When you’re doing a geological investigation for a tunnel, only having vertical or traditional angled core holes only gives you a very limited picture of geology of the rock mass through which you’re going to be tunneling. Even with multiple holes there is a huge potential for missing information, even entire geologic domains that could make a tunneling project a financial and/or technical disaster.
In the September 2009 Trenchless Technology Magazine, there is an article by Rune Lindhjem describing using directional drilling technologies for drilling a geotechnical borehole along the anticipated tunnel alignment to better sample the rock mass and provide information related to rock mass quality and anticipated fluid flow. Apparently the technology has been used quite a bit in Norway and Hong Kong for tunneling projects, and on a few projects in Boston, New York and Detroit in the U.S. The article also describes the application of the technology on a project in Detroit. The author mentions International Directional Services, LLC as a U.S. Company that performs this type of work. (Figure from TrenchlessOnline.com)