Characterizing the Deformation Evolution with Stress and Time of Biocemented Sands

Soil biocementation represents an emerging technique that has dominated the development of sustainable and innovative geotechnics during the past decade. Despite numerous studies focusing on peak strength behavior and the ensuing softening response, less is known about the stress- and time-dependent deformability of biocemented geomaterials even though it remains central for most envisaged applications. This study comprised an experimental campaign on two microbially induced carbonate precipitation (MICP)-treated sands with different initial characteristics. Samples with various calcite contents were subjected to uniaxial and incremental loading and to long-term monotonic loading to evaluate their compressibility and capture the principles of their deformation. Furthermore, the use of the porosity-to-cement ratio, originally developed for artificially bonded soils, is herein evaluated as a parameter to capture and express the behavior of MICP-treated sands. Observations from the incremental loading campaign revealed that for a range of calcite contents between 3% and 8% and for applied stress levels up to 1,000 kPa, MICP treatment significantly enhanced the stiffness properties of the geomaterials and reduced their overall deformability. Medium-grained sand required lower bond contents to achieve a similar compressibility to fine-grained sand and was more compatible with the porosity-to-cement ratio. The effects of time dependency were also assessed under different sustained monotonic loads over a long time ($>75\mathrm{days}$). Under sustained high stresses exceeding the apparent preconsolidation stress, the coefficient of secondary compression reached up to a threefold increase compared with the untreated state. Based on the behavioral characterization, stress and time considerations were shown to be interdependent. The cementation achieved by the treatment shifted a portion of immediate settlement, which was released as delayed deformation after bond breakage took place, depending on loading configuration and bond quality (deposition and imperfections), as determined via microstructural observations.