[1] Evidence of fossilized microorganisms embedded within mineral veins and mineralfilled 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, and 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.

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 CaCO3 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.

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 to 22 μm, corresponding to a transmissivity reduction of 1.71 × 10−5 to 8.75 × 10−9 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 3 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.

In this paper, we explore the mechanical performance of colloidal silica grout to assess its potential for ground stabilisation and hydraulic barrier formation during decommissioning of major industrially contaminated sites. We consider two colloidal silica -soil systems: sand grouted with colloidal silica and kaolin clay mixed with colloidal silica. The aims of the paper are to evaluate the drained stress-strain behaviour (1-D compression and shear resistance) of colloidal silica-soil systems and to determine the particle interactions between soil and colloidal silica at a micron-scale so as to provide an understanding of the macroscopic mechanical behaviour. Two different colloidal silica-soil interaction mechanisms have been found: formation of a solid, cohesive matrix for the case of grouted sand, and increase of the clustering of clay particles for the case of clay mixtures. This paper illustrates for the first time that even under drained conditions colloidal silica can provide mechanical improvement.Colloidal silica-grouted sand showed an increased stiffness and enhanced peak friction angle, while still having a very low hydraulic conductivity (~10 -10 m/s), typical of intact clay.Similarly, clay-colloidal silica mixtures showed reduced volumetric deformation, increased stiffness for low values of stress (~100kPa), and increases in both the peak and the ultimate shear strength. Our results show that colloidal silica could be deployed in environments where not only hydraulic containment is critical, but where reduced deformation and enhanced resistance to shearing would be beneficial, for example in landfill capping or in the outer fill , Persoff et al., 1999, Manchester et al., 2001, and (iii) for preventing water ingress in 16 the tunnelling and underground construction industry (Bahadur et al. (2007), Butrón et al. 17 (2010. 18However, CS also provides some level of mechanical improvement. Indeed, a field test 19 by showed that "CS imparted sufficient structural strength to the matrix 20 to permit 10ft high vertical sections of the matrix (characterized by very loose, friable, and 21 heterogeneous materials) to stand without collapsing". CS has also been investigated as a 22 means of increasing resistance to liquefaction in loose sands (Gallagher and Mitchell, 2002,

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