An analytical model for the control of silica grout penetration in natural groundwater systems

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 10-7 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

Experimental Evidence of the Effectiveness and Applicability of Colloidal Nanosilica Grouting for Liquefaction Mitigation

“…The viscosity of the fluid in this state is about 10 m Pa • s, i.e., 10 times higher than the viscosity of water, and this makes the suspension extremely suitable for injection in lowpermeability sands. When mixed with an activator consisting of a 10% sodium chloride water solution (pH ≈ 7), the pH reduction and the presence of salt destabilises the colloids and lead to polymerize complex and highly hydrophilic silica chains (Iler 1979;Hench 1998;Pedrotti et al 2017) grouped in the form of spherical silica particles (SOLS). The gel structure, analyzed by X-ray diffraction (XRD) tests, can be defined as amorphous because the few peaks in Fig.…”

Comparison of Nanomaterials with Other Unconventional Materials Used as Additives for Soil Improvement in the Context of Sustainable Development: A Review

“…The gelation of CS originates from the bonding of H 4 SiO 4 molecules through the siloxane (Si-O-Si), as shown in Figure 11 [ 90 ]. Moreover, the gelation process can be motivated and promoted by adding electrolyte (i.e., NaCl, CaCl 2 et al) [ 91 , 92 ]. However, the microstructure of laponite after gelation presents elongated cells through Cryo-SEM observation, which is different from CS [ 93 ].…”

State-of-the-Art of Colloidal Silica-Based Soil Liquefaction Mitigation: An Emerging Technique for Ground Improvement

“…Wong proposed that a 1.7 mol/L NaCl solution should be hand mixed with CS in a 1:5 ratio by volume to obtain a 1 h gel formation time [50]. The required salt concentrations to obtain the targeted gel time can be determined with the analytical model proposed by Pedrotti et al (2017) [51].…”