HDD Construction Challenges up a Steep Slope

This paper presents a case history of an HDD construction project with 50 feet of elevation difference between entry and exit locations. This paper will present pre-construction challenges, present lessons learned during construction, and suggest future design strategies to mitigate the challenges encountered during this project. This HDD project involved installation of approximately 500 ft of new 16-in. diameter HDPE water transmission line. The HDD bore started in a field on the low side of a 2-lane state highway, went under a gas line in the near shoulder of the highway, and started the new water line with a tie-in to the existing water line in the far lane of the highway. From the point of tie-in to the existing system, the HDD alignment traversed up-slope and crossed under a ditch line on the far side of the highway, a wetland mid-slope, and a rockery farther up-slope. Near the top of the slope, the HDD alignment went between two houses, followed a private drive, and exited into a cul-de-sac. The original design documents (designed by others) showed the HDD stopping midway up the private drive and transitioning to open cut at a 22.5-degree bend and thrust block. The contractor and the contractor’s engineer (the author) proposed post-award design modifications to deepen the alignment, lengthen the installation, and realign the installation. The alignment was deepened to provide greater depth of overburden to reduce the likelihood of inadvertent fluid returns, the installation was lengthened to eliminate the 22.5-degree bend and thrust block in a private driveway, and the alignment was moved laterally to avoid another private driveway that bisected the field at the HDD entry side. During construction there were unexpected challenges. These challenges included: Drilling fluid was lost to the surface in the wetland area due to the presence of springs that fed the wetland, there were steering challenges because of loose soil at the vertical curve to turn into the cul-de-sac, and there were areas of borehole collapse. Based on these construction challenges, the design strategies that will be discussed include communicating construction concerns and design limitations prior to construction, discussing and agreeing on contingency plans prior to construction, and the importance of obtaining permits from key stakeholders during design.