FOCUS ON... Stormwater Management
City of Richmond lightweight storm sewer system By Phil Carroll and Scott Groves
In the spring of 2003, the City of Richmond in British Columbia
identified the existing 240 m long open ditch adjacent to Westminster Highway North as a candidate for a
these parameters: • 320 kPa pipe stiffness at 5% deflec tion - essential given the poor ground conditions.
• 6 m nominal length between joints. • Bell and elastomeric rubber gasket joint certified to CSA B 182.8-02. • Handling weight of 73 lbs per lineal
ditch enclosure and installation of a
metre.
600 mm diameter storm sewer by the City Operations. Upgrading stormwa ter conveyance from an open ditch to a storm sewer is normally a routine task, except in this area of Richmond, which is characterized by: • Low bearing capacity subgrades. • High groundwater table/flooding. • Aggressive soil chemistry and agri
• Bedding and haunch gravel- 19 mm diameter clear granular crush gravel. • Backfill above bedding and haunch es to 300 mm over pipe - river sand to take advantage of its light weight. • Native light peat organic mix to sur
cultural run-off.
Given these ground conditions, the City of Richmond specified a light weight storm sewer pipe, an open cor rugated smooth inner wall HOPE pipe commonly known as BOSS 2000 with
SEWAGE
face.
The City worked with local suppli ers to look at ways of reducing the weight of each of the three manholes, measuring 1.49 m to 2.3 m deep. As a result, the following were installed: • 1,067 mm diameter corrugated HOPE manhole risers, weighing 88 lbs per lineal metre.
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12 Environmental Science & Engineering,September 2004
• Integral fusion welded HOPE stubs fitted with bell connections.
• Pre-installed ladder rungs in each HDPE manhole riser.
• Bases were cast-in-place concrete poured in the field to British Columbia's MMCD Specification Drawing SI (Standard and Sump Manholes). • Lids were pre-cast concrete resting directly on the compacted granular material surrounding the riser - this would enable the lid to carry the live load of traffic in the road shoulder.
The estimated weight comparison for the riser components, was approxi mately: - Concrete Riser (1,600 lbs per lineal metre of height)) - HDPE Riser (88 lbs per lineal metre of height) Field drain tie-ins were completed with insertable tees ranging in size from 150 mm to 300 mm diameter,
each fitted with a rubber gasket, PVC lead and pipe clamp. This allowed for simple installation and a flexible leak resistant joint. At the inlet of this storm sewer sec
tion, City Operations installed a light weight polymer composite headwall, commonly known as Pro-Eco-Lite headwall. The headwall was fitted with
a polymer composite (non-rusting) trash rack and factory-installed pipe stub. With an overall height of 1.3 m, the headwall weighed approximately 230 lbs - light enough to be supported on the soft, saturated subgrade. The innovative lightweight storm sewer was successfully installed by City Operations over a three week time period from start to finish. Recent inspections of the site indicate that the design has proven to be a success. Phil Carroll, P.Eng., is Region Engineer (Western Canada), Armtec Limited and Scott Groves, P.Eng., is Project Engineer, City ofRichmond. Contact e-mail: pcarroll@anntec.com.