Field performance of four vibrating-wire piezometer installation methods

Vibrating wire piezometers provide a number of advantages over the traditional hydraulic piezometer design. There are currently many methods and configurations for installing vibrating-wire piezometers, the most common being: single piezometers in sand packs (SP), multilevel piezometers in sand packs (MLSP), and fully-grouted multilevel piezometers using either bentonite (FGB) or cement-bentonite grout (FGCB). This study assesses the performance of these four different installation methods at a field site possessing complex stratigraphy, including glacial and marine sediments. To accomplish this objective, pore pressure data recorded between December 2017 and July 2019 were analyzed. Data indicate that SP, MLSP, and FGB piezometers performed most reliably, based on the fact that piezometers installed at the same depth with these methods recorded similar pressure variations that were coherent with the hydrogeological setting. Of the two fully-grouted installations using cement-bentonite grout, one installation failed completely due to a hydraulic short circuit, likely caused by preferential flow occurring along the wires of the embedded instruments. The lack of a standard method for mixing cement-bentonite grout at the time of construction likely contributed to the failure of the FGCB installations, as the grout mixture used in this study was likely too viscous to provide a suitable seal.

Experimental study on penetration of bentonite grout through granular soils

Permeation grouting using bentonite grouts is one of the effective methods to improve the engineering properties of granular soils. However, the low penetrability of bentonite grouts into soils limits their practical application in permeation grouting. This study presents a new approach to control the penetration length of bentonite grouts through granular soils using an ionic additive, sodium pyrophosphate (SPP). It is hypothesized that the chemical modification changes both rheological and physicochemical properties of the bentonite grout, and thus affects its penetration length through soils. The rheological properties (yield stress and apparent viscosity) of bentonite grouts with weight ratios (ratio of water to dry bentonite, W/B) of 19, 12.3, 9, and 7.3 were controlled by the addition of 1%–4% SPP by weight of dry bentonite. The bentonite grouts were also injected into sand columns prepared at various experimental conditions to evaluate the effect of each experimental parameter on their penetration lengths. The results show that the penetration length of bentonite grouts decreases with a decrease in W/B ratio and an increase in yield stress and apparent viscosity. Moreover, the penetration length increases with the increase of the normalized effective grain size and injection pressure, but the increase of fines content reduces the penetration length of the grouts. While the existing analytical equation produces good agreement with the measured penetration lengths for the grouts having high yield stress (>26 Pa), it significantly overestimates the penetration lengths of the SPP modified bentonite grouts due to filtration (especially, the grouts having low yield stress and low W/B ratios). Therefore, a new empirical correlation is proposed to predict the penetration length of the bentonite grouts based on filtration and rheological blocking.