1 Japanese Geotechnical Association for Housing Disaster Prevention, 1622, Oshikiri, Shimizu-ku, Shizuoka 424-0008, Japan.
2 Department of Geotechnical Engineering, Institute of Civil Engineering, Warsaw University of Life Sciences, Nowoursynowska 159, 02-776 Warsaw, Poland.
3 Department of Building, Civil and Environment Engineering, Concordia University, 1455 de Maisonneuve Blvd. W., Montreal, QC H3G 1M8, Canada.
4 Fudo Tetra Co., 7-2, Koami-Cho, Nihonbashi, Chuo-ku, Tokyo 103-0016, Japan.
5 Chubu Sokuchi Research Institute Co., 801-1 Konami, Suwa City 392-0131, Japan.
6 Sanko Kaihatsu Co., Ltd., 1320 Gokanjima, Fuji City 416-0946, Japan.

Research on MICP technology for ground improvement began in the early 2000s, and since then, it has been considered as innovative research. The field of applications is showing signs of expanding from sandy soil stabilization to remediation. However, the research has not always progressed, because it is extremely difficult to evaluate the ability (viability rate) related to microorganisms and how to handle them quantitatively. In fact, this problem hinders the consensus of research results in terms of quantitative evaluation of microorganisms and the cross-comparison (evaluation) and use of MICP technology research. The crucial disadvantage of using bacteria is that their properties are not constant due to changes over time and in the surrounding environment. Therefore, for engineering purposes, we used the carbonate formation rate (CPR), instead of urease activity, as a function of the microbial mass (OD) with viable bacteria. Thus, the standard OD-CPR relationship was defined experimentally, and the estimation method of viability was established. The required amount of microorganisms for testing was given by OD*, and the relationship "OD = Rcv OD*" was defined to convert from OD* to OD. Rcv was defined as the viable bacterial rate. It was found that the Ca²⁺/OD ratio controls the inhibition behavior in MICP. At a Ca²⁺/OD ratio of >8.46 M, then inhibition occurs, while at Ca²⁺/OD = 8.46 M, CPR = 8.46 OD and the CPR is proportional to the viable OD, Rcv, and OD*. We show that it is possible to perform an experiment using OD* with aged bacteria, obtain Rcv from the standard OD-CPR and OD*-CPR relationships, convert OD* to OD and to perform a unified evaluation without actually determining the viability rate.