Environmental Science & Engineering - www.esemag.com - September 2004
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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.
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