IIT Mandi researchers develop sustainable methods of soil stabilization using bacteria

NEW DELHI: Researchers at Indian Institute of Technology (IIT), Mandi, have developed sustainable techniques for soil stabilization using a harmless bacteria called S Pasteurii that hydrolyse urea to precipitate calcite. The process does not involve hazardous chemicals, and natural resources can be used sustainably. The study will help in designing microbial methods to improve soil shear strength at the field scale to protect the soil from erosion in hilly areas and during geo-disasters, the researchers claimed.

The findings of the research team have been recently published in the journal "Geotechnical and Geo-environmental Engineering of the American Society of Civil Engineers (ASCE)". Soil stabilization is the process of conferring long-term permanent strength to the soil by artificial means. It is used when construction work must be carried out on unstable grounds or to protect soil from erosion.

Traditionally, mechanical processes such as compression and chemical processes such as the injection of chemical grout fluids into the soil are used for soil stabilization. According to the team, in the past decades, an eco-friendly and sustainable soil stabilization technique – Microbial Induced Calcite Precipitation (MICP) – is being investigated worldwide. In this method, bacteria are used to produce calcium carbonate (calcite) within soil pores, which cements the individual grains together, thereby enhancing the soil strength.

"Our study will be helpful in designing microbial methods to improve soil shear strength at the field scale, to protect the soil from erosion in hilly areas and during geo-disasters. We are also working on the microbe-driven production of construction material from quarry waste," said KV Uday, assistant professor, school of engineering. "While there are studies worldwide on the development of MICP techniques for soil stabilization, the factors that affect the efficacy of the process are still not completely understood," he added.

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The researchers used a harmless bacteria called S Pasteurii that hydrolyse urea to precipitate calcite. The use of urea is particularly attractive because it does not involve hazardous chemicals and natural resources can be used sustainably. Their experimental setup consisted of a column of sand through which was percolated a mixture of the bacteria and cementing solution comprising urea, calcium chloride, nutrient broth, etc.

"The Calcite Precipitation Efficiency (CPE) depends on a number of parameters including and not limited to the concentration of the cementing solution, its flow rate through the column, the supply rate, applied pore volume, and the sand grain characteristics. We set out to understand the effects of various parameters on the CPE,"said Deepak Mori, research scholar, IIT Mandi.

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Since there were numerous parameters involved, researchers used the Taguchi method to analyse the effects of these various parameters on the strength enhancement of soil by MICP. This method involves the use of orthogonal arrays to organize the parameters affecting any process and the levels at which they should be varied. The Taguchi method enables the analysis of many influential parameters without the corresponding need for many experiments. "The team found that the amount of calcite formed is not as important as the size and location of the calcite grains formed in the pores during the process.

The higher cementing solution concentration resulted in higher strength improvement. Similarly, the flow rate and the supply rate of the cementing solution also affected the enhancement of strength. "There was an optimum value for these parameters that produced maximum calcite deposition and thereby best strength enhancement," Mori said.