Application of pore fluid engineering for improving the hydraulic performance of granular soils
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Over the past years, levee failures during floods have caused significant losses of lives and properties in the nation. Majority of these failures were induced by seepage through granular foundation soils underneath the top soil on which the levees were built. One of methods to mitigate this phenomenon includes the treatment of the granular soil deposits with an engineered fluid delivered by permeation (permeation grouting), resulting in a less pervious deposit. Since the conventional cement-based suspensions and chemical solutions may cause groundwater contamination due to long term reaction with groundwater, clay suspension such as bentonite suspension can be an alternative in terms of environmental friendliness and long-term safety. Moreover, the suspensions, after being permeated, are expected to be stable in the pore space due to the thixotropic nature of bentonite. The main challenge in this approach is being able to permeate a concentrated suspension through the pores of a granular material. To achieve a significant reduction in the hydraulic conductivity, concentrated bentonite suspensions should be used; however, concentrated suspensions can have low mobility, resulting in a low penetration depth and little practical application. The main objective of this study is to investigate the permeation of concentrated bentonite suspensions by controlling their rheological properties. The first portion of this research focuses on measuring the rheological properties of the various engineered bentonite suspensions over time. The second point of focus of this research is the parameters affecting the flow of the bentonite suspensions through granular soils, and the final focal area is determining the hydraulic performance of the grouted granular soils. In order to achieve these objectives, an experimental program was developed in this research. First, rheological tests were performed with the bentonite suspensions with and without various concentrations of sodium pyrophosphate (SPP); SPP is an ionic additive that is used to reduce the initial yield stress and viscosity of bentonite suspensions. A stress controlled test with the vane geometry produced rheological parameters with a minimal disturbance. Suspensions were stored in sealed cups and tested at various times to measure the long term thixotropic changes in yield stress and viscosity. Second, the various concentrations of the bentonite suspensions were injected at a constant pressure through clean sands which were prepared at various conditions (relative density, fine contents, and grain size) in order to investigate soil and suspension parameters affecting the flow of the bentonite suspensions. The results from these experimental tests were utilized to develop a groutability criterion of bentonite suspensions for practical purposes. Finally, the saturated hydraulic conductivity of the treated soils was measured using falling and rising head method. The traditional concept of “clay void ratio” was re-examined. The results from this study showed that the modified bentonite suspensions could be used as an alternative grout in permeation grouting to improve hydraulic performance of the permeable granular soils.