There were a stack of Drainage pipes piled up behind the Visitors’ Center on Isla Nublar near the Generator Building. They were Meter pipes, which were very narrow, barely wide enough for a grown man to fit in. As he was escaping from the Velociraptors, Robert Muldoon managed to squeeze himself into one of the pipes. The pipe was wide enough for him to wield his rocket launcher from within it.
Jurassic Park by Michael Crichton page 308 (First Edition, Thirty Fourth printing)