Anthony Smith

Twelve tenon-crowned leg components from Hahn Plastics Ltd’s biofilter floor system were subjected to compressive strength testing in order to investigate if their performance was affected by twelve weeks of exposure to extremely-low pH acid solutions. Of the twelve leg samples, four samples were not exposed to any acidic solutions in order to provide control samples. A further four samples were immersed during the twelve weeks in a pool of acid solution, while a final four samples were sprayed twice-daily for twelve weeks with acid solution in order to create streaks that would run down the sides of the legs, forming slight pooling at the bottom. Upon completion of the twelve week period, all twelve samples were subjected to identical compressive strength testing in a Denison loading frame, in accordance with the methodologies stipulated in BS EN ISO 604:1997.

Microbially induced calcite precipitation (MICP) is an emerging solution to issues faced by geotechnical engineers that has yet to turn its attention to strengthening fine particle clays, including lateritic soil. The lateritic clays found in tropical regions have long been used as a low cost construction material for earth roads linking rural village clusters. However, earth roads are exposed to prolonged tropical wet seasons and become inundated with rainwater, deteriorating their ability to bear traffic. MICP soil strengthening may provide a low cost, sustainable solution that would allow earth roads to remain usable. This paper presents the first phase of geotechnical strength related tests undertaken on a lateritic soil, prior to any MICP treatment, including plasticity index, Proctor compaction, Californian bearing ratio (CBR) and unconfined compressive strength (UCS). They have been undertaken to provide the baseline data against which future MICP treated samples can be assessed. The results indicate that the lateritic sample was a low plasticity clay, which may be prone to turbulent shearing when past its semisolid/plastic limit of 12 %. When tested at 12.5 % moisture content, the values of CBR and UCS fell by 96.4 and 87.4 %, respectively, when compared to samples tested at 7.5 % moisture content.

Lateritic soils are frequently utilised in tropical areas of the developing world as an engineering material in the construction of rural earth roads, usually in the form of engineered natural surface (ENS) roads. The heavy, seasonal rainfalls common to the tropics results in ENS roads becoming quickly saturated with rainwater, and no longer accessible to motorised transportation. Microbially induced calcite precipitation (MICP) has been successfully used as a treatment process to decrease the permeability of clean, cohesionless sands by studies trying to impede the movement of groundwater, and any pollutants they may contain. In order to see if MICP treatment can also reduce the susceptibility of ENS road lateritic soils to rainwater saturation, this study has treated a Brazilian sample extracted from an ENS road in Espirito do Santo, Brazil, using the MICP bacterium Sporosarcina pasteurii contained within a urea-calcium chloride solution inoculum. Investigation, by means of a Rowe cell, of the post-treatment permeability, to untreated control samples, has shown an average decrease in the vertical coefficient of permeability of 83%, from 1.15 × 10-7 m/s for the untreated control samples, to 1.92 × 10-8 m/s in treated samples.