Performance assessment of on-grade horizontal directional drilling

Although horizontal directional drilling (HDD) has been widely accepted and used for installing gas lines, waterlines, and telecommunication cables, its applications to gravity-flow sewer lines have been limited. One primary reason is the difficulty with maintaining critical grades essential for gravity-flow sewer systems when using the conventional HDD tracking methods. Recently, advancements made in tracking systems, electronics, and operational techniques have made this HDD technology a technically feasible and an economically attractive method for installing on-grade underground pipes in many situations. This paper presents a newly developed HDD technology in terms of its technical capabilities and economic feasibilities compared with the traditional open-cut method on the basis of two real-scale case studies. Installation errors are quantitatively analyzed and discussed, and a general set of boundary site conditions for the application of this new technology is also discussed. The detailed analysis of the two case studies demonstrates that the on-grade HDD method could be a very attractive alternative construction method for gravity-flow sewer pipelines when some favorable site conditions are available.