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.

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