Assessing Feasibility of Hazard Reductions of Fine Grain Soil Using Biocementation in Sistan Plain

Biocement is a method that stimulates native soil bacteria to connect soil grains through a procedure known as microbially-induced calcite precipitation (MICP). It can use microorganisms to produce a tough and renewable building material with the slightest impact on the environment. MICP has been exhaustively explored using different types of bacterial species to enhance the compressive strength of cement composites such as mortar. Sporosarcina pasteurii is the most studied species of Bacillus bacteria for MICP utilizations on cement composites. S. pasteurii are urease-creating bacteria that have the prowess to induce sufficient calcium carbonate precipitation for bio-cementation to materialize in concrete structure. The mechanism of bio-cementation of the bacteria is regulated by urea hydrolysis.

Bacillus megaterium and Sporosarcina pasteurii bacteria were chosen in in this research study with a focus on the Sistan silt soil area (South East of Iran). Bacterial solution consists of Nutrient broth and bacteria. Nutrient broth consists of Peptone, Yeast extract and sodium chloride. In this method, the temperature to be maintained in the oven is 37 for 24 hours for the solution and then the experiment was conducted at a temperature of 17°-20°c atmospheric pressure. This study aimed at achieving a more realistic environmental condition for the application of MICPA control specimen containing commercially bought Bacillus megaterium which was used to compare the MICP efficiency of the indigenous bacteria. A blank specimen (without any bacteria) was subjected to the same temperature, pH and the cementation solutions.
Initially, bacteria was added to the fine soil and was mixed properly, which was followed by addition of the cementation reagent. The amount of bacteria and cementation reagent to be added. Direct shear and ultrasonic tests were performed on the silty soil. Proper mixing was ensured for proper fixation and distribution of bacteria in the soil. Fine Soil was compacted and was tested. Since the treatment duration was a parameter, soil samples of given bacterial and molar concentrations were allowed for curing or treatment duration of 8 weeks. The treatment duration was managed in order to provide sufficient time period for the chemicals to react and further allow CaCO3 precipitates to develop.

Two microorganisms (S. pasteurii and B. megatarium) were proposed for soil improvement by MICP technique. In this particular treatment, fine soil as clayey sandy silt or loam in Sistan plain, S. pasteurii was evidenced to be the most effective microorganism for MICP treatment.
The MICP-treated soil specimen exhibited moderate improvement in shear strength. This improvement is estimated for a specified density of soil specimen. Density on strength improvement by MICP indicated little increase.
The maximum amount was observed for the soil specimens treated with the microorganism S. pasteurii and B. megatarium after the biocementation; the velocity of ultrasonic waves increased and the internal angle of friction in bacterial sandy samples increased as well. Tests were conducted to evaluate the feasibility of using ultrasonic testing for stabilization applications. The results indicated the fact that some calcite forming microorganisms were present in the original soil.

A series of direct shear and ultrasonic tests were conducted to examine the behavior of the biocemented silt specimens of Sistan plain. The tests were directed towards the stiffness of biocemented silty specimens with different agents. The specimens cemented by two different microbial products and non-bacterial soil were prepared and tested under 25, 50 and 100 kPa vertical stresses. The variation of the deviatoric stress with the local vertical stress was registered.
 The shear stress behavior of the specimens significantly changed with different times of microbial products for cementation from the starting point of samples preparation. A significant increase in the shear stress values velocity of Ultrasonic Waves was observed as the duration of biological treatment increased.
The researchers consider that the biological treatment of the specimens under confining pressure plays an important role in increasing the strength and internal angle of friction. The fact is that an increase in the duration of the biological treatment process, which corresponds to the number of urea-calcium chloride cycles, resulted in an increase in the amount of calcite precipitating on the soils grains. These results were verified via increasing velocity of Ultrasonic wave. It seems that the amount of Ca increases in the treated samples as the treatment period increases.
A slight decrease followed by a sharp decrease in the stiffness of biocemented specimen was observed, while a gradual decrease in the stiffness of grain soil was observed. The sharp decrease, due to the bonding breakage, is a distinctive behavior of a typical artificially-cemented silt.
In general, velocities increased with curing time, and the increase from the day of compaction to 7 days was more significant than the increase from 7 days to 21 days. As the curing progresses, further reactions occur between the soils and the stabilizing agents. These reactions generally result in increases in the stiffness of the soils. The velocity of wave propagation increases with the increased stiffness of the soils. Therefore, MIBC can be an effective method for the stabilization of silt soil in Sistan plain.