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This study investigated the performance of microbially induced calcite precipitation (MICP) in reinforcing road bases reconstituted with calcareous or silica sands. Four testing models were prepared by the surface percolation method at two cementation levels. A series of tests for California bearing ratio (CBR) and unconfined compressive strength (UCS) were performed to examine the strength of biotreated samples at the model and element scales. Scanning electron microscopy (SEM) tests were conducted on specimens sectioned from the four road base samples to investigate the microstructure of the biotreated sands. The test results showed that the strength of the biotreated road bases increased and then decreased with an increase in soil depth due to clogging in the upper layer. Calcareous sand samples demonstrated better biotreated performance than silica sand samples. The SEM results showed that the calcium carbonate (CaCO3) produced by MICP treatment can coat and bond grains and fill voids, which is more apparent in the upper portion of the bases. The morphologies of the CaCO3 precipitate in the calcareous and silica sand samples were different; this probably resulted from differences in the grain roughness and minerology of the two sands. [ABSTRACT FROM AUTHOR]