Comparative evaluation of hydraulic conductivity of compacted bio-cemented lateritic soil using nature-inspired optimization algorithms

Abstract

Until recently, the use of microorganism to improve the Engineering properties of soil have not received the needed attention of researchers in the related areas.  This study evaluates the laboratory results of hydraulic conductivity (k) of compacted Sporosarcina pasteurii (S. pasteurii) treated lateritic soil for landfill application. Specimens prepared using stepped S. pasteurii suspension density of 0, 1.50 x 10⁸, 6.0 x 10⁸, 1.20 x 10⁹, 1.80 x 10⁹ and 2.40 x 10⁹ cells/ml at -2 % and +4 % moulding water contents (MWCs) relative to optimum moisture content (OMC) and cured for 12 hours at 24 ±2^oC before saturation were considered. A modelling software (Gene Expression Programming GeneXpro tools 5.0) was used to formulate the model equation (fitness function). The objective function was targeted for maximum permissible k value of 1.0 x 10^-9 m/s for landfill. The fitness function with coefficient of determination R² value = 98.22 % was then incorporated into Bacterial Foraging Optimization (BFO), Particle Swam Optimization (PSO) and the Smell Agent Optimization (SAO) codes in Matrix laboratory 2016 version to predict the minimum k values. Scanning electron microscopy (SEM) indicated the presence of calcite precipitate and bio-film which reduced k values. A strong correlation exists between the measured and the predicted k values. Results of the 1^st to 1000^th iteration showed k values lower than the maximum permissible value. The 200^th, 500^th and 1000^th iterations recorded the least k value of 0.00E+00 m/s considered to be the best combinations. The three optimization algorithms considered recorded good outputs in the order PSO > SAO > BFO for the laboratory generated k data. Therefore, it is recommended that compacted lateritic soil - S. pasteurii mixture for all the suspension densities considered be used in landfill and waste containment applications.