Geoenvironmental Engineering

Numerous contaminated sites exist worldwide that contain a mixture of organic and heavy metal contaminants. Very few technologies are proven to be efficient to address the problem of such mixed contamination. Most of these technologies are energy-intensive and expensive and they can disturb the natural ecosystem. Phytoremediation has the potential to be a green and sustainable approach to decontaminate and restore the contaminated sites, maintaining the biological activity and physical structure of the soils. However, its effectiveness for mixed contaminants is not well understood. This study presents a series of laboratory experiments conducted to investigate the effects of initial contaminant concentration on phytoremediation of mixed contaminated soils. A silty clay (typical field soil) was spiked with naphthalene, phenanthrene (representative organic contaminants), lead, cadmium and chromium (representative heavy metals) in different concentrations. Two plant species - specifically, Avena sativa (oat plant) and Helianthus annuus (sunflower) - were grown in these contaminated soils as well as in uncontaminated soil for comparison purposes. Results showed that the increase in contaminant concentrations in the soil negatively influenced the growth and biomass of the plants. Helianthus annuus showed lower germination, survival, growth rates, and biomass under increasing contaminant concentrations compared to Avena sativa.

When exposed to contaminants, soil-bentonite mixture tends to form a flocculate structure, leading to an increase in permeability while decreasing in performance of mitigating contaminants. To solve this problem, dispersants can be added to the soil-bentonite mixture to maintain a dispersed structure, relatively low permeability, and homogeneous. This study focuses on the viscosity property of soil-bentonite suspensions amended with dispersants. Three commercially available phosphate dispersants were used, which include sodium hexametaphosphate (SHMP), sodium tripolyphosphate (STPP) and sodium pyrophosphate decahydrate (TSPP). The designed bentonite content was 0, 5, and 10 %, whereas that of dispersant was 0, 0.01, 0.05, 0.1, 0.5, 1, 1.5, and 2 %, respectively. Both are based on the dry-weight of kaolin-bentonite powder. The result shows that with an increase in the content of dispersant, the viscosity of the mixture decreased and then tended to remain stable, irrespective of the type of dispersant. Three types of dispersants resulted in marginal differences in the measured viscosity regardless of the bentonite contents. Bentonite content of backfills can be improved by the addition of dispersants. In addition, optimum dosages of dispersants are 0.05, 0.1-0.5, and 0.5 %, corresponding to the mixtures with bentonite content of 0, 5, and 10 %.

Soil-bentonite (S-B) vertical cutoff walls are effective and economical as engineering barriers for contaminant containment. Water content higher than liquid limit (w_L)is needed for S-B backfills to achieve a target slump, ranging from 100 to 150 mm, for the purpose of improving constructability. A series of one-dimensional consolidation tests was performed to evaluate the compressibility and hydraulic conductivity of S-B backfills. Kaolin and sand were selected to simulate in-situ soils. The results indicate that BC critically controls the compressibility and hydraulic conductivity (k) of S-B backfills. The effect of grain size of sand on compression index (C_c) and k is insignificant. C_c increases with an increase in initial water content (w₀). However, the effect of w₀ on the compressibility can be eliminated by using compression ratio (CR). In addition, values of k are further discussed based on Nagaraj's generalized state parameters (e/e_L) and an empirical method for predicting k using void ratio (e) and w_L.

A series of tests was performed to investigate the air flow patterns in saturated sands during air sparging. Test results show that air flow patterns are affected by the particle size of the sand as well as the air flow rate. Two types of gas-phase flow patterns have been observed, namely, channel flow and bubble flow. Basically, the channel flow pattern develops when the sand particle diameter is less than 1.0 mm; whereas, the bubble flow pattern occurs when the sand particle size is larger than 2.0 mm. It has been observed that a transition flow state exists when the sand particle size changes from 1.0 to 2.0 mm. The results indicate that air-phase saturation degree of the sand mass increases with the increased air flow rate. When the air flow rate exceeds a certain threshold value, the air phase saturation tends to reach a steady state. Given that the other conditions remain unchanged, the air-phase saturation of the soil mass increases with the decrease of the soil particle size. A pretty good linear relationship was found between air flow rate and air pressure. Based on the extrapolation method, the minimum air injection pressure is derived, which is an important parameter in design practice. Finally, the effect of sand particle size on the estimated capillary pressure is discussed.

This paper describes the development of a numerical model, called CSTCRS1, for one-dimensional coupled contaminant transport and large strain consolidation under constant rate of strain (CRS) loading conditions. Numerical simulations using CSTCRS1 indicate that concentration boundary conditions can have an important effect on contaminant transport during CRS consolidation. For the conditions considered, zero concentration gradient and reservoir boundary conditions yielded the same contaminant mass outflows regardless of the transport mechanisms (i.e., diffusion, mechanical dispersion, and sorption). However, these transport mechanisms become important when a zero concentration boundary condition is specified. Additional simulations indicate that applied strain rate also has an important effect on the coupled CRS consolidation and contaminant transport. A higher strain rate will generally yield more non-uniform local strain profiles, smaller contaminant mass outflow, and a larger concentration gradient within specimen.

The presence of preferential flowpaths (PFP) causes high heterogeneity of soil. The complex contaminant transport through highly heterogeneous soil has become a key point on predicting pollution of the soil and groundwater system. In this study, a generalized model of preferential flow paths has been established based on a dual-domain model which reflects contaminant transport and the dynamic transfer between two domains, and quantitatively analyzes the difference between advection-dispersion model (ADM) and dual-domain mass transfer model (DDM) in pore scale. The preferential ratio and scatter number are proposed to reflect the impact of PFP physical parameters. Results show that contaminant mass flux increases significantly with the increase of preferential ratio, specifically when: (1) the preferential ratio is 0.15 and the mass flux is 4.5 times that of homogeneous at 150 a; and (2) the scatter number is 0.22 and the solute flux is 8.5 times that of homogeneous at 150 a. Finally, the impact of saturation change for solute flux was also analyzed in the research.

The resistivity piezocone penetration test (RCPT) is a fast, repeatable and cost-efficient in situ method to detect soil engineering properties. In geo-environmental site detection, soil properties - such as soil resistivity (ρ), effective angle of internal friction (φ'), constrained modulus (M) and relative density (D_r) - can be obtained. This paper analyzed engineering properties of soils contaminated by different pesticide, which were obtained by RCPTU in one pesticides factory in Eastern China. The results showed that soils contaminated by nitidine chloride and organophosphorus had lower resistivity (ρ), effective angle of internal friction (φ'), constrained modulus (M) and relative density (D_r) as compared with uncontaminated soil. Nitidine chloride-contaminated soil had higher properties than organophosphorus-contaminated soil. Both contaminated and uncontaminated soils were identified in Robertson (1990) soil classification chart (Fr-Qt). It is noted that different pesticides have similar influence on soil classification. They are all located in lower areas than the uncontaminated ones on then soil classification chart.

Sedimentation and consolidation of soil particles in dredged slurry is very complex since there are no explicit rules. A modified experiment method - multilayer extraction sampling - is adopted in this study. The volume distribution of soil particles can be determined first, and the average velocity of soil particles for different size groups of soil particles is calculated using the volumetric flux function method. The result reveals that there are three different zones existing during the sedimentation and consolidation of soil particles in dredged slurry -hindered settling zone, flocs compression settling zone, and consolidation zone. During the first hour and the following 6 hours of preliminary sedimentation period, the velocity of soil particles is much smaller than the velocity calculated from Stokes equation in the hindered settling zone. There is no relative displacement of soil particles in flocs compression settling zone and self-consolidation zone. All soil particles in these two zones belong to the sediments. The density of flocs compression settling zone is about 1.04 g/cm³ in the experiment.

This paper reports on the results of a study on the performance and effectiveness of acrylic copolymer stabilization in improving the strength, water retention capacity and dust resistance of copper mine tailings (MT). Samples were taken from both untreated and acrylic copolymer-treated TSF. Fall cone, water retention and wind tunnel tests were conducted on the samples to evaluate their strength, water retention capacity and dust resistance. SEM imaging was also performed to evaluate their microstructure. The results indicate that acrylic copolymer treatment improves the strength, water retention capacity, and dust resistance of MT. The improvement is because acrylic copolymer application enhances the agglomeration of MT particles and forms coating on the MT particle surface, leading to a denser microstructure. The formation of a loose white powder on the surface of MT was observed in the study. FTIR and XRD analyses were performed to characterize this material because it may have an adverse effect on dust resistance of MT.

The direct shear test and the compression test of municipal solid waste are carried out by using large-scale test equipment to study the shear strength properties of municipal solid waste. The municipal solid waste materials are divided into three parts and an appropriate proportion is selected for them. The relationships between shear stress, shear strain, compression strain, shear strength and time are studied. Research results show that the shear stress does not vary linearly with the shear strain, and the curve of the shear stress versus shear strain is a strain hardening curve, even the shear strain exceed 20%. The curve of the vertical strain versus logarithmic time can be fitted into a fold line, and the relationship between the initial void ratio and logarithmic time can also be fitted into a fold line. Besides, the curve of the shear stress and the logarithmic shear strain can be fitted into a linear line, and the slope of the line is at the same value of0.97. On the basis of the experiments, a new model has been presented which is simple and has less parameters.

Methane (CH₄) is generated during anaerobic biodegradation of municipal solid waste in landfills and can be collected as an energy source. CH₄ generation rate (k) and potential CH₄ generation capacity (L₀) are two key parameters of the EPA LandGEM model used to estimate CH₄ generation in full-scale landfills. These parameters can be measured in laboratory batch and simulator tests. The relationships of these parameters with waste composition - specifically, the percentages of biodegradable constituents - were investigated. k was positively correlated to the percentage of biodegradable constituents. L₀ correlated well with volatile solids inthe waste. A k-L₀ chart was developed to delineate the boundaries of CH₄ generation rate and potential CH₄ generation capacity during MSW biodegradation in laboratory tests. Waste composition and heterogeneity, moisture content and temperature likely contributed to the differences of k and L₀ values obtained between batch and simulator tests.

The majority of municipal solid waste (MSW) generated in Sri Lanka is disposed in open dump yards. Compaction of wastes at such open dumps is the main factor that controls short-term density and resulting placement efficiency of wastes in the dumps. Maximizing waste density allows reducing dump yard space requirements. Density influences the stability of dump yards with high shear strengths. Combined moisture-density characteristics influences the hydraulics response and compressibility of wastes. Further, a lot of civil engineering construction in the Colombo city area commenced on the lands previously used for dumping of waste. Construction on already-covered dump wastes will create a lot of problems such as high settlements, slope instability, and shear failure. Four dump yard locations - Bloemandhal, Seeduwa, Negombo and Madampitiya - were selected for the study and the moisture-density relationship was studied. Some of these locations are now abandoned and the land is being used for construction activities and some are still in the process of dumping. It was observed that the compaction behavior of waste was similar to that of soils with waste; but the shape of the compaction curve has deviated from the standard bell shaped curve established for soils.

A numerical two-phase flow modeling was performed for leachate recirculation in bioreactor landfills using horizontal trench (HT) system to evaluate the moisture distribution, pore water and capillary pressures generation, and the resulting impacts on the stability of a simplified bioreactor landfill. A staggered geometric configuration of HT system with known horizontal and vertical spacing was modeled based on the results from previous study. The lateral distance of HT system from side slope (setback distance) was varied in the range of 15-30 m, as recommended by USEPA. The waste was considered as heterogeneous and anisotropic with varied unit weight and saturated hydraulic conductivity. Leachate was injected at high injection pressure, both continuously and intermittently. Unsaturated hydraulic properties of MSW were based on the van Genutchen model. It was found that staggered HT system, located near side slope, could significantly affect the moisture distribution and lead to landfill slope failure under continuous injection as a result of excessive pore fluid pressures as compared to the HT systems, located relatively far away from side slope. However, intermittent injection sequencing with injection in alternate HT layers proved effective in maintaining stable slope without generating excess pressures, and resulting in an optimal environment for biodegradation.

Consolidated undrained triaxial compression tests with pore water pressure measurement were carried out on 150-mm-diameter samples of municipal solid waste (MSW) recovered from a waste site in Delhi. While understanding that the mechanical response of municipal solid waste has evolved significantly in the recent years, most of the previous work has studied the behavior of MSW under drained conditions. However, there are several situations in which the undrained behavior of MSW may be important, particularly for uncontrolled dumps and bioreactor landfills where the MSW may be in a saturated or near-saturated condition. MSW samples were reconstituted at their in-situ composition and reconstituted with only a less-than-20-mm fraction (the soil-sized fraction) for comparison. The stress strain response of specimens at the in-situ composition showed a strain hardening type behavior, with the mobilized shear strength continuing to increase without reaching peak strength at strains in excess of 20 percent. Specimens with only the soil sized fraction exhibited a peak response, clearly demonstrating the role/importance of fibrous waste constituents in the mechanical response of MSW. In both cases, shear strength parameters established based upon several different failure strain criterion indicated an effective friction angle in excess of 45 degrees.

This paper, for the first time, presents a practical case where the probabilistic slope stability approach has successfully been used to evaluate a Municipal Solid Waste (MSW) landfill slope that failed due to a combination of steeper slopes and high liquid levels. The results of this evaluation will benefit practicing landfill engineers in applying probabilistic theory to landfill design and operational practice. Conclusions are presented based on deterministic factors of safety (FS), reliability indices (β), and probability of failure (P_f) values. The β and P_f values are compared with the US Corps of Engineers' recommended values for slope performance. Results show that probabilistic methods can capture the behavior of highly heterogeneous slopes (such as MSW slopes) and can be used by landfill engineers to make design, construction, and operational decisions.

A three-dimensional pseudo-static analysis method is developed to conduct seismic stability analysis for potential failures of a landfill along the liner system. In this calculated model, assuming that the horizontal coefficient k_h and vertical coefficient k_v are the same and known everywhere, the factor of safety and yield acceleration can be calculated based on the developed equations. Effects of the most significant parameters considered are investigated in this 3D model. Finally, the permanent displacement based on the calculated yield acceleration is obtained using the Newmark method. From the analysis results, it can be concluded that simply ignoring the size of a landfill will lead to over estimating the factor of safety and permanent displacement of landfills. Therefore, a 3D analysis considering the geometrical configuration of landfills will be more suitable for its seismic stability evaluation.

In this study, the relative performance of two soils - after amending them with suitable matrices in retaining the heavy metal ions - has been evaluated. Al-Ghat soil and Al-Qatif soil of Saudi Arabia were mixed with different percentages of lime. Heavy metal ions (such as copper and chromium) were used to carry out sorption studies. Experimental data was used to plot adsorption isotherms. Langmuir isotherm was found to be more suitable than Freundlich isotherm for all soil mixtures. It can be concluded that Al-Qatif soil lime mixture can be used as a filter material and Al-Ghat soil lime mixture as the main liner material to attenuate selected heavy metals.

Improved soil quality and increased crop yield are commonly reported benefits of amending soil with biochar. This study is to address the potential of using biochar as an organic matter-rich material to amend soil in order to enhance crop production. Pot experiments were conducted in Chicago using agricultural soil amended with wood-based biochar for a period of two months. The soil was amended with biochar before the sowing of two kinds of bush beans, Tendergreen Improved (TI) and Blue Lake 274 (BL). Soil and plant samples were collected while the yield was measured when harvested. The changes in soil properties, as well as crop yield, were examined with comparison between the pots treated with and without biochar. Results show that biochar increased bean growth for TI. However, no negligible effects were observed for plant growth of BL. The yield of TI was increased over the control under biochar treatment. Effects of biomass yield were different for two kinds of beans due to the different dependence on fertilizers of the crop, which was partially proved by significant increase of soil fertility of TI and inconsistent nutrients change of BL. These results demonstrated a possible potential of biochar treatment on soil improvement and crop productivity enhancement in agricultural soil, although the effects varied with plant species.

A series of physical and chemical (as well as batch adsorption) tests were carried out on compacted residual lateritic soil treated with up to 10 % cement kiln dust (CKD), an industrial waste, to evaluate the attenuative capacity of the mixture when used as a liner material. Batch adsorption tests were carried out to determine the sorption capacities of CKD-treated lateritic soil. Specimens were prepared at stepped CKD concentrations of0, 2.5, 5.0, 7.5 and 10.0 % by dry weight of soil in a 1:4 lateritic soil-solution ratio. Assessments of the adsorption of ions present in the municipal solid waste leachates used were carried out. Tests results gave trends of chemical sorption by the lateritic soil - cement kiln dust mixtures in the order, potassium, calcium and sodium (i.e., K⁺ > Ca²⁺> Na⁺). A 2.5 % CKD treatment of lateritic soil recorded peak retardation factor and increased sorption of the selected contaminant species. In general, CKD treatment of lateritic soil, which is a beneficial disposal of an industrial waste, has significant advantages over the untreated lateritic soil and, therefore, can be used as landfill liner material.

This study was aimed at investigating the effects of moisture content (MC) on the methane transport and adsorption through wood-pellet biochar (CE-WP2) and comparing those results with that of a more conventional adsorbent, granular activated carbon (GAC). Laboratory column adsorption tests were conducted under dry conditions and at two levels of moisture content at three different landfill gas inflow rates. The breakthrough curves from column studies were modeled using the 1-D advection-diffusion equation by accounting for both the transport and adsorption processes affecting the system under different levels of MC. The presence of moisture negatively affects the methane adsorption onto both materials with more pronounced influence in the case of GAC. The methane diffusion coefficients were modeled under dry and wet conditions for CE-WP2 and GAC. Results from this study show that the modeled methane diffusion coefficients for CE-WP2 were higher than that of GAC. Overall, this study provides critical information to design a novel biocover for landfills by incorporating biochar as a sustainable cover material to mitigate current CH₄ emissions.

The maximum particle size of dewatered sludge from the quarry is 2 mm, with the water content of 24- 25 %, the liquid limit of 38% and the plastic limit of 17%. It is usually disposed into the landfill. However, there is a possibility in authors' minds to recycle the dewatered sludge as a barrier material. The soil with fine particles indicates a lower permeability providing appropriate compaction over the one with the coarse particles. In this study, firstly, compaction tests, CBR tests, cone penetration tests and permeability tests were conducted for the row dewatered sludge and the dewatered sludge mixed with crushed stone. As a result, it is found that the dewatered sludge mixed with crushed stones having a size between 5 and 13 mm at a ratio of 20 % in dry mass has the sufficient traffic ability and efficient coefficient of permeability as a barrier material. Then, cutting geonets were added into the dewatered sludge mixed with crushed stones in order to prevent a shrinkage crack from progressing. Consequently, the mixing ratio of 0.8 % of the cutting geonets is the best for preventing the drying shrinkage crack effectively.

In relating to the feasibility of geological disposal of high-level nuclear waste, numerous experimental investigations and numerical simulations have been conducted to guarantee the safety and the efficiency of the geologic sealing structure in a long time period. The thermo-hydraulic-mechanical (THM) coupling problem is considered as one of the most important factors in evaluating the whole process of the geological disposal of nuclear waste. In this paper, a heating experiment (Munoz, 2006) has been simulated to investigate the THM problem in the unsaturated ground using a program of finite element method (FEM) named SOFT. The program is based on the finite element-finite difference (FE-FD) scheme in fully coupled soil-water-air, three-phase field theory under non-isothermal conditions. In the simulation, a rational constitutive model that takes Bishop-type effective stress and the degree of saturation as the state variables, is adopted. The model can describe the behavior of saturated and unsaturated soil automatically. According to the simulation results, it is known that the present simulation can properly describe the THM behaviors observed in the heating experiment such as the hydration of water, the evolution of temperature, and the evolution of pore water pressure to some extent.

In order to utilize reclaimed gypsums (gypsum board waste) derived from waste plasterboards as geo-materials, thermal behaviors of the reclaimed gypsums and reagent gypsum are investigated in this paper. As a result, it was found that the dihydrate gypsum is changed into the hemihydrate gypsum under thermal condition of 90°C and the hemihydrate gypsum is changed into anhydrite under that of 120°C with heating of 24 hours. The followings are clarified in this paper:(1) The cement density measuring method was available in order to measure the density of reclaimed gypsums; (2) that density was dependent on the drying conditions of the reclaimed gypsums; and (3) the density measuring method was appropriate to estimate the gypsum hemihydrate content on reclaimed gypsums.

Shredded tires mixed with sand are used as a backfill material for mechanically stabilized earth (MSE) walls and as an embankment fill material. Use of tire shred-soil mixtures as backfill materials in mechanically stabilized earth (MSE) walls has several advantages over other backfill materials: (1) good drainage; (2) high shear strength; and (3) low compacted unit weight. Estimation of pullout resistance of reinforcement embedded in tire shred-sand mixtures is essential for the design of these structures. In this paper, the results of large-scale laboratory pullout tests on ribbed-metal strips in tire shred-sand mixtures are presented. The effects of tire shred size, tire shred-sand mixing ratio and confining pressure on the interaction between the ribbed-metal strip and tire shred-sand mixtures are also evaluated. Three sizes of tire shreds are considered: tire chips (with 9.5-mm nominal size); tire shreds 50-to-100 mm and 100-to-200 mm in length with mixing ratios of 0, 12, 25; and, 100% of tire shreds in the mixtures (by weight). For the test conditions considered in the study, the pullout resistance factors of metal strips placed in sand only ranged from 0.87-1.49, whereas, for tire shred-sand mixtures, this factor ranged from 0.75-1.25, 0.57-1.08, and 0.32-0.54 for the 12/88 mix, 25/75 mix and 100/0 mix, respectively.

Investigation of aging effect is important due to examination of long-term influence of contaminants on the landfill liners. The effect of Na⁺ and Ca²⁺ ions on the engineering behavior of soils were examined and the aging effect on engineering properties of kaolin and bentonite samples in the presence of ions were observed. The pore size distribution, swelling and shear strength parameters were determined under the same conditions after approximately 60, 190 (shear strength), and 250 (swelling) days. There is no difference at the swell index values of kaolin in the presence of DI water and 0.5 M NaCl. However, Ca²⁺ ions decreased the swell index of the kaolin sample. In the presence of Na⁺ ions, there is no difference at the swell index values of the bentonite sample. However, in the presence of Ca²⁺ ions, swell index decreased from 0.035 to 0.029. During aging, the reverse effect occurs at bentonite sample when compared with kaolin sample. It is expected that, as aging progresses, bonds between particles increase; therefore, the swell index of the soils decrease. However, the results of this study have shown that this is not true for bentonitic samples. When the results of 0 day, 60 and 190 days samples were compared, the \xQ' values of kaolin and bentonite samples decreased by time.