Sewer relocation over rail right-of-way poses construction challenges
The City of Edmonton was required to relocate over 300 metres of existing 250 mm sanitary sewer that crosses a CN Rail right-of-way to allow installation of new rail bridge abutment foundations. The new abutments were required to extend the span of two existing rail bridges over Highway 16 to make room for a new on-ramp to be constructed under the Province of Alberta’s Northeast Anthony Henday Drive (NEAHD). The NEAHD is part of a major ring road around the city to provide much-needed capacity to Northern Alberta’s highway network; the Gateway to the North. Much of the Anthony Henday Drive has been completed, with the exception of the northeast stretch.
Sewer relocation challenges
Relocating the sanitary sewer was necessary to allow CN Rail to construct their bridge abutments and extend the span of both bridges, a task required before construction of the new on-ramp under the NEAHD project could proceed. The sewer relocation was one of the first components that had to be completed to permit the road construction near the CN Rail bridges. There was significant urgency in relocating the sewer to avoid delays to CN Rail bridge construction and the overall NEAHD project.
Future infrastructure that the sewer relocation had to avoid was still in the design phase, and those designs continued to evolve. Future infrastructure included the CN Rail bridge abutments and expansion to the existing bridge piers (both with deep pile foundations), the future highway on-ramp, storm drainage swale, and several retaining walls and their foundations. The uncertainty about location of future infrastructure meant that there was uncertainty regarding which structures the relocated sanitary sewer had to bypass and selection of a firm alignment for the relocated sewer was difficult. Several design iterations were completed to keep up with the fluidity in the design of other infrastructure by other stakeholders. Decisions had to be made to determine the most feasible alignment and construction method (open trench and trenchless options) in an area with railway and pipeline right-of-ways, a major provincial highway and private land. Coordination with multiple stakeholders and numerous contractors was required to ensure there were no sewer relocation delays that would in turn cause schedule postponements to the rail bridge and NEAHD projects.
A number of alignment options were considered for the relocated sanitary sewer. After much discussion with stakeholders regarding the possible alignments and the realization that there was no ideal alignment for the relocation, it was decided that a temporary sanitary sewer bypass would be constructed. This solution would allow CN Rail to proceed with their bridge construction on time, and would buy additional time for the design team to determine the best permanent alignment and obtain any required easements.
Initially the temporary bypass system was believed to be a relatively inexpensive solution. However, as design proceeded, the problems got more complex and projected costs increased. Eventually, the projected cost and risks associated with the temporary bypass increased enough that the design team considered permanently relocating the sanitary sewer along the alignment where the temporary pipe was designed.
The preferred alignment had significant challenges, including construction under one of Alberta’s busiest highways, close proximity to existing pile foundations under the CN Bridge piers and Hayter Road overpass abutment, a shallow depth at the downstream end, several oil and gas pipeline crossings and limited working space adjacent to the highway.
HDD – the preferred method
Concurrent to the evaluation of the alignment options was the consideration of potential construction methods. Construction using Horizontal Directional Drilling (HDD) under and parallel to Highway16 was preferred for most of the alignment options and was attractive due to the depth of the existing sanitary sewer and relatively small laydown areas available at the upstream and downstream ends of the relocation.
Construction of the HDD section was completed using a pilot hole and one reaming pass prior to pulling the 300- mm high density polyethylene (HDPE) with pressure rating DR11 into place. Drilling was completed from the east end, which was approximately 14 metres higher than the west end. Connecting the relocated sanitary sewer constructed using HDD to the existing sanitary sewer was challenging.
The CN Railyard Sanitary Sewer Relocation posed many challenges throughout the design and construction phases. Perhaps the most significant construction challenge was the proximity to Highway 16, which is the main east-west arterial highway running through Edmonton. Alberta Transportation required that the highway remain open at all times and that only one lane could be closed and only during off-peak hours. These requirements had to be accounted for in scheduling the drilling because the walk-over method was used to track the drill bit and the drill path was located along the north edge of the highway.
The shallow depth of the relocated sanitary sewer also presented a significant risk of hydrofracture, and constructing the downstream tie-in to the existing live sewer was not considered feasible because of the steep pipe slope at this location and resulting high velocity and turbulence. As a result, temporary flow bypassing was used during the downstream tie-in connection. The upstream tie-in to the existing sewer was completed without flow bypassing using a 7 to 10-metres-deep trench.
Another challenge was maintaining the design grade of the relocated sewer during drilling. The relocated sanitary sewer had to be installed at just under a two percent grade (grade is important because it is a gravity sewer) and with several horizontal bends, in addition to the vertical bend that was required at the entry location. The horizontal bends were required for the relocated sewer to bypass south of the existing Hayter Road overpass and CN Rail bridge piers. The drill path was approximately 1.2 metres from the CN Rail bridge pier pile foundation and approximately 2 metres from the Hayter Road overpass pile foundation, so there was little room for error. Highway 16, which the relocated sewer is parallel to and under, is also aligned in a bend at this location requiring additional horizontal bends. No vertical bend was required at the downstream end because the drill path terminated in an open excavation.
The result of this project was that over 300 metres of sanitary sewer was successfully relocated using horizontal directional drilling (HDD) construction technology with short sections of open trench to connect to the existing sewer at each end. The new sewer is located under and parallel to the north edge of Highway 16. Grade was within the acceptable tolerance. Construction was completed on time, allowing CN Rail to proceed with bridge construction. The sewer relocation was completed without disrupting sewer service to the upstream industrial customers and with only minor disruption to traffic on Highway 16.
Copyright 2015 by the North American Society for Trenchless Technology. Original presentation given at NASTT's 2015 No-Dig Show. All rights reserved. www.nastt.org.
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