Three-dimensional behaviour of a prototype radioactive waste repository in fractured granitic rock

An investigation of the three-dimensional coupled thermohydromechanical behaviour of a prototype repository in fractured granitic rock is presented. The pre-placement behaviour of the repository is first considered, making use of a full three-dimensional simulation of the problem. An effective continuum approach, augmented with discrete features with a high hydraulic conductivity, is employed. The method adopted is found to be able to simulate accurately the highly anisotropic flow regime observed at the pre-placement phase. The major features of the full repository experiment under applied heating conditions were then successfully simulated. The range of buffer hydration rates, the thermal response of the repository, and the associated mechanical response were successfully simulated. Approaches to capture both the transient microstructural behaviour of the compacted bentonite (MX-80 type) and a MX-80 pellet material are incorporated. The repository behaviour was shown to be strongly influenced by complex coupled processes, including interactions between the system components. The adoption of a three-dimensional modelling approach proved to be essential to realistically simulate the behaviour of a repository incorporating anisotropic flow behaviour. Finally, potential impacts of the processes considered on performance of the barrier system and in safety assessment are considered

Recovery technologies for materials in landfills

Europe hosts more than 500,000 landfills of which 90% are non-sanitary and around 80% essentially contain Urban Solid Waste (https://www.eurelco.org/infographic). Urban landfills (UL) and extractive (mining and metallurgical) industry residues (EIR) are potential sources of materials that, if recovered, can contribute to the circularity of economy. Among other factors, technology plays one essential role in the viability of landfill mining projects (Krook, et al, 2012). The methods for mapping landfills, sampling and characterizing waste, the readiness of technologies, the optimization of technologies and their combination in treatment and recovery schemes, their applicability, costs and environmental impacts effect the valorization of waste from landfills. This report addresses Deliverable 1.1 “Recovery technologies for materials in landfills” developed by Working Group 2 of COST Action “Mining the European Anthroposphere” (MINEA). MINEA aims to quantify and assess the material resources and reserves in the Anthroposphere and consolidate existing knowledge related to the exploration, evaluation, classification and recovery of materials in anthropogenic deposits and waste flows. This report integrates the activities of the MINEA WG2 in the 1st Grant Period (May 2016 to April 2017). The following documents were developed: (1) Literature Review Report on practices and technologies for waste valorization from landfills (Calvo, 2016) and (2) MINEA WG2 Workshop on technologies in the landfill-mining sector, which resulted in an overview on landfill mining projects and on state-of-the-art as well as enhanced recovery technologies (Workshop on “Technologies for material recovery from landfills and mining residues”, Book of abstracts, 2016). This report also profits from the non published report on “Science and technology in enhanced landfill mining” (EURELCO, 2016), which has been developed by the Working Group II of the European Enhanced Landfill Mining Consortium (EURELCO). Both activities examine current practices, knowledge transfer and recovery technologies across European countries, research fields and disciplines. This information is essential to assess the availability of secondary material from landfills and the viability of landfill mining projects in the context of circular economy

Hydrologic behavior of model slopes with synthetic water repellent soils

In the natural environment, soil water repellency decreases infiltration, increases runoff, and increases erosion in slopes. In the built environment, soil water repellency offers the opportunity to develop granular materials with controllable wettability for slope stabilization. In this paper, the influence of soil water repellency on the hydrological response of slopes is investigated. Twenty-four flume tests were carried out in model slopes under artificial rainfall; soils with various wettability levels were tested, including wettable (Contact Angle, CA 90°). Various rainfall intensities (30 mm/h and 70 mm/h), slope angles (20° and 40°) and relative compactions (70% and 90%) were applied to model the response of natural and man-made slopes to rainfall. To quantitatively assess the hydrological response, a number of measurements were made: runoff rate, effective rainfall rate, time to ponding, time to steady state, runoff acceleration, total water storage and wetting front rate. Overall, an increase in soil water repellency reduces infiltration and shortens the time for runoff generation, with the effects amplified for high rainfall intensity. Comparatively, the slope angle and relative compaction had only a minor contribution to the slope hydrology. The subcritical water repellent soils sustained infiltration for longer than both the wettable and water repellent soils, which presents an added advantage if they are to be used in the built environment as barriers. This study revealed substantial impacts of man-made or synthetically induced soil water repellency on the hydrological behavior of model slopes in controlled conditions. The results shed light on our understanding of hydrological processes in environments where the occurrence of natural soil water repellency is likely, such as slopes subjected to wildfires and in agricultural and forested slopes

A reformulated hardening soil model

The hardening soil (HS) model is an advanced soil plasticity model which incorporates many features including stiffness stress dependency, hardening from initial loading, and soil dilatancy. In this paper, the HS model is explored in depth, and two improvements are proposed. The first is a new shear yield surface and hardening rule that have been reformulated to remove singularities. The second is a robust implicit return mapping scheme. Options for improving the global convergence of finite element analyses are also explored. Single elements tests replicate results from experimental triaxial data and previous versions of the HS model very closely, and at excellent convergence rates. In addition, a slope stability analysis is performed using the ϕ-c strength reduction method in 2D plane-strain. Results from the slope analysis showed good agreement with analytical and graphical slope stability methods. A 3D slope stability analysis was also conducted with modified boundary conditions, in order to demonstrate the 3D capabilities of the model

Cover systems with synthetic water repellent soils

A cover system is a crucial component of engineered landfills, to minimize water percolation into the underlying waste. Capillary barriers are an alternative cover system, which has been widely used in the arid and semiarid regions as no cohesive, low‐permeability materials are used. However, the performance of capillary barriers in tropical climate has been unsatisfactory (breakthrough observed). In recent years, synthetic water‐repellent granular materials have drawn increasing attention due to their distinctive hydraulic behavior (inhibited water infiltration and high water entry pressure), suggesting they may also be used to improve the performance of cover systems. In this study, flume tests were conducted with inclined model slopes under artificial rainfall. By monitoring the surface runoff, lateral diversion, and basal percolation and conducting water balance analysis, the performance of monolithic cover, conventional capillary barrier, and water‐repellent cover systems were evaluated. The study revealed that (a) the barrier effect and diversion capacity were significantly strengthened by induced water repellency, providing a promising solution to extend the application of capillary barrier covers; and (b) cover systems can be formed using one raw material to decrease the construction cost, by using synthetic water‐repellent soil as the underlying layer

Erodibility of synthetic water repellent granular materials: adapting the ground to weather extremes

Granular materials with synthetic water repellent coatings have great potential to be used in ground interfaces (ground-atmosphere-vegetation and ground-structure) as infiltration barriers, due to their altered hydrological properties (suppressed infiltration and decreased sorptivity). However, very few studies have evaluated the impact of synthetic soil water repellency on soil erosion. This paper investigates the effect of water repellency on soil erosional behavior, including splash erosion and rill processes. Twenty-four flume tests were carried out on model slopes under artificial rainfall; soils with three wettability levels were tested, including wettable (contact angle, CA  90°). Various rainfall intensities (230 mm/h, 170 mm/h, 100 mm/h and 40 mm/h) and grain sizes (Fujian sand and sand/silt mixture) were adopted. Erosional variables, including splash erosion rate, average sediment concentration, peak sediment concentration and time to peak sediment were measured to quantitatively analyze the behavior. This study confirms the impact of water repellency on soil erosion and unveils the possibility to reduce infiltration at ground-atmosphere interface with controlled soil erosion. The results revealed that: (1) synthetic water repellency does not necessarily lead to increased soil erosion yield; its impact is dependent on grain size with the soil erosion loss increasing for Fujian sand, but decreasing for sand/silt mixtures; (2) splash erosion is positively correlated to soil water repellency and high rainfall intensity, regardless of grain size; (3) the erosion processes for sand/silt mixtures are particle size selective and not affected by soil water repellency, whereas this phenomenon is not observed with Fujian sand

Can wood waste be a feedstock for anaerobic digestion? A machine learning assisted meta-analysis

Anaerobic digestion is widely employed to process various organic wastes while generating renewable energy and nutrient-rich digestate. However, lignocellulosic wastes, especially wood waste, suffer from the recalcitrance associated with high lignin content, thereby adversely impacting on biogas production. It remains unclear whether wood waste is suitable as a feedstock for anaerobic digestion and to what extent pretreatment techniques could affect its biochemical methane potential. In this paper, 769 datasets on methane production from wood waste were collected for meta-analysis. The results showed an average 146 % increase in methane production for other organic wastes compared to wood waste when pretreatment techniques were not applied, but this gap could be mitigated to 99 % when pretreatment techniques were considered, indicating that pretreatment techniques could be more effective for wood waste. A further analysis of different pretreatment techniques showed that pretreatment significantly increased the methane production of wood waste by 113 % and that a combination of pretreatment techniques was more effective than a single method. Finally, three machine learning algorithms were applied to explore the relationship between methane production and selected variables. The results showed that the random forest method yielded better predictive performance for methane production (R2 = 0.9643) than artificial neural networks and support vector regression. Feature importance analysis found that particle size had a higher influence than temperature or feedstock composition. Overall, this study gives insight into the potential of utilizing wood waste as a feedstock for anaerobic digestion and the importance of employing suitable pretreatment methods. This work also reveals correlations between methane production and critical variables, which could serve as a guide for optimizing operational adjustments during anaerobic digestion

Hydro-geomechanical characterisation of a coastal urban aquifer using multiscalar time and frequency domain groundwater-level responses

Hydraulic properties of coastal, urban aquifers vary spatially and temporally with the complex dynamics of their hydrogeology and the heterogeneity of ocean-influenced hydraulic processes. Traditional aquifer characterisation methods are expensive, time-consuming and represent a snapshot in time. Tidal subsurface analysis (TSA) can passively characterise subsurface processes and establish hydro-geomechanical properties from groundwater head time-series but is typically applied to individual wells inland. Presented here, TSA is applied to a network of 116 groundwater boreholes to spatially characterise confinement and specific storage across a coastal aquifer at city-scale in Cardiff (UK) using a 23-year high-frequency time-series dataset. The dataset comprises Earth, atmospheric and oceanic signals, with the analysis conducted in the time domain, by calculating barometric response functions (BRFs), and in the frequency domain (TSA). By examining the damping and attenuation of groundwater response to ocean tides (OT) with distance from the coast/rivers, a multi-borehole comparison of TSA with BRF shows this combination of analyses facilitates disentangling the influence of tidal signals and estimation of spatially distributed aquifer properties for non-OT-influenced boreholes. The time-series analysed covers a period pre- and post-impoundment of Cardiff’s rivers by a barrage, revealing the consequent reduction in subsurface OT signal propagation post-construction. The results indicate that a much higher degree of confined conditions exist across the aquifer than previously thought (specific storage = 2.3 × 10−6 to 7.9 × 10−5 m−1), with implications for understanding aquifer recharge, and informing the best strategies for utilising groundwater and shallow geothermal resources