We introduce the application of microbial-induced calcite precipitation via the ureolytic soil bacterium Sporosarcina Pasteurii in freeze-dried form, as a means of enhancing overall MICP efficiency and reproducibility for geotechnical engineering applications. We show that the execution of urea hydrolysis and CaCO3 precipitation persist as a “cell-free” mechanism upon the complete breakdown of rehydrated cell clusters. Further, strength and stiffness parameters of bio-cemented sands are determined. Medium-grained bio-cemented sand yields compressive strengths up to 12 MPa while, surprisingly, fine-grained sand yields up to 2.5 MPa for similar bond contents. To understand the observed discrepancies, we undertake a systematic study of the bio-cemented material’s microstructure, by combining a series of microstructural inspection tools. The study extends beyond conventional qualitative and textural characterization and provides with new insight into the material’s peculiar 3D micro-architecture. We apply a new methodology towards quantifying crucial microscopic characteristics such as the particle sizes of the crystalline bond lattice, the bond-grain contacts and particle orientations. Bonds are found to exhibit distinctive geometries and morphologies when MICP applies to different base materials. We thus contribute to the debate on the importance of factors affecting: (i) MICP efficiency, (ii) the mechanical response and (iii) peculiar micro-architecture of bio-improved geo-materials.

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Investigation of the enhanced mechanical response of treated soils through microbially induced calcite precipitation requires extensive understanding of the pore structure. Depending on the applied treatment conditions, the resultant material exhibits distinct changes in the solid phase. In this paper, the focus is on the microstructural characteristics of bio-cemented soils derived from different treatment patterns. The objective is to clarify the predominant fabric features as a function of the adopted treatment conditions. For this purpose, microstructural observations with scanning electron microscopy are used to analyse the structure during the treatment and post-treatment. Mesocrystalsthat is, aggregates of single particlesare identified as a distinct form of precipitate that provides the crucial grain-to-grain contact surfaces. The cemented samples are subsequently subjected to undrained triaxial shear test to investigate the response in three typical cases of bio-cemented samples produced under laboratory conditions: (a) one of very low calcite content (2•5% w/w), (b) one with inhomogeneous distribution of calcite and (c) for the case of homogeneously distributed calcite mass. Improved characteristics are obtained in all three cases. KEYWORDS: fabric/structure of soils; failure; ground improvement ICE Publishing: all rights reserved NOTATION c′ cohesion (kPa) E precipitation efficiency (%) E ur unloading-reloading Young's modulus (MPa) e 0 initial void ratio OD 600 optical density measured at 600 nm p′ mean effective stress (kPa) q deviatoric stress (kPa) ε 1 axial strain (%) σ c confining pressure (kPa) φ′ friction angle (deg) Ω saturation index Ø grain diameter (mm)

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Efficiency of energy piles is strongly affected by soil saturation conditions: low water contents considerably decrease their performance thus limiting the possibility to extend their application to arid environments. This paper investigates the microbially induced calcite precipitation (MICP) technique as a potential means of enhancing the soil–pile heat exchange rates by improving the thermal properties of soil. The study puts the focus on measuring the thermal conductivity of untreated and treaded sand at various degrees of saturation. Experimental results clearly show a significant improvement of the thermal conductivity of soil especially for low degrees of saturation. This enhancement is attributed to the mineralised calcite crystals acting as ‘thermal bridges’ between the soil grains, offering a larger surface area for heat exchange compared with the untreated material in which exchanges occur through smaller contact points.