Bio-improved Soils

BIOGEOS

The given soil’s strength and intrinsic properties are major parameters that guide the decisions during the conception of a construction work. Soil improvement techniques offer engineers a framework of tools that allows interfering into soil’s structure in order to enhance its properties.

A whole new field in geotechnical engineering has therefore been developed aiming at the conception and implementation of soil improvement techniques targeting to specific areas and foreseen improvements. One such technique is cement grouting and it is being implemented in several applications including slope stability works, embankments, marine structures, foundations of typical buildings etc. Nevertheless, cement-grout has aroused concerns mainly around the potentially pollutant chemical agents employed. The foreseen improvement passes through an erosive process entailing the injection of viscous fluids under high pressures, thus destroying the initial state of the soil to replace it with a mixture of cement and grains forming a cement column. The improved area is limited to the diameter of this column and in this sense the technique needs to be repeated multiple times at a given area to scale-up its positive effects.

A bio-cemented sample

Biologically induced calcite mineralization has been recently brought into focus as an alternative cementation mechanism for soils. The whole process lies on the metabolic activity of unicellular microorganisms that are responsible for generating those conditions that allow for the formulation of calcium carbonate crystals to take place. The technique has its base at two chemical reactions; the hydrolysis of urea catalyzed by the enzyme urease, produced by the bacteria strain Sporosarcina pasteurii, and the calcite precipitation. This knowledge is put to use in an emerging grouting technique called microbial induced calcite precipitation (MICP). By temporarily regulating the concentration of bacteria and chemical constituents in a soil, a new engineering material can be generated through the nucleation of calcite crystals inside the soil matrix. Understanding, controlling and predicting this alternative environmentally friendly soil reinforcement technique, exposes innovative applications, such as restoration of weak foundations, seismic retrofitting, erosion protection, seepage flow or pollution mitigation and construction of floating beaches.

The bacteria strain S. Pasteurii that catalyzes the urea hydrolysis

Research around this promising technique at the Laboratory of Soil Mechanics, EPFL, focuses on the conception of a geo-mechanical model to describe the enhanced behaviour of the bio-treated soil and on the adaptation of the teto quantify the resulting bonding effect with respect to the calcite content.