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Investigation of the Effects Of Microbially-Induced Calcium Carbonate Precipitation on the Soil-Water Retention Curve and Changes of Small Strain Shear Modulus of Unsaturated Sand During Drying Path

Jahanbakhsh, Yashar | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53351 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Pak, Ali; Khosravi, Ali; Ghobadi Nejad, Zahra
  7. Abstract:
  8. Microbially-induced calcium carbonate precipitation is a low-cost, environmentally friendly, novel ground improvement technique that has received much attention in recent years. Most studies on biological soil stabilization have investigated the mechanical and hydraulic behavior of soils in dry and saturated conditions. However, the behavior of soils stabilized by this method in the unsaturated state should be of immense concern through the extent of unsaturated soils in nature. Therefore, this study aims to investigate the effects of microbially-induced calcium carbonate precipitation on the soil-water retention curve and changes of small strain shear modulus of unsaturated cemented and uncemented sand under mean net stress of 24 kPa during the drying path. For this purpose, a modified triaxial cell was used to inject bacteria into the soil sample and measure the shear wave velocity. Sporosarcina pasteurii, a non-pathogenic soil bacterium, was injected into the soil sample by a peristaltic pump. A pair of bender elements embedded in the pedestal and top cap of the triaxial cell was utilized to examine the soil cementation process. Afterward, when the shear wave velocity reached to its desired value, the desaturation process was performed by a flow pump system, and the values of the degree of saturation and small strain shear modulus at different matric suctions were measured. Finally, the acid washing technique was used to calculate the weight percentage of precipitated calcium carbonate. Moreover, scanning electron microscopy (SEM) images were taken from the cemented soil to observe how calcium carbonate crystals have been formed between the soil grains. The results show that the water retention property of cemented soil is higher than uncemented soil through the formation of calcium carbonate crystals. Besides, the analysis of the results revealed that the air entry value of soil did not change, which can be attributed to the small amount of calcium carbonate precipitation. The experimental results also indicate that, with increasing matric suction, the small strain shear modulus for both cemented and uncemented soil states first increases and after reaching a maximum value at a suction in the range between air entry value and residual suction, follows a decreasing trend with further increases in matric suction. The results also show that although the changes of small strain shear modulus of cemented and uncemented soil against matric suction are similar to each other, the values per each suction are much higher in cemented soil than uncemented soil
  9. Keywords:
  10. Ground Improvement ; Unsaturated Soil ; Soil-Water Retention Curve ; Maximum Shear Modulus ; Microbially-Induced Calcium Carbonate Precipitation

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