Investigation of critical hydraulic gradient and its application to the design and construction of bentonite-grout curtain

Abstract

The application of salt–bentonite (Salt–Bent) grouting for the design and construction of grout curtain to form vertical barriers (VB) to serve as seepage remediation measures has been investigated. Recent experiences on the high cost of grout injection materials when conducting a grouting campaign have necessitated the authors to consider the use of clay of the montmorillonite (Mt) type (Wyoming bentonite), which is less expensive; readily available naturally occurring; yet very effective and environmentally friendly grouting material for the construction of grout curtain. Grout curtain is formed by injecting grout slurry into a series of boreholes, drilled through the weathered zone of the subsurface, to form VB. Vertical barriers play a significant role in geo-environmental engineering for the control of horizontal groundwater flow and contaminant spread into the unsaturated–saturated zone of the subsurface. The design and construction of effective grout curtain is key to the overall performance of the barrier and this is dependent on the accurate determination of a design parameter, the critical hydraulic gradient (CHG). A laboratory investigation of the CHG was conducted through grout injection into fractures of aperture sizes 100–60 µm and river sand compacted to different porosities (0.33 ≤ n ≤ 0.40), to determine the safety thickness of a bentonite-grout curtain to withstand a given hydraulic gradient without failure. The study finds that, the CHG due to the fractured and porous media ranges between 30 and 35; and 180 and 200, respectively. The resulting permeability values obtained from the grouted samples were in the order of 10−7cm/s or less. It can be inferred from the results that, bentonite-grouting can provide the required permeability for an effective barrier. The results also suggest that, accurate determination of the safety thickness of the barrier is of great importance for sustainable barrier system.