Effect of Treatment Compositions on the Plasticity of Tropical Red Soil Treated with Bacillus Coagulans. Paul Yohanna 1 *, Kolawole Juwonlo Osinubi 2 , Oshioname Adrian Eberemu 2 , Thomas Stephen Ijimdiya 2 , John Engbonye Sani 3 1. Department of Civil Engineering, University of Jos, Jos, 930001, Plateau State, Nigeria 2. Department of Civil Engineering, Ahmadu Bello University Zaria, 810001, Kaduna State, Nigeria 3. Department of Civil Engineering, Faculty of Engineering, Nigeria Defence Academy, Kaduna, Nigeria E-mail: paulyohanna45@yahoo.co.uk (Corresponding author) Received: 28 December 2021; Accepted: 21 January 2022; Available online: 10 February 2022 Abstract: The study evaluated the effect of four treatment compositions on the plasticity of tropical red soil (Lateritic soil) admixed with Bacillus coagulans (B. coagulans). Samples for Atterberg limits test were prepared using four treatment compositional variables. They include 25% B. coagulans suspension and 75% cementation reagent, (25%B /75%C); 50% B. coagulans suspension and 50% cementation reagent (50% B /50%C); 75% B. coagulans suspension and 25% cementation reagent, (75%B /25% C) with the above three being in equivalent volumes of the corresponding liquid limit(LL) and 50% of the optimum moisture content (OMC) of compaction, to be both B. coagulans suspension and cementation reagent (i.e. 50% OMC B /50%OMC C) of the natural soil. Results showed that the LL for; 25%B /75% C, 50% B /50% C and 75% B /25%C generally increased from 0 up to peak values at 1.8 x 10 9 cells/ml and then declined at 2.4 x 10 9 cells/ml. In the case of samples treated with 50% OMC B/50% OMC C, the LL initially decreased from 0 up to 6.0 x 10 8 cells/ml and thereafter increased significantly. Plastic limit (PL), Plasticity index (PI) and Linear shrinkage (LS), recorded improvement. Regression analysis for the best treatment composition (i.e 75%B /25%C) has regression coefficient of 91.8%. Based on the four treatment compositions considered, 75%B/25%C enhanced the soil workability significantly and is suggested for geotechnical engineering applications such as road pavements that are lightly trafficked. Keywords: Atterberg limit; B. coagulans; Cementation reagent; Lateritic soil; Micro analysis; Regression. 1. Introduction Lateritic soil is a reddish tropical soil found in abundance in Nigeria which in most case is not fit for usage as construction material in its natural form due to high fines content, high water absorption, swelling and shrinkage problems during wetting and drying and so on. Many of such soils are deficient for engineering applications and hence need improvement prior to field application. Past researches [1-4] used well known additives such as cement, agro-industrial waste, lime, and other pozzolanic materials for soil improvement. These practices are either more expensive or some of them are not ecologically pleasant and therefore not viable as a means for soil improvement. Over the years, several techniques of soil improvement have emerged to enhance engineering properties of deficient soils. Van Paassen [5] first reported on a research in Australia someplace bacteria were utilized to reinforce sand and repair monuments. However, further investigations by Australian research group reported that after treatment with bacteria suspension, the sand turned to columns of calcareous sandstone [6]. The bio-cemented columns of sand were then broadly analyzed and recorded a decline in permeability and increase in strengths. This finding gave rise to microbial-induced calcite precipitation (MICP). MICP is a new sustainable and environmentally friendly soil improvement procedure that makes the use of soil micro- organisms (such as B. coagulans) to precipitate calcite through urea hydrolysis. Report of research by Burne and Chen [7] on urea hydrolysis involves a sequence of chemical reactions (see eqs. (1) – (6)) that end result is the formation of ammonium and carbon dioxide. The role of microbes (i.e B. coagulans) is the release of urease enzymes that activate the urea hydrolysis needed for the bio-cementation and bio-clogging of the treated soil. The chemical reaction of urea hydrolysis is shown in eq. (1): ( 2 ) 2 +  2  =2 3 +  2 (1) 56 https://doi.org/10.32732/jcec.2022.11.1.56 Journal of Civil Engineering and Construction 2022;11(1):56-64