GEOTECHNICAL EVALUATION OF BIOCEMENT TREATED LATERITIC SOIL

Abstract

Bio-cementation is one of the newest techniques used in soil stabilization, which involves the use of eco-friendly microorganisms to improve engineering properties of soil. This research focused on the treatment of lateritic soil with microbial–induced calcite precipitate (MICP). Thirty-two Bacillus strains were screened for urease, an enzyme which initiates calcite precipitation and six strains were observed to produce urease enzyme reasonably. Bacillus strain (CE9) had the highest urease enzyme production at pH 7. The unconfined compressive strength (UCS) shows that the lateritic soil specimens treated with urease producing strains (UPS) had more strength compared with the untreated lateritic soil sample (control). Bacillus strain (CE8) at pH 7 had the highest UCS (329.6 kPa) while Bacillus strain (KA7) had the least UCS (223.9 kPa) at pH 7. The best two Bacillus strains:CE8 ( Bacillus spp ) and CE11 ( Bacillus mycodies_MTO47265) with the highest UCS were further tested, singly and combined. The soil specimens were prepared in polyvinyl cylindrical molds of diameter 50 mm and 150 mm height. 120 soil specimens were prepared, in which 30 control specimens were untreated and not cured. 15 of the treated specimens were also cured at varying ages by covering with Polythene bags. This was done to evaluate the effect of age and curing on strength development and permeability in bio-cemented lateritic soil. The dominant minerals present in the lateritic soil used are Quartz (47%), Muscovite (21%), Kaoline (13%) and Geothite (8%). The peak strength attained for cured treated specimens was 497.9 kPa after 28 days of curing. Bacillus mycodies_MTO47265 was found to improve the performance of the tested soil specimens when compared with other strain and the consortium used. The percentage strength increases of 138.33 % was recorded at 3 day of curing age when compared with the control specimen (CON) of the same age. Maximum percentage decrease in the coefficient of permeability of 42.92 % was achieved on 28 days of curing in measure with the value obtainedfor control specimen of the same curing age of 28 days. Increase in elemental concentration of calcium and carbon, after treatment with Bacillus mycodies_MTO47265, was found to be 0.32 and 1.02 respectively. The specimen treated with Bacillus mycoides_MTO47265 shows fully clustered crystals morphology of calcite cementing the soil matrix when compared with that of untreated specimen.