Cyclic behavior of bio-cemented soils using relatively large grains

Consequences of liquefaction during earthquakes can be disastrous and fatal. During the past decade, bio-mediated methods have gained researchers and engineers' attention, and have been developed to strengthen soils and mitigate liquefaction. Among these methods, a commonly used method ― the microbial induced calcite precipitation (MICP) method ― has proved to be a potential alternative technique due to its sustainability and relatively environmentally friendly character, with lower carbon emissions compared to traditional methods. In most of the existing studies on cyclic behavior of MICP-treated soils, uniformly graded fine sands like Ottawa 50-70, with a maximum diameter of 1 mm and a narrow grain size distribution, were used. As liquefaction can also occur in soils with larger grain size, it is quite important to study the applicability of MICP method to these soils and to know their behavior after treatment. In this study, we used coarser soils with a maximum grain size of 5 mm. Undrained cyclic triaxial tests were applied to MICP-treated and untreated soils. Cyclic stress ratios (CSR) and number of cycles from 0.25 (300 cycles) to 0.3 (100 cycles), 0.35 (100 cycles), 0.4 (100 cycles), up to 0.5 were applied one after another as long as liquefaction did not occur. The results obtained on one of these soils showed that the untreated soil samples liquefied at a CSR equal to 0.25 after 42 cycles, with an axial deformation around ± 4 %. For the MICP-treated samples with 8.6 % calcium carbonate content, cyclic resistance increased slightly to 63 cycles at 0.25 CSR, while the axial deformation was in one direction and relatively lower rate of increment. With around 3 % more calcium carbonate content (11.5 %), the MICP-treated soil was like cement which could withstand a much higher cyclic stress ratio (up to 0.5) with many more cycles compared to the untreated soil.