Abstract Innovative soil improvement methods providing more environmentally friendly solutions for liquefaction countermeasure techniques have emerged in recent years. One such technology is the enzymatically induced calcite precipitation (EICP). This study presents comprehensive experimental results to provide better understanding of the undrained cyclic behavior of an EICP-treated sand. Accordingly, undrained cyclic triaxial tests were performed on the EICP-treated sands with testing parameters including particle size of the sand, confining pressure, calcite contents (CC), and degree of saturation during the precipitation. The effects of these factors on the liquefaction resistance and small-strain stiffness are systematically investigated. The results show two underlying mechanisms that help in enhancing the liquefaction resistance of the calcite-precipitated sand. First, the precipitated calcite binds the sand grains, which directly contributes to improving the mechanical properties. However, this beneficial effect of the calcite bonding between the sand particles is lost if the double-axial amplitude is in the order of 0.5%. Second, an enhanced dilative nature is observed because of the relative angularity provided by the calcite crystals or the ratio of the crystal size to the grain size of the sand. The number of cycles required to obtain DA = 5% after achieving DA = 0.5% depends significantly on the relative angularity. It is also confirmed that the amounts of urea and CaCl2 needed to obtain a given liquefaction resistance can be significantly reduced by decreasing the degree of saturation during curing. The shear-wave velocity or small-strain stiffness (G0) of the calcite-precipitated sand increased with increases in the calcite content and confining pressure. Unlike the liquefaction resistance, the effect of the particle size of the sand on the G0 of the treated sand is insignificant. Finally, the relationship between the liquefaction resistance and normalized shear-wave velocity obtained in this study is compared with the empirical one that has been developed and used in practice for evaluating the liquefaction potential of soils in the field. It is found that the existing liquefaction charts are not applicable for assessing the liquefaction resistance of the calcite-treated ground.