Doctor of Philosophy
Civil and Environmental Engineering
Peat and mine tailings present significant challenges in geotechnical engineering due to their adverse mechanical properties. Peat, rich in organic materials, demonstrates considerable compressibility and permeability, leading to significant settlements during construction. Consequently, building on peat is fraught with risks because of its low bearing capacity, with non-uniform settlements posing a threat to structural integrity. Mine tailings, remnants of mining activities, frequently contain harmful elements. It is imperative to store them within tailings dams' saturated layers, isolating them from the environment and averting potential contamination. However, when stored, these tailings can undergo liquefaction in response to both monotonic and cyclic loads. Given these challenges and the increasing scrutiny on traditional treatments like Portland cement for environmental reasons, an alternative solution becomes imperative. This study advocates for Microbially-Induced Calcite Precipitation (MICP) as a viable technique to enhance peat and mine tailings' mechanical properties. In this research, these materials are subjected to treatments and subsequent Direct Simple Shear (DSS) testing under different loading conditions, drained and undrained under monotonic and cyclic loads. The results were used to evaluate parameters such as density, yield and residual strength, brittleness, state parameter, stiffness, compressibility, liquefaction resistance, and dynamic characteristics. The outcomes highlighted pronounced improvements in these parameters upon successful MICP application. Additionally, X-ray diffraction (XRD), scanning electron microscopy (SEM) images, and energy-dispersive X-ray spectroscopy (EDS) are employed to scrutinize the microstructural and compositional transformations in the materials post-MICP treatment.
Summary for Lay Audience
Geomaterials exhibiting unfavorable mechanical properties, particularly low strength, present substantial challenges in the realm of geotechnical engineering. These materials, under certain loading scenarios - such as seismic events - can jeopardize even the most robustly constructed structures. Specifically, mine tailings and organic soils are of notable concern. The former is prone to significant strength degradation due to liquefaction, while the latter is characterized by a low bearing capacity and heightened compressibility. In response to these challenges, this research introduces the use of a distinct bacteria adept at hydrolyzing urea, subsequently elevating the environmental pH. This mechanism instigates the formation and precipitation of calcite particles and is known as Microbially-Induced Calcite precipitation (MICP). Such precipitations not only bind soil particles cohesively but also fill pore spaces, thereby improving the soil's mechanical characteristics. Within this context, the initial segment of this research delves into gauging the improvement in strength of two types of tailings (fine-grained and coarse-grained) subjected to monotonic loads in static environments. This exploration is imperative, given the historical instances of tailings storage facility collapses under analogous load conditions. The subsequent section pivots towards assessing the efficacy of MICP in bolstering the strength of both fine-grained and coarse-grained tailings during seismic events. Such evaluations are crucial to discern the potential of MICP in forestalling liquefaction and the concomitant strength deterioration under cyclic loads. Additionally, given that strengthening tailings modifies their seismic wave transference capability from the bedrock to the surface, it is indispensable to evaluate MICP's implications on the dynamic characteristics of treated tailings, a topic addressed in this research. Last but not least, this study evaluates MICP's influence on the strength parameters and compressibility of organic soils. Findings from this research suggest that MICP commendably improves the strength of both tailings and peats. Nonetheless, it is paramount to recognize that such strength improvement might diminish soil damping, potentially amplifying seismic waves as they traverse from the subsurface to the ground level.
Behzadipour, Hamed, "The Application of MICP for Strengthening of Mine Tailings and Peats" (2023). Electronic Thesis and Dissertation Repository. 9776.
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