Study on hydraulic conductivity behaviour with regard to soil characteristics in landfill performance

Municipal solid waste generation continues to increase in Malaysia, placing greater demands on landfill containment systems to effectively prevent leachate migration and protect groundwater resources. The engineering performance of compacted landfill liners depends largely on the hydraulic conductivity achieved after compaction, which is governed by the combined influence of soil physical properties and compaction behaviour. However, the hydraulic performance of landfill-sourced residual soils in Malaysia remains insufficiently understood, particularly regarding the interaction between soil characteristics and hydraulic conductivity under realistic moisture and loading conditions. Therefore, this study investigates the relationship between hydraulic conductivity and residual soil characteristics obtained from the Tanjung Dua Belas (T12) and Air Hitam (AH) landfill sites to evaluate their suitability as compacted landfill liner materials through the development of a preliminary empirical hydraulic conductivity prediction model. Laboratory investigations were conducted in accordance with BS 1377:1990 to determine the physical properties, compaction characteristics and hydraulic conductivity of the investigated soils. Both soils were classified as Silty SAND, with the Tanjung Dua Belas soil exhibiting higher plasticity and moisture sensitivity, whereas the Air Hitam soil exhibited higher particle density. Standard Proctor compaction tests established the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD), followed by specimen preparation at 3% below OMC, OMC and 3% above OMC to simulate field moisture tolerance. Tanjung Dua Belas achieved an MDD of 1.50 g/cm³ at an OMC of 18.56%, while Air Hitam attained an MDD of 1.88 g/cm³ at an OMC of 11.75%. Hydraulic conductivity was evaluated using a modified oedometer under incremental vertical loading from 25 kPa to 200 kPa. At 200 kPa, hydraulic conductivity ranged from 1.241 × 10⁻⁸ cm/s to 3.256 × 10⁻⁸ cm/s across the investigated moisture conditions. Specimens compacted wet of optimum consistently exhibited lower hydraulic conductivity than those compacted at optimum and dry of optimum, while the Air Hitam soil recorded lower hydraulic conductivity than the Tanjung Dua Belas soil due to its denser compacted structure and lower air void content. Statistical analyses using Microsoft Excel and Minitab, including descriptive statistics, Pearson correlation, regression analysis and paired t-test validation, confirmed that hydraulic conductivity is governed by the combined influence of moisture condition, dry density, particle density and air void. Regression analysis further produced a preliminary empirical prediction model based on the air void parameter, integrating dry density, moisture content and specific gravity into a single predictive relationship for the preliminary estimation of hydraulic conductivity. All measured hydraulic conductivity values satisfied the commonly adopted landfill liner requirement of 1 × 10⁻⁷ cm/s, confirming that both investigated residual soils are suitable for landfill liner applications, with the Air Hitam soil demonstrating comparatively superior hydraulic barrier performance. Overall, the findings provide valuable site-specific geotechnical data for Malaysian landfill residual soils, establish the engineering relationship between soil characteristics and hydraulic conductivity, and provide a practical preliminary predictive approach to support material selection, compaction control and landfill liner design.