TECHNICAL AND ECONOMIC EVALUATION OF SOIL STABILIZATION USING RICE HUSK ASH (RHA) AND NATURAL LIME (NL) AS AN ALTERNATIVE TO CUTTING AND FILLING IN ROAD CONSTRUCTION

Abstract

Pavement construction in areas where expansive clay forms the bulk of alignment soil is expensive. The common practice in Kenya is to remove the undesirable material, and fill the space with more stable materials, the cost for such construction methods are unnecessarily very high. The present study has evaluated the technical and economic viability of stabilization of problematic expansive clays soils using Rice Husk Ash (RHA) and Natural Lime (NL) through experimentation designs and cost benefit analysis for subgrade class S2 pavement construction. The aim of this study was to assess the technical and economic viability of using RHA and NL to stabilize expansive clay soil under Kenyan conditions. Expansive soil specimens from Kitengela, Kajiado County, were stabilized by adding varying quantities of RHA and NL. Chemical and geotechnical tests were performed on untreated and stabilized samples to assess the effect of RHA and NL on the properties of expansive clays. The bearing strength of the clay soil increased significantly from a California Bearing Ratio (CBR) value of 2% to 9% with addition of 20% RHA and 2% Lime. A CBR of 9% met the minimum requirement for a road subgrade layer. Increasing RHA content in expansive clay soils resulted in increased Cation Exchange Capacity. With the addition of stabilizers, free swell dropped from 2% for untreated material to as low as 0.6% which is within specified limits of below 1%. With the addition of 20% RHA and 2% NL the soil Plasticity Index (PI) reduced from 56% to 8%, compared to a maximum allowable PI of 50%. This study demonstrates that: a minimum of 20% RHA and 2% NL will stabilize expansive clay soil for subgrade construction; the cost of Stabilization of Expansive Clay soil (SEC) technology, using RHA and NL, is relatively low compared to Conventional Methods (CM) which involve cutting and filling and the SEC technology can save up to 17% of the cost of earth works in road construction and up to 39% of the cost of sub-grade construction. The study therefore recommends 20% RHA and 2% NL as optimum proportions of stabilizers required to give expansive clay soil the desired engineering properties to support road pavement.