Browsing by Subject "Permeation grouting"

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Application of pore fluid engineering for improving the hydraulic performance of granular soils
(2011-12) Yoon, Jisuk; El Mohtar, Chadi Said; Gilbert, Robert B.; Zornberg, Jorge G.; Ferron, Raissa D.; Balhoff, Matthew T.
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.
Influence of sand friction angle, grout stiffness, and confinement on the behavior of acrylamide and sodium silicate grouted sand
(2021-08-02) Foong, Wai Joon; El Mohtar, Chadi Said; Gilbert, Robert B
In the geotechnical engineering field, grouting is a commonly used option within the plethora of ground improvement methods. Yet, the practice of grouting relies heavily on empirically developed models to estimate strength and predict grouted soil behavior. Collectively, these models consider many factors including soil and grout properties, but do not incorporate the soil friction angle, which is also a commonly used soil parameter in geotechnical engineering. Thus, the objective of this study is to investigate whether the friction angle will influence the strength of a grouted soil – particularly 3 sand types of different densities, gradation, and shapes, and 2 grout types with different stiffness magnitudes – to predict the strength of a grouted soil mass. To achieve the stated objective, a series of tests were performed to characterize the friction angle of the different sands at the same void ratio/porosity. In a separate set of tests, friction angle values were controlled and grouped to investigate the influence of sand void ratio. Then, properties of the grouts at pre-gelation and post-gelation stages were determined. Finally, sand columns built with longitudinally split molds were permeated with grout and left to cure prior to compression or triaxial tests to determine the grouted sand properties. Following the procedures developed in the testing program, it is found that the prediction of grouted sand strength using sand friction is appropriate when the failure strain of grout exceeds at least 4 times the failure strain of ungrouted sand, due to strain compatibility. Additionally, acrylamide-grouted sand unconfined compressive strength models developed from 25 compression tests are presented using the grouted sand stiffness and complex modulus, along with ungrouted sand properties such as friction angle and gradation. Finally, the results showed that instead of using unconfined compression tests, confined compression tests on grouted sand will be more representative of grouted sand behavior in the field because it is demonstrated in this study that there is a crossover between the Mohr-Coulomb failure envelopes of acrylamide-grouted and ungrouted sands. Consequently, the efficacy of strength improvement of in-situ soils by means of acrylamide grouting is not constant with depth.
Kinetics of microfine cement filtration during constant rate permeation through sands
(2021-07-30) Ward, Katie Kristine; El Mohtar, Chadi Said; Zornberg, Jorge G
Permeation grouting is a cost-effective ground improvement technique that involves the injection of grout into the voids of geological material to improve the engineering properties by reducing the permeability and/or increasing the strength. Permeation grouting can be performed with either cementitious suspensions or chemical solutions. However, over the last several decades, cementitious suspensions have become popular for permeation grouting due to their cost-effectiveness, finer particle size distribution, and minimal impact on the environment. Nevertheless, the particulate nature of the microfine cement increases the susceptibility to filtration: a phenomenon that reduces the penetration performance of the grout and the uniformity of the delivered cement through the injected length of the soil material. The penetrability of a cementitious grout suspension is highly dependent on the stability and the filtration tendency of the cementitious grout, while the filtration of cement particles in the voids of the soil material is dependent on both the grain size distribution of the porous medium and the rheological properties of the grout itself. The research in this thesis consisted of various permeation grouting experiments constructed with a poorly graded mortar sand at variable lengths and permeated with four different grout mixes: two microfine cement grouts at water to cement (w:c) ratios of 2:1 and 3:1 and two microfine cement grouts at w:c ratios of 2:1 and 3:1 with 2 percent by weight of superplasticizer. During the permeation, the pressure buildup at the bottom of the sand columns was measured and the effluent was collected to determine the cement concentration of the grout with time for the duration of the permeation. After permeation, the sand columns were de-constructed and samples were taken to measure the cement distribution, in terms of w:c ratios, along the height of the column. The results of this research highlight the impacts that the stability and the filtration tendency of cementitious grouts, with and without superplasticizer, have on the pressure buildup at the bottom of a soil column, the cement concentration outflow for the duration of permeation, and the cement distribution along the injected length of a soil column.
Towards better characterization and understanding of internal stability of ultrafine grouts
(2018-08) Miller, Anna Kate; El Mohtar, Chadi Said
Internal stability of a grout is critical in determining whether a grouting job is successful or not. A highly stable grout would be able to permeate through formations/fractures without losing any significant percentage of its solids fraction to filtration. Having an unstable grout would result in non-uniform distribution of the cement along the grouted distance, with a higher concentration of cement near the injection point (above the design value based on the grout concentration) and a much weaker cementation at the end of the grouted zone. Formations grouted with unstable grouts end up with non-uniformly enhanced properties and increase the local heterogeneity. Due to the many problems associated with unstable grout use, it is undesirable in most cement grouting operations (Rosquoët et al. 2003, Naudts et al. 2004, Tan et al. 2004, Bremen 1997). This Thesis presents a comprehensive study on internal stability of a micro-fine cement (UltraFine) and its impact on grouted sand through an experimental investigation. The hydration and particle size distribution of the Ultrafine cement is determined through a physio-chemical characterization to help understand the observed behavior during stability tests. Grouts with a wide range of w/c ratios are then tested using the traditional internal stability characterization methods (Column Bleed Test and API Filter press test). These tests are index tests and do not represent the state of the grout during its application in the field, among other limitation. Additionally, image analysis is used to better understand grout stability and the movement of solids within a standing column. The digital analysis approach allows for characterization of the bleed test beyond measuring the height of free standing water on top by accounting for the change in grout concentration with depth. A new testing procedure (Dynamic Stability Test) is developed to investigate the impact of continuous shearing on the internal stability of the grouts. All the current methods for determining the stability of a grout are performed under static condition. However, for its first few hours after mixing in the field, the grout is never under static. The new method measures the change in the rheology of a grout using Physica MCR 301 rheometer (Anton Paar, Graz, Austria). The rheological measurements are performed under different shearing conditions between measurements to evaluate impact of shearing/flow on internal stability of the grouts and the results showed that grouts are more stable when continuously sheared, implying that the static tests can underestimate internal stability of grouts. Last, a new Grout Filtration Test is proposed in this study that is a modification of the filter press to better model the grout performance in the field. Two testing procedures and analysis methods (Simplified field and Laboratory) are presented to quantify the internal stability of grouts. Results from the current and newly proposed testing are used to better understand the internal stability of the grout and identify the most efficient way to measure grout internal stability