Feasibility of a soft biological improvement of natural soils used in compacted linear earth construction

Increasing demand for infrastructures requires innovative, cheap and environmental sustainable practices in construction. The soils which are available on site for linear embankments often need to be improved to satisfy the necessary performance and strength requirements. A bio-improvement is evaluated here, for use in compacted earth construction. To the aim of sustainability and cost reduction, a soft technique was chosen by the industrial party, which consisted in adding bacteria to a superficial soil retrieved in situ, and letting them precipitate calcium carbonate with the aid of the nutrients available in the organic matter of the soil and in the compaction water. The effects of the soft biological treatment on silty clayey sand were studied systematically in a comprehensive laboratory investigation, focused on the properties mostly affecting the performance of earth constructions: compaction energy, water retention, hydraulic conductivity, small-strain shear stiffness, collapse potential and shear strength. Mercury intrusion porosimetry tests and scanning electron microscopy were performed to help in providing a comprehensive picture of the consequences of the soft biological treatment on the natural soil. Lack in artificial nutrients reduces the efficiency of the biological treatment with respect to other cases reported in the literature. Nonetheless, organogenic aggregates and bonds are created during mixing and ageing, as detected from small-stiffness measurement during the curing time lapse. The bio-cemented bonds are mostly broken during compaction, while the aggregated structure remains, and the fine fraction generated by broken bonds ends in acting as filler of some inter-grain and interaggregate porosity. Eventually, the effects of the adopted technique on the hydro-mechanical behaviour of the compacted soil can be described in a coherent picture as the result of bio-filling of an aggregated compacted soil fabric.