Horizontal Directional Drilling (HDD) technology can be used to conduct ground investigation programs along the planned alignment of a tunnel. Since HDD is conducted along a horizontal orientation, it can provide geomechanical and geophysical data along and within the entire length of the excavation trajectory. The geomechanical data can be generated as fast as the hole can be drilled without additional operations or equipment. This information can be used for soil and rock characterization in traditional geological terms, but it is also directly transferrable for use in TBM design and production estimates. Following completion of the borehole, the HDD rig can run a broad suite of geophysical instruments through the hole. This paper explores the geomechanical data that can be generated using HDD and how that data can be used directly in TBM design and production estimates. An overview of the operational procedure for conducting the geophysical program is provided. Finally, the economics associated with conducting a ground investigation program using HDD is presented.
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