HDD stands for horizontal directional drilling.  It is a minimal impact trenchless method of installing underground utilities such as pipe, conduit, and/or cables.  These products are installed in a relatively shallow arc or radius along a prescribed underground path (that runs generally parallel to the surface of the ground), using a surface-launched drilling rig.  This technique is routinely used when conventional trenching or excavation is not practical or when minimal ground surface disturbance is required. Horizontal Directional Drilling is a surface-launched trenchless technology for the installation of pipes, conduits, and cables.  HDD creates a pilot bore along the design pathway and reams the pilot bore in one or more passes to a diameter suitable or the product, which is pulled into the prepared bore in the final step of the process.  (NASTT HDD Good Practices Guidelines)