Use of a resistivity cone for detecting contaminated soil layers

A cone-penetration technique was developed to detect contaminated soil layers with electrolytes and NAPLs. In this study, laboratory and field experiments were performed to apply the technique. In the laboratory, the change in resistivity was quantitatively examined by adding salt or oil to soil samples. The results showed that the resistivity measurement was varied with an order of ppm for the electrolyte concentration in soil. It was found that the resistivity of sand increased with increasing oil concentration. The effect of oil content was stronger for lower water content of sand.

The results obtained from field experiments showed that the resistivity cone can be used for detecting the contaminated layer in soils whose background values are known. Particularly, the instrument can be used effectively for examining the effects of remediation, by measuring the resistivity of the ground before and after remediation, unless the soil fabric changes significantly during remediation procedures.     

Assessment of the leachate movement in a sealed landfill using geophysical methods

This paper describes a case study concerning the use of integrated geophysical methods applied to environmental assessment. The study is focused on an old municipal solid waste sealed landfill site, located in Gaeiras, Central Portugal. The problem is related with leachate overproduction in this domestic and industrial waste landfill that became an environmental problem with urgent assessment, so that a solution could be planned. Due to the lack of accurate information regarding the shape, history and development of the landfill, the use of a set of classical geophysical methods was the option, since they are non-invasive and non-destructive. The available area was small, almost restricted to the landfill area. To conduct this assessment, electromagnetic RF-EM and Geonics EM34, spontaneous potential (SP), vertical electrical soundings (VES) and magnetic prospecting surveys were planned to understand the various problems that could be related with the leachate overproduction. The joint use of these classical geophysical methods was targeted to investigate bedrock depth and structure (RF-EM, EM34 and VES), waste and leachate characteristics (EM34 and magnetics) and groundwater flow (SP) in the landfill. Geophysical results were correlated with hydrogeological information, integrated and interpreted, using geographic information system tools. The results obtained were important to understand the geological mechanisms that are responsible for leachate overproduction and to suggest remedial measures.     

Relationships between electrochemical properties of leachate and resistivity of strata in the landfill site consisting mainly of combustion residuals

There are many landfill sites consisting mainly of combustion residuals in Japan. The disposed materials are chemically unstable. Therefore, temporal changes of waste deposit conditions must be monitored for environmental management of landfill sites. Electrochemical properties of leachate and the relationships between the leachate properties and the resistivities of strata were studied in such landfill sites. In addition, incineration ashes immersed in distilled water were prepared and the temporal changes of electrochemical properties of the solution were examined. In composition, the ashes used in the experiment are similar to those disposed in the above sites. From the above results, the electrochemical properties of waste deposits consisting mainly of combustion residuals have been studied and the efficiency for managing landfill sites by resistivity measurements has been established.     

剪切对非饱和土土-水特征曲线影响的探讨

用净平均应力和吸力等于常数、偏应力增大的三轴排水剪切试验，探讨了剪切对非饱和土水量变化的影响。试验结果表明，土-水特征曲线依赖于偏应力，改进了非饱和土中水量变化的本构关系，建立了含水量．吸力．净平均应力．偏应力形式的土．水特征曲线。     

Modelling the effect of temperature on unsaturated soil behaviour

A simple thermohydromechanical (THM) constitutive model for unsaturated soils is described. The effective stress concept is extended to unsaturated soils with the introduction of a capillary stress. This capillary stress is based on a microstructural model and calculated from attraction forces due to water menisci. The effect of desaturation and the thermal softening phenomenon are modelled with a minimal number of material parameters and based on existing models. THM process is qualitatively and quantitatively modelled by using experimental data and previous work to show the application of the model, including a drying path under mechanical stress with transition between saturated and unsaturated states, a heating path under constant suction and a deviatoric path with imposed suction and temperature. The results show that the present model can simulate the THM behaviour in unsaturated soils in a satisfactory way.     

Adsorption capacity of iron oxide-coated gravel for landfill leachate: simultaneous study

Landfill leachate is a high-strength wastewater. If it is not managed properly, it can pollute surrounding environment. The aim of this study is to determine the simultaneous adsorption capacity of iron oxide-coated gravel for metals such as Cd(II), Cu(II), Fe(II), Ni(II) and Zn(II) in high-strength leachate sample. Different operating conditions such as pH, time, and dosages were investigated to determine the kinetics and mechanism of adsorption process. Coating with iron oxide changed the external surface of gravel. The adsorption capacities increased with increased pH, and the optimum pH was found to be 7. High removal rates were observed in a short period of time. The Freundlich model fitted reasonably well to the experimental data, indicating multilayer adsorption process and the heterogeneity of the surface (R ² ranging 0.57–0.94). The Temkin model fitted well to the experimental data as well (R ² ranging 0.67–0.98), indicating that the adsorption is an exothermic process. The adsorption of ions was found to obey second-order kinetics, indicating one-step, surface-only adsorption process. The degree of metal adsorption on iron oxide-coated gravel at pH 7 was in the order Cu(II) > Cd(II) > Fe(II) > Zn(II) > Ni(II).     

Estimation of soil weathering degree using electrical resistivity

In this study, the electrical resistivity of soil having different chemical weathering index (CWI) was measured, and the correlation between CWI and the electrical resistivity was estimated. The electrical resistivity of soil varies with CWI of soil. The difference in the electrical resistivities of soils having different weathering degrees is clear at lower water contents. At the volumetric water contents estimated in this study, CWI could be described by a linear equation of electrical resistivity with the constants related to the volumetric water content. The findings in this study suggest that the electrical resistivity could be used as an effective alternative for estimating the weathering degree of soil.     

基于电阻率法的膨胀土膨胀性评价研究

目前膨胀土的膨胀率一般只能通过室内试验获取,无法通过简单快速的测试得到。结合课题组前期的相关膨胀土电学特性试验研究发现,膨胀土吸水膨胀会改变土体内部结构,而土体内部结构的变化会引起电阻率的改变。通过对取自合肥的膨胀土进行电阻率、膨胀率等相关性能的试验,分析电阻率指标,并对膨胀土的吸水膨胀特性进行预测评价。研究发现,膨胀率随着初始电阻率的增大而增大,且随着上覆荷载的增大而减小;竖向电阻率与实时膨胀率基本呈直线增加关系,直至土体电阻率达到孔隙液电阻率为止。根据膨胀土基本电学试验和膨胀土吸水膨胀及相应横纵向电阻率实时测试试验研究发现,膨胀土的膨胀性能与电阻率结构参数吻合很好,因此,可以通过电阻率相关指标对膨胀土的膨胀特性进行预测。由于相应膨胀率为无量纲指标,故通过黏性土电阻率结构参数去除孔隙率影响,建立了无量纲综合电阻率指标Er与膨胀率之间的关系,以供后期膨胀土的膨胀率预测。     

Experimental study of steady seepage in unsaturated loess soil

The complexity of unsaturated soil means that its permeability coefficient and seepage law are important topics in soil mechanics and geotechnical engineering. In this study, a new type of equipment for measuring the seepage of unsaturated soil was used to study the steady seepage of unsaturated loess soil collected from northern Shaanxi province in China. The entire process of the steady seepage of unsaturated soil under different seepage hydraulic gradients was obtained, and the permeability coefficient in the steady seepage state was measured. When the soil water content was low, the soil moisture remained only in small pores or at the junction of soil particles, the effective seepage area of the sample was reduced, and the seepage path was extended. Therefore, the lower the water content, the smaller the permeability coefficient of unsaturated soil and the longer the time required for the seepage to reach a steady state. The matric suction of the unsaturated loess soil was measured under different water contents. With decreasing water content, water remained only in the smaller pores, which provided shrink films with a smaller radius of curvature. This allowed the shrink films to withstand a larger pore air pressure and transmit a smaller pore water pressure, so that the matric suction of the loess soil increased with decreasing soil water content. The Gardner model provided an accurate fit of how the matric suction varied with the volumetric water content of the unsaturated soil. The seepage velocity and seepage hydraulic gradient were correlated well with a linear relationship, indicating that the seepage law of unsaturated loess soil accords with Darcy’s law. The experimental results show that the new unsaturated soil permeameter has good performance and controllability, thereby providing support for studying the steady seepage of unsaturated soil.

     

Simulation modelling of pollutant transport from leachate in Shiraz landfill

A one-dimensional mathematical model based on convection–dispersion equation in unsaturated porous media is presented to compute inorganic total solid concentration in the soil column under the Shiraz landfill. In addition, a dynamic mathematical model is formulated to simulate concentrations of ions such as Ca²⁺ , Mg²⁺, Fe²⁺, K⁺, Na⁺, Cl⁻, SO₄ ²⁻ and HCO₃ ⁻ as well as PH and EC in soil profile under the Shiraz landfill. Leachw model was applied to simulate water flow, water content and hydraulic conductivity in soil depth. The model was calibrated and verified by using different sets of data collected from several segments of soil depth in the study area. The numerical solution obtained using finite element method. The simulated values for the parameters were compared with measured values as well as analytical solution. The simulated results are in good agreement with measured values. This model could be applied to field scale problems for the landfill management.     

Analyses on a high leachate mound in a landfill of municipal solid waste in China

The leachate levels in the landfills in southern China are generally high. Field monitoring was carried out in the Suzhou landfill to investigate the leachate mound. The saturated hydraulic conductivity and soil–water characteristic curve (SWCC) of municipal solid waste were measured using samples taken from different depths of the landfill. Field monitoring reveals that a perched leachate mound and a substantial main leachate mound existed in the landfill. The saturated hydraulic conductivities of wastes in shallow, middle and deep depth were measured to be 4.81 × 10^?2, 3.50 × 10^?3 and 3.56 × 10^?4 cm/s, respectively. The results of SWCC tests show that the SWCC curve was steep when matric suction was low, and the shallower the waste the steeper would be the curve. In addition to the field and laboratory tests, an unsaturated–saturated seepage analysis was conducted to simulate the development of the high leachate mound and to calculate the annual leachate production. The simulated volumetric water content in the unsaturated zone was about 40 %, which agreed well with the test result. The calculated leachate mound was consistent with the field measurement. The calculated annual and daily leachate productions were all more reasonable than the results of the HELP model.     

Rainfall-triggered landslides in an unsaturated soil: a laboratory flume study

Extreme and/or prolonged rainfall events frequently cause landslides in many parts of the world. In this study, infiltration of rainfall into an unsaturated soil slope and triggering of landslides is studied through laboratory model (flume) tests, with the goal of obtaining the triggering rainfall intensity–duration (I–D) threshold. Flume tests with fine sand at two different relative densities (34 and 48%) and at slope angle of 56.5° are prepared, and rainfall (intensity in the range of 18 to 64 mm/h) is applied via a mist sprinkler system to trigger landslides. Soil water characteristic curve and hydraulic conductivity function of the fine sand are also presented. In flume tests, suction in the soil is measured with tensiometers, the progress of wetting front with time and deformations in the soil are also measured. Some of the findings of this study are: for the fine sand used in this study (a) the failure mechanism is infinite-slope type (mostly translational), and the failure surface is generally coincident with the wetting front or is in its vicinity, (b) the deformations leading to a landslide occurred abruptly, (c) both relatively high-intensity–short-duration rainfalls and relatively low-intensity–long duration rainfalls triggered landslides, (d) the shape of the I–D threshold is demonstrated to be a bilinear relation in log intensity–log duration plot, (e) below a certain rainfall intensity landslides are not triggered, (f) the effect of relative density of the soil on the I–D threshold is demonstrated by physical laboratory tests (as the relative density of the soil increases, the triggering rainfall intensity–duration threshold moves to larger rainfall events). The results of this study could be useful for accurate numerical modeling of rainfall-triggered landslides.     

赤泥渗沥液对黏性土和固化赤泥影响的试验研究

赤泥渗沥液具有强碱性,首先对赤泥渗沥液的碱性进行分析,利用碱溶液模拟赤泥渗沥液,研究了山西某赤泥堆场黏性土样在强碱的腐蚀作用下物理性质的变化及其微观结构特征,重点考察不同的作用时间效应的影响。对利用水泥(或粉煤灰)固化的赤泥样的受碱腐蚀所引起的强度变化规律进行研究表明,在碱溶液作用下不同的固化时间时固化样的强度变化过程主要体现在整个过程的早期,固化样在经过较长时间的凝结过程和腐蚀效应的共同作用后强度逐渐趋于稳定,无侧限抗剪切强度不会有明显的衰减,利用水泥和粉煤灰对赤泥进行固化处理是可行的。     

Water infiltration characteristics of unsaturated soil slope and its effect on suction and stability

Rainfall has been considered the cause of the majority of slope failures and landslides that happened in regions experiencing high seasonal rainfalls. The mechanism of the failures was mainly due to the lost of matric suction of soils by rainwater. This paper presents the results of a laboratory model study on the effect of slope angle and surface cover on water infiltration into soil and soil matric suction. A field infiltration test is carried out for comparison. A parametric study is also done to examine the effect of permeability ratio, development of perched water table and rainfall intensity on the factor of safety against instability of a soil slope. Results of the model study show that different surface covers on slopes have an effect on the water infiltration. Generally the covered surface (grass or geosynthetic net) has a lower infiltration rate compared with the bare (no cover) surface. On the effect of slope angle, it was observed that water infiltration decrease with increase in the slope steepness. With regards to the movement of the wetting front, it appears that water infiltration is more at the toe compared with the top of the model slope. Based on the parametric study, it is found factor of safety of the slope against instability drops for slope with higher ratio of permeability for the permeable and impermeable stratum. As the perched water table is formed, the factor of safety decreased. The rainfall intensity also has a marked effect on the slope factor of safety. The higher the intensity of the rainfall, the higher is the infiltration rate into the soil, hence the lower is the factor of safety against slope instability.     

Laboratory tests on soil conditioning of clayey soil

Tunnelling in difficult and challenging conditions such as soft soils in urban areas is increasing. In this condition, it is important to minimise the possible negative effect of the tunnel excavation, such as settlement or, in the worst case, collapses. To achieve this result, earth pressure balance machines are commonly used. One of the key parameters that must be considered for an optimal management of the EPB-TBM excavation is soil conditioning since the excavated muck must properly transmit the pressure to the tunnel face. Soil conditioning is also necessary to reduce the effect of the problems, such as clogging in clay layers, that can occur during the excavation and that can affect the performance of the tools and of the entire tunnelling process. For this reason, in the last decade, much research has been carried out to understand how to deal with and reduce the effects of clogging and stickiness, using different conditioning additives. These studies have proposed several different test procedures to evaluate the effect of the conditioning on the adhesion of the soil on the metallic parts of the machines. The present research has been carried out with the aim of proposing a new approach and new devices to study clay conditioning with laboratory tests, and the results of many tests carried out with the proposed device are presented and discussed.     

3D behaviour of contamination in landfill sites using 2D resistivity/IP imaging: case studies in Portugal

Aiming at defining a valid spatial contamination model, resistivity and induced polarization (IP) measurements were used to investigate contamination plumes in the vicinity of two municipal landfills (Ovar and ílhavo). Previous geophysical surveys and underground water samples confirmed the contamination. However 2D resistivity/IP surveys enabled in obtaining a more accurate spatial model. The Ovar survey consisted of two profiles with nine Wenner soundings each; the ílhavo survey was carried out along two individual lines using a Wenner standard pseudo-section. In both situations, negative IP values were found associated with positive IP values, which can be explained mainly by 2D or 3D geometric effects caused by the presence of the conductive plumes. The data were modelled using a 2D inversion program (RES2DINV) and the resulting resistivity and chargeability distributions were displayed as pseudo-sections. The resistivity and chargeability pseudo-sections define the contamination plumes and the sedimentary structure. These case studies illustrate the advantages of 2D resistivity/IP surveys for the mapping of shape and dimension of contamination associated with landfills.     

Modelling transient heat and moisture transfer in unsaturated soil using a parallel computing approach

A parallel numerical model, employing a finite difference explicit scheme for the analysis of coupled heat and moisture transfer in unsaturated soil, is employed to simulate a laboratory experiment of heating of medium sand. The model, written in a two-dimensional polar co-ordinate formulation, is programmed in the concurrent language Occam and executed on a parallel computing network of transputers. Parallelization is adopted as a means of overcoming computing difficulties, which limited numerical solutions to those at steady state, to enable transient behaviour to be simulated. The parallel algorithm was found to be very efficient, enabling a full solution of transient behaviour to be obtained. An investigation of the ability of the model to accurately simulate the complex, interrelated coupled nature of both two-dimensional transient and steady-state behaviour yielded very good correlation between experimental and numerical results. It can therefore be concluded that overall the results obtained provide confidence in the validity of the approach proposed.     

Fate and transport of pollutants through a municipal solid waste landfill leachate in Sri Lanka

This study focuses on the characterization of leachate generated from Gohagoda dumpsite in Kandy, Sri Lanka, assessment of its spatial and temporal variations, and identification of subsurface canals and perched water bodies in the wetland system affected by the leachate flow. Leachate samples were collected monthly throughout dry and rainy seasons from different points of the leachate drainage channel over a period of 1 year and they were tested for quality parameters: pH, temperature, electrical conductivity, total dissolved soils, alkalinity, hardness, total solids, volatile solids, total suspended solids, volatile suspended solids, biochemical oxygen demand (BOD₅), chemical oxygen demand, nitrate-nitrogen, nitrite-nitrogen, phosphates, ammonium-nitrogen, chloride, dissolved organic carbon, total organic carbon and heavy metals. Sequential soil extraction procedures were performed for the characterization of leachate-affected local soil. A geophysical survey using direct current resistivity technique was conducted at locations downstream of the dumpsite. Leachate characteristics indicated that the leachate is in the methanogenic phase and the results strongly suggest that the leachate may be polluting the river where the leachate is discharged directly. Leachate exceeds the allowable limits of Sri Lankan wastewater discharge standards for many of the parameters. Significant difference (P < 0.05) was observed for most of organic and inorganic parameters among all sampling locations. Many parameters showed a negative correlation with pH. The affected soils showed high heavy metal concentrations. Resistivity study confirmed a confined leachate flow at the near surface with few subsurface canals. However, no separate subsurface plume movement was observed. The results of this research can effectively be used for the establishment of an efficient and effective treatment method for the Gohagoda landfill leachate.