بهبود خواص مقاومتی و نفوذپذیری ماسه‌بادی‌ به روش بیولوژیکی

Due to population growth and land scarcity, especially in big industrial cities, many ground improvement projects are required annually for new developments. Moreover appropriate ground improvement techniques are also required to encounter dust storms and desert expansion which are common environmental problems in many countries. Thus looking for more efficient and comprehensive methods in the field of soil improvement seems to be an essential necessity. Although a lot of improvement techniques are in use around the world, they have their own advantages and disadvantages. Chemical, physical, mechanical, biological and electrical techniques may be named as the common methods of soil improvement. Some of the methods, particularly those using cement and other toxic chemical grout, may cause environmental problems which limit their usage. The biological stabilization seems to be a promising technique to control the expansion of dune sand deserts and in turn encountering the problem of dust storms. This paper reports the likely potentials of application of biological treatment on dune sand samples taken from Kerman deserts. As an environmental friendly method, biological improvement presents an innovative, novel technique in which microorganisms present in the natural soil are employed to initiate biochemical processes leading to deposition of calcium carbonate. This procedure bonds soil particles to each other and improves soil physical and strength properties. Microbial-induced calcite precipitation (MICP) is an innovative technique that harnesses bacterial activities to modify geotechnical properties of soils. In microbial-induced carbonate precipitation method, urea is hydrolyzed by bacteria and calcium carbonate precipitate is formed by a network of biochemical reactions. The bacteria acts as a biochemical reaction network controller and so power supply of bacteria is very important. Nutrients needed by the bacteria are CO_2,N,P,K,Mg,Ca,Fe generally. In this research study a hybrid microorganism was prepared in the laboratory and injected into cylindrical sand samples of 100mm length and 47mm diameter. In this context, mixed culture performance was compared with that of single culture bacteria in terms of the treatment efficiency regarding strength enhancement of dune sand samples. Sporosarcina urea bacteria was used as single culture and Sporosarcina urea+Bacillus subtilis were used as hybrid culture. Cementation solution by dissolving 1 mole of urea and 2 moles calcium chloride per liter of distilled water were prepared. Unconfined compression test results as an indicator of the strength properties showed that the microbial-induced carbonate precipitation method increases the unconfied strength of samples. Unconfied strength of the improved samples by single and mixed culture were obtained 527.7 kPa and 771.8 kPa, respectively, that these amount is 16.6 and 23 times of unconfied strength of sandy samples. Falling head test results as an indicator of the physical properties showed that the microbial-induced carbonate precipitation method decreases the permeability factor of samples. Permeability factor of the improved samples by single and mixed culture compared to sandy samples has decreased 50.5% and 60%, respectively. increasing unconfied strength and decreasing permeability factor of improved samples by mixed culture to single culture is for this reason that Bacillus subtilis increases urea hydrolysis rate and the rate of precipitation of calcite. Finally precipitated calcium carbonate has been shown by SEM.