Essential Upgrade: Slave Lake Raw Water Intake Part of Wildfire Recovery Efforts
The Slave Lake region is located within the Canadian boreal forest, approximately 250 km north of the City of Edmonton, Alberta. The region is home to more than 9,000 people within a mix of rural and urban (Town of Slave Lake) development. In 2011, the region was hit with one of the worst wildfires in Canadian history. The fire resulted in significant widespread damage within the Town and surrounding area.
As a result of the wildfire, the Town of Slave Lake, Municipal District of Lesser Slave River and Sawridge First Nation created the Tri-Council Recovery Coordination Group to collectively plan and implement the needed recovery efforts for the area.
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Water Supply Improvement
An assessment was completed on the water infrastructure for the area and various items were identified for upgrading. These upgrades were recommended to increase the system’s redundancy and resilience, improve fire protection capacity and reduce the risk of contamination of the raw water supply.
The raw water supply was identified as being at risk primarily due to its location. The intake, located at the discharge of the Lesser Slave Lake, was historically prone to sedimentation effectively cutting off supply during low lake levels. The area required dredging on several occasions to ensure supply to the intake and water to the community. Additionally, the intake occasionally experienced poor water quality due to sediment, algae growth, and risked contamination from recreational and transport contamination. Replacement of the intake was identified as Phase 4 of the recovery effort program.
Selection of the intake’s new location was decided after water quality was measured at various locations within the lake. A total of nine sites, four primary and five secondaries, were monitored over a 14-month period. Locations were assessed on water quality, construction feasibility, access and workspace, land availability, operations and maintenance and costs. The design included two intake lines, to provide redundancy, and a new pump station capable of using one intake at a time or both simultaneously.
The location of new intake and pump station was selected to be within a small community roughly 18 km outside of the Town of Slave Lake. The dual 550 mm diameter high density polyethylene (HDPE) lines extended 580 m into the lake to maintain adequate depth below ice levels.
Installation Method Selected
The design team developed two options, horizontal directional drilling (HDD) and open cut using coffer dams and requested contractor bid on their preferred method. During tendering in 2017, two contracting teams submitted bids for the HDD option, and one submitted for the open cut. The project was awarded to an HDD contracting team to complete the installation.
Use of HDD was an obvious choice to limit the disturbance within the lake. Ground conditions within the geotechnical investigation showed alternating clay/clay till layers with a large sand seam along the alignment. To mitigate the risk posed by the sand the decision was made to design the installations to a length of around 340 m and complete the remaining portion of the intake pipe by laying HDPE pipe along the lake bottom with concrete weights. An alternate option for open cut, including coffer dam construction, was developed to allow contractors to select the best method for the installation.
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During the design, permits and approvals were gathered which would allow the successful contracting team to choose the time of year they wished to complete the work. This would potentially include working on the frozen surface of the lake to stage and prepare for the HDD or weighted pipe sections. While initially planning for the work to be done within the winter months, the contractor elected to complete the intake work in the summer due to concerns related to shifting ice during high winds.
Preparation for summer construction started with the building of a temporary dock where a barge would load to move materials. The barge also served as a base for the divers while on the lake. Equipment and materials included reamers, pipe weights and a mini excavator used to lift the drill string in and out of the water for tooling changes. Pipe string preparation was completed with the two HDD and two weighted pipe sections being pulled onto the lake and anchored on the shore.
Working on the Lake
One of the biggest challenges of the project was working with the weather. Weather on the lake could change rapidly and maintaining the barge as a working site on the water was a challenge. One of the environmental challenges of working within the lake was the use of silt curtains, which were needed to surround the exit area and contain the turbid water generated during drilling. On days of high winds, 2-m high waves would pull the curtain away from the working area and made it a challenge to navigate the barge. Days were lost retrieving and placing the curtains during the drilling despite the contractor’s best efforts to secure them with concrete anchors.
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Prior to drilling, marker buoys were placed along the alignment to give the driller the line needed while locating. Steering records provided by the driller showed that despite the challenges of locating and steering within the water, the alignment was maintained quite well.
Drilling proved to be relatively straight forward despite the challenging ground conditions and some issues maintaining circulation. The tooling change process within the water was especially challenging with the turbid conditions within the isolation zone of the silt curtain. The turbidity lengthened the time for locating and connecting of the drill stem to be lifted out of the water. Having the proper equipment on the lake was essential to working as efficiently as possible.
Pipe pull was successful for both installations without concerns or issues with boats controlling the pipe on the lake. The weather and lake conditions were calm for the pulls reducing the risks to the installations, as well as the boats controlling the drag sections. The contractor staged the work such that the second pipe pull was completed while the weighted pipe was also being installed. Once the weighted pipes were connected to the HDD installations the intake screens were floated out, lowered and leveled.
This project is an example of how trenchless technology can be used to help municipalities upgrade their infrastructure.
Chris Lamont P.Eng. is a project engineer/trenchless designer at Associated Engineering.