Horizontal Directional Drilling (HDD) is increasingly being used as a technique to install pipeline through challenging conditions. With this increased use, several post-installation geotechnical issues have, quite literally, surfaced, often many months or years following the original installation. These issues include sinkhole development around the entry/exit points for the HDD operations and settlement of the surface above the HDD bore path. Both can be attributed to two major factors, those involving unfavourable ground conditions and those involving problematic installation procedures. Several examples of each of these factors are described along with mitigation measures designed to prevent both sinkhole and settlement from occurring following HDD installation. Case histories from two large HDD crossings are subsequently presented which illustrate the potential magnitude of these issues and the steps that are often required for repair; the first from a crossing of the Fraser River outside of Vancouver, Canada, and the second from a crossing of a major river in north-central Argentina. In both of these cases, large sinkholes formed behind the HDD exit points, resulting in property damage and possibly threatening neighboring utilities. Site investigation and design techniques implemented to minimise the potential for sinkhole development and settlement are discussed, and several remediation options used in the cases histories are presented.

In August and September of 2003, Terasen Pipelines (Trans Mountain) Inc. successfully completed the Horizontal Directionally Drilled [HDD] replacement of it’s NPS 24” pipeline crossing at the Fraser River from Surrey to Coquitlam in southwestern British Columbia. The pipeline replacement was necessary to mitigate the high seismic risk from liquefaction induced lateral spreading of soils on the north and south shores of the Fraser River. The high level of urban development and the close proximity of major linear infrastructure on both sides of the river created spatial restrictions which increased the complexity and risk of the HDD crossing. Measures to reduce construction risks and limit Terasen’s exposure to claims for changed subsurface conditions, environmental damage due to frac-outs, delays, pipe and coating damage, and inability to complete the crossing, were applied. Important strategies used during planning and construction of the crossing included: proper selection of the pipeline route and laydown corridor, site specific geotechnical investigation, HDD annular pressure monitoring, HDD electronic drilling recording, and a contractor pre-qualification process. The final result was that the contractors successfully drilled and installed the 1293m long HDD crossing over a period of one month, without incident. This paper presents the challenges and solutions implemented by the project team to bring the project to its successful conclusion.