One of the techniques used for the construction of underground infrastructure is horizontal directional drilling (HDD). This trenchless method is complicated when crossing gravel deposits as a borehole in coarse gravel tends to collapse, causing the drill pipe to get stuck or the failure of installation of the product pipeline due to exceeding pull forces. In order to find a solution for the problem of borehole instability, the Biogrout process was adapted for borehole stabilization in gravel. In the Biogrout process, loose sand is converted into sandstone by injection of a dedicated mixture in the underground, which stimulates micro-organisms to catalyze chemical reactions leading to the precipitation of calcium carbonate (CaCO3) crystals. These crystals form ‘bridges’ between the grains, increasing the strength and stiffness of the material. After a first successful test on lab scale in 2008 in which gravel was cemented, a 3 m3 container was treated after which a hole successfully was drilled through it using HDD equipment. Following the success of this container test, two field applications were performed as part of the installation of two 48 inch steel gas pipelines with a length of 600 and 900 meter near Nijmegen NL. During these field applications twice a volume of 1.000 m3 gravel was stabilized in only 7 days each time using the Biogrout technique, after which a HDD was performed successfully.
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