Abstract
Most engineering infrastructures, including municipal solid waste containment facilities, are situated in unsaturated zone. The use of chemical additives for soil improvement has been linked to the adverse environmental issues being experienced globally. Research into more sustainable methods of soil improvement has led to the discovery of a novel and innovative method known as microbial induced calcite precipitation (MICP). Lateritic soil was treated with stepped Sporosarcina pasteurii (S. pasteurii) suspension density up to 2.40 x 109 cells/ml at moulding water content (MWC) -2, 0 and +2 % relative to optimum moisture content (OMC) before compaction using Reduced British Standard light (RBSL), British Standard light (BSL), West African Standard (WAS) or Intermediate and British Standard heavy (BSH) energies. Compacted specimens were saturated with cementation reagent in three cycles with 1/3rd pore volume at 6 hours’ interval to induce calcite precipitation at an ambient laboratory temperature of 24±2°C. The soil-water characteristics curves (SWCC) of the specimens were determined using pressure plate extractor over a pressure range of 10-1500 kPa. Unsaturated hydraulic conductivity values were predicted for the S. pasteurii suspension densities considered using Brooks-Corey, van Genuchten as well as Fredlund and Xing curve fitting prediction models. Results show that the unsaturated hydraulic conductivity values of specimens prepared at lower, MWC relative to OMC were generally higher than at higher MWC regardless of compactive efforts and S. pasteurii suspension densities considered. The Brooks-Corey model gave the best predicted unsaturated hydraulic conductivity values which fit the requirements for waste containment facilities over the matric suction range and S. pasteurii suspension densities considered, than the van Genuchten and Fredlund – Xing, when compared with measured values.
Highlights
Geotechnical engineering technology and practice as reported in the literature was developed in the temperate climate where the soils are mostly saturated
In the studies of unsaturated soil, much emphasis was placed on the flow of water in the soil, the constitutive equations developed which have become generally accepted in geotechnical engineering consist of the following : volume change, shear strength, as well as flow in the unsaturated soil (Fredlund and Rahardjo, 1993a) [2]
Some of the key difficulties that exist in the measurement of theproperties of unsaturated soils include unsaturated soil laboratory tests being time-consuming and expensive, problems linked with the measurement of negative pore-water pressure in situ in unsaturated soil, the training of technical personnel to handle the test, etc. (Phoon et al, 2008 ; 2010 [5,6] ; Mahmood and Kareem, 2010 [7] and Wang et al 2015) [8]
Summary
Geotechnical engineering technology and practice as reported in the literature was developed in the temperate climate where the soils are mostly saturated. It was much easier and fruitful to direct a substantial research interest towards problems involving saturated soils with positive pore-water pressure (Fredlund and Rahardjo, 1993b) [1]. The study of unsaturated soils is an essential tool for the analysis of soil behaviour because the soils used in engineering construction are unsaturated (Fredlund and Rahardjo, 1993a) [2]. In the studies of unsaturated soil, much emphasis was placed on the flow of water in the soil, the constitutive equations developed which have become generally accepted in geotechnical engineering consist of the following : volume change, shear strength, as well as flow in the unsaturated soil (Fredlund and Rahardjo, 1993a) [2]. Some of the key difficulties that exist in the measurement of theproperties of unsaturated soils include unsaturated soil laboratory tests being time-consuming and expensive, problems linked with the measurement of negative pore-water pressure in situ in unsaturated soil, the training of technical personnel to handle the test, etc. (Phoon et al, 2008 ; 2010 [5,6] ; Mahmood and Kareem, 2010 [7] and Wang et al 2015) [8]
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