The effect of Bacillus Pumilus on the free swell and Atterberg limits of black cotton Soilfor use in road subgrades

Abstract

The study evaluated the potential utilisation of Bacillus pumilus (B. pumilus) induced calcite precipitate for the improvement of the engineering properties of black cotton soil in road construction which is an eco-friendly technique and forms part of what is referred to as green engineering. The effect of B. pumilus on the free swell and Atterberg limits of black cotton soil for use in road subgrades was studied using microbial induced calcite precipitation (MICP) method. Five stepped B. pumilus suspension density of 0, 1.5 x 10⁸, 6.0 x 10⁸, 1.2 x 10, 1.8 x 10⁹ and 2.4 x 10⁹ cells/ml, respectively, were utilized for the treated specimens, while only cementitious reagent acted as the control specimen. Three (3) mix ratios of % Bacteria suspension (B) - %Cementitious reagent (C) (i.e., 25B - 75C, 50B - 50C and 75B - 25C) were adopted. It was observed that free swell treated with 25B - 75C decreased from 70 % and 50 % for the natural and control specimens, respectively, to minimum value of 46 %, with increasing B. pumilus suspension density at 2.4 × 10⁹ cells/ml. Atterberg limits results showed liquid limit values decreased from 53 % and 62 % for the natural and control samples, respectively, to 43 % for samples treated with 25B - 75C at B. pumilus suspension density of 2.4 x 10⁹ cells/ml, while the plastic limit increased from 28.2 % for the control soil sample to 30 % for the 75B - 25C mix ratio at B. pumilus suspension density of 2.4 x 10⁹  cells/ml. Tests results indicated a considerable improvement with 25B - 75C mix ratio at B. pumilus suspension density of 2.4 × 10
cells/ml which significantly reduced the free swell, while the Atterberg limits fell below Nigerian General Specifications requirements of not greater than 35% Liquid limit, 12% Plasticity index and 35% passing No.200 sieve for utilization as road subgrades. However, it could be used as a subgrade material in low-volume roads.