1,721,002 research outputs found
Reductions in Hydraulic Conductivity of Sands Caused by Microbially Induced Calcium Carbonate Precipitation
No full textMicrobially induced calcite precipitation (MICP) modifies soil behavior and properties through the precipitation of calcium carbonate (CaCO3) in the pore space. It has gained prominence as one strategy for biologically induced soil improvement. This study investigates the effect of MICP on hydraulic conductivity reduction and presents permeability reduction models for MICP-treated sands. Four column experiments, each with a different size of poorly graded sand, were subject to low-concentration equimolar MICP treatments while monitoring hydraulic conductivity reduction and precipitated CaCO3 distribution. Multiple MICP treatments produced homogeneous distributions of CaCO3 and caused a gradual reduction in hydraulic conductivity of 50%-90% until a CaCO3 content of similar to 10%-15% was achieved. The high-resolution X-ray computed microtomography (CMT) and scanning electron microscopy (SEM) imaging reveals that the pore-scale precipitation behavior changes from a contact-cementing pattern in fine sands to a mixed pattern of contact-cementing and surface-coating precipitation in coarse sands as the grain size increases. The Kozeny-Carman type of permeability models appear to well capture the hydraulic conductivity reduction caused by MICP as a function of volumetric pore fraction of CaCO3. The experimental results presented in this study advance our understanding of the pore-scale CaCO3 precipitation patterns in different sizes of sands and their effect on hydraulic conductivity. Additionally, this study provides unique and reliable hydraulic conductivity data that can be used to develop hydraulic conductivity models for MICP-treated sands.
Interface load transfer degradation during cyclic loading: a microscale investigation
No full textThe shaft capacity of piles in sand subjected to cyclic (wave) loading has been observed to decrease significantly with loading cycles (Poulos, 1989). A number of researchers (Boulon and Foray, 1986; Tabucanon et al., 1995; Shahrour et al., 1999) have replicated the characteristics of the load transfer degradation behavior in the laboratory through cyclic interface shear testing with a constant normal stiffness confinement condition (Vesic, 1972). However, no consensus currently exists as to the primary microscale mechanisms that govern cyclic interface shear behavior and load transfer degradation. A research program was undertaken to quantify the contribution of soil properties, cementation, confinement condition, and displacement mode, in load transfer degradation. Monotonie and cyclic interface shear tests were performed using a modified interface direct shear device with a Perspex side window. The specimen particle displacement fields were quantified during selected cycles by capturing high resolution digital images (1600 × 1200 pixels) and using Particle Image Velocimetry (White et al., 2001a). Results indicate that the confinement condition, which is intended to replicate the elastic response of the far-field soil, is of primary importance as it allows for normal stress relaxation with soil contraction adjacent to the interface. The displacement magnitude, particle characteristics, and particle-particle cementation were also observed to affect the magnitude and rate of degradation. It is anticipated that these findings will provide a fundamental rationale to identify field conditions where shear stress degradation is likely to occur and a basis from which more rigorous models may be developed.</p
Microscale observation and modeling of soil-structure interface behavior using particle image velocimetry
No full textThe shearing behavior of a soil-structure interface governs the response of many geotechnical systems, in particular piled foundations. The shaft resistance of piled foundations is known to degrade with cyclic loading, although the governing mechanism is not well understood. This paper presents the results of a laboratory soil-structure investigation in which internal specimen deformations were obtained using particle image velocimetry (PIV) and the normal confining stress was permitted to vary according to a constant normal stiffness (CNS) condition. The PIV measurements showed the shear deformation and volume change to be concentrated within a shear band with a thickness of 5-7 particle diameters adjacent to the interface. During a single cycle the volume change within the shear band began with an initial contraction, followed by dilation to the failure envelop. For the cycling amplitude investigated this response led to a net specimen contraction. The benefit of quantifying the thickness and contraction of the shear band using PIV is that the progressive decrease in void ratio of the shear band can be linked to the limiting value imposed by the minimum void ratio. This provides a framework in which the contraction of the specimen depends on the potential contraction expressed as the difference between the current and minimum void ratio. A model for this contraction is presented, and linked to the decay in normal stress and the limiting loss of interface friction. This framework clarifies the mechanism of friction fatigue during installation and loading of displacement piles in sand.</p
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Monotonic and Cyclic Resistance of MICP Cemented Silica and Carbonate Sands
Get PDFEarthquake-induced cyclic loading poses a concern to infrastructure founded on liquefiable soils, often resulting in severe damage like foundation bearing failure and lateral spreading. To mitigate these risks, ground improvement methods can be implemented to increase the soil's resistance to liquefaction triggering and thus any ensuing deformations. This research investigates the behavior of biocemented soils using a triaxial device, focusing on how monotonic and cyclic responses change with varying levels of Microbially Induced Calcite Precipitation (MICP) treatment, soil composition, and loading conditions. Previous studies have explored MICP at different scales, but few have utilized triaxial testing, which offers control over stress conditions and allows for localized specimen response. This study builds on prior work by conducting twenty-one monotonic and twenty-nine cyclic tests on carbonate and silica sands, with mixed sands also tested to examine the influence of carbonate content. The results indicate that MICP treatment enhances cyclic resistance significantly, with shear wave velocity measurements providing novel insights into fabric changes. Uncemented specimens behaved as expected, showing rapid pore water pressure generation and liquefaction triggering, while cemented specimens exhibited increased strength and stiffness, particularly under cyclic loading. The findings suggest that MICP treatment is more effective for improving cyclic resistance than monotonic strength, underscoring its potential as a method for liquefaction mitigation
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The Effect of Sand Gradation on the Dynamic Performance of Embankments
Get PDFResearchers have recently explored the effects of gradation on earthquake-induced liquefaction behavior, showing that broadly graded sands have a distinct response compared to poorly graded sands. However, current state of practice applies liquefaction procedures developed from poorly graded sands to liquefiable natural alluvial deposits that are much more broadly graded. The goal of this research is to better understand and quantify the systematic effects of sand gradation on embankment performance and the consequences of liquefaction. The soil’s coefficient of uniformity (Cu) is used as a proxy for gradation as it is a single parameter that affects multiple index and density metrics, which in turn generate distinct responses.The testing series evaluated herein incorporates results from two centrifuge tests, including four different soil types with Cu values ranging from 2 to 12. The centrifuge testing was performed on the 9 m-radius centrifuge at the Center for Geotechnical Modeling (CGM) at UC Davis. The model container was designed with two identical parallel sloping embankments of different gradations, prepared to the same relative density (Dr) of 40%, and configured with arrays of accelerometers and pore pressure transducers. High speed cameras mounted outside the model container recorded embankment plane-strain deformations of dyed soil columns through the clear polycarbonate sidewalls. The models were subjected to four strong motion events of increasing intensity. The results reveal that at the same relative density, horizontal displacements and, strains decreased by over 50% with increasing Cu, whereas settlements reduced to zero and even resulted in net upward displacement due to the strong dilatancy of the broadly graded sands. Despite similar numbers of cycles to liquefaction triggering, the rate of dissipation of excess pore pressures occurred 3-4 times faster for the more broadly graded sands, reducing the duration of soil matrix instability. The accelerations in the broadly graded embankments were also approximately 3 times more amplified than the poorly graded embankments. Together, these results show that gradation plays a significant role in the consequences that proceed from liquefaction triggering, which should be explicitly considered in the evaluation of liquefaction susceptibility and consequences
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MICP Technology Upscaling
Get PDFGeotechnical engineering practices for soil improvement are oftentimes material and energy intensive and thus new technologies are being developed to be environmentally conscious. Many researchers are taking advantage of microbial and bio-mediated pathways. Microbially Induced Calcite Precipitation (MICP) is one of these technologies and is a method that uses ureolytic bacteria to bind soil particles together. In this dissertation, laboratory experiments and physical modeling through centrifuge experiments are used to advance the understanding of MICP for future field implementation. This research program begins with using the knowledge from previous researchers Montoya (2012) and Gomez (2017). The MICP process starts with the establishment of ureolytic activity either through augmentation or through stimulation. Augmentation involves cultivating ureolytic bacteria ex-situ and is then introduced to the soil. In contrast, stimulation involves injecting a selective media into the soil to enrich the in-situ ureolytic bacteria. The beginning chapters of this dissertation focuses on stimulated MICP such as treatment optimization, spatial control assessed through uniformity and treatment extent, and its viability in marine conditions. Small column experiments were performed to evaluate the effect of changing the urea concentrations during the stimulation and cementation phase to reduce the environmental and financial costs associated with MICP without affecting the engineering performance of the improved soil. This study was then used to tailor the treatment formulation for the 3.7-meter scale horizontal columns that were designed to improve spatial uniformity. In addition to reduced chemical usage, a strategy of lowering the ureolytic activity through yeast extract concentrations was tested. It was hypothesized that lowering the ureolytic activity would reduce reactions during transport, which would then allow for unreacted constituents to reach the full length of the column. The 3.7-meter-long columns were also used to compare stimulation and augmentation, address ammonium by-product management, and evaluate the stimulation process in different soils. The sites of interest for MICP implementation also expand to coastal environments leading to another set of column experiments to show the viability of stimulation under various brackish conditions. Lastly, a set of centrifuge experiments were performed to add to our understanding of the dynamic performance of MICP. For this research program, discrete MICP treated zones were tested in the 1-m radius centrifuge rather than the full extent of the model container. The depth of improvement was varied while maintaining the same cross-sectional area with treatment extents of 100%, 75%, and 37% of the liquefiable sand layer depth. The responses from these models were compared to two baseline conditions: (1) uncemented and (2) fully cemented. The results from this test showed a base isolation effect from the underlying loose uncemented sand. This effect decreased the demand on the overlying MICP treated sand, resulting in minimal degradation to the cementation integrity. In addition, MICP improved zones showed an increased resistance to liquefaction
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Efficacy of Microbially Induced Calcite Precipitation in Carbonate Rich Soils
Get PDFThere is a growing interest in microbially induced calcite precipitation (MICP) treatment in carbonate rich soils. Research has found MICP bio-augmentation and bio-stimulation to be effective in carbonate rich soils with potential differences dependent on chemical formulation, treatment time, soil composition and soil fabric. This study focused on lab-scale MICP stimulation applications on carbonate rich soils in preparation for large scale tests and field trials. This test program investigated the robustness of bio-stimulation in carbonate particles with respect to the effectiveness of treatment solution concentrations, the effectiveness of commercial grade chemicals, and the effectiveness of byproduct removal in carbonate rich soils. Despite significant differences in urea degradation between treatment solution concentrations, all soil columns resulted in improvement in shear wave velocity (Vs ) and increase in calcite contents. Results showed commercial grade chemicals can be as effective as laboratory grade chemicals in bio-stimulation applications and ammonia byproduct removal methods decreased aqueous ammonia concentrations by four to five orders of magnitude. The results of this study suggest MICP bio-stimulation is successful for laboratory scale and may be successful in field scale applications
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Evaluation of Soil Disturbance due to Sonic Drilling using Instrumentation and Cone Penetration Test Measurements
Get PDFSonic drilling is an efficient and versatile drilling technique used to continuously core soils and soft rock, and it is increasingly used for geotechnical site characterization. During sonic drilling, the sonic head and attached drill string are vibrated to advance the drill string into the subsurface. These vibrations are transmitted to and propagated through the surrounding soil, causing some amount of disturbance to the soil. However, the magnitude of disturbance as a function of the zone of influence (radial and vertical distance) and soil type has not yet been quantified. This thesis presents the results of two field studies that quantify the sonic induced soil disturbance with an array of installed instrumentation. In addition, as an indicator of the possible disturbance to in-situ tests or soil sampling below the bit, changes in baseline measured CPT parameters are evaluated when a CPT sounding is performed into soils immediately below the sonic casing
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Experiments, Simulation, and Optimization of Tree Root Inspired Anchors and Foundations
Get PDFGeotechnical foundation and anchorage system designs must be adapted to growing and changing loading demands, and the consumption of materials (i.e. steel and concrete) and energy must be reduced. In this dissertation, a bioinspired design approach is developed and employed to seek new solutions to society’s foundation and anchorage needs through the study of the mechanical performance of tree root systems.Beginning with a survey of recent developments in the broader field of bioinspired design, aspects particularly relevant to the exploration of new geotechnical engineering innovations are highlighted. A deeper understanding of a biological system in terms of forms (physical structures), behaviors (processes and mechanisms), and principles (why the forms and behaviors are effective in their context) facilitates the extraction of more general principles that can be transferred between the biological and engineering contexts, circumventing context-specific constraints.
Field vertical pullout tests of three-year-old natural root systems (of Lovell, Marianna, and Myrobalan rootstocks) were performed to develop this understanding of tree root system forms, behaviors, and principles. Direct measurements of force, trunk displacement, and ground displacements, along with photogrammetric reconstruction of 3D root system structure and soil characterization enabled detailed description of the root systems’ mechanical performance. Results showed that the root systems studied are 6-10 times as efficient as a conventional micropile system in developing tensile capacity on a per material volume basis. The 3D root system models were skeletonized as nodes and root branch segments to perform statistical characterization of the architecture of mechanically important structural roots, enabling the development of L-system based tree root system architecture simulation capabilities.
A spectrum of architecture complexity is constructed, ranging from a simplified root analog to an L-system simulation of a root system and a model of an extracted field tree root system specimen. The models are 3D printed and tested in vertical pullout in a dry sand using geotechnical centrifuge modeling to generate information on the features that are most critical for development of efficient anchorage. The most important structural feature affecting vertical pullout capacity was vertically projected area. The force-displacement response of the simplest structure exhibited high initial stiffness and rapid softening, while the more complex models exhibited high peak resistance and residual capacity, indicating tradeoffs between capacity, stiffness, and efficiency in anchor design.
In recognition of the importance of structure deformation mechanisms in the progressive mobilization processes of tree root systems and tree root inspired designs, a soil springs model is developed to investigate the impact of shape and flexibility of embedded structures on pullout behavior. Results reveal a coupling between mobilized bearing and tensile resistances in terms of the rate of development and magnitude. Finally, taking the soil springs model environment and defining the structure curvature as a control input. Optimized nonlinear structure shapes are identified for a broad range of inputs, revealing principles of how to maximize vertical pullout capacity of a nonlinear, flexible embedded structure as a function of the performance demands
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The Evaluation of Energy Loss Along Drill Rods Using an Instrumented SPT
Get PDFThe Standard Penetration Test (SPT) N60 value is one of the most common measurements used to estimate in-situ soil strength and density of sands. During SPT testing, a sampler is attached to a series of rods and driven through the bottom of a borehole. A hammer strike at the surface generates energy that propagates through the rods and into the soil at a sampler head. The amount of energy driving the sampler through the soil is dependent on the hammer type, drill rig, and wave propagation along the rod. However, the magnitude of energy loss as a function of the rod length has not been well characterized at depths greater than 15 meters. This study presents the results of four field studies that attempt to quantify the amount of energy loss that can be attributed to increasing rod length. Two SPT rods were instrumented with a series of strain gauges and accelerometers which yielded force and velocity measurements. Measurements acquired from these sensors were integrated to determine the energy loss along the sampler rod. Drilling was performed to depths of up to 85 meters across various sites and soil types to develop the relationship between rod length and energy loss. The study revealed that approximately 1.4% of the theoretical hammer energy is lost for each 3 m of rod added to the sampler rod
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