Improvement of sand re-liquefaction resistance by microbially induced calcite precipitation under cyclic undrained triaxial loading conditions

Re-liquefaction of sandy ground during successive earthquakes can cause extensive damage to infrastructure. This study investigates the potential of microbially induced calcite precipitation (MICP) as an environmentally sustainable technique to enhance the re-liquefaction resistance of sand. Cyclic undrained triaxial tests were conducted on Toyoura sand specimens with and without MICP treatment to examine the changes in liquefaction resistance through three consecutive liquefaction–reconsolidation cycles under an effective confining pressure of 50 kPa. The MICP-treated specimens exhibited a nearly uniform calcite precipitation ratio of approximately 2%. The test results show that MICP treatment significantly suppressed double-amplitude axial strain compared with untreated sand, with the difference becoming more pronounced after repeated liquefaction. While cohesion increased in the treated sand, the internal friction angle showed little change, indicating that the improvement in strength was mainly due to interparticle cementation by precipitated calcite rather than frictional effects. Even when the excess pore pressure ratio reached 0.95, MICP-treated specimens exhibited limited deformation, suggesting that not only densification after the liquefaction, but also calcite bonding maintained the integrity of the soil skeleton structure under nearly zero effective stress conditions. The cyclic resistance ratio corresponding to 20 loading cycles (RL20) increased through repeated liquefaction in treated specimens, whereas it decreased in untreated ones. These findings demonstrate that MICP treatment effectively enhances not only the liquefaction resistance but also the re-liquefaction resistance of sand by densification of pore structure and stabilizing interparticle bonding.