Balancing rheology and filtration : a novel approach for optimizing suspension flow and sustainability in granular media




Kundu, Ritika

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A significant amount of research (at laboratory scale) has been performed in recent years on the use of chemical and suspension grouts for ground improvement, however; field implementation has always been hindered by the in situ soil heterogeneity and discontinuities. In the field, there are many practices of grouting such as compaction grouting, jet grouting and permeation grouting; the exquisiteness of permeation grouting is that it can be performed without disturbing the existing facilities in close vicinity. However, permeation grouting of granular soils in the field face two main challenges: how to maximize the penetration depth of grout propagation in sands and how to control flow and ensure the stability of grouts in gravels. Consultants and site engineers design grouting work mostly based on their experiences and some empirical correlations. These empirical correlations are mainly based on particle size of grout and soil, and in some cases, include pressure, yield stress, and concentration of suspensions. Even when these correlations indicate that a soil can be penetrated by a specific grouting material, there is little information about the maximum injection depth, appropriate grout mixture or allowable injection pressure.

The flow of suspension grouts through porous media is a very complex phenomenon owing to the diversity of the mechanisms involved. It is controlled by the rheology of the suspensions and its filtration as it flows through the porous media. Filtration is the process of particles being filtered from a suspension grout by soil grains leading to the simultaneous change in the physical properties of porous media and the grout with permeation. The rheology of a grout is dependent on the grout mix as well as the flow rate and size of voids in the porous media, all of which change with time and space as filtrations occurs.

The objective of this study is to investigate the grout flow through porous media by balancing its rheological and filtration properties. Rheological properties of non-Newtonian suspension grouts were determined for a better understanding of its flow behavior through porous media. The rheology of thicker suspension was modified using additives to lower its initial viscosity and get better permeation. One dimensional constant flux permeations tests were performed to determine the effect of different parameters (such as bentonite concentration, injection flow rate, the percentage of additives, apparent viscosity, and grain size) on filtration and ultimately, groutability and permeation depth.

A simple laboratory test was introduced to study the filtration behavior of clay and cement suspensions permeating through porous media. The filtration function and permeability reduction function were determined to simulate the flow of suspension grout through the granular soil. The proposed filtration model predicts the permeation depth and the distribution of suspension particles along the permeation length to evaluate the performance of the grouted specimen. The rate of particle deposition depends on the rheology and concentration of grout, particle size and shape of suspended particle, grain size distribution of porous media, and physio-chemical interaction of grout with the porous medium through which it is flowing. The novelty of the newly proposed model is that it accounts for the changes in the rheology of grout and the porosity and permeability of the porous medium as a function of the deposition of particles, none of which has been considered in the past.

Another research focus is to evaluate the homogeneity of prepared sand specimens in the laboratory. The distribution of local void ratio is more pertinent to the mechanical and hydraulic properties of the soil than a single average value for the whole specimen. A simple method is proposed and evaluated to determine the spatial variability of pore space within the soil specimen. The new method composes of a simple experimental water flushing sequence that can be performed on specimens assembled in most existing geotechnical testing setups. For the specimens tested in the this study, it was found that local porosity can be off by up to 10 percent to the average porosity of the whole specimen (despite special care being taken to produce uniform specimens).

The final research focus in this dissertation is the development of a simple, inexpensive and quick method, to characterize the unsaturated hydraulic properties of sandy soils. This method combines a multistage constant flux water flushing experimentation with inverse modeling using Hydrus 1D to obtain the unsaturated hydraulic properties of a soil. The unsaturated hydraulic properties obtained from the proposed method were comparable to those obtained using conventional hanging column test without the need for any specialized equipment. Additionally, the new method covers a wider range of soils as can be seen by the results obtained on a uniform sand with a very lower suction that is couldn’t be tested using the hanging column test.


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