February 17, 2015The Regional Municipality of Wood Buffalo has expanded its water and sewer major infrastructure network by completing the Saline Creek Plateau Offsite Water and Sewer Servicing project.
The project provides water and sewer infrastructure to service the new Saline Creek Plateau 20,000-person residential development, Highway 69 commercial and industrial development and the Fort McMurray Airport Expansion.
It’s been nearly five years since the project was initiated, which included preliminary design, an extensive geotechnical investigation, risk assessment workshops, detailed design and construction. The project has now made it to the final commissioning stage and is estimated to be completed in 2015.
The overall project scope is summarized as follows:
• 7,000 m of 762-mm Diameter Water Supply Line
– Open-Cut Installation: 4,700 m
– Horizontal Directional Drilling: 2,300 m
– Pipe Ram: 90 m
• 2,000 m of 600-mm Diameter Water Transmission Main
– Open-Cut Installation: 1,000 m
– Horizontal Directional Drilling: 1,000 m
• 3,000 m of 250-mm Diameter and 650-mm/700-mm Diameter Double Barrel Sanitary Sewer Siphon
– Open-Cut Installation: 1,700 m
– Horizontal Directional Drilling: 1,300 m
– Pipe Ram: 90 m
The detailed design and construction phases faced many challenges, including a complex topography, proximity to existing infrastructure and the presence of water courses along the alignment. As a result, the project required extensive use of trenchless installation technology, including pipe ramming, auger boring, and horizontal directional drilling (HDD). The following is a summary of the four key crossings along the alignment:
Hangingstone River Pipe Ram and HDD
The Hangingstone River has a relatively short span, however, the underlying geotechnical conditions consist of a deep layer of sand and gravel overtop of limestone. Telescopic pipe ramming was considered; however, it was determined that large ramming equipment and heavy wall steel pipe was a more effective solution. The ramming equipment was set up 10 m below the existing ground elevation and 6 m below the river elevation. This required an extensive well point dewatering system and shored excavation to protect the river banks. Geotechnical and tie-in limitations required two 900-mm diameter and one 600-mm diameter steel pipes to be installed over a span of 95 m via pipe ram. Initially, the 600-mm diameter steel pipe ram was attempted. However, due to the geotechnical conditions and the unexpected presence of what was thought to be railwayold timbers, the 600-mm pipe ram was aborted and the 450-mm HDPE carrier pipe was successful installed on a tight radius 150 m length HDD.
Saline Creek Valley HDD
The Saline Creek Valley has two steep and unstable valley slopes, which span 600 m in length between the top of the banks. The geotechnical conditions along the valley slope consist of clay till, sand, gravel, silt, clay/silt in varying thicknesses overlying bedrock. The bedrock consists of clay shale, siltstone and sandstone overtop of oil sand. Due to the unstable nature of the valley slopes, combined with the required top of bank setbacks and limiting bend radius geometry of the steel piping, the total crossing length was 1,000 m. Two pipes, 600-mm and 750-mm diameter steel, were installed in parallel over a period of 132 days. The drilling is slow in the oil sands. The thick oilsands material builds up on the shaker screens and slows the entire process. Additives would normally be used for cutting this oil; however, since the crossing was beneath a water course, concern with toxicity to fish, in the event of a frac out, prevented the use of such additives. Pipe string layout and exit pit location was excellent and accommodated the two pipe string and pull back activities very well. The entry pit location was tucked away on a small triangle of municipal land within a commercial industrial development. Although the entry location was confined to a smaller than ideal area, the contractor was able to arrange activities and equipment layout and optimize access in and out with materials and personnel.
Tower Site Auger Crossings
This augured crossing faced challenges with a narrow right of way and the presence of clustered communication towers, requiring the installation of 4- to 70-m length auger crossing underneath the tower guy wires. This auger consisted of two 1,500-mm, 8-m deep on grade (1 percent), auger crossings for the sanitary infrastructure and two 1,050-mm, 5-m deep, auger crossings for the watermain infrastructure. The ground conditions consisted of gravel with large boulders. The boulders presented steering challenges and damage of auger flights and steering head and large rocks were continually removed from the face of the casing.
Clearwater Valley Slope HDD
The most challenging of the trenchless installations was the HDD installation of a 300-mm, 700-mm and 750-mm steel pipes underneath the unstable Clearwater valley slope. The drill path was 80 m below the ground surface, a total of 1,300 m in length and had a 130-m elevation difference between the entry and exit point on the Saline Creek Plateau. Oil sands were encountered for a large portion of the drill path, which posed challenges with maintaining drilling fluid properties and recycling fluid. Deep layers of silts and sands at the entry point required more than 100 m of surface casing and steeper entry angles, for all three drills. The pipeline drag alignment required threading the piping between the top of bank slope to the west and congested communication towers to the east. This necessitated comprehensive planning and placement of additional above-ground support equipment to facilitate tight radiuses. The exit was challenging. To reduce the exit angle and overall height of the pipe breakover, the drill path was flattened. This caused a sinkhole from continual cleaning at the change in drill angle. Pullback operations proved to also be challenging on the larger diameter lines. Pipe momentum, due to the downhill gradient causes the swivel to buckle at the pipe to drill connection. Pull force was added to counter the drill pull and maintain tension.
The challenging geotechnical conditions and varying terrain required designing four multiple pipe trenchless installations with various diameters, lengths, and methods. Prior to this project, the Municipality had limited experience with trenchless installations outside of the typical short span highway auger crossing or small diameter and length HDD. The Municipality is quickly expanding with new development areas that are separated by major creeks and valleys. Trenchless installation of infrastructure within these areas of challenging geotechnical conditions is required to reduce the risk to the infrastructure being installed.
The trenchless installation work was completed by four different, reputable and experienced contractors. This led to the successful completion of the work on time and without major disruptions. To limit risk to the project, the project team decided the drilling contractors must be pre-qualified in advance of public tender to ensure that the work would be completed by experienced contractors. This approach proved to be valuable in retaining competent contractors to complete the work. The pre-qualifying process also allowed the Municipality and its consultants to evaluate the expertise of the contractors based on past performance and personnel and not solely on tender pricing.
The Clearwater Valley slope and Saline Creek Valley piping was installed through rich oilsands deposits that contaminated the drill cuttings. Drilling fluids were contained onsite within temporary holding cells lined with HPDE liners prior to permanent disposal. Considering that the HDD alignment extended the Clearwater River valley from plateau to floodplain without crossing the Clearwater River, the use of a tar control additive was acceptable. This product slowed the accumulation of tar on steel surfaces and shakers, though constant physical washing and replacement of screens was still required. The proposed additives were not able to be used for the Saline Creek crossing and, therefore, the contractor was forced to continually replace screens. The final disposal of nearly 20,000 m3 of contaminated drill cuttings proved to be challenging. However, varying levels of treatment were developed by the project team, owner and contractor to ensure environmental standards were met.
The Saline Creek Plateau Offsite Water and Sewer project has provided the Municipality with the confidence it needed to move forward with future trenchless installations. As a result, additional designs for installations have been implemented on other projects based on the success of this Saline Creek Plateau Offsite Water & Sewer Servicing project. The design effort, coordination and collaboration between the contractors, owner and engineering consultant has created greater opportunities in the municipal market for future large diameter installations.
Owen Mierke, P.Eng, is a project manager for Associated Engineering’s Northern Alberta Infrastructure Division, based out of their Edmonton office.