Investigation into the collapse potential of subbase material in the tropics region case study: the greater Kampala metropolitan area

Abstract

Road construction in tropical regions such as the Greater Kampala Metropolitan Area (GKMA) faces persistent challenges from hydro-consolidation, which can weaken subbase layers and compromise pavement durability. This study aimed to evaluate the collapse potential of subbase materials in the GKMA and recommend measures to enhance their performance for sustainable road infrastructure. Representative samples were subjected to physical-mechanical tests [Maximum Dry Density (MDD), California Bearing Ratio (CBR), Atterberg limits and Particle Size Distribution (PSD)], chemical analysis (chloride, sulphate, and pH content) and conventional oedometer testing under wetting conditions to determine hydro-consolidation behavior. The oedometer test results showed that subbase soils at Natural Moisture Content (NMC) exhibited medium to high collapse potential, exceeding 3% under higher applied stresses, indicating susceptibility to hydro-consolidation when saturated under load. However, samples compacted at Optimum Moisture Content (OMC) displayed collapse potentials below 2%, demonstrating that proper compaction significantly reduces collapse risk. These results confirm that collapse potential is stress-dependent and highlight the importance of achieving optimum moisture content during construction to enhance subbase stability. Based on the findings, the study recommends further evaluation of subbase materials under higher applied stresses to better simulate traffic conditions, expanding investigations beyond the GKMA to capture broader geological variability, and examining other road failure mechanisms such as drainage efficiency and construction practices. Overall, this research provides a practical evidence base on hydro-consolidation behavior of subbase materials, enabling engineers and policymakers in Uganda to design, construct, and maintain pavements that are durable, resilient, and cost-effective in tropical environments.