Use of waste streams and microbes for in situ transformation of sand into sandstone

In the BioGrout process, sand is strengthened to sandstone with a strength, which is controllable from 0.3 to 30MPa (unconfined compressive strength) using biobased methods in which calcium carbonate (calcite) is precipitated in situ. The spectacular increase in strength, coupled to a limited reduction in porosity and permeability, makes the method a promising alternative to chemical grouting methods. The product is applicable in many geo- and civil-engineering applications, like strengthening of dykes, the production of underwater reefs or reducing risk of piping. A first generation of the process based on the hydrolysis of urea has been applied on a 100 m³ scale. Denitrification is one of the microbial processes which can be used as a BioGrout process. In this process, calcium nitrate and calcium-fatty acids are converted to form calcite by denitrifying microbes. These organisms are already present in the subsoil in low numbers, but are selectively enriched upon addition of the substrates the required substrates can be produced from chalk, manure and waste streams from food industries or tanneries. When nitrate is completely reduced, nitrogen gas is the only side product, emphasizing the sustainability of this new ground improvement method. In this contribution, the governing principles behind the method are elucidated and applications are discussed.