In early August DTD completed a HDD crossing of Slab Cabin Run in State College, Pennsylvania. Slab Cabin Run is a tributary to the famous trout fishery of Spring Creek. The crossing is part of a much larger innovative system (Click
The project has been in the planning for over 10 years and has the goal of allowing the UAJA to meet the treatment needs of the growing community while not only minimizing ecological effects, but improving the local ecosystem through the creation of new wetlands and stream augmentation.
The HDD crossing underneath Slab Cabin was particularly difficult because of the local geologic conditions. The small stream is underlain by the Bellefonte Formation (a very hard limestone) that is difficult to drill due to its extreme hardness (>25,000 psi), fractured nature, and pinnacled bedrock surface. The bedrock either outcrops at the ground surface or is overlain by soils varying from clay to cobbly gravel. Test pits near the entry and exit of the bore showed the top of rock in along our bore path varied from 2′ to 5.5′. And while no test pits were dug at the stream, if the bedrock surface was consistent, the depth to rock was about 3′ below the stream.
The 140′ bore was completed with a Vermeer 2440, and was drilled with water. The water minimized the negative effects that could otherwise be created by the loss of clay-based drilling fluid into the stream (inadvertent returns). The bore was designed to trend downward until encountering the top of rock, and then across. As drilled, the bore passed beneath the stream at a depth of 4.9′, directly on the top of rock. The 6″ pilot was then reamed to 10 inches in diameter, and the 8-inch HDPE pipeline pulled back.
The project duration was less than 24 hours, and the bore completed with no inadvertent return to Slab Cabin Run. The HDD avoided the potential damage to the wetland and stream that would have been created by open trenching.