Microbially mediated sand solidification using calcium phosphate compounds

To evaluate the potential utility of a new calcium-phosphate-compound (CPC)-based biogrout (CPC biogrout), we conducted unconfined compressive strength (UCS) tests and scanning electron microscope (SEM) observations of sand test pieces cemented with CPC biogrout. The CPC biogrout was produced using (1) soil extracts that contained microorganisms derived from one of two soils, which had different pH values, and (2) one of three amino acids or urea as a pH-increasing reactant. A temporal increase in pH was observed in slightly acidic soil by the addition of ammonia sources. On the other hand, there was no significant increase in pH in slightly alkaline soil except for that due to urea. In most cases, the UCS of the test pieces cemented with CPC biogrout produced using soil extracts from acidic soil along with an ammonia source was higher than that of the test pieces cemented with CPC biogrout produced without the addition of ammonia sources. SEM observation of test pieces with UCS of over 50 kPa showed the presence of whisker-like CPC crystals. These results suggest that CPC biogrout affords sufficient strength as a countermeasure for soil liquefaction and that amino acids can be used as new pH-increasing reactants for CPC biogrout. In addition, they suggest that either CPC biogrout or CPC chemical grout alone, or a combination of the two grouts, can be used depending on the various properties of grounds and soils.

► This study evaluated the feasibility of a new CPC-based biogrout.
► CPC biogrout and chemical grout provide sufficient UCS to counter soil liquefaction.
► Whisker-like CPC crystals were identified by SEM in high-UCS samples.
► Amino acids can be used as pH-increasing reactants for CPC and other biogrouts.
► CPC biogrout and chemical grout can be applied independently or in combination.