Analytical assessment of microbially induced calcite precipitation for liquefaction mitigation in Indonesian volcanic ash deposits: seismic demand modelling, cyclic resistance improvement, and life cycle environmental comparison

Indonesia hosts over 130 active volcanoes, producing widespread volcanic ash and lahar deposits that exhibit extreme liquefaction susceptibility under seismic loading. The 2018 Palu earthquake (Mw 7.5) triggered catastrophic liquefaction-induced lateral spreading in Sulawesi, causing over 4,300 fatalities; geologically similar volcanic-origin deposits underlie densely populated settlements in East Java proximal to Semeru, Merapi, and Kelud. Conventional ground improvement methods -- vibro-compaction, stone columns, and cement deep soil mixing -- carry significant carbon burdens and logistical constraints in remote volcanic terrain. Microbially induced calcite precipitation (MICP) offers a bio-mediated alternative in which ureolytic bacteria precipitate calcium carbonate at grain contacts, increasing intergranular bonding and cyclic resistance. This study presents the first analytical assessment of MICP-based liquefaction mitigation specifically applied to Indonesian volcanic ash deposits, integrating: (i) site characterisation from published geotechnical investigations of Semeru and Merapi lahar deposits; (ii) liquefaction triggering analysis using the Boulanger and Idriss (2014) SPT-based simplified procedure with hazard parameters from the Indonesian National Seismic Code (SNI 1726:2019); (iii) MICP cyclic resistance improvement modelling from published CRR multipliers; and (iv) life cycle environmental comparison against four conventional methods. For saturated loose volcanic ash (specific gravity 2.50, void ratio 1.30, fines content 10%, water table at surface) subjected to East Java seismic demand (PGA = 0.3-0.4g, Mw = 7.5), computed factors of safety against liquefaction range from 0.18 to 1.10 for untreated deposits at SPT-N = 4-20 at PGA = 0.4g. MICP treatment achieves FS ≥ 1.2 for SPT-N ≥ 15 at PGA = 0.4g with 2.2-4.4% calcite content, but is analytically insufficient (≤5% calcite required) for very loose deposits (SPT-N ≤ 12), identifying a feasibility boundary with direct design implications. A practitioner design chart quantifying required calcite content as a function of initial SPT-N and PGA is the study's primary deliverable. The MICP carbon footprint is estimated at 41.9 kg CO2-eq/m3 of treated ground, which is 3.6 to 6.0 times lower than cement deep soil mixing (150-250 kg CO2-eq/m3) though higher than vibro-compaction (15-25 kg CO2-eq/m3), which is inapplicable in volcanic ash with fines content exceeding 8-10%.