A Quantitative Approach to Assessing the Technical and Economic Performance of Source Containment Options for Contaminated Aquifers

The containment of contaminant plumes to protect groundwater from pollution is recognized as a frequent need in brownfield redevelopment. Plume containment can be physical, with slurry walls, jet grouting etc., or hydraulic, with wells capturing the subsurface flow that crosses the contaminated front (Pump & Treat), or a combination of both types. The choice of the most suitable technique is a difficult task, since various aspects must be taken into consideration. In this paper, we present a framework for evaluating barriers in terms of effectiveness and efficiency, along with a simplified approach for the evaluation of capital and operational costs. The contaminant mass discharge escaping from the containment system is a robust indicator of its effectiveness, and can be derived from modelling results. The abstracted water flowrate is a key indicator of the efficiency and sustainability of each option, especially in the long term. The methodology is tested in a simplified case study and in a real one, highlighting the relevance of modelling results in guiding the choice and design of contaminant source containment systems.

Stress-Strain Characteristics of a Cement-Bentonite Mix for a Barrier System The Implication of Time and Curing

For containment and water exclusion purposes, cement-bentonite (CB) barrier walls are usually built, with certain design criteria specified for the wall to fulfill intended purposes. While permeability is believed to be the most important criterion, it can be impacted by the stress-strain properties of the wall, which define the strength and stiffness. This study investigates the influence of curing time, confinement, rate of axial deformation, and quantity of cement (mix of Portland cement (PC) and ground granulated blast furnace slag (GGBS)) on the stress-strain properties of CB slurry walls. An unconfined compressive test supported by (i.e. UCT) undrained triaxial (UUT) tests were carried out on specimens prepared from two mix-designs (differentiated by the proportion of GGBS in the cement component) and cured 7, 14, 28, 60, and 90 days. Two rates of deformation (1.0mm/min and 1.2mm/min) were examined, using a range of confining pressure (i.e., 50 - 200 kPa) in the UUT. The results reveal that varying rates of deformation and the range of confining pressures have no clear influence on the mechanical properties (e.g., deviator stress, shear strength, and stiffness) of the CB mix-designs. However, increased curing time, and the proportion of GGBS significantly improved these properties. An increased proportion of GGBS enhances early strength. Thus, further work needs to be done to establish a balance between adequate strength and adequate flexibility of CB walls in order to not compromise permeability.

QUALITY ASSURANCE OF DIAPHRAGM AND PILE WALLS BY GEOPHYSICS

Underground structures like diaphragm and pile walls are constructed to organize waterproof curtains, protect pit sides, and transfer loads from the structures. Violations of the construction technological process can lead to the formation of defects. To prevent adverse consequences, before excavation, it is necessary to control the integrity of the slurry walls using non-destructive geophysical methods. A review of geophysical slurry wall quality control methods based on the excitation and registration of physical fields through access tubes installed in the reinforcement cage, in wells drilled near the structure or on the surface of the structure, is presented. The main capabilities and limitations of the methods are given. Cross-hole ultrasonic logging was used to study the section of the slurry wall during the construction of the Moscow Metro station. The results of parallel soundings made it possible to identify anomalous zones, interpreted as defects. The geometric dimensions of one of the major defects were refined by ultrasonic tomography. For the first time in Russian testing practice, thermal integrity profiling was applied to study the diaphragm wall at the base of a residential building. The results of temperature monitoring during the concrete hardening are presented. According to the method, a major flaw, excess of the design mark of the wall bottom and bulging of the structure were revealed. The results of thermal integrity profiling were verified by ultrasonic logging. The combined use of thermal and ultrasonic methods can be recommended as a reliable tool for integrity testing of diaphragm and pile walls. To carry out the measurements, the access tubes shall be included in the reinforcement cage of the structure at the design stage.

Clay minerals in drilling fluids: functions and challenges

The addition of clay minerals in drilling fluids modifies the dispersion's viscosity. In this article, scientific advances related to the use of clays and clay minerals (bentonite, palygorskite, sepiolite and mixtures of clay minerals) in drilling fluids are summarized and discussed based on their specific structure, rheological properties, applications, prevailing challenges and future directions. The rheological properties of drilling fluids are affected by the temperature, type of electrolytes, pH and concentration of clay minerals. Bentonites are smectite-rich clays often used in drilling fluids, and their composition varies from deposit to deposit. Such variations significantly affect the behaviour of bentonite-based drilling fluids. Palygorskite is suitable for use in oil-based drilling fluids, but the gelation and gel structures of palygorskite-added drilling fluids have not received much attention. Sepiolite is often used in water-based drilling fluids as a rheological additive. Dispersions containing mixtures of clays including bentonite, kaolin, palygorskite and sepiolite are used in drilling fluids requiring specific features such as high-density drilling fluids or those used in impermeable slurry walls. In these cases, the surface chemistry–microstructure–property relationships of mixed-clay dispersions need to be understood fully. The prevailing challenges and future directions in drilling fluids research include safety, ‘green’ processes and high-temperature and high-pressure-resistant clay minerals.

Preliminary investigation on the water retention behaviour of cement bentonite mixtures

Cement bentonite mixtures are often used to build slurry walls for the containment of both aqueous and non aqueous pollutants, due to their quite low hydraulic conductivity and relatively high ductility and strength. Although their hydro-mechanical behaviour in saturated conditions has been studied in the past, a part of the slurry wall is expected to rest above the groundwater level. The hydraulic characterization in unsaturated conditions is then particularly relevant to evaluate the performance of the barrier, especially when it is aimed at containing non aqueous pollutant liquids which are lighter than water (LNAPL). These non wetting fluids rest above the water table and their penetration is possible just if the barrier is unsaturated. This paper presents some preliminary results of a laboratory characterization of the water retention behaviour of three different cement bentonite mixtures. The mixtures, prepared at cement – bentonite mass ratios ranging from 4:1 to 6:1, were immersed in water and cured for 28 days. Their water retention behaviour was then determined along drying and wetting paths through different techniques, namely axis translation, filter paper and vapour equilibrium. In the high suction range, the water content – suction relationship was found to be independent of cement-bentonite ratio. In the low suction range, the water content at a given suction was found to decrease for increasing cement bentonite ratios.

Monitoring drying and wetting of a cement bentonite mixture with Electrical Resistivity Tomography

Cement bentonite slurry cutoff walls are used to encapsulate pollutants within contaminated areas, so avoiding their spreading in the environment. In both temperate and arid climates, at shallow depths, slurry walls are exposed to interaction with the atmosphere and thus to relative humidity values which might induce desaturation and significant shrinkage. This note presents the main results of a study aimed at investigating the impact of drying processes on the integrity and the hydraulic performance of cement bentonite slurry walls. Cement bentonite samples were cured under water for different times (1 months, 2 months and 4 months) and then dried naturally by exposing them to the laboratory environment (T = 21 °C, relative humidity approximately 38%). Once dried, the bottom of the samples was placed in contact with a thin layer of water to induce wetting. The distribution of the electrical conductivity within these samples was evaluated through Electrical Resistivity Tomography measurements, and electrical conductivity maps were converted then into maps of water contents on basis of a phenomenological relationship. The reconstructed water contents compared very well to the measured ones. Drying induced a limited cracking of the samples, which might affect to some extent the hydraulic performance of the barriers.

MODEL RESEARCH ON THE EFFECTIVENESS OF TECHNICAL METHODS OF LIMITING THE EXPANSION OF THE DEPRESSION CONE CAUSED BY DEWATERING OF THE PLANNED GUBIN OPEN-PIT BROWN COAL MINE

Due to the planned exploitation of the Gubin lignite deposit by opencast method, a numerical hydrogeological model was built. The main goal of the model was to forecast the development of the cone of depression caused by the future mine dewatering. Due to the location of the mine near the Nysa Łużycka River, a drainage system will be developed. The main task of this system is to eliminate the development of the cone of depression in the upper Quaternary aquifer in Germany, as well as to protect the Natura 2000 areas located in the immediate vicinity of the mine against harmful effects on groundwater-dependent ecosystems. The modelling studies allowed analyzing the efficiency of slurry walls, deep barriers of recharge wells, and shallow soakaway systems for water from the future mine dewatering. It has been shown that in case of dewatering of a multi-layer aquifer complex with many hydraulic contacts, the use of slurry walls may be insufficient to limit the development of the cone of depression. However, the combination of slurry walls with the systems recharging the aquifers enables effective counteracting the negative impact of dewatering on the environment.