Abstract: A system and method for emplacing a relatively large casing under and spanning an obstacle such as a river is disclosed. The casing is emplaced along the arcuate path of a previously bored and placed existing pilot string running beneath the obstacle between positions near ground level on opposite sides of the obstacle. A seal is provided between the outer surface of the leading portion of the casing and the reamed pilot hole so that the drilling mud containing the entrained cuttings flows into the leading end of the advancing casing. In one preferred embodiment a washover pipe circumscribing the pilot string serves the dual function of rotating a case-leading reamer and driving a positive displacement pump-typically of the moineau variety-to evacuate drilling fluid and entrained cuttings. This evacuation can occur through the drill string.