Nanosilica Grout Permeation in Sand: Experimental Investigation and Modeling

Abstract

Low-pressure injection of nanosilica aqueous suspensions is often adopted to either waterproof or increase the liquefaction resistance of granular soils. The basic principle behind this ground improvement technique consists in filling the soil pores with a low-viscosity fluid that changes its consistency with time, first into a gel, then into a solid. From an application point of view, the simulation of the time-dependent permeation process is crucial to relate the in situ distance covered by the grout to the operational parameters. A comprehensive investigation was performed, combining laboratory experiments with theoretical approaches, to characterize the phenomenon and then derive predictive relations useful for designing treatment executions. The time-dependent rheological properties of different nanosilica aqueous suspensions were first quantified by means of rheometric tests, then described with Bingham’s law. Grout permeation in granular media was then simulated by suitably modifying Darcy’s law to incorporate the temporal evolution of Binghamian grout rheology. After validating the modified Darcy’s law employment for nanosilica grout flows with respect to laboratory experimental data, simplified analytical equations, capable of predicting the temporal evolution of the distance covered in situ by the grout and the flow rate–injection pressure relation, are provided. Nanosilica aqueous suspensions are environmentally nontoxic materials with the consistency of a low-viscosity fluid, suitable for injections into fine-graded soils, but, when mixed with a sodium-chloride solution, they transform with time into a gel of solid consistency. Thanks to these properties, they are frequently adopted to provide a fast remedial against piping induced by excavation, seal contaminants or reduce the liquefaction potential of sands. Nanosilica grout is commonly injected at low pressure, leading to filling soil pores during seepage. The previously mentioned transient evolution of the suspension rheological properties, controlled by nanosilica and sodium-chloride proportions, starts during the injection-seepage phase, playing a paramount role in affecting the geometry of the treated soil domain. The present work provides an accurate description of the time-dependent grout rheology and a predictive seepage analytical tool to simulate the diffusion of nanosilica grouts, characterized by variable compositions and injected from sources of different geometrical layouts into homogeneous soils with different grain size distributions. This tool allows tailoring of the injection parameters (pump pressure, nozzle spacing, injection time) on in situ soil hydraulic properties and rheology of the selected nanosilica suspension.