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