INNOVATIVE ENVIRONMENTALLY RESPONSIBLE TECHNIQUES OF GROUND IMPROVEMENT STIMULATING NATURAL PROCESSES

摘要

The increase in the earth's population is bringing about the need to build on sites and subsoil conditions that are inadequate in their present state. This need is exacerbated by the gradual depletion of natural aggregate resources. It is therefore becoming increasingly important to improve in situ soil rather than replacing it with imported, more suitable natural aggregate. Established methods of ground improvement have a number of serious drawbacks related to costs, disturbance and environmental impacts (e.g. toxic injection formulae used for grouting, noise and dust production, impact of compaction to plant roots etc). An emerging alternative approach is to stimulate natural biological processes using specific environmentally safe microorganisms as a controlled ground improvement method that can change the physical and mechanical properties of originally unsuitable soils. An advantage of this technique is its non-toxicity, whereas many equivalent chemical substances used for ground improvement (e.g. those based on acrylamides, lignosulfonates, and polyurethane), are toxic and environmentally harmful. The novel technique follows a natural process, therefore consuming less energy and potentially reducing CO2 emissions linked to major construction techniques. It also has the potential of leading to cost effective, non-intrusive, and waste-free processes and solutions for infrastructure-related problems.Along these lines, the present research studied the feasibility of using two different urea-hydrolysing bacteria for the biocementation of different soil mixtures of silt and kaolin clay. The leading idea was that such bacteria would mediate a reaction resulting in the precipitation of calcium carbonate in the form of crystalline calcite on the surface of the soil particles, thus creating a cementation bond between adjacent particles (and hence an increased soil strength and stiffness). In addition to their potential of inducing biocementation the selection of these bacteria was also made on the basis that they would be harmless to the environment. A potentially suitable growth medium for the bacteria was selected and the successful growth of bacteria in time was monitored. A large number of soil samples were prepared at the same moisture content and identically compacted: these included samples that contained only water, and samples that contained the growth medium and each bacterium respectively. The samples were left to cure under different conditions. Consequent tests investigated the undrained shear strength evolution of the soil samples with time, in parallel with the changes in moisture content. Both bacteria show evidence of potential biocementation of the soil, however their response is variable. Further work is being undertaken to verify the present results and to further refine- the experimental procedures, while extending the testing to investigate other salient factors affecting the cementation process.