Saturated Water Permeability Research Articles

Reliability analysis of vegetated slope considering spatial variability of soil and root properties

Vegetation acts as a natural engineering approach that can enhance the stability of shallow soil slopes. However, the intrinsic uncertainties of vegetation, particularly the randomness of root features, have rarely been considered in vegetated slope stability analysis. To address the issue, a modified random field model is proposed to simulate the spatial variability of root parameters (e.g., root volume ratio, Rv) with different root orientations and architectures. The proposed random field model, combined with the finite element method (FEM) and limit equilibrium method (LEM), is utilised to investigate the effects of root architecture, orientation, and depth on vegetated slope reliability, considering spatial variability of soil and root properties simultaneously under rainfall infiltration. It is found that the proposed model effectively characterises the spatial variability of root parameters with different root orientations and architectures, as validated against theoretical and field-measured data. Ignoring root orientation during simulation can lead to a misestimation of the slope reliability index by up to nearly 17.1%. Moreover, without considering the spatial variability of Rv within the root zone or spatially variable soil properties (i.e., void ratio, saturated water permeability, and soil water retention curve) beyond the root zone, the vegetated slope reliability index can be overestimated by up to 4.8 times. It is recommended to adopt a triangular root system, concentrating most of the roots in the shallow depth zone and extending horizontally, which can reduce slope stability uncertainty and improve overall slope reliability.

Probabilistic analysis of a sustainable landfill cover considering stress-dependent water retention model and copula-based random fields

Material properties, crucial design parameters in landfill cover systems, can exhibit spatial variability due to non-uniform particle size and uneven compaction, affecting cover performance. Additionally, cover thickness, another key design parameter, can induce stress variation and influence soil-water retention capability. Previous designs always ignored these uncertainties and stress effects, potentially leading to unreasonable design schemes. This study aims to provide design recommendations for a three-layer landfill cover system under heavy rainfall, considering these factors simultaneously by the random finite element method (RFEM). The stress effects are captured using a stress-dependent soil-water retention curve (SDSWRC) and the hysteresis in the SDSWRC is considered in probabilistic analyses. For practical applications, uncertainties in two easily controllable particle size parameters, D10 and D60, are depicted by copula-based cross-correlated random fields. Based on empirical equations, the spatial variability of SDSWRC and water permeability function (WPF) can be further characterised by D10 and D60 random fields. It is found that considering SDSWRC and its spatial variability (e.g., shape) can overestimate percolation by at least 1.7 times compared to deterministic analysis using SWRC. The spatial variability of SDSWRC shape has a more significant impact than spatially variable saturated water permeability on cover system performance. Among candidate copulas, the CClayton copula (survival copula of Clayton copula) yields the most conservative results due to its upper tail dependence. The particle size and thickness of the bottom layer influence the cover system performance more than the upper two layers. A bottom layer with D10≤ 0.009 mm and D60/D10≥ 7.54, and thickness ≥ 0.8 m is recommended to achieve a satisfactory performance level with a 100-year design life.

Three-year field study on grass growth and soil hydrological properties in biochar-amended soil

Field monitoring was conducted to investigate and quantify the long-term effects of peanut shell biochar on soil-grass interaction over three years. Three 10 m × 5 m grassed plots were constructed in completely decomposed granitic soil. Two of them were amended, respectively, with 5% and 10% biochar contents (m3/m3) for grass growth, while the third was without biochar amendment. During the three-year monitoring, plant characteristics, saturated water permeability (ks) of grassed soil and soil suction were measured. The monitored results show that the grass leaf area index (LAI) and root length density (RLD) with biochar amendment were improved by 38% and 200%, respectively. In the grassed plot without biochar, a threshold RLD existed with a value of 1.7 cm/cm3, beyond which ks raised pronouncedly. The threshold RLD increased by 52% when biochar content increased from 0% to 10%. This implies that biochar may restrict the increase in ks of grassed soil due to the rise in the threshold RLD. The presence of biochar and grass can retain over 100% higher suction after heavy rainfalls, while 54% lower peak suction under evapotranspiration (ET) compared with the non-amended plot. Biochar can alleviate the negative effects on hydraulic properties caused by plant growth and reduce ET-induced excessive water loss. A 5% peanut shell biochar content is recommended for the long-term management of vegetated earthen infrastructures.

Three-dimensional reliability analysis of unsaturated soil slope considering permeability rotated anisotropy random fields

The spatial variability is an inherent characteristic of soils. In addition to isotropic and transversely anisotropic soils, rotated anisotropic soils can also be observed in nature owing to deposition and weathering. Recent studies revealed that the spatially variable soil strength parameters can significantly affect rotated anisotropically deposited soil slope stability. However, the influence of spatial variability of saturated water permeability (ks) on this type of slope stability has been not fully understood, especially for the three-dimensional (3D) slope analysis. This paper aims to investigate unsaturated soil slope reliability with 3D ks rotated anisotropy random fields under rainfall infiltration. Due to the low efficiency of the covariance matric decomposition method (CMD) in simulating large-scale 3D rotated anisotropy random fields, a modified CMD method is proposed to save the computational cost. Moreover, comparisons between 2D and 3D random finite element analyses are also discussed. The results indicate the modified CMD method can markedly improve computational efficiency by more than 3.3 times compared with the CMD method. Only applying variations of pore water pressure (PWP) of a section or groundwater table (GWT) cannot sufficiently account for the uncertainty of slope stability. The cross-dip slope, where the dip direction of the slope is perpendicular to the dip direction of strata, has a larger reliability index (1.3−3.0 times) than the dip slope and reverse slope at the same soil bedding plane orientation. The most unfavorable soil bedding plane orientation for the cross-dip slope and the dip/reverse slope is 0° (horizontally deposited) and 30° (slightly larger than slope angle), respectively. It is attributed to the relatively low ks along these two orientations being more likely to form the aquiclude, affecting overall variations of PWP, leading to a larger standard deviation of the factor of safety and a lower reliability index. Furthermore, the reliability of cross-dip slope can only be studied in 3D, while the dip slope and reverse slope reliability can be studied with 2D analysis to decrease computational burden when the slope length does not exceed 50 m.

Long-term field performance of non-vegetated and vegetated three-layer landfill cover systems using construction waste without geomembrane

This paper compares the field performance of non-vegetated and vegetated three-layer landfill cover systems utilising construction waste but without geomembrane at the Shenzhen Xiaping landfill site in humid climates over a monitoring period of 54 months. The top layer of each cover system was constructed using coarse-grained completely decomposed granite (CDG). The middle and bottom layers were compacted with coarse recycled concrete (CRC) and fine-grained CDG, respectively. Numerical analyses were also carried out. During the 54-month monitoring period, the cumulative rainfall recorded was 9800 mm. Retained pore water pressure in the vegetated cover was close to that in the non-vegetated cover after the heaviest rainfall recorded in Shenzhen during the monitoring period. The pore water pressure retained in the vegetated cover can be higher than that in the non-vegetated cover due to the increased saturated water permeability (ks) induced by plant growth over 3 years. Surface runoff and water storage capacity in the vegetated cover were about five times and 9% higher than those in the non-vegetated cover, respectively. However, the long-term behaviour of the cover system in respect of these two aspects was influenced by the increase of ks due to the growth of grass roots. The middle CRC layer diverted infiltrated water by up to 51% of total rainfall. The measured average annual percolation of the non-vegetated and vegetated covers was about 23 mm and 21 mm, respectively. The measured data are supported by numerical analyses and they meet the US recommended criterion.

Pore Fractal Characteristics of Hydrate‐Bearing Sands and Implications to the Saturated Water Permeability

AbstractPermeability mainly governs fluid flow through hydrate‐bearing sediments, and its theoretical models play a primary role in the efficiency prediction of gas recovery from hydrate reservoirs by using numerical simulators. Most of these numerical simulators rely on empirical or semiempirical permeability models largely due to the lack of suitable parameters that well quantify the evolution of pore structures. In this study, X‐ray computed tomography scans are conducted on methane hydrate‐bearing sands, followed by extractions of the maximal diameter and fractal dimensions for the pore space occupied by fluids. These extracted parameters, including area and volume pore‐size fractal dimensions, tortuosity fractal dimension, and the area maximal pore diameter, are further extended to develop fractal theory‐based models for predictions of the hydraulic tortuosity and the saturated water permeability in hydrate‐bearing sands. Results show that the pore space occupied by fluids within hydrate‐bearing sands is inherently fractal. The area pore‐size fractal dimension decreases but the tortuosity fractal dimension changes little with increasing hydrate saturation, and the sum of these two fractal dimensions can be used to predict the volume pore‐size fractal dimension. In addition, the area maximal pore diameter decreases with increasing hydrate saturation, and a semiempirical model is proposed. The fractal theory‐based permeability reduction model agrees well with available experimental data, and it can capture the essential physics of saturated water permeability reduction in hydrate‐bearing sediments during hydrate formation.

Effect of coal on mine tailings’ water permeability and water retention

For safe and efficient mining operations to occur the management of waste materials is required, which often takes the form of geotechnical structures constructed from this waste. The safe use of these structures requires a number of resources, one of these being sufficient information about the waste material properties. For example, the drying process of a tailings dam is predicted with the water retention and permeability of the tailings. When considering coal tailings, which are comprised of coal and mineral soil particles (typically), the presence of coal may be problematic. The localised hydrophobicity of coal molecules may have a unique effect on water permeability and retention; this is relevant to geotechnical analysis where hydrophilic behaviour is assumed. To explore the possible effect of localised hydrophobicity, mine tailings were obtained from a coal mine of the Hunter Valley, NSW, Australia, and the coal fraction was separated via density separation. After this, three materials were available: unchanged mine tailings and a coal and mineral fraction of tailings. The goal was to characterise the three materials and allow deeper insight on what effect the addition of coal has on retention and hydraulic properties. Characterization involved measuring particle size distribution, pore size distribution, soil water retention curve, and saturated water permeability. The results show that there are distinct differences in the water retention and permeability properties of each material, and a number of these differences could be explained by the differing particle/pore sizes observed in each material. However, the coal containing materials desaturated at low suctions (< 10 kPa) compared to the mineral fraction, which could not be explained by particle/pore size differences and points towards localised hydrophobicity as a possible cause.

Effects of nano-activated carbon on water and gas permeability and hydrogen sulphide removal in compacted kaolin

Many soil amendments have been proposed to exclusively enhance the removal of hydrogen sulphide (H2S; one of the major odorous gas) and reduce water percolation/landfill gas emission in a municipal solid waste landfill. There is no reported data about any single amendment that can simultaneously reduce (i) gas permeability (kg) and saturated water permeability (kw) and (ii) H2S emission. The present study investigated the possible use of nano-activated carbon to amend the physico-chemical properties of kaolin, including kw, kg and H2S adsorption capacity. It was revealed that adding nano-activated carbon of 6% (by mass) could reduce kw and kg by about 60% and more than an order of magnitude, respectively, while increasing the H2S adsorption capacity by more than four times.

Influence of poultry litter and biochar on soil water dynamics and nutrient leaching from a very fine sandy loam soil

Soil amendments affect various soil properties that influence water and nutrient dynamics in the soil. Thus, appropriate understanding of their influence on some of these soil properties will be helpful to improve water and nutrient use efficiency, while minimizing environmental pollution. In this study, the effect of poultry litter (PL) and pinewood biochar (PBC) on soil water holding capacity (SWHC), plant available water (PAW), unsaturated and saturated water permeability (WP), evaporation, leaching losses of dissolved organic C (DOC), inorganic-N and metals from a very fine sandy loam soil were evaluated. Addition of both PL and PBC significantly increased SWHC, while PAW was increased by PBC and decreased by PL. Amending of soil with PL at 2.5 and 5% w/w increased unsaturated WP rate, while10% PL decreased WP rate. At all three levels PL decreased saturated WP rate of soil by up to 67% compared to control. Pinewood biochar improved unsaturated WP by up to 2.5 times and saturated WP of soil by up to 2.2 times as compared to control. Despite the black color of biochars that increases thermal absorption, PBC treatments had lower evaporation rates than the control. Both PL and PBC extended the drought tolerance of ryegrass compared to control. Soil amended with PL lost appreciable amounts of DOC, inorganic-N and metals. Amending soil with PBC along with PL benefited by significantly minimizing the leaching losses of DOC, NH4+ and heavy metals. Overall, application of PL along with PBC could benefit crop production by improving SWHC, PAW, and drought tolerance while minimizing the non-point source pollution compared to PL only application for very fine sandy loam soils.

Analytical solutions of pore-water pressure distributions in a vegetated multi-layered slope considering the effects of roots on water permeability

New analytical solutions of pore-water pressure (PWP) distributions in a vegetated multi-layered slope are derived considering the effects of roots on water permeability. PWP distributions are significantly affected by root-induced reductions in water permeability due to increases in the root volume ratio (Rv). Under drying conditions, negative PWP increases with Rv. The negative PWP induced by root water uptake increases with the desaturation coefficient (αi) or the ratio of the transpiration rate to saturated water permeability (Tp/ksi). As the overall water permeability underneath the vegetation layer decreases, the negative PWP induced by root water uptake increases.

Effect of phenolic acids on the formation and stabilization of soil aggregates

ABSTRACTTo examine the effects of phenolic acids, which are generated by the decomposition of cell walls in plant residues, and other constituents on the stability of soil aggregates, phenolic acids and carbohydrates were mixed into three different types of soil. After a 1-month incubation, the plot containing soil mixed with phenolic acids showed the greatest mean weight diameter of all the soils. In the treated soils, before incubation, the decline of saturated water permeability during continuous water percolation was mitigated in the plot containing soil mixed with phenolic acids compared with that in the other plots. Soil aggregates were synthesized with the addition of phenolic acids and carbohydrates using two methods (mixing and surface brushing) and were incubated for 153 days. The aggregate stability was greatest in the plots surface-brushed with phenolic acids for Andosol and gray lowland soil, whereas the aggregate stability was most stable in the plots mixed with phenolic acids for yellow soil. This difference in the effectiveness of application methods is rationalized by the densities of the active Al and Fe contents, the carbon content, and the specific surface area of the soils. The phenolic acids also affected sandy soil. In a similar experiment using a gray lowland soil, mixing a portion of p-coumaric acid into synthetic aggregates was found to shift the molecular weight distribution of substances to larger molecular weights, as determined by size exclusion chromatography of the liquid extracted from the aggregates, which was likely accompanied by an increase in aggregate stability. The effects of fungi and bacteria on soil long-term stability were not greater than those of phenolic acids. Our findings and previous results show that microorganisms aid in soil-aggregate formation during the early stages, and phenolic acids not only aid in the formation of aggregates but also strongly stabilize them.

Bodenveränderungen und Typisierung von Fahrspuren nach physikalischer Belastung | Changes in the soil and the classification of ruts according to physical loading

In Swiss environment legislation, protection of the soil is defined by reference to the long-term maintenance of soil fertility. To fulfil this commitment, long-lasting damage to the soil must be prevented, respectively reduced to a minimum. When the forest floor is driven over with heavy logging machinery, this can lead to profound and long-lasting changes in the soil structure in the ruts thus formed. Based on driving trials under controlled conditions in the Heiteren region of the forest near Bern, investigations were made to find out whether the classification of ruts into three distinct types showing morphologically determined changes in the soil could be substantiated with the help of known values in soil physics. It was shown that the non-compacted reference soils could be clearly distinguished from all three types of rut by comparison of the stratification density, the total pore space and the saturated water permeability. In addition, the three types significantly differed from each other. Damage to the soil can be reduced to a minimum through consequent planning of skidding tracks and by paying due attention to the prevailing humidity of the soil at the time when vehicles are used. Thanks to the connection established between the character of the ruts and soil functionality, the classification of rut types provides a practically relevant and objective instrument for effective physical soil protection.

Predicting Soil‐Water and Soil‐Air Transport Properties and Their Effects on Soil‐Vapor Extraction Efficiency

AbstractAccurate prediction of water and air Iran sport parameters in variably saturated soil is necessary for modeling of soil‐vapor extraction (SVE) at soil sites contaminated with volatile organic chemicals (VOCs). An expression for predicting saturated water permeability (kl,s) in undisturbed soils from the soil total porosity and the field capacity soil‐water content was developed by fitting a tortuous‐tube fluid flow model to measured water permeability and gas diffusivity data. The new kl,s expression gave accurate predictions when tested against independent kl,s data. The kl,s expression was implemented in the Campbell relative water permeability model to yield a predictive model for water permeability in variably saturated, undisturbed soil. The water permeability model, together with recently developed predictive equations for gas permeability and gas diffusivity, was used in a two‐dimensional numerical SVE model that also included non‐equilibrium mass transfer of VOC from a separate phase (nonaqueous phase liquid [NAPL]) to the air phase. SVE: calculations showed that gas permeability is likely the most important factor controlling VOC migration and vapor extraction efficiency. Water permeability and gas diffusivity effects became significant at water contents near and above field capacity. The NAPL‐air mass transfer coefficient also had large impacts on simulated vapor extraction efficiency. The calculations suggest that realistic SVE models need to include predictive expressions for both conveciive, diffusive. and phase‐partitioning processes in natural, undisturbed soils.

A high pressure triaxial cell with improved measurement sensitivity for saturated water permeability of high performance concrete

The measurement of the saturated water permeability of concrete is of great interest, but with the rapid improvements in properties of high performance concretes, the most common problem is the ability and accuracy of measuring the very small flow volumes. A high pressure triaxial cell with improved measurement sensitivity, capable of continuously measuring saturated water permeability of the order of < 10 −15 m/s, is presented in this paper.