Impact of permeation and gelling times on the strength of N-sodium silicate grouted sands

Agopian, Hratch Antoine
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Grouting is a technique used frequently to alter in-situ engineering soil parameters, mainly strength, deformability, and permeability. Chemical grouting in particular refers to injection of one or multiple fluids in grouting holes, with the aim of permeating the volume desired before the grout sets. Engineers require crucial parameters pre-, during, and post-gelation to quantify the economics and the efficacy of the treatment. The field of chemical grouting is highly reliant on laboratory and field pilot tests, due to the complexity of its mechanisms and processes. This study aims at understanding the behavior of Ottawa sand grouted using N-sodium silicate grout neutralized by dibasic ester. Sodium silicate grouting is one of the oldest methods in chemical grouting; its survival in the field is largely due to its economics. This type of grout is among the least expensive in its family, and pertains desirable properties such as flexibility in controlling gelling time, and ease of penetration in finer soils. Sodium silicate is also environmentally friendly, a quality much desired in this modern era. This research aims at finding the impact of the gelation time, relative to grout permeation time, on the properties of the grouted sand. The rheological properties of various mixes were first tested using a rheometer to identify the impact of the different mix components on the viscosity and set time. The various components of the grout mix were altered to determine the optimal mix. Select mixes were then subjected to a series of two syneresis tests on the gel itself and the grouted mass. The former showed a 70% loss in mass of the gel over the first year compared to non-measurable changes in volume in the latter. Grouted mass syneresis testing also showed that the lost mass is controlled by the exposed surface area, rather than total mass or volume. It was also concluded that constant mixing of the grout during the permeation phase is crucial in obtaining optimal results. A series of unconfined compressive strength tests revealed that sodium silicate grouts experience sedimentation and filtration during and post the grouting process (and prior to gelling). In an effort to show and overcome the effect of sedimentation, laboratory tests were conducted where the start of grouting was delayed such that the permeation was concluded at the onset of its gelation. The permeated samples with delayed start exhibited a higher strength than those permeated without a delay at a similar permeated number of pore volumes. The difference in strength decreased as the number of permeated pore volumes increased (and thus the time delay becomes smaller). A similar trend was seen in the moduli of the two series of tests with different gelling times. Additionally, a decrease in strength was observed within the grouted mass as we move away from the injection point; this variation in strength is mostly associated with filtration. Tests on 6” specimens with delayed permeation (reduced impact from sedimentation) showed a linear increase in the strength of the samples as the number of permeated pore volumes increased.