CSCE 2021 Annual Conference Inspired by Nature – Inspiré par la Nature
26-29 May 2021
IQALUIT, NUNAVUT - SEWER MAIN REPLACEMENT IN PERMAFROST Crawford, I1, Johnson, K1,4, Plourde, S1, Turner, S2, Harasimo, J2, Sithole, R3 , Lafleur, M1 , and Keung, C1 1
EXP, Canada City of Iqaluit, Canada 3 Colliers Project Managers, Canada 4 ken.johnson@exp.com 2
Abstract: The water and sewer systems in Iqaluit generally consist of shallow buried, urethane foam insulated, high density polyethylene pipe. A trunk sewer through the Lower Base neighourhood conveys sewage from approximately 20 percent of the community, and because of capacity and condition issues, this sewer needs replacement. The replacement is a complex project because of the technical issues associated with the permafrost ground conditions, the existing alignment of the pipes, the remedial work required on the access points to the system, and the logistics of providing temporary servicing. The permafrost ground conditions have an active layer that may extend as deep as 3 metres and produce very wet and unstable ground conditions. The existing alignment of the services includes a water main, a water recirculation main, as well as the sewer main, which should be appropriately separated horizontally and vertically. In addition, all 3 pipes run through insulated metal access vaults. The replacement neighourhood is a commercial district of Iqaluit which will require temporary services for domestic use as well as fire flow. The design development has included options for replacing the water and sewer services within the same trench, and alternately, the development of an independent new sewer system. An independent system was selected as the most appropriate solution, which will allow continuous operation of the existing water and sewer system, provide an emergency bypass for future maintenance works, reduce the risk of watermain contamination and negate the need for temporary services during construction. 1
BACKGROUND
The construction of the accommodation and sealift area of the Crystal II Air Base, now known as the City of Iqaluit, was completed in 1941 as part of the original series of airfields used for ferrying aircraft from North America to Europe along what was called the Crimson Route. The original level of service of this area, referred to as Lower Base, was trucked water and sewage until 1985, when piped services were constructed, which included the piped services from Access Vault (AV) 205 to Access Vault (AV) 211. Shallow buried sewers are a common practice for the construction of piped sewers in Iqaluit and Nunavut. At the time of the installation of the original buried sewer and water system, placing buried services within the active layer was thought to be an appropriate construction approach. However, with time, it was discovered that shallow buried services could significantly deteriorate from freeze-thaw forces experienced within the dynamic active layer, and in several cases destroying the pipe by crushing it and ultimately constricting the flow. In 1995, with only 10 years of service life, the flows within the sections of sewer between AV205 and AV211 were severely constricted and causing significant surcharging in the sewer line.
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Figure 1. Project limits of sewer replacement Access Vault 205 to Access Vault 211 in Lower Base A replacement project was advanced in 1996, which designed the replacement of the 3 segments of the sewer between AV205 and AV211. The execution of the 1996 project only replaced the segment between AV207 and AV208 in Phase 1 of the work. However, due to budget limitations, the second phase including the replacement of segments AV205 to AV206, and AV210 to AV211 never advanced.
Figure 2. Typical water supply service connection in Iqaluit
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Figure 3. Sewer installation in 1995 between AV207 and AV208 with insulated 400 mm HDPE pipe The observations made during the replacement of segment between AV207 and AV208 in 1996 suggested that the ground conditions in the active layer were poor, and special design provisions were needed for future construction in the neighbourhood. These provisions included attention to the dewatering of the construction trench, and supplementary insulation in the trench to make sure that the installed pipe stayed independent from the active layer and was not prone to freeze thaw dynamics of the active layer. 2
GEOTECHNICAL CONSIDERATIONS
Although there does not appear to be any large areas where bedrock or large boulders are present, the terrain of Iqaluit is highly variable and encountering bedrock or large boulders during construction is not uncommon. This is particularly significant in areas of new construction where excavation and blasting of rock may be required to achieve the appropriate depth of installation of the pipe and the AVs. However, no rock was encountered during the installation of the 1985 sewer or the replacement work in 1996. It is also very possible that the soil in the construction area is contaminated with hydrocarbons. This is a characteristic of the entire Lower Base area, as a legacy of the military fuel storage activity in this area associated with the airfield operations almost 80 years ago. Provisions should be made for construction in fuel-contaminated soils to control risks associated with potential removal and remediation work.
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Figure 4. Ground conditions in 1995 with exposed water main, water recirculation main, and sewer main 3
SEWAGE FLOW ESTIMATES
The design sewage flow is one of the most important calculations incorporated into the design criteria. The appropriate sewer pipe sizing is based upon current estimated sewage flows from the upstream neighborhoods of Iqaluit and includes recent developments, as well as the anticipated development within the service area. A conservative approach was applied by estimating a maximum potential loading from anticipated future developments, as well as the addition of existing buildings to the piped service. Future flows were estimated using growth projections, which applied a conservative estimate of future flows based on background information and current knowledge of the City’s development plans. Based on the City of Iqaluit General Plan, three population projections were used in the design criteria (low= 2.04%, medium= 2.87%, high= 3.38%). 4
SEWER DEPTH, ALIGNMENT AND PIPE STRENGTH
Iqaluit‘s municipal guideline recommends a minimum cover of at least 3.0 m from the finished grade to the top of pipe. This is intended to reduce the influence of active layer thawing dynamics acting on the pipe, which can cause pipe deformation and complete collapse due to external pressures developing during freeze back of the active layer. Climate change and the associated changes in the active layer may further influence the stability of buried services in Iqaluit in the future. The existing sewer profile provides only between 2.0 to 2.5 m of cover from the finished grade to the top of the pipe. One major constraint to the possibility of deepening the sewer profile is the downstream connection near AV211, which requires crossing over a 2300 mm diameter culvert. Going under the existing culvert is also not an option for gravity conveyance because there is insufficient grade to connect to the AV 211. Therefore, to maintain minimum grades and hydraulic flow, the invert elevation at AV 211 will dictate the upstream profile of the new sewer. The existing water and sewer mains occupy the south edge of Mivvik Street. Replacement of the sewer along this existing alignment conveys a multitude of constructability issues since this would require working within a very wet area with existing water supply and water recirculation mains in addition to the GEN 451-4
sewer main, as well as multiple water and sewer service connections to the adjacent buildings. An opportunity exists to reduce these construction issues and achieve a significant savings in construction costs by constructing the new sewer main in its own separate trench on the north edge of the street. Along the north edge of Mivvik Street there is sufficient space in the right of way and no conflicting buried utilities that would contribute to the construction issues on the south side. This opportunity could be considered along the entire length of new sewer segments along Mivvik Street or for just a portion of the new sewer segments. The municipal guidelines require the use of DR11 (ratio of pipe outer diameter divided by the wall thickness) polyethylene pipe with a minimum pressure rating of 1100 kPa (160 psi). However, since the depth of burial may be under the recommended 3.0 m, to provide adequate pipe strength, and freeze protection, additional design considerations are necessary which include: installation of DR9 polyethylene pipe with a minimum pressure rating of 1380 kPa (200 psi) complete with 50mm applied polyurethane insulation and FRP jacket, and; the installation of additional board insulation over the pipe-zone to maintain the permafrost within the pipe zone. 5
PIPE ZONE AND GENERAL BACKFILL
In situations where ice-rich or silty soils are encountered within 0.5 m below the specified pipe base, over excavation of at least 450 mm may be necessary to remove ice-rich soil that could thaw and contribute to pipe movement. The pipe zone material for dry trench conditions should consist of well-graded, natural sand and/or fine gravel. For wet trench conditions, a coarser material such as a well-graded, rounded to sub-rounded, natural gravel is preferred to stabilize the trench. The municipal guidelines require a minimum 230 mm horizontal spacing between pipes (outside insulation to outside of insulation) and 300mm from obvert (sanitary) to invert (watermain) vertically from any waterline to allow for proper compaction and to minimize cross contamination. As the new sewer lines require a pipe diameter of at least 450 mm, there may not be sufficient space in the existing trench to maintain the spacing requirements between the proposed sewer and the existing watermains. In addition, in the situations where there are 3 pipes in a trench, namely water supply main, water recirculation main, and sewer main, it is desirable to align the sewer on the outside portion of the trench to further reduce the risk of cross contamination. 6
ACCESS VAULT CONSIDERATIONS
Access vaults (AVs) are prefabricated, double walled, insulated metal units, installed at least 0.5 m below the base of the pipes. The typical temperature within the of the AVs will be between 5 and 10°C, which is the result of the heat radiated from the sewer main going through the AV. Although the base of the AVs are insulated to provide a thermal break, the above freezing ambient temperature with the AV may cause some long term thaw below the AV. If thawing occurs, the stability of the AV may be compromised because of differential movement and can cause additional stresses on the pipe connections at the outside of the vaults. Breaks in the piping connected to the AVs have been observed in the past. To counter this effect, the thaw stability of the soils underlying the AVs must be considered. Two additional considerations relating to potential differential movement of the AVs structures, are the insulation beneath the steel base of the AV, which protects the permafrost below the AV from thawing, and the use of flexible couplings in the piping connect to the AVs to accommodate the differential movement.
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Figure 5. Typical insulated sewer and water Access Vault (AV) used in the Arctic Strategically placed board insulation beneath the steel base will help protect the permafrost, which provides a stable base for the AVs and resists differential movement. Couplings will also accommodate some degree of differential movement (should it occur) without imposing significant stresses on the pipe connections to the AVs. Another consideration to minimize the potential movement of the AVs is installing AVs deeper into the permafrost. Access vaults, which are structures that may cost more than $125,000 each, form an essential part of the sewer system. Therefore, opportunities to maintain the existing AVs could provide potential substantial savings. However, considering hydraulic performance, existing conditions of the AVs, construction factors and realignment of existing flows, the replacement of the AVs in conjunction with the work may be necessary.
Figure 6. Existing Access Vault 208 with hydrant built into vault GEN 451-6
The existing access vaults have been in service for more than 30 years. Visual inspections of the condition support the replacement of the AVs as the existing structures have been subjected to the inevitable deterioration, including corrosion, that occurs during normal service. Maintaining the existing AVs during construction also poses multiple challenges. The new sewers will require a larger pipe diameter of at least 450 mm, upsized from the existing diameter of 200mm which presents several space constraints in some of the existing AVs. Creating new penetrations in the old AV walls increases the risk of breakage and the susceptibility of leakage. Additionally, during the excavation works, permafrost degradation will occur around the structure. This will increase the risk of potential post construction shifts which could lead to water and sewer breaks.
Figure 7. Inside Access Vault 205 showing deterioration 7
CONSTRUCTION CONSIDERATIONS
As the site is in the downtown core of Iqaluit, this project becomes significantly more challenging from a construction execution standpoint. The contractor will be required to safely contend with maintaining access and service to existing businesses, overhead electrical wires, excavation adjacent to and across existing roadways, managing heavy vehicular and pedestrian traffic, and limited workspace within the Right of Way. Additionally, if the desired alignment of the new sewer will be along the existing sewer alignment, this will require removal and replacement of the existing sewer and likely the watermain piping and thus, the project challenges, anticipated construction duration, and project/construction costs will be significantly increased. This is due to the necessity of designing and maintaining a continuous sewer bypass, the design, installation, and removal of temporary water mains, and maintaining existing service connections. The temporary water servicing will require significant effort to make sure continued service of the existing water services. Hydraulic calculations will be necessary to make sure adequate fire protection is maintained for all users on the loop. Details will need to be prepared for temporary connections at both the main as well as along the service piping. The sewer bypass details will need to be able to handle a large volume of flow from the upstream catchment areas as well as capturing the sewage generated by the existing users. The flow rates of the bypass will need to be controlled to make sure that downstream surcharging does not occur. Alternatively, constructing a separate trench for the sewer main on the north site of the Mivvik Street would significantly reduce the project complexity, construction duration, service disturbances, and costs. As an order of magnitude estimate, installing the sewer main in the north side of Mivvik Street may generate a savings of up to 50 percent of the project costs. GEN 451-7
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RECOMMENDATIONS
The following design criteria are recommended for capacity upgrades to the sewer main between AV 205 to AV211, as well as other potential improvements to other related infrastructure within the system including access vaults, water main and recirculation lines. The specific criteria include. 1.
Current and Future Sewage Flows
Previous sewer capacity assessments were used to calculate current (2020) sewage flows. To estimate future flows in 2050, the current flows were extrapolated on a high growth rate scenario (high= 3.38%). 2.
Sewer Profile, Alignment and Pipe Sizing
Due to the culvert crossing at AV211, the existing outlet elevation is to be retained. Based on the estimated sewage flows and slopes, the new sewer will likely require pipe diameters of between 450 to 500mm. To reduce the complexity, risk, schedule and cost of the sewer replacement, the option of constructing a separate trench in a new alignment for the sewer main on the north side of Mivvik Street is recommended. 3.
Pipe Specification
Since the depth of burial may be under the recommended 3.0 m, to provide adequate pipe strength, and freeze protection, it is recommended that DR9 insulated polyethylene pipe be installed, with a minimum pressure rating of 1380 kPa (200 psi) complete with 50mm applied polyurethane insulation and FRP jacket. 4.
Pipe Trench
The anticipated shallow installation depth of the pipe and the anticipated dynamics of the active layer along the pipe alignment suggest that dewatering and over excavation in the pipe zone will be needed, along with the installation of additional board insulation over the pipe-zone to maintain the permafrost within the pipe zone. 5.
Replacement of Existing Access Vaults
Considering hydraulic performance, existing conditions of the AVs, construction factors and realignment of existing flows, the replacement of the AVs in conjunction with the AV205 to AV211 work is recommended in conjunction with the AV205 to211 sewer replacement works. 6.
Soil and Sub Surface Water Contamination
Considering the likelihood that some soil and subsurface water within the construction area may be contaminated with legacy contaminants dating back to the military base established in 1942, provisions should be included for management of the contaminants to avoid any terrestrial discharges.
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