Performance of Red Soil Stabilized with Gravel and Lime in Construction of Low-Volume Roads in Nyeri County, Kenya

Abstract

Red clay soil is abundant in Kenya’s central highlands but is often regarded a poor road construction material. The objective of this study was to stabilize red clay soil using natural gravel and hydrated lime to generate an efficient, affordable and sustainable material for construction of low volume road in Nyeri County. The soil was blended with natural gravel at 20% increments varying from 0 to 100% to give six soil-gravel admixtures. A predetermined amount of hydrated lime was also added to each of the six admixtures. The soil, gravel and lime were subjected to advanced mineralogical and chemical analytical methods, which showed that red soil and gravel consisted mainly of silica at 40.7 and 50.8% respectively while hydrated lime consisted of 72.5% calcium oxide. Kaolinite was also the predominant clay mineral in red soil. The gravel classified as stone Class A after testing for toughness. Three test specimens were prepared and tested in all cases according to applicable standard testing procedures for particle size, specific gravity, consistency limits, activity, free swell, compaction, and California Bearing Ratio (CBR). Both red soil and natural gravel were well-graded according to particle size and classified as laterite soil and lateritic gravel after silica-sesquioxides ratio analyses. Specific gravity dropped marginally for all soil admixtures with increasing gravel content and with addition of lime. Hydrated lime content varied with gravel content from 4.7 to 2.2% that further decreased consistency limits. Red soil had 74.3 % liquid limit and 30.1% plasticity index, and differed from natural gravel with a margin of 36.4% liquid limit, 22.8% plastic limit, 23.5% plasticity index, and 12.4% linear shrinkage. Free swell index ranged between 0 and 6.3% for all admixtures but with a peak of 12.7% in soil-gravel-lime admixtures. All admixtures compacted fairly well and maximum dry density increased by 744 kg/m3 while optimum moisture content decreased by 22.4% with increasing gravel content in red soil. Similarly, the corresponding soaked CBR rose by 121.7% and swelling factor dropped marginally by 0.55%. Red soil classified as MH and A-7-5 after Unified Soil Classification System (USCS) and American Association of State Highway and Transport Officials (AASHTO) system, and as subgrade class S3. The respective classification of natural gravel was GW and A-2-7. The study successfully yielded optimal mixes applicable for most paved roads. This comprised about 80% red soil, 16% gravel and 4% lime for improved subgrade, and 32% red soil, 65% gravel and 3% lime for subbase of low volume sealed roads (LVSRs).