Time Dependent Rheological Behavior of Modified Bentonite Suspensions

Abstract

Time dependent rheological behavior of modified bentonite suspensions was measured to investigate their applicability in permeation grouting. Permeation grouting of bentonite suspensions has been investigated as an effective way to deliver fine particles into soils for ground improvement. The ideal grout for this application needs to have low enough initial flow properties, such as yield stress and apparent viscosity for permeation, followed by rapid buildup of stiffness to obtain the desirable post grouting performance and durability. In order to achieve these properties, concentrated bentonite suspensions were modified with an ionic additive, Sodium Pyrophosphate (SPP). The addition of SPP improves the penetrability of the concentrated bentonite suspensions through soils while maintaining the thixotropic nature of the bentonite suspensions resulting in a gel-like structure in pores with time. In this study, stress ramp tests were conducted, immediately after mixing, to examine the initial reduction in yield stress and apparent viscosity of the modified suspensions at 7.5, 10 and 12% bentonite suspensions with 1 to 4% SPP (by dry weight of bentonite). In addition, oscillatory shear tests (strain sweep) were performed to investigate the time dependent variation in stiffness (critical storage modulus) at the 7.5 and 12% bentonite suspensions with 1 and 4% SPP and resting times (0 to 120 hours). The results showed that both flow parameters and storage modulus of the modified suspensions was significantly reduced immediately after mixing (liquid state), but storage modulus rapidly increased by two (2) to three (3) orders of magnitude within short resting times (gel state). The proposed modification of bentonite suspensions would be an effective method of enhancing permeation of concentrated bentonite suspensions through sands by improving the mobility of the suspensions, and then forming gel-like structures with time after being placed in pore space to resist washing out.