Effect of Optimum Moisture Content on the CBR of Subgrade Soil

Abstract

This research thesis presents a comprehensive geotechnical investigation into the physical and mechanical characterization of granular subgrade materials intended for high-capacity heavy-haul infrastructure. The primary objective of this study was to evaluate the structural integrity and load-bearing performance of soil samples through rigorous laboratory testing protocols, strictly adhering to the IS: 2720 standards. The research methodology encompassed a systematic assessment of Grain Size Analysis (GSA), Modified Proctor compaction characteristics, and California Bearing Ratio (CBR) testing under controlled moisture conditions. The initial phase of the research focused on determining the optimal geotechnical parameters at the established Optimum Moisture Content (OMC) of 6.69%. Under these optimal conditions, the experimental testing yielded a Maximum Dry Density (MDD) of 1.989 gm/cc and an average soaked CBR value of 12.75%, which confirms the material's suitability for high-performance subgrade applications. However, recognizing that real-world field conditions often deviate from laboratory-perfect environments, the study conducted a critical moisture sensitivity analysis. By intentionally varying the moisture content to 8.77% (representing an OMC + ~2% condition), the research observed a detrimental shift in material behavior. The MDD decreased to 1.972 gm/cc and more significantly, the average soaked CBR value witnessed a substantial decline to 10.33%. This comparative analysis quantifies a moisture-dependency factor, revealing a nearly 19% reduction in the structural bearing capacity of the subgrade due to a marginal 2% increase in moisture. Furthermore, the research delves into the correlation between static compaction methods and the resulting internal matrix interlocking, providing insight into why dense, well-graded granular materials react strongly to moisture fluctuations. These findings underscore the imperative necessity of implementing stringent field moisture monitoring and strict quality control protocols during the construction of subgrade and blanket layers. Ultimately, this thesis provides a technical framework for optimizing subgrade design by demonstrating that the target CBR is not merely a material property, but a function of precise moisture management.