Seton Storm Trunk: Conventional and Trenchless Installation

The Seton Storm Trunk and Outfall to the Bow River were designed to convey clean stormwater flows from the Rangeview and Ricardo Ranch neighborhoods being developed in southeast Calgary, Alberta, Canada. The Seton Storm Trunk has a total length of 2.3 km with pipe diameters ranging from 900 to 1,500 mm and provides storm outlet servicing for a total area of 614 ha. Approximately 2.0 km of the storm trunk was installed using conventional open-cut installation. The main challenge was when the storm trunk reaches the top edge of the Bow River escarpment where the ground elevation drops more than 50 m in a horizontal distance of approximately 175 m. Several groundwater springs along the toe of the escarpment where it meets the Bow River floodplain make the installation even more challenging. There are also several known areas where soil creep is occurring to the east of the storm trunk alignment, likely due to the shallow groundwater table in combination with surface water drainage paths. A review of available options and a triple bottom line analysis, where economic, social, and environmental issues are evaluated equally, determined that Horizontal Directional Drilling (HDD) was the most viable and cost-effective option that would have the least environmental and social impacts on the escarpment slope. To meet the hydraulic requirements of the project, it was determined that a 900 mm HDPE pipe installed at a slope of just over 14% was required for capacity. At this slope, an energy dissipation structure was required at the bottom prior to the flow being discharged through the outfall to the Bow River. The energy dissipation uses an innovative design that uses hydraulic jumps to dissipate the energy at the bottom of the escarpment. The drill entry was located approximately 135 m back from the top of the escarpment, 60 m north of where the gravity storm trunk connects to the HDD section through a specially designed hydraulic drop structure. The structure and piping include air management features to avoid air entrapment in the pipe, thus compromising the hydraulic capacity of the system. This paper presents an overview of the challenges, both geotechnical and hydraulic, that the design team had to overcome, which resulted in the very complex HDD plan necessary to complete the work.