Definition - What does Pilothole mean?
A pilot hole is a small diameter bore that is first drilled during horizontal directional drilling (HDD) before the hole is enlarged for the pipe installation.
A pilot hole is drilled from the entrance pit to the exit pit and is directionally controlled to cover the desired route. Once the route is established using a pilot hole, the pipe can be laid accurately.
Trenchlesspedia explains Pilothole
Horizontal directional drilling (HDD) is a multiple stage process, the first of which is critical to laying out the route of the pipe from its start point to the destination. It is easier to control a smaller drilling rig and to adjust the direction of the pilot bore than it is to try and execute the entire project in one pass.
Once the pilot bore has been established, the drill string is used to pull a reamer back along the bore to enlarge the diameter.