Dissertations/Theses - Department of Civil Engineering
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Item Experimental investigation of thixotropic hardening of selected clays(Department of Civil Engineering (CE), BUET, 2024-12-24) Raihan, Muhammad; Shariful Islam, Dr. MohammadThis study investigates the thixotropic hardening behavior of reconstituted clay soils from Gazipur, Savar, and Mohakhali in Bangladesh, examining the impact of thixotropic aging on their strength recovery, deformation characteristics, and microstructural changes over 48 days. Utilizing advanced analytical techniques, including X-ray Diffraction (XRD), X-ray Fluorescence (XRF), and Scanning Electron Microscopy (SEM), this research comprehensively assesses the role of clay mineralogy, particle arrangement, and water content in enhancing the engineering properties of these soils, with significant implications for geotechnical design and construction. The methodology involved collecting both disturbed and undisturbed soil samples using wash boring and Shelby tube sampling techniques. These samples were then naturally dried, ground, and sieved to prepare reconstituted specimens at their respective liquid limits. A series of laboratory tests, including unconfined compressive strength (UCS), triaxial compression, and one-dimensional consolidation, were conducted at various aging intervals up to 42 days. These tests were complemented by detailed mineralogical and microstructural analyses to determine specific gravity, Atterberg limits, and particle size distribution, revealing variations in fines content ranging from 89.6% to 98.63% and specific gravity values between 2.63 and 2.7. Significant findings from the study highlight a pronounced time-dependent strength recovery, especially notable in Mohakhali soil, which demonstrated the highest increase in unconfined compressive strength, escalating from 60.8 kPa to 87.5 kPa. The research introduced and utilized the Thixotropic Strength Ratio (TSR) and Thixotropic Regain Strength Ratio (Bt) to effectively quantify the recovery, capturing the reformation of particle structures and bond enhancement post-disturbance. Triaxial test results showed a remarkable 144% increase in deviator stress under a consolidation pressure of 120 kPa over 28 days. Microstructural analyses via SEM revealed densification and improved particle alignment, enhancing soil mechanical properties. XRD and XRF identified mineral variations influencing strength recovery, with higher illite in Mohakhali soil enhancing cohesion and thixotropy. Elevated alumina and iron oxide further improved particle bonding and strength regain. These findings highlight the critical role of mineralogy and microstructure in thixotropic behavior, providing valuable insights for geotechnical applications in clay-rich environments.Item Determination of liquid limit and plastic limit of clays using cone penetrometer(Department of Civil Engineering (CE), BUET, 2024-09-01) Ullah, Ahad; Islam, Dr. Mohammad SharifulThis study employed an alternative method to determine soils' two significant geotechnical index properties, the liquid limit (LL) and the plastic limit (PL). As the traditional methods are time-consuming, need much human labor, and have faced criticism for being subjective, the study attempted to use cone penetration methods as an alternative to determine the LL and PL of soils. The study aims to reduce the operator variability and improve consistency in determining these two geotechnical properties of soils. The study also examined the relationship between soil minerals and soil type to determine further insights into soil behavior. Soil samples were collected from 20 locations across Bangladesh and then tested using ASTM standard procedures. The LL was determined using the Casagrande method, and the PL was determined through the hand-rolling method (ASTM D4318). The LL obtained from the cone penetrometer method was then compared with the Casagrande method. To determine the PL by cone penetrometer, first, the standard penetration depth for plasticity was determined using the 30-degree standard cone utilizing the hand-rolling method. Then, the standard penetration depth was used to determine the PL of the soils, which was subsequently compared with the standard method. Modifications were made to conventional penetrometer cones, with two additional cone angles of 20 and 16 degrees fabricated alongside the standard 30-degree cones to determine the liquid and plastic limits. The study findings revealed that the cone penetrometer method measurements of LL and PL, well enough and matches with the standard method. For LL determination, the 30-degree cone penetrometer measurement deviates from -10% to +7% for low to high plasticity soil by comparing with the standard Casagrande method. For soil with very high plasticity, the deviation was higher (>25%). For PL determination, the results showed much better, with the measurement deviation varying between -3.28% and +6.63% by comparing with the standard hand-rolling method. The standard 30-degree cone proved to be the most reliable for the simultaneous determination of liquid and plastic limits. Among the soil minerals identified from the X-ray diffraction (XRD) analysis, the Illite clay mineral was the most abundant (11.21% to 25.50%) for most of the soil samples, correlating with plasticity and grain size results. The study's findings are expected to be an alternative to the standard methods in determining the liquid and plastic limits of the soil, which will reduce human labor and the complicacy of the standard procedure.Item Stabilization of sandy soil with bio-stimulation based microbially induced calcite precipitation(Department of Civil Engineering (CE), BUET, 2024-11-13) Nazmus Sakib, Mirza Md.; Islam, Dr. Mohammad SharifulThis research explores the innovative integration of Microbially Induced Calcite Precipitation (MICP) with traditional cement stabilization to address the strength and durability challenges of alluvial sandy soils. The primary objective is to enhance soil properties while introducing a sustainable, bio-inspired technique for compressed earth block (CSEB) fabrication. Sandy soil samples were sourced from Anowara, Chattogram, and mixed with 10% garden soil to support bacterial activity. The study examined the impact of varying content ratios, curing periods, and mellowing periods of MICP treatment on the micro-mechanical properties of the soil. Analytical techniques, including Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), and Thermogravimetric Analysis (TGA), were employed to evaluate microstructural and chemical transformations, alongside 16S metagenomic sequencing to analyze bacterial community dynamics. The mechanical tests demonstrated substantial improvements in soil strength, with dry unconfined compressive strength, split tensile strength, and flexural strength increasing by 113%, 95.84%, and 133.86%, respectively. MICP treatment enhanced water absorption resistance by 59.2% and improved durability under dry-wet cycles. SEM and EDS analyses revealed reduced voids and the formation of calcite crystals and cement hydration products that bridged soil particles. FTIR results confirmed stronger calcite and hydration-related spectral peaks, correlating with observed strength and durability enhancements. DSC and TGA results indicated significant calcite content, with a notable weight loss above 600°C. A 101.53% increase in calcite content was achieved in 5-day treated samples with 8% cement content. 16S sequencing highlighted the enrichment of urease-positive bacteria, particularly Sporosarcina and Bacillus genera, with Firmicutes dominance in MICP treated cement-stabilized blocks. Life cycle analysis (LCA) further revealed that CSEBs are 14.69% more cost-effective and environmentally sustainable compared to fired clay bricks (FCBs). Optimal results were achieved with 6% cement content by weight and a 5-day mellowing period. This study underscores the potential of combining MICP bio-stimulation with traditional cement stabilization to establish an effective protocol for partial cement replacement, advancing sustainable construction practices.