Fungal-induced water repellency in sand

Water infiltration into granular soils and the associated pore water pressure increase and reduction in shear strength can trigger landslides, instability of vertical cuts and failure of retaining walls. Water-repellent soils can reduce infiltration to maintain soil suction. Recent research has demonstrated the creation of synthetic water-repellent soils using chemical methods. This paper investigates a biological treatment for creating water-repellent sand by way of the growth of the fungus Pleurotus ostreatus. Water repellency was assessed using: (a) the water drop penetration test; (b) the molarity of ethanol drop test; and (c) the modified sessile drop method with contact angle (θ) determination by way of image analysis. Fungal-induced water repellency was found to be ‘extreme’ (θ > 110°) up to 4 weeks and ‘severe’ (θ > 105°) up to 12 weeks, even with no further supply of moisture or nutrients. A water-repellent layer was formed and maintained in saturated conditions, which is difficult to achieve using chemical methods

Reducing hazard in spent fuel removal using colloidal silica gel

Nuclear site decommissioning involves the retrieval and handling of radioactive waste. Waste removal from nuclear reactors and/or storage facilities, such as spent fuel pools and storage silos, represents a potential hazard in terms of radiation exposure for the workforce and the surrounding environment. This study explores the suitability of colloidal silica grouting around radioactive waste to reduce radiation exposure during retrieval operations. Previous work on colloidal silica gel has proved its potential to form low-permeability hydraulic barriers, and to inhibit the diffusion of radionuclides through the gel, making it a promising material for spent fuel recovery applications. This work provides experimental evidence that colloidal silica hydrogel can maintain its integrity upon exposure to temperatures typical of the nuclear waste stored within pools and silos, both in standard conditions (up to 60 °C), and during loss of cooling/loss of coolant accidents (>100 °C)

Development of a reactive transport model for field-scale simulation of microbially induced carbonate precipitation

Microbially induced carbonate precipitation (MICP) is a promising technique that could be used for soil stabilization, for permeability control in porous and fractured media, for sealing leaky hydrocarbon wells, and for immobilizing contaminants. Many further field trials are required before optimum treatment strategies can be established. These field trials will be costly and time consuming to \carry out and are currently a barrier to transitioning MICP from a lab-scale process to a practical field-scale deployable technology. To narrow down the range of potential treatment options into a manageable number, we present a field-scale reactive transport model of MICP that captures the key processes of bacteria transport and attachment, urea hydrolysis, tractable CaCO 3 precipitation, and modification to the porous media in terms of porosity and permeability. The model, named biogroutFoam, is implemented in OpenFOAM, and results are presented for MICP treatment in a planar fracture, three-dimensional sand media at pore scale, and at continuum scale for an array of nine injection/abstraction wells. Results indicate that it is necessary to model bacterial attachment, that bacterial attachment should be a function of fluid velocity, and that phased injection strategies may lead to the most uniform precipitation in a porous media

'Microbial mortar'- restoration of degraded marble structures with microbially induced carbonate precipitation

To evaluate a restoration strategy for highly degraded marble structures, microbially induced carbonate precipitation (MICP) has been employed to reduce porosity and permeability in a column filled with coarse crushed marble. A 3D X-ray tomography scan revealed the spatial variation in porosity throughout the column and tracer breakthrough curves, recorded at intervals during treatment, enabled derivation of core-scale fluid transport properties and their alteration by precipitating carbonate. Micro-continuum scale flow modelling based on the X-ray data indicated that treatment led to changes in the pore network structure with flow increasingly focused into a smaller number of faster flowing open channels

Microscale analysis of fractured rock sealed with microbially induced CaCO3 precipitation : influence on hydraulic and mechanical performance

Microbially induced CaCO3 precipitation (MICP) has shown great potential to reduce permeability in intact rocks as a means to seal fluid pathways in subsurface ground, for example to secure waste storage repositories. However, much less is known about how to apply MICP to seal fractured rock. Furthermore, there is limited information on the hydraulic and mechanical properties of MICP filled fractures, which are essential criteria to assess seal performance. Here, MICP injection strategies were tested on sandstone cores, aimed at obtaining a homogeneous porosity fill that reduced permeability by 3 orders of magnitude. The injection strategy resulting in the most homogenous calcite distribution was then applied to fractured granite cores, to yield transmissivity reduction of up to 4 orders of magnitude. Microscale analysis of these sealed granite cores using X‐ray computed tomography and electron microscopy showed that > 67% of the fracture aperture was filled with calcite, with crystals growing from both fracture planes, and bridging the fracture aperture in several places. Shear strength tests performed on these cores showed that the peak shear strength correlated well with the percentage of the fracture area where calcite bridged the aperture. Notably, brittle failure occurred within the MICP grout, showing that the calcite crystals were strongly attached to the granite surface. If MICP fracture sealing strategies can be designed such that the majority of CaCO3 crystals bridge across the fracture aperture, then MICP has the potential to provide significant mechanical stability to the rock mass as well as forming a hydraulic barrier

Rock fracture grouting with microbially induced carbonate precipitation

Microbially induced carbonate precipitation has been proposed for soil stabilization, soil strengthening and permeability reduction as an alternative to traditional cement and chemical grouts. In this paper we evaluate the grouting of fine aperture rock fractures with calcium carbonate, precipitated through urea hydrolysis, by the bacteria Sporosarcina pasteurii. Calcium carbonate was precipitated within a small-scale and a near field-scale (3.1 m2) artificial fracture consisting of a rough rock lower surfaces and clear polycarbonate upper surfaces. The spatial distribution of the calcium carbonate precipitation was imaged using time-lapse photography and the influence on flow pathways revealed from tracer transport imaging. In the large-scale experiment, hydraulic aperture was reduced from 276 μm to 22 μm, corresponding to a transmissivity reduction of 1.71x10-5 m2/s to 8.75x10-9 m2/s, over a period of 12 days under constantly flowing conditions. With a modified injection strategy a similar three orders of magnitude reduction in transmissivity was achieved over a period of three days. Calcium carbonate precipitated over the entire artificial fracture with strong adhesion to both upper and lower surfaces and precipitation was controlled to prevent clogging of the injection well by manipulating the injection fluid velocity. These experiments demonstrate that microbially induced carbonate precipitation can successfully be used to grout a fracture under constantly flowing conditions and may be a viable alternative to cement based grouts when a high level of hydraulic sealing is required and chemical grouts when a more durable grout is required

Hydrodynamic coupling in microbially mediated fracture mineralization : formation of self-organized groundwater flow channels

Evidence of fossilized microorganisms embedded within mineral veins and mineral-filled fractures has been observed in a wide range of geological environments. Microorganisms can act as sites for mineral nucleation and also contribute to mineral precipitation by inducing local geochemical changes. In this study, we explore fundamental controls on microbially induced mineralization in rock fractures. Specifically, we systematically investigate the influence of hydrodynamics (velocity, flow rate, aperture) on microbially mediated calcite precipitation. Our experimental results demonstrate that a feedback mechanism exists between the gradual reduction in fracture aperture due to precipitation, and its effect on the local fluid velocity. This feedback results in mineral fill distributions that focus flow into a small number of self-organizing channels that remain open, ultimately controlling the final aperture profile that governs flow within the fracture. This hydrodynamic coupling can explain field observations of discrete groundwater flow channeling within fracture-fill mineral geometries where strong evidence of microbial activity is reported

Preliminary observations of the shear behaviour of fungal treated soil

This paper presents results of an investigation into an entirely novel technique for ground improvement involving the use of fungal hyphae. Fungal hyphae (long filamentous branches) are known to contribute to soil aggregation and soil hydrophobicity, and are hypothesised to also influence the hydro-mechanical behaviour of soil. We present here preliminary observations of the mechanical behaviour of sands treated with the fungal species Pleurotus ostreatus (P. ostreatus). Direct shear tests were carried out on sand containing different percentages of organic substrate (the nutrient source for fungal growth) and treated with P. ostreatus. The stress-strain behaviour of fungal treated and untreated soil was investigated. Results show that irrespective of the percentage of organic matter, fungal treated specimens tended to show a loss in the peak behaviour characteristic of the untreated control specimens and an associated transition towards a more contractive volumetric response. The limited experiments conducted to date appear to indicate that the main factor responsible for the differences in behaviour between treated and untreated specimens is due to lubrication of the grains by the fungal hyphae and exudates. Further investigation is required to fully elucidate the mechanisms influencing the mechanical behaviour of fungal-treated soils

Performance of colloidal silica grout at elevated temperatures and pressures for cement fracture sealing at depth

Hydrocarbon well decommissioning requires the long-term sealing of abandoned wells. Current plug and abandonment (P&A) operations are not always able to address all potential fluid migration pathways, resulting in the possible upwards migration of hydrocarbons from formations penetrated by the wellbore. The development of innovative materials to improve well sealing remains a major challenge. This paper presents a proof of concept for the use of colloidal silica (CS)-based grout to improve the sealing performance of P&A operations. CS is a non-toxic suspension of silica nanoparticles (<100 nm) undergoing gelation upon destabilisation. Due to its excellent penetrability and controllable gel time, CS has the potential for repairing fine-aperture cracks within the cement sheath, at the cement/casing interface, or within a cement plug, where the penetration of cementitious grouts is restricted due to their relatively large particle size. In this study, the suitability of CS grout for deployment up to 1500 m depth was successfully demonstrated. Firstly, a range of CS grout mixes were investigated to test the feasibility of grout emplacement considering a timescale of 2 h for pumping operations from the surface to depth. Secondly, to investigate the sealing performance, the CS grout was injected into fractured cement cores (0.2 and 0.5 mm fracture aperture) and exposed to pressure and temperature conditions simulating downhole scenarios up to 1500 m depth (based on gradients for North Sea, UK). Fracture permeability upon water injection was assessed pre- and post-treatment. This work found that permeability values after treatment were reduced by three orders of magnitude, thus confirming the potential of CS grout for repairing fine-aperture cracks

Desiccation behaviour of colloidal silica grouted sand : a new material for the creation of near surface hydraulic barriers

This paper considers the mechanism of cracking in colloidal silica (CS) grout subjected to drying and wetting cycles, with the aim of testing its use for the creation of near-surface low hydraulic conductivity barriers for applications in nuclear decommissioning. The advantage in this context of CS over more traditional materials, such as clay liners or geotextiles, are its capability to permeate into in situ soils at low injection pressures and injectability within and beneath regions of existing contamination, thus reducing cost and removing any requirement for excavation and disposal of hazardous materials. In near-surface applications, hydraulic barriers are exposed to natural climatic variations, in the form of cycles of drying and wetting, which can result in cracking of the barrier material and a subsequent increase in the hydraulic conductivity. Ultimately, this reduces their ability to perform their end function. The aims of this paper are to study the mechanism of crack formation in colloidal silica grout barriers when exposed to severe drying and wetting cycles and to determine the effect on its hydraulic properties. To achieve these aims, grouted soil samples were created and exposed to severe drying and re-wetting. Samples were tested for hydraulic conductivity at each stage and 3D images of the pore structure were obtained from micro X-ray CT scanning. On drying, nanoscale cracks form within the CS matrix, which are 10s of nanometres in width, these have an associated air- entry value of ~20,000 kPa. Additional meso-scale cracks can also form in CS filled pores when surrounded by sand grains, due to conditions of restrained shrinkage. These cracks are typically hundreds of microns in width and have an associated air-entry value of ~200 kPa. X-ray CT analysis of the connectivity of this meso-scale pore space, filled by air after drying, indicates that although cracks form, a connected network does not, thus explaining the observation that even after severe drying the CS grouted sand retains a very low hydraulic conductivity