Evaluation and Design of Horizontal Directional Drilling for Water Transmission Main Installation

Early in the conceptual design stages of a water transmission main design project, areas where standard open-cut installation would be difficult or costly were identified and trenchless methods to install segments of the proposed 400 mm (16-inch) diameter main were evaluated. Three areas along the proposed route were identified as conducive to trenchless installation: a major street crossing which has traffic impact considerations and requires numerous concrete encased utility crossings; a brook-box culvert adjacent to a reservoir and a railroad track crossing. Trenchless methods were also considered for crossing Interstate Highway Route 95. However, it was determined during design that sufficient space inside the existing bridge utility bay would eliminate the need to install the main by trenchless methods at this location. Trenchless technologies considered included horizontal directional drilling, jacking, microtunneling and boring. The use of a single method of trenchless technology was the design objective. This would allow for one contractor to perform the three installations and minimize costs. Space and construction impact considerations resulted in the selection of horizontal directional drilling to install the proposed transmission main in the three locations. The necessity of installing a sleeved main under the railroad right-of-way resulted in an innovative design in which the sleeve would be installed by horizontal directional drilling and the main will be placed inside by traditional installation methods. The sleeve would be shorter than the actual length of directional drill required to install it under the track. However, upon review of the proposed installation by railroad authorities, directional drilling was not allowed at this location and installation by jacking was utilized in final design. This paper will present the methods evaluated, the design issues encountered, utility survey and geotechnical investigations.