THEME:
CROSS
CONNECTION
CONTROL
CRYOFRONT: News, Views and Muse from the Far North
CCC in the close quarters of a northern water and sewer access vault By Ken Johnson, Stantec
manholes were problematic from the start because this technology was not suited to the demanding environment of the active layer with the ground movement and the seasonal groundwater flow. Problems that commonly occurred included water infiltration into the vaults; physical and moisture damage to the internal urethane insulation; difficulty of access to appurtenances; and freeze breakage of piping and appurtenances. The innovation to solve this problem in the 1980s was the introduction of the Access Vault as a replacement for manholes. The access vault provides access to both the sewer and water system, which are essential for maintenance activities of cleaning and the unique nothern requirement of draining and thawing. The proximity of the water and sewer provides a heat source from the sewer, but the sewer also creates the potential for cross connection. The cross connection potential demanded a robust mechanical design, which is a mechanical marvel. Another part of the use of these systems was the definition for regulatory purposes. Manholes were for sewage only, whereas access vaults, by definition could contain water and sewer lines. To ensure no cross connection can occur, the sewer lines are completely sealed within the vault and are only accessible through a normally bolted access
Inside access vault with hydrant (vertical section of pipe)
30 | Western Canada Water | Spring 2013
hatch, which is sealed with a flexible gasket. Installation efficiency was also substantially improved with access vaults, and commissioning was less challenging as the vaults had been tested prior to shipment from the factory. Studies have been undertaken to investigate alternate, less costly concepts to the insulated steel water and sewer acccess vaults used, however the robust design needed for the harsh arctic environment has not been matched. These design features include: access for sewer clean out; access for thawing sewer main; access for draining water main; access for thawing water main; access to operate, maintain and repair appurtenances; freeze protection for hydrants incorporated into vaults; resistance to all uplift forces; resistance to thaw settlement; and prevention of all ingress of water; and accommodation of thrust forces due to expansion and contraction of pipe. The insulated steel access vault that is presently used has rectified all of these problems associated with the previous designs. However, these vaults may cost in excess of $100,000, which includes supply and installation of all fittings and appurtenances. The cost of the access vaults may represent 30% of the contract price for the piped utilities system, but the performance of the vaults over the past several decades has justified their expense.
Several access vaults awaiting installation
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Photos courtesy of Steve Burden, exp
C
ross connections may not have the same importance in the North by the fact that most of the communities do not have piped services. If my math is correct, of the 85 or so communities in the northern territories (Yukon, NWT and Nunavut), only 16 or 20% have piped systems. The remainder have trucked systems, which pose unique problems unto themselves. Back in the 1960s, water and sewer mains were constructed using asbestos cement piping. The above ground portions were installed in sheet metal boxes, which were filled with vermiculite insulation. The innovation introduced during this iteration of servicing was the concept of recirculation and reheating. Heat was added at specific points in the system and recirculation was provided by parallel small diameter copper pipes. The next great evolutionary step was the introduction of buried servicing. This innovation brought a host of benefits, most significantly the placement of the pipe in the constant temperature of the ground rather than the extreme cold of the winter air. Buried installation was possible due to improved materials such as piping that was pre-insulated with polyurethane foam. Along with buried pipe came heated concrete manholes for system access. Concrete