Evaluation of Time-Dependent Yield Stress Using Dynamic Rheological Property of Bentonite Suspensions

摘要

Permeation grouting using weak grouts such as bentonite grout is one of the effective methods to mitigate liquefaction in loose saturated sand deposits. While a flow parameter of the grout such as yield stress determines its penetrability into the deposit and resistance to groundwater flow, a dynamic parameter such as critical storage modulus evaluates post-grouting performance of the treated soils under cyclic loading condition. However, the yield stress and critical storage modulus should be obtained through two different types of rheological tests: drag and oscillatory shear test. Although previous research has suggested a method to evaluate yield stress from an oscillatory shear test, the conventional method does not consider the time-dependent nature of bentonite grout, which is one of its crucial properties as a grout. In this study, flow and dynamic rheological properties of bentonite suspensions were measured using drag (stress ramp) and oscillatory shear (strain sweep) tests with a vane geometry for various weight fractions of bentonite suspensions (5, 7.5,10, and 12 %) and resting times (0 to 480 h). At different resting times, elastic and crossover stresses from strain sweep tests were compared to yield stresses obtained from stress ramp tests. The results showed that both the elastic and crossover stresses from strain sweep tests were significantly lower (40 %-60 %) than the yield stresses measured by stress ramp tests. The comparison also showed a dependency on particle fractions. In order to evaluate yield stress from the oscillatory shear test, a time-independent relationship between yield stress and critical storage modulus was proposed. This study suggests an economical approach to evaluate an important design parameter ("undisturbed" yield stress) in permeation grouting using bentonite grout.