A reamer is a piece of equipment attached to a directional drill. The purpose of the reamer is to break up an old pipe as it is pulled along by the directional drill or else to enlarge a pilot hole to accommodate the required pipe size. The reamer creates the space for the final pipe to be pulled into position.
Trenchlesspedia Explains Reamer
The reamer is attached to a directional drill on one end and to the replacement or new pipe on the other. The drill pulls the reamer through the old pipe or pilot hole, which is ground up into small pieces by the action of the rotating reamer, while the new pipe is pulled into position behind the reamer. A reamer is normally about 1.5 x the diameter of the pipe to be installed.
Where an old pipe is being replaced, the broken down pieces of pipe are extracted along with drilling fluid by sucking it out of the recovery pit.