Seismic-prone project area: only HDPE will do
A new retail power center in Burnaby, British Columbia is quite literally floating on a bed of wood, gravel and concrete. In an area where construction must always consider the impact of seismic activity, the potentially fluid nature of this sites foundation meant that for underground contractor TAG Construction Ltd. of Langley, British Columbia, the choice when it came to the construction of the underground water and sewer lines was crystal clear.
"In this case it was readily apparent that only HDPE pipe would be able to sustain the flexible nature of the site," said professional engineer Michael Grant of TAG.
The site of the 302,000 square foot Big Bend Crossing shopping center had been a waste dump for building demolition material for many years. Then it was preloaded for construction using building excavation material from sites throughout the lower mainland. "This made for very fluid soil conditions," explained Grant. "The design using HDPE mains and services for water and sanitary was based on the "flexible" nature of the site, given the presence of the wood wastes below and their ground water content, and the need to ensure the integrity of the pipe in the moving ground."
Unlike other pipe products such as ductile iron or PVC, HDPE pipe can be bent to a radius 25 times the nominal pipe diameter for mid range pressure pipe, 20 times for heavy wall pipe, and 30 times for low pressure pipe. (Example: 12" HDPE can be cold formed in the field to a 25ft radius). In addition to eliminating many fittings required for directional changes in a piping system, the "flex" factor makes HDPE pipe ideal in areas of high seismic activity.
HDPE -- or polyethylene pipe -- is becoming recognized worldwide for its flexibility in active environments in addition to its durability, sustainability and environmental and health strengths, explained Brad Warning of Prolite Systems in Maple Ridge, British Columbia, the specialists who provided the pipe, fittings and fusion expertise for the Big Bend Crossing project. Unlike products that require gaskets or external joints, PE pipe is joined using a heat-fuse process with machines such as the Connectra Fusion 14-inch hand pump machine that was used in this project. The fusion process creates an area at the joint that is even stronger than any other point along the length of pipe, and is therefore equally resilient when bent.
In places such as Europe and Asia PE pipe has been used for years, and in-situ results back up PEs touted superior performance. The Great Hanshin - Awaji Earthquake in Japan reported damage to all water pipe systems with the exception of the areas using PE pipes. A report on failure rates for the Swedish water pipe network stated per 10 kilometer failure rates of 0.3 percent for PE, while cast iron and steel pipes failed at a rate of 1.9 and 3.3 percent respectively.
In America, municipalities and states in quake-prone regions such as southern California have been installing PE pipe at increasing rates. A report on damage rates to gas distribution lines following the Northridge, California earthquake in 1994, stated that while PE makes up 41 percent of the gas piping system in the area, only 27 of the 709 leaks (or 3.8 percent) occurred in PE pipes.
Environmentally the argument is ironclad. Heat fused PE joints provide a zero allowable leakage rate, conserving a substantial amount of water as compared to other pipe systems such as ductile iron, concrete and PVC, that have allowable leakage rates from the point of installation, which tend to increase with corrosion over time. This in turn means surrounding soil contaminants are not leaching in. "British Columbia has always been a leader environmentally in North America," said Warning, "so its not surprising to see B.C. developers and municipalities specifying HDPE in their projects in order to both conserve and provide safe water for its citizens." Add on the seismic potential in the region, and PE pipe was the clear choice, he added.
For the Big Bend project, new water mains for the project area were specified to be HDPE DR11 DI OD and were 460 meters in length. Laterals to the site structures accounted for an additional 800 meters of pipe ranging from 150 to 250 diameters. Sanitary sewer lines were all low pressure force mains and constructed using 400 meters of HDPE DR26 along the main road and site right of ways and included four flush-out chambers on the main and three additional flush-out chambers on the laterals to the site structures adjacent to the right of way. Sanitary servicing to the remainder of the site, not adjacent to the right of ways, required an additional 165 meters of HDPE DR26, and PE series 75, including the laterals to the individual units.
The building construction program was on-going throughout the $730,000 utility installation, creating an even more challenging work environment for the installation crew. In order to ensure access to the site for other contractors, only certain lengths of pipe string could be pre-joined. This meant that some of the HDPE fusion welds had to be done in the trench, requiring a fusion machine that could perform complicated ditch welds and fusion of fittings. In addition, the flush-out chambers on the sanitary lines required a riser that could be accessed by a flush truck for maintenance purposes, creating a space problem: some of the HDPE fittings had to be customized and re-welded in order to fit inside the specified 1500 mm diameter barrel of the chambers. A Connectra Fusion 14-inch hand pump machine was used both in and out of the trench to perform all of the 230 required fusion welds for the project. "There is no question this was a complicated job when it came to joint fusion," said Warning. "The Connectra Fusion machine was perfect for the job because of its adaptability and ease of use in tight spaces."
In total, 2800 meters of pipe ranging from 2-inch to 10-inch DIPS were fused. All the water and sanitary mains were installed by standard trenching with a depth of cover of approximately 1.5 meters and imported bedding and granular backfill were used.
Download High Resolution Images: