Improvement of engineering properties of black cotton soil for road construction using PROBASE soil stabilizer
Black cotton soils, also referred to as expansive soils expand when wet and shrinks when dry causing cracks which on re-wetting, bulks out and causes damage to structures built on them. Research conducted on black cotton soils of Maiduguri in Nigeria indicated clay minerals such as montmorillonite with claystones, shales, sedimentary and residual soils. It is believed that the presence of montmorillonite contributes to the high shrink/swell potential because of its many isomorphic substitutions (Mg2+, Fe2+ and Fe3+), high surface area and large cation exchange capacity, hence requiring the use of stabilizing agents to improve the soil engineering properties. Chemical stabilization including PROBASE has been known to strengthen sub-grades and weak road materials by proper mixing of chemical or bio-chemical admixtures with such soils (Wu et al, 2009; Ujjval et al., 2011). This research investigates the use of PROBASE (SH-85 and TX-85) stabilizers on black cotton soils from Kitgum – Orom Road (Kitgum District) in Northern Uganda using standard laboratory testing methods by applying dosages ranging from 0 to 8% by weight of dry soil sample. The soil samples were subjected to physical and mechanical analysis to determine gradation, plasticity index (PI) and mineralogical composition, whereas PROBASE soil stabilizers (SH-85 and TX-85) were subjected to mineralogical composition analysis to determine its chemical composition. The gradation test indicated class A-7-6 against suitable threshold of A-1 to A-3 confirming that the soil was highly plastic and unsuitable for road sub-grade construction. Analysis of Plasticity Index (PI) for a neat sample gave a value of 43.2% which was way above the acceptable limit of 25% for G15 or 30% for G7 materials. Tests conducted on stabilized samples exhibited a significantly higher mean value of PI (46.80%) for TX-85 and 37.8% for SH-85. The reduction in PI for SH-85 stabilized samples, though not significant, indicated a slight improvement in the soil properties although it remained above the maximum PI value of 30% for G7 materials. On the other hand, the increase in PI for TX-85 treated samples were attributed to cation exchange inside the soil that influences the surfaces of the soil area due to its water-based nature. British Standard light compaction was used to determine the MDD and CBR at one level of compaction (93%). The optimum values of MDD were 1426Kg/m3 for SH-85 and 1399 Kg/m3 for TX-85 whereas optimum values for soaked CBR were 5.85% for SH-85 and 1.1% for TX-85 at 2% dosage respectively. Statistical analysis indicated no significant improvement on MDD and soaked CBR for both SH-85 and TX-85 stabilized samples. A comparative analysis on the effect of SH-85, TX-85 and lime indicated that lime performed better than PROBASE in respect to MDD and SCBR. A chemical analysis of black cotton soils from Kitgum District revealed Iron (51.34%), Calcium (0.05%), Magnesium (0.29%), Aluminium (0.88%), Carbon (4.96%) and Silicon (5.91%). For stability purposes, the ratio of Calcium to Magnesium should be at least 0.5. For this study, the ratio of Calcium to Magnesium was 0.17 indicating that the soil was weak, unstable, high swelling potential making it unsuitable for use as a road subgrade material. Analysis of PROBASE mineralogy indicated Quartz, Calcite, Clay minerals of Sodium Aluminium Silicate, Potassium Aluminium Silicate, Microcline, Aragonite, Halloysite and Vaterite which are key players in cation exchange indicating that PROBASE and black cotton soil can be mixed. PROBASE therefore was not recommended as a suitable stabilizer for black cotton soils from Kitgum District on ground that the results for gradation, PI, MDD and CBR fell below the standards recommended by Gravel Road Design Manual (MoWT, 2010) and General Specification for roads and bridges (MoWT, 2005).