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The capillary barrier effect (CBE) is employed in a large number of geotechnical applications to decrease deep percolation or increase slope stability. However, the micro-scale behaviour of CBE is rarely investigated, and thus hampers the scientific design of capillary barrier systems. This study uses microfluidics to explore the micro-scale behaviour of CBE. Capillarity-driven water flow processes from fine to coarse porous media with different pore topologies and sizes were performed and analysed. The experimental results demonstrate that the basic physics of CBE is the preferential water movement into the fine porous media due to the larger capillarity. The effects of CBE on water flow processes can be identified as delaying the occurrence of breakthrough into the coarse porous media and increasing the water storage of the fine porous media. The CBE can impede the increase of the normalised length and decrease the normalised width of the water front, suggesting that the two normalised parameters are potential indicators to assess the performance of CBE at the micro scale. CBE can be formed in square and honeycomb networks with the ratio of coarse to fine pore throat width larger than 2·0 when gravity is neglected, and its performance can be affected by pore topology and size.

Landfill leachate-contaminated groundwater is widespread all over the world. In order to study the organic contaminant removal efficiency of landfill leachate-contaminated groundwater under oxygen micro-nano-bubble (MNB) aeration, a series of lab-scale experiments of oxygen MNB aeration as well as common bubble (CB) aeration were conducted. Firstly, the difference in mass transfer, microbial activity enhancement, and contaminant removal efficiency between MNB and CB aeration was estimated. Then, the composition variations of dissolved organic matter (DOM) in groundwater treated by MNB or CB aeration were characterized by ultraviolet-visible (UV-VIS) absorption spectrum and fluorescence excitation-emission matrix (EEM). The test results showed that the oxygen utilization efficiency and volumetric oxygen transfer coefficient of MNB aeration were 10 and 50 times that of oxygen CB aeration, respectively. On the 30th day after MNB aeration, the dehydrogenase activity (DHA) of groundwater increased by 101.25%. Compared with CB aeration, the chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD

In order to study the flushing efficiency of different leaching agents on the mixed contaminated soil in landfill, five surfactants (SDS (sodium dodecyl sulfate), SDBS (sodium dodecyl benzene sulfonate), Tween 80, RL (rhamnolipid), and saponin) and three chelants (EDDS ([S,S]-ethylene-diamine-disuccinic acid), EDTA (ethylene- diaminetetraacetic acid), and CA (citric acid)) were selected to carry out batch tests and one-dimensional column tests. In addition, the influence of injection conditions on the flushing efficiency of saponin was also evaluated. The results show that the saponin with low COD and degradability performed the best removal efficiency of the selected eluents. The extraction rates of total Cr, Cr (VI), ammonia nitrogen, and COD increase with increasing saponin concentration. In the batch test, the extraction rates of total nitrogen were less than 40%, and it was not sensitive to the concentration. Saponin can simultaneously extract heavy metals (total Cr and Cr (VI)), total nitrogen, ammonia nitrogen, and organics from the contaminated soil. The leaching effect of saponin is obviously different under different injection modes. For contaminated soil dominated by total Cr or Cr (VI), continuous injection is recommended. If total nitrogen or ammonia nitrogen is the priority target, the step-gradient injection can demonstrate better removal efficiency. Muti-pulse injection is preferred when organics are the main factor in mixed contaminated soil.

The amount of municipal solid waste (MSW) has increased significantly in developing countries. Landfilling is the commonly used treatment for the disposal of MSW. The MSW contains more food waste in developing countries than in developed countries. This work analyzed the degradable components of MSW with different food contents. A theoretical model was introduced to analyze the coupled hydro-mechanical-chemical interactions in the landfilled MSW. The impacts of the degradation on the properties of high-food-waste-content MSW were reviewed, including the compression behavior, strength parameters, and hydraulic conductivity. The major cause of geoenvironmental issues in high-food-waste-content landfills are rapid leachate and landfill gas generation. A practical model for analyzing leachates and gas production was presented. Landfills in China were used as an example to describe engineering measures for leachate drainage and landfill gas collection. These methods proved successful in solving geoenvironmental issues of landfills with high leachate levels. The experiences are useful for engineers who face similar issues with high-food-waste-content landfills in developing countries.

With the rapid development of urbanization, a large amount of construction spoil was stockpiled around cities and formed extensive dumps. Construction spoil is one of the main construction and demolition (C&D) waste and municipal solid waste (MSW). Once the construction spoil dump becomes unstable, it will bring great risks to the surrounding residents. A catastrophic dump failure occurred on 20 December 2015 in Guangming New Strict, Shenzhen, China. Approximately 2.51×106 m3 of construction waste slid out from the dumpsite, destroying 33 houses and causing total 77 casualties. This paper attempts to analyze the failure probability of the construction spoil dump using Monte Carlo simulation considering the spatial variability of soil properties, and to quantify the dynamic human risk considering the increasing urbanization. Influence of urbanization on the human element at risk is analyzed by referring to multi-temporal remote sensing images. A quantitative human risk assessment model is employed to determine the landslide human risk referring an assessment criteria curve between frequency of number fatalities and number of fatalities (F-N curve). It is found that the societal risk at daytime was 0.078, 0.088, and 1.432 in 2002, 2014, and 2015, respectively. Meanwhile, the societal risk at night was 0.034, 0.037, and 0.611 in 2002, 2014, and 2015, respectively. The quantitative method was benchmarked by the other landfill failure. It implies that the human risk increased with the development of urbanization and its value at daytime was approximately twice as much as at night. The new approach for the human risk assessment provides guidance for modern MSW landfills and highlights the obvious influence of urbanization on the human risk in other areas.

A new apparatus and an interpretation method simplified from existing solutions of spherical air flow in soil were developed to measure in situ air permeability (ka) of unsaturated soil. By pumping air with a controlled volumetric flux (qv) into a spherical aeration bulb and measuring the corresponding pseudo-steady-state air gauge pressure (Δp0), ka was determined based on the slope of a qv–Δp0 relationship. The method was applied to measure ka in a full-scale landfill cover. The ka measurements were verified by finite-element modelling of air flow. The measurements show good agreement with the computed results. Sensitivity analyses reveal that using a smaller bulb radius (r0) would increase the measurement accuracy by minimising the boundary effects associated with atmospheric conditions and contrasting ka between neighbouring soils. Yet, the use of a small bulb would result in more prominent effects of preferential flow and hence substantial overestimation of ka. For a small bulb size (r0 = 0.01 m), the measurement accuracy of ka could be within 15% (i.e., 2.4 × 10−13 to 6.3 × 10−13 m2) of the true values when the ratio of vertical-to-horizontal ka is between 0.4 and 1.

Geosynthetic clay liners (GCLs) are widely used in landfill and heap-leach facility cover system for mitigating rainfall infiltration and gas migration into atmosphere. Laboratory tests were conducted to investigate methane diffusion and advection through GCLs. Gas permeability coefficient of GCL for the case with moisture content = 47.5% is one and two orders of magnitude greater than the cases with moisture content = 68.5% and 80.9%, respectively, when 20 kPa vertical stress was applied. The batch adsorption tests indicated that adsorption of methane onto bentonite is negligible. The concentration variation for the adsorption of methane onto bentonite can be neglected. However, methane concentration decreased by 14.2% for the test of methane adsorption onto GCL during the first 2–3 days. This is because methane was adsorbed by the geotextiles rather than by the bentonite in GCL. The large porosity and surface area of geotextiles provide lots of micropores for methane adsorption. Analytical model was then developed to analyze the performance of GCL-based liners system with respect to methane transport. The results indicate that methane emission fluxes for the case with SL + GCL are 7.8 and 5.1 times less than the cases with SL + CCL when the moisture contents were 25.9% and 35.1%, respectively. The methane emission fluxes for both of the SL + GCL and SL + CCL can be neglected when they are fully saturated. GCL is recommended to be used in arid and semi-arid regions rather than CCL. GCL is recommended to be used in arid and semi-arid areas rather than CCL. Advection plays a more important role in methane migration through SL + GCL and SL + CCL than that of diffusion. With moisture contents = 25.9% and 32%, methane emission flux attributed to advection accounts for more than 90% of the total emission flux for both cases of SL + GCL and SL + CCL. With the increase of moisture content of SL, the effectiveness of SL in reducing methane emission increases. The saved space for using GCL + SL composite cover compared with using a single SL cover is 0.7 m when the moisture content equals 25.9%, which is 0.5 m greater than the case when moisture content equals 32%. GMB plays a dominant role in inhibiting methane migration and reducing methane emission flux. When moisture content equals 25.9%, the methane emission fluxes for SL + GMB + GCL and SL + GMB + CCL are 343 times and 2643 times less than the cases with SL + GCL and SL + CCL, respectively.

A triple-layer composite liner consisting of a geomembrane liner (GMB), a geosynthetic clay liner (GCL) and a compacted clay liner (CCL) is commonly used at the landfill bottom liner system to isolate the contaminated leachates. In this paper, one-dimensional quasi-steady-state small deformation model (SDSS) was developed to investigate the behavior of organic chemicals transport in landfill composite liner system considering coupled effect of consolidation, diffusion and degradation. The first and second type bottom boundary conditions are used to derive the analytical solutions. The generalized integral transform technique (GITT) is adopted to derive the analytical solutions. The effect of consolidation on the performance of GMB/GCL/CCL with intact or leaking GMB is investigated. The triple liner under double drainage boundary condition (DDBC) has better performance compared to the case under single drainage boundary condition (SDBC). This is because the velocity induced by consolidation under DDBC is lower than that under SDBC. The effect of GCL consolidation shows an opposite trend compared to CCL consolidation. Considering GCL consolidation can increase the breakthrough time. The effective diffusion coefficient of GCL can be two magnitude orders smaller after consolidation, which provides a better diffusion barrier for the chemical transport. The effects of adsorption and degradation have been analyzed as well. Increasing the adsorption capacity of a deforming composite liner can increase the steady-state bottom flux, which shows the opposite tendency compared to the case without considering consolidation. This is due to the fact that for the case of a deforming composite liner, the advection induced by consolidation includes a new term due to the solid velocity. This velocity will result in the increase the mass of chemical migration through the composite liner.

The geosynthetic-encased vertical column and geosynthetic-embedded horizontal cushion are recognized as the effective methods to reduce the settlement of the soft subgrade. This paper investigated the settlement behavior of a soft subgrade reinforced by geogrid-encased stone column and geocell-embedded sand cushion using the finite element analysis method (Plaxis 2D). The simulating settlement was in good agreement with the field monitoring data, indicating the reasonability of the designed model and adopted parameters. After that, the factors, geocell layer in sand cushion, encasement length around stone column, and standing time between embankment filling stages, were employed to study their influences on the subgrade settlement. The results showed that the embedment of geocell reduced construction settlement, postconstruction settlement, and differential settlement is attributed to the increase in stiffness of sand cushion and therefore the uniform distribution of additional stress on subgrade surface. When the encasement length of stone column increased from 1D (one time the column diameter) to 8D (full encasement), the settlement in construction stage and postconstruction stage decreased by 32.2% and 35.1%, respectively, which is benefited from the increase in the compression modulus of the column. The maximum lateral deformation occurred at the position of about 2D from the top of the stone column, and it decreased more significantly when the encasement length increased from 1D to 4D than that from 4D to 8D. The encasement length up to 4D is found to be adequate in reducing the subgrade settlement and the column lateral deformation based on the consideration of performance and economy. The extension of the filling interval increased the construction settlement caused by soil consolidation, while it decreased the postconstruction settlement.

This paper presents a study on stochastic evaluation of leakages through holes in wrinkle networks of composite liners. The statistical parameters of wrinkles are used as indexes to describe the spatial distributions of wrinkles in a wrinkle network and the wrinkle density is modeled as a random field in the proposed approach, which allows the construction of a database about how wrinkles may be distributed in different conditions and provides input parameters for leakage evaluation at the design stage when the site has not been constructed yet and the aerial image of the wrinkle network (AWN) is unavailable. Statistical analyses were performed on wrinkle geometric parameters and wrinkle density of wrinkles from three sites reported in the literature. The procedures of generating random wrinkle networks (RWNs) based on the statistical parameters of wrinkles are introduced. The proposed approach was applied to typical examples and showed sufficient accuracy when compared to the evaluated leakages based on the corresponding AWNs. Wrinkle density is recommended to be modeled as random field.

The methane emission in a loess-gravel capillary barrier cover (CBC) in winter and summer was investigated by constructing a full-scale testing facility (20 m × 30 m) with a slope angle of 14.5° at a landfill in Xi’an, China. Weather conditions, methane emission, gas concentration, temperature, and volumetric water content (VWC) in the CBC were measured. The temperature and moisture in the CBC showed a typical seasonal pattern of warm and dry in summer and cold and wet in winter. Accordingly, the maximum methane oxidation rate and methane emission were higher in summer. The mean methane influx and methane emission decreased significantly as the VWC increased beyond 40% (i.e., a degree of saturation 0.85) at a depth of 0.85 m, which was near the loess/gravel interface. At this depth, more water was presented in the loess layer in the downslope direction due to capillary barrier effects, which increased the upslope methane emission. More dominant methane emission in the middle- and upper-section of the CBC occurred in summer than in winter as there was less soil moisture to facilitate methane transfer. The LFG balance showed that a significant fraction of the loaded LFG was not accounted in the flux chamber measurements due to the preferential flow along the edges of the CBC. The maximum methane oxidation rate was 93.3 g CH4 m−2 d−1, indicating the loess-gravel CBC could mitigate methane emissions after landfill closure.

In this paper, three sets of laboratory tests were conducted on high-food-waste-content (HFWC-), no-food-waste-content (NFWC-) and decomposed (D-) MSWs to characterize their compression behaviors. The immediate compression ratios C'c were 0.30, 0.23 and 0.18 for HFWC-MSW, NFWC-MSW and D-MSW respectively, and tended to increase with the increasing food waste content of MSW. The release of intra-particle water contained in food waste contributed over 23.6-29.2% to immediate compression for HFWC-MSW. The mechanical creep ratios C'sc were 0.02, 0.015 and 0.01 for HFWC-MSW, NFWC-MSW and D-MSW respectively. A prediction model for C'sc was proposed which incorporated the effects of moisture content, dry unit weight and organic waste content. The bio-compression ratios C'sbI, C'sbII and C'sbIII in response to degradation stage I, II and III were 0.12, 0.10 and 0.02 for HFWC-MSW, and were 0.01, 0.15 and 0.01 for NFWC-MSW. Bio-compression is dominant in stage I and II and mechanical creep is the major contributor in stage III for HFWC-MSW, but to NFWC-MSW, mechanical creep is dominant in stage I and III, and bio-compression takes the main position in stage II. The bio-compression tended to increase linearly with leachate draining rate for HFWC-MSW, and the release of intra-particle water contributed 61.9-65.6% to bio-compression. A new model was proposed that can well capture the highly non-linear behavior of bio-compression for both HFWC-MSW and NFWC-MSW. Based on the above findings, the settlement behavior of HFWC-MSW and NFWC-MSW landfills was compared, and suggestions for technique-efficient and cost-effective design of a NFWC-MSW landfill were discussed.

To limit the oxidation of waste rocks that originates from mining operations and the subsequent leaching of acidic solutions with high concentration of metal ions, a tailing–rock–clay triple layer capillary cover system was developed to prevent rainwater infiltration in humid climatic regions. The fine grained soil (FGS) layer consists of mine tailing and a hydrodesulfurization (HDS) clay from waste-water treatment with a 95:5 mass ratio. The coarse grained soil (CGS) layer consists of local waste rock granules with a size of 1–10 mm. Methyltrimethoxysilane (MTMS), an oxidation-inhibiting agent with strong hydrophobicity, was passivated on the rock grains to further reduce water infiltration and leaching of metal ions. Prototype-scale column tests were performed with matric suction and water content measurements under 680 min rainfall of 60 mm/h, the most severe annual precipitation case scenario for the Dexing Copper Mine (Jiangxi Province, China, 28.95° N, 117.57° E, humid climate). Both the uncoated and the coated covers exhibited zero leakage throughout the experiment. The passivation on rock granules in the coated cover increased the water entry value (WEV) of the CGS layer to −0.56 kPa. This led to a 15 mm water storage increment in the overlain FGS layer as compared to that in the uncoated cover, and induced lateral drainage (5% of the precipitation) in the FGS layer, which was not overserved in the uncoated cover. The concentrations of the leached Fe2+, Cu2+, Zn2+, Mn2+ and Mg2+ cations drained from the CGS layers of the uncoated cover were 0, 0.4, 0.8, 73.5, and 590.5 mg/L, which are all within the regulation limits of industrial discharge water standards. The concentrations of Cu2+, Mn2+ and Mg2+ cations drained from the coated CGS layer were reduced by 1–3 orders of magnitude. The abovementioned laboratory studies validated the water retention and leaching prevention abilities of the proposed three-layer capillary covers and the MTMS coating, which hold promises in engineering applications.

Disposal of the excess excavated soft soils produced in construction is a contemporary engineering challenge. A win-win solution could be found through utilising much excavated soil during reclamation of abandoned mines, and an evaluation model is necessary to realise this solution. This study aimed to propose a decision-making framework for large-scale disposal of excavated soft soil during mine reclamation using a Bayesian network (BN). Utilising multi-facet information, a decision-making path considering engineering and economic factors, and engineering risks was established. Thereafter, the practicability of backfilling much excavated soft soil into 215 abandoned opencast mines located in Wenzhou, China was evaluated. Based on the decision-making path and information of the 215 mines, a BN including 22 nodes was established with the practicability of large-scale soil disposal in mines as the decision node. Consequently, 40 valid mines were evaluated to be able to absorb 5 × 107 m3 soil, around 50% of the total excavated soil from the municipal districts of Wenzhou during 2020–2024. The BN evaluation showed high reliability with accuracy of all the nodes >70% and that of the decision node >90%. Five input nodes were identified to be high-impact for evaluation of the practicability and total fill volume.

To promote environmental protection and sustainability, the use of plants and recycled wastes in geotechnical construction such as landfill covers is recommended. A landfill cover field test was conducted at the Shenzhen Xiaping landfill site, located in a humid climatic region of China. The main objective was to validate the field performance of a novel vegetated three-layer landfill cover system using recycled construction waste without the need of geomembrane. Unsieved completely decomposed granite and coarsely crushed concrete was used for the top and intermediate layers while sieved completely decomposed granite was used as the lowest layer. One section was transplanted with Bermuda grass while the other section was left bare. To assess the landfill cover performance, pore-water pressure, volumetric water content, percolation, and atmospheric parameters were measured for a period of 13 months under natural climatic conditions. The cumulative rainfall depth was about 2950 mm over the entire monitoring period. During rainfall, the presence of grass led to lower pore-water pressure (or higher suction) and volumetric water content in the three-layer landfill cover system. At the end of monitoring, the cumulative percolation was about 27 and 20 mm for the bare and grass-covered landfill covers, respectively. It is evident that the vegetated three-layer landfill cover system using recycled concrete without geomembrane can be effective in minimizing percolation in humid climates.

Flow properties of soils can impact a wide range of geotechnical, agricultural, and geophysical processes. Few studies focus on the physical understanding of how pore characteristics can affect flow properties in soils. In this paper, pore network modeling is utilized to investigate intrinsic permeability, water pressure distributions, flow patterns, and critical flow paths in soils with pores varying in size, connectivity, and anisotropy. The results show that increased mean diameter, decreased standard deviation, and increased coordination number of the pores can lead to an increase in intrinsic permeability in soils. Non-uniform water pressure and flow rate distribution will more likely occur in soils with a larger pore size variability. A higher coordination number mitigates the pressure localization but slightly exacerbates non-uniform flow. With the increase in the coefficient of variation (COV) of pore diameters, the percolation path becomes more tortuous and carries more flow. When COV increases from 0 (homogeneous) to 1 (large pore size variability), the tortuosity increases from 1.00 to ∼1.71 and the flux carried by the percolation path in soils increases from 2.0% to 7.8%. Pronounced preferential flows may take place in soils with uniformly distributed pore sizes, in which the percolation path can carry as much as 9.2% of the total flux. The anisotropy in pore throat sizes also increases the flow tortuosity and the fraction of flux carried by the percolation path. The permeability anisotropy Kh/Kv increases linearly as the pore throat size anisotropy µdh/µdv increases logarithmically. These results provide insight for designing soil barriers for either uniform flows or exacerbated preferential flow for fast transport.

Settlement prediction of municipal solid waste (MSW) is a key issue in the design, construction, and post-closure development of landfills. As MSW is a highly heterogeneous material with compressible and degradable elements, the factors causing compression and settlement are complex. This paper explores the mechanisms and timescales for the settlement of land-filled MSW, and develops a rational taxonomy and framework of understanding. Conceptual models are reviewed and discussed, and the relative importance of different causes of settlement is assessed with reference to new, high quality, datasets and data from the literature.

Settlement behaviors of high-food-waste-content (HFWC) and low-food-waste-content (LFWC) municipal solid wastes (MSWs) respectively from developing and developed countries were characterized and compared based on a great number of experimental datasets from references. Fresh HFWC-MSW generally has larger primary compression ratio compared to fresh LFWC-MSW, due to the release of a large amount of intra-particle water contained in food waste under additional stresses. The slopes of strain-logarithmic time curves with respect to the three secondary compression phases are characterized as “slight-steep-slight” for LFWC-MSW and “moderate-moderate-slight” for HFWC-MSW. It is difficult to distinguish the first two phases of the secondary compression in strain-logarithmic time curves for HFWC-MSW. The entropy method was built to evaluate the performance and applicability of nine published settlement models based on the settlement datasets of four large-scale experiments. The computational simplicity, the fitting performance, the prediction performance and the parametric stability were taken as the four criterions in the entropy method. Based on the evaluation results, the models proposed by Sowers et al. (1973) and Gourc et al. (2010) are recommended for predicting settlement at LFWC-MSW landfills, while the hyperbolic model and the Chen et al. (2010) model are recommended for HFWC-MSW landfills.

Microbial aerobic methane oxidation (MAMO) has been considered as an environmental-friendly method for mitigating methane emission from municipal landfill sites. Soil column has in a landfill cover under one-dimensional (1-D) condition. However, most of the published soil column tests failed to simulate 1-D heat transfer due to the use of thermal conductive boundary at the sidewall. In the present study, a heavily instrumented soil column was developed to quantify the effects of thermal boundary condition on the methane oxidation efficiency under different ambient temperatures in landfill cover soil. The sidewall of the soil column was thermally insulated to ensure 1-D heat transport as would have been typically expected in the field condition. Two soil column tests with and without thermal insulation were conducted at a range of controlled ambient temperatures from 15 to 30°C, for studying how soil moisture, matric suction, gas pressure, soil temperature and gas concentration evolve with MAMO. The test results reveal that ignoring thermal insulation in a soil column test would result in a greater loss of soil heat generation by MAMO and hence oxidation efficiency by up to 100% for the range of temperature considered. When the ambient temperature increased to 30°C (but less than the optimum temperature for MAMO), the MAMO efficiency increased abruptly at first but then decreased substantially with time, and this is likely due to the accumulation of biomass generated by MAMO.