The use of eggshell powder as calcium source in stabilizing expansive soil using Bacillus subtilis

Expansive soils are commonly characterized by low strength and high swelling potential. Thus, to use this type of soil as foundation, stabilization process is needed. This research aimed to improve the stability of expansive soils by using Bacillus subtilis. The bacteria have been reported for its ability to form calcium carbonate precipitation which could bind soils particles, hence, increase the soil strength. In this study, the bacteria were grown in medium containing eggshell as calcium source. Then, the bacterial liquid culture was injected into expansive soil samples, followed by curing period of 30, 60 and 90 days. The swelling potential of the stabilized soil samples were evaluated with free swell index (FSI) test. Meanwhile, the soil strength was analysed with unconfined compressive strength (UCS) as well as direct shear (DS) tests. The results showed that Bacillus subtilis could use eggshell as calcium source to improve the stability of expansive soils. The use of higher bacterial cell concentration and longer curing period showed higher improvement in soil strength. After 90 days of curing period, there was a 30% decrease in the FSI, 74.32% increase in the UCS, and 77.27% increase in the cohesion of soil samples.