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1. Influence of the coefficient of uniformity on bio-cemented sands: a microscale investigation

   Reed, Marlee; Montoya, Brina M. (September 2023, Proceedings of the 8th International Symposium on DEFORMATION CHARACTERISTICS OF GEOMATERIALS)

   Viana da Fonseca, António; Ferreira, Cristiana (Ed.)

   Microbially induced carbonate precipitation (MICP) is a bio-mediated ground improvement technique that can increase soil stiffness and produce cohesion within granular material. Most experimental investigations on MICP-treated soils are performed on idealized granular materials. Evaluating a narrow range of particle sizes dismisses the potential influence of soil fabric on MICP treatment efficiency. Therefore, little is known regarding the influence of soil fabric on the level of improvement achievable post-MICP treatment. We investigate the influence of the coefficient of uniformity (Cu) on the level of improvement that can be obtained from MICP treatment. This study couples unconfined compression testing with microscale observations obtained from x-ray computed tomography (CT) of two sand mixtures with different Cu values. A soil column and CT specimen of each sand mixture were prepared and received the same number of MICP- injections. The shear wave velocity (Vs) of the soil columns was monitored to evaluate the increase in soil stiffness over time. After MICP treatment, the bio-cemented columns were subjected to unconfined compressive strength testing. Results indicate that for a similar mass of carbonate, the soil with a larger Cu experienced a greater increase in Vs but a lower maximum unconfined compressive strength. Through CT imaging, the soil with a smaller Cu was observed to have a more uniform distribution of carbonate within the sand matrix whereas the soil with a larger Cu has more sporadic MICP trends. This study elucidates the influence of soil fabric on the level of improvement that can be achieved through MICP treatment and assesses the reliability of x-ray CT scanning of MICP-treated sands with moderate carbonate content. 

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2. The impacts of biomineralization and oil contamination on the compressive strength of waste plastic-filled mortar

   https://doi.org/10.1038/s41598-022-25951-3  

   Rux, Kylee; Kane, Seth; Espinal, Michael; Ryan, Cecily; Phillips, Adrienne; Heveran, Chelsea (December 2022, Scientific Reports)

   Abstract Researchers have made headway against challenges of increasing cement infrastructure and low plastic recycling rates by using waste plastic in cementitious materials. Past studies indicate that microbially induced calcium carbonate precipitation (MICP) to coat plastic in calcium carbonate may improve the strength. The objective of this study was to increase the amount of clean and contaminated waste plastic that can be added to mortar and to assess whether MICP treatment enhances the strength. The performance of plastic-filled mortar was investigated at 5%, 10%, and 20% volume replacement for cement. Untreated, clean plastics at a 20% cement replacement produced compressive strengths acceptable for several applications. However, a coating of MICP on clean waste plastic did not improve the strengths. At 10% replacement, both MICP treatment and washing of contaminated plastics recovered compressive strengths by approximately 28%, relative to mortar containing oil-coated plastics. By incorporating greater volumes of waste plastics into mortar, the sustainability of cementitious composites has the potential of being improved by the dual mechanisms of reduced cement production and repurposing plastic waste. 

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3. Sustainable Solution on Desiccation Crack Mitigation with Recycled Glass Sand

   https://doi.org/10.1088/1755-1315/1337/1/012050  

   Zhang, Bin; Wen, Kejun; Li, Junjie; Huang, Wei (May 2024, IOP Conference Series: Earth and Environmental Science)

   Abstract Desiccation cracking is a frequent natural phenomenon that occurs in drying soil and has a significant negative impact on the mechanical and hydraulic properties of clay or geomaterials in various engineering applications. In this study, recycled glass sand (RGS) was used to reduce the plasticity of clay soil and mitigate desiccation cracks in clay soils. The effect of the RGS particle size and content was investigated using a desiccation crack observation test. Digital image processing technology was used to evaluate the crack rate, length, width, and area during the observation test. The results reveal that the cracking rate was inversely proportional to the RGS content and directly proportional to the RGS particle size. For instance, the cracking rate of clay soil treated with 25% RGS with a particle size of 0.15 mm was reduced to 0.17% compared with untreated soil. The strengths of the untreated and RGS-treated soils were evaluated through unconfined compression tests. The unconfined compressive strength of the RGS-treated clay soil decreased slightly with the addition of RGS. In general, the addition of RGS has great potential for mitigating desiccation cracks in clay soils. 

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4. Vetiver Grass Performance on a Distressed Highway Slope of High-Plastic Clay under Excessive Rainfall

   Shuman, Nur Mohammad; Nobahar, Masoud; Khan, Mohammad Sadik; Alzeghoul, Omer; Chia, Henry Kini. (March 2022, Geo-Congress 2022 GSP 336 268)

   With climate change frequency of extreme disasters is increasing day by day. Excessive rainfall causes shallow landslides during such extreme events, very commonly on highway slopes containing highly plastic clay. Mississippi results in a perfect area to study the Vetiver System (VS) performance for shallow slope failure under excessive rainfall, with an average annual rainfall intensity higher than other neighboring states in the south. A highway slope in Mississippi containing high-plastic clay is explored throughout the year 2019, where time-dependent slope movement along the depth is monitored with rainfall variation. A 9.1 m slope inclinometer along with a rain gauge and an air temperature sensor have been installed at that section. The slope has experienced a shallow movement at the slope surface with a depth of 1.9 m with already observed rainfall intensity. A 6 m2 area of the slope is selected and reinforced with Vetiver grass. Nevertheless, quantitative studies of how much contribution the VS provides to slope stabilization in the field are still relatively scarce. The current paper presents a comparative study of the VS performance on the shallow slope movement in expansive soil observed from inclinometer data for the last two years. It is observed that due to Vetiver grass, the slope movement rate has reduced to 2% since plantation of Vetiver grass from that of 10% before Vetiver grass. Later, this result is further verified through a numerical investigation where Vetiver grass is simulated by changing soil properties. Numerical analysis indicates root reinforcement from the root dimensions increases slope stability up to 30%. This study outlines an approach for the study of VS in terms of contribution to slope stabilization, including field observation and numerical analysis. 

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5. The Effect of Level of Cementation and Geometry on Stability of Cemented Coastal Bluffs and Slopes

   Pegah Ghasemi and Brina M.Montoya (March 2023, Geo-Congress 2023; eISBN 9780784484661)

   Coastal bluff erosion and recession are among the common mechanisms altering the geomorphology of the coastline in California. The accelerated erosion rate increasingly threatens the stability of structures located on these bluffs. Previous researchers have investigated the effect of material properties and strength on the generation of the shear plane and failure modes of coastal bluffs and cliffs. Monitoring the morphology of the moderately cemented coastal bluffs with time has indicated that a comparison of material strength with the expected insitu minor principal stress distribution can be used as a criterion to assess bluff stability. However, the effect of varying factors such as cementation levels and bluff geometry and dimensions on stress distribution patterns and material properties that determine bluff failure susceptibility requires further investigation. While bond breakage and disturbance during sampling and transportation undermine the quality of recovered soil samples, artificial cementation methods (e.g., Portland cement) may not properly replicate the natural formation processes. Instead, microbially induced carbonate precipitation (MICP) is a ground improvement method that simulates the cementation processes that occur in natural geological settings. This method harnesses the activities of bacteria to generate cementitious precipitation among soil particles. The formation of the cementing agent improves the mechanical properties of the soil. In the past two decades, extensive studies have been devoted to understanding the cementation formation mechanism and the improvement of mechanical properties that can be used as a proxy for natural cemented soil for stability analysis. In the study presented herein, a series of FEM models were developed in SIGMA/W software. The effect of the different cementation levels and variation of bluff geometry on minor principal stress distribution was investigated. Results of the study demonstrated that although the cementation level of the materials determines the failure mode, the stress distribution mainly depends on the bluff geometry. The obtained results offer further insights into the failure mechanism of coastal bluffs as well as MICP-treated slopes for future field implementation of this soil improvement method. 

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