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,
Over the last three decades, colloidal silica has been investigated and more recently adopted as a low viscosity grouting technology (e.g. for grouting rock fractures within geological disposal facilities nuclear waste). The potential of colloidal silica as a favourable grouting material exists due to: its initial low viscosity; its low hydraulic conductivity after gelling (of the order of 10-7 cm/s); the very low injection pressures required; its controllable set/gel times (from minutes to several days); the fact it is environmentally inert; its small particle size (less than hundreds of nanometres) and its cost-effectiveness. Despite the documented success of colloidal silica based grouts for hydraulic barrier formation, research has not translated into widespread industrial use. A key factor in this limited commercial uptake is the lack of a predictive model for grout gelling which controls grout penetration: whilst data are available to underpin design of a grouting campaign in laboratory conditions, little research has been done to underpin applications in natural environments. Here we develop and validate an analytical model of colloidal silica gelling in groundwaters with varying pH and background electrolyte concentrations. This paper presents an analytical model that accounts for changes in pH, electrolyte concentration, cation valency and molar mass, silica particle size and silica concentration giving predictive capability without the need for site-specific calibration. The model is validated against experimental observations for gel times of 32 minutes to 766 minutes, the model accurately predicts the log(gel time) with an average error of 4% which corresponds to an R2 value of 0.96 The model is then applied to a hypothetical case study to demonstrate its use in grout design, based on published in-situ groundwater data from the Olkiluoto area of Finland. The model successfully predicts the required accelerator concentration to achieve a grout gel time of approximately 50 minutes, taking into account the cations already present within the synthetic groundwater
This paper provides upper- and lower-bound limits of stiffness improvement observed from the treatment of infilled solution features in chalk as part of the Central 1 contract being delivered by the Align JV, which is part of the UK's High Speed 2 Phase 1 rail link. Infilled solution features were treated using rapid impact compaction (RIC) to achieve a sufficiently stiff subgrade beneath a wide range of temporary foundations and reduce the risk of collapse settlement. Improvement by RIC treatment was sufficient to ensure subsequent foundation performance or reduce the extent of compaction grouting subsequently required beneath the most heavily loaded foundations. The depth of improvement observed was up to 10 m and the improvement in elastic modulus observed was up to five times the pre-treatment value. Over 40 cone penetration tests were conducted before and after RIC. Typical lower- and upper-bound improvement curves are presented based on the observed minimum and maximum post-treatment stiffness. The degree of stiffness improvement was observed to generally reduce at greater than 5 m depth.
The decline in reserves from conventional reservoirs, paired with the technological advances made in the drilling, stimulation and production areas over the last two decades have placed unconventional reservoirs in the limelight. Shale plays, in particular, have become increasingly attractive prospects and production from these reservoirs has increased significantly during this period. Most of the petrophysical characterisation techniques routinely used in the laboratory were originally developed for rocks with relatively high porosity and permeability, making some of them unsuitable for tight rocks. Gas permeability measurements in shales can be particularly challenging due to their small pore and pore throat sizes, and even the validity of Darcy’s law under these conditions needs to be evaluated. The steady-state technique is generally unsuitable for measuring gas permeability in shales due to technical limitations of the instruments required, so the quasi steady-state method is proposed as an alternative. This paper presents the results of gas permeability measurements conducted on two shale core plugs using the quasi steady-state technique. Although the effect of variations in ambient conditions is not usually significant for tests performed on cores from conventional reservoirs, our results indicate that it should not be overlooked when experiments are conducted on shale samples. Furthermore, the length of the core plugs should be minimised to reduce the time required to measure gas permeability.
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