Abstract
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 10−7cm/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 32–766min, 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 50min, taking into account the cations already present within the synthetic groundwater.
Highlights
Over the last three decades, colloidal silica has been investigated and more recently adopted as a low viscosity grouting technology
The potential for colloidal silica to be deployed as a low pH grout within nuclear waste repositories in crystalline rock has been investigated by the Chalmers University of Technology (Axelsson, 2006; Funehag and Fransson, 2006; Funehag and Gustafson, 2008; Butrón et al, 2009)
This paper presents an electrochemically inferred model that (i) is able to predict the gelation time and the change in viscosity for a given pH, electrolyte, silica particle size and silica concentration, and (ii) forms a useful tool for the design of grout mixes using colloidal silica that accounts for in situ groundwater conditions
Summary
With increasing use of underground space, and redevelopment of subsurface infrastructure, comes the requirement for adequate groundwater control. The model presented could be used by grouting contractors to design their own mix of the components to achieve their desired gel time, for example reducing the colloidal silica content in order to increase the gel time and achieve greater penetration. This would greatly extend the engineering applications in which colloidal silica could be deployed. This paper presents an electrochemically inferred model that (i) is able to predict the gelation time and the change in viscosity for a given pH, electrolyte, silica particle size and silica concentration, and (ii) forms a useful tool for the design of grout mixes using colloidal silica that accounts for in situ groundwater conditions. The model is applied to demonstrate grout design using published in-situ groundwater data from the Olkiluoto area of Finland (Ollila, 1999)
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