed in drainage lysimeters (1.52m diameter, 1.52m depth) using dredged sediment amended with organic material and consisted of immature (I, plants seeded onto soil) or mature (M, plants transferred from a restored tall-grass prairie) plant mixtures. The water balance for the ET covers was monitored from June 2009 to June 2011, which included measured precipitation and percolation, and estimated soil water storage and evapotranspiration. Precipitation was applied at a rate of 94cmyr(-1) in the first year and at rate of 69cmyr(-1) in the second year. During the first year, covers with the M plant mixture produced noticeably less percolation (4cm) than covers with the I plant mixture (17cm). However, during the second year, covers with the M plant mixture produced considerably more percolation (10cm) than covers with the I plant mixture (3cm). This is likely due to a decrease in the aboveground biomass for the M plant mixture from year 1 (1008gm(-2)) to year 2 (794gm(-2)) and an increase for the I plant mixture from year 1 (644gm(-2)) to year 2 (1314gm(-2)). Over the 2-year period, the mean annual rates of percolation for the covers with the M and I plant mixtures were 7 and 8cmyr(-1), which are below the OEPA standard. The results suggest the application of ET covers be extended to northwest Ohio and other humid regions.
Article
Riparian vegetation has a number of effects on the mechanisms by which streambanks fail, some positive and some negative. Previous research has shown that the effect of mechanical root-reinforcement on soil stability can be considerable, and can be successfully quantified and included in streambank stability models. Root networks contained within a soil-matrix, however, also have effects on the hydrologic and hydraulic processes acting on a streambank, and although these effects are often discussed they have generally been difficult to quantify. This paper summarizes the results of field data collection, laboratory testing and computer simulations carried out to better quantify the effects of riparian vegetation on hydrologic and hydraulic processes occurring along streambanks. First, the evapotranspiration potentials of different riparian species were isolated by setting up an experiment to grow young riparian trees and switch grass in separate soil columns, each instrumented with tensiometers at 30 cm and 70 cm depths, and compared against bare control columns. The hydrological reinforcement provided to the soil from increased apparent cohesion as a result of enhanced matric suction was estimated to range from 1.0 to 3.1 kPa in spring when bank stability was most critical and up to a maximum of 5.0 kPa in the summer. Second, a vertical jet-test device was used to measure rates and volumes of scour in soils permeated by switch grass roots. Results showed that the volume of soil scoured during a test declined non-linearly with increasing root volume, per unit volume of soil, and with increasing root length density (RLD) and increasing root biomass. Calculation of relative soil detachment rates (RSD) showed that with the highest rooting densities measured in the field jet-tests, eroded soil volume was 10% of that in the tests with no roots. Third, the effects of enhanced matric suction from evapotranspiration, and decreased soil erodibility because of the presence of plant roots were modeled using BSTEM 5.1 to quantify the effects on streambank factor of safety (Fs), and to compare with the effects of mechanical root-reinforcement. The sensitivity analysis showed that the change in soil matric suction from evapotranspiration provided the greatest potential benefit to Fs but only during the summer months. During the winter and spring months, root-reinforcement remained the most important contributor to Fs. The sensitivity analysis conducted here also showed that whilst roots are capable of reducing the volume of hydraulic scour, the resulting effect on streambank geometry did not increase Fs as much as changes in soil matric suction and/or mechanical root-reinforcement.
Article
A compacted soil liner (CSL) has been widely used as a single barrier layer or a part of composite barrier layer in the landfill final cover system to prevent water infiltration into solid wastes for its acceptable hydraulic permeability. This study was conducted to test whether the CSL was also effective in prohibiting landfill gas emissions. For this purpose, three different compaction methods (i.e., reduced, standard, and modified Proctor methods) were used to prepare the soil specimens, with nitrogen as gas, and with water and heptane as liquid permeants. Measured gas permeability ranged from 2.03 x 10(-10) to 4.96 x 10(-9) cm(2), which was a magnitude of two or three orders greater than hydraulic permeability (9.60 x 10(-13) to 1.05 x 10(-11) cm(2)). The difference between gas and hydraulic permeabilities can be explained by gas slippage, which makes gas more permeable, and by soil-water interaction, which impedes water flow and then makes water less permeable. This explanation was also supported by the result that a liquid permeability measured with heptane as a non-polar liquid was similar to the intrinsic gas permeability. The data demonstrate that hydraulic requirement for the CSL is not enough to control the gas emissions from a landfill.
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Use of waste materials and industrial by-products in concrete construction
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