Stabilisation of weathered limestone surfaces using microbially enhanced calcium carbonate deposition

Abstract Surface weathering of limestone may result in rock embrittlement via internal or surficial mineral alterations, potentially triggering serious rock falls, slope failure, etc. Traditional stabilisation techniques have relied on physical approaches such as anchoring and retaining walls, though these can require maintenance and even restoration as their effects are not permanent, requiring the development of novel ground-restoration techniques with self-organising restoration functions and environmentally-friendly properties. For example, bio-mediated calcium carbonates may be useful as a bio-protection material for weathered rocks. In this study, the author used field observations and laboratory experiments to assess a weathered limestone outcrop near the Giza cliff in Okinawa, Japan, found to be widely covered by such material, and examined the feasibility of bio-protection technology based on photoautotrophic microorganisms. In the presence of water, calcium carbonate grew around biofilms and other fibrous materials and on limestone particles; the relevant microorganisms were more active in terms of their growth in water with ~100 ppm of calcium ion concentration than at higher concentrations. The Giza site had naturally optimal environmental conditions for these processes, suggesting that bio-protection techniques based on photoautotrophic microorganisms would be useful for protecting weathering limestone surfaces where surface and/or spring water is present.