A pipe assembly has a hollow inner member nested within a hollow outer member. A series of pipe assemblies are connected end-to-end to form a dual-member drill string useful in horizontal directional drilling operations. The drill string has mutually exclusive first and second fluid paths. Fluid seals, interposed between adjacent inner members of the string, isolate the first fluid path from the second fluid path. Compressed air is delivered into the first fluid path from above ground level, and routed to an underground boring tool. The expelled air and spoils are returned to above ground level by way of the second fluid path. One or more baffle elements are supported on the drill string adjacent the boring tool. The baffle elements are configured to prevent compressed air and spoils from flowing between the walls of the borehole and the drill string.