Rheological properties of sodium pyrophosphate modified bentonite suspensions for seepage control

Abstract

The placement of plastic fines such as bentonite into pore spaces of granular soil deposits is one of the effective methods to control the seepage problem. For such an application, bentonite needs to be delivered into soils in forms of concentrated suspensions without disturbing the original soil matrix using low injection pressures (permeation grouting). However, the low penetrability of the concentrated suspensions through soils limits their practical applicability for large-scale seepage control in the field. In order to increase the penetration distance, the initial mobility of the suspensions should be increased; however, the suspensions need to maintain their thixotropic nature to reduce the washout probability of the suspensions from the treated soils over time. The objective of this study is to investigate the rheological properties of concentrated bentonite suspensions, modified with sodium pyrophosphate (SPP), for evaluating a possible application of the modified suspensions in seepage control through permeation grouting. In this paper, yield stress and apparent viscosity of the SPP modified bentonite suspensions measured by the stress ramp technique are presented. Bentonite suspensions with the clay contents of 5, 7.5, 10, and 12% (by total weight of suspension; W/B (weight ratio of water (W) and bentonite (B))=19, 12.3, 9, and 7.3) were tested at various SPP concentrations (0 to 7% by weight of dry bentonite). Moreover, the time-dependent behavior of the suspensions was evaluated through measuring the yield stress at various resting times (0 to 45days). The results show that the initial yield stresses are reduced to approximately zero and apparent viscosities decrease approximately 70–90% with the addition of 2, 3, and 4% SPP for the 7.5, 10, and 12% suspensions, respectively. The reduced yield stresses increase gradually with resting times, reaching approximately 90 to 180% of the yield stresses of the unmodified suspensions after 45days. However, the required resting times for full recovery of yield stress increase with the increase of percentage of SPP used.