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 ditch enclosure and installation of a 600 mm diameter storm sewer by the City Operations. Upgrading stormwater 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 agricultural run-off.

Given these ground conditions, the City of Richmond specified a lightweight storm sewer pipe, an open corrugated smooth inner wall HDPE pipe commonly known as BOSS 2000 with these parameters:

• 320 kPa pipe stiffness at 5% deflection – essential given the poor ground conditions.
• 6 m nominal length between joints.
• Bell and elastomeric rubber gasket joint certified to CSA B182.8-02.
• Handling weight of 73 lbs per lineal metre.
• Bedding and haunch gravel – 19 mm diameter clear granular crush gravel.
• Backfill above bedding and haunches to 300 mm over pipe – river sand to take advantage of its light weight.
• Native light peat organic mix to surface.

The City worked with local suppliers 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 HDPE manhole risers, weighing 88 lbs per lineal metre.
• Integral fusion welded HDPE 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 S1 (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 approximately:

• 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 section, City Operations installed a lightweight 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 Scott Groves, P.Eng., is Project Engineer, City of Richmond. Contact e-mail: pcarroll@armtec.com.