Innovative reinforced concrete pipe drop structure used to construct stormwater outfall
By Albert Wissa and Kent Campbell
Assembly of a standard 1200 mm diameter maintenance hole from the transition structure to the surface.
Development of tableland,
north of Sheppard Avenue
and east of Meadowvale
Road in the Scarborough
Community Council Area of the City
of Toronto, has been planned for residential
use for decades. A major concern
addressed throughout the planning
process has always been the fragile
nature of the steep bluffs dropping
sharply to the Rouge River. Closely
associated with the nature of the bluffs
is the sensitivity of the river and valley
ecosystem to the impacts of development.
Environmental studies and reports
detailed the constraints to be imposed
on any development of the tableland to
protect the Rouge River Valley and all
of its elements. Proactive citizens and
local residents organized to be sure
that environmental issues were
addressed throughout the development
application process, and solutions to
mitigate potential environmental damage
were thoroughly reviewed before
approvals for construction were granted.
Mattamy (Rouge) Limited was one
of the development applications that
required approvals to discharge
stormwater from its site to the Rouge
River. A stormwater management pond
had to be constructed on the tableland,
outside of a 30-metre buffer from the
edge of the steep slope to the river. A
storm sewer outfall at the base of the
steep valley slope was constructed by
trenchless technology so that the
impact of the outfall within the valley
was limited to the headwall and
approximately 50 metres of stonelined
open channel to the river.
Stormwater is now collected from
the housing development in a network
of storm sewers and discharged into a
stormwater management pond. From
there, the water drains to a 975 mm
diameter concrete pipe outlet and is
conveyed by a 31.3-metre deep drop
structure (900 mm diameter) to the
base of the slope. At this point, the
flow is directed from a specially engineered
energy dissipater box, through
a jacked 1200 mm diameter concrete
pipe to the Rouge River. By the time
the stormwater reaches the river, its
velocity is minimized to prevent any
erosion of the riverbanks.
Niran Construction of Toronto was
awarded the contract for sewer and
watermain services. The
contractor immediately
proposed an alternative to
the design of the drop
structure that would comprise
a system of precast
concrete maintenance hole
components readily available
from Hanson Pipe &
Products Canada, Inc. of
Cambridge. Using a precast
concrete system of standard
gravity pipe, jacking pipe,
and maintenance holes
would result in the least
impact on the environment
and construction cost savings
to the client.
The concept for the drop
structure included the following major
elements:
4600 mm diameter metal plated
working shaft extending vertically
31.3 metres to accommodate an access
maintenance hole shaft, transition box,
maintenance hole risers housing the
drop structure, and an energy dissipater
at the base connecting to a jacked
outfall to the river;
6.4 metre access maintenance hole
section from ground level on the tablelands
to the top of a transition box;
2400 mm x 1200 mm transition
riser with specially designed, benched
elliptical pipe connected to the riser
and 975 mm diameter inlet pipe from
the stormwater pond, complete with
precast weir;
maintenance shaft comprised of
eight 2400 mm x 1200 mm risers,
incorporating drop structure comprised
of 900 mm diameter reinforced
concrete pipe risers cast integrally to
the 2400 mm x 1200 mm risers; and,
3000 mm diameter riser, transition
slab and base with energy dissipater
sump box, and connection to 1200 mm
diameter jacking pipe.
Niran commenced excavation of the
4600 mm diameter working shaft in
mid February, 2004. The silty clay till
did not present any major obstacle to
construction and there was very little
groundwater seepage.
The process for the dig involved an
open pit excavation of about 1.0 to 1.5
metres per day. A steel liner plate was
constructed each day as the pit progressed.
The first section of liner was
about 4 metres deep with one metre
raised above ground level to provide a safety barrier for workers and any visitors to site. At one point in the dig, small amounts of explosives were required
to loosen the soil. A Bobcat was lowered into the shaft and
used for excavating the earth. Once the invert elevation of
the shaft was reached, the Bobcat was removed and replaced
with tunnel jacking equipment.
Throughout the dig, all safety precautions were taken
including the use of fans for ventilation, ladders, safety platforms,
and lighting.
Tunnel outfall to channel: Outfall to Rouge
River using jacked 1200 mm diameter
reinforced concrete pipe.
The shaft was required so that 95.5 metres of 1200 mm
diameter jacking pipe could be jacked from the 3000 mm
diameter maintenance hole riser at the base of the drop
structure. The jacked pipe connects to the maintenance hole
by means of a “doghoused” opening in the first 3000 mm
riser at the base of the shaft to provide an outlet to the creek.
An internal granite box was specially designed by Niran to
accommodate the velocity and weight of freefalling
stormwater and dissipate its energy. Once the outfall pipe
had been jacked into position and the 3000 mm diameter
riser installed, the base of the drop structure was ready for
construction of the 2400 mm x 1200 mm maintenance shaft
that contained the 900 mm diameter concrete drop structure.
The idea for the internal drop structure came from
Hanson’s engineers. There was some concern that an external
concrete drop structure might shift and crack. In addition,
an external drop structure (concrete or PVC) would
have required a steel lined working shaft to accommodate
pipe and equipment. The construction cost could have been
much higher. With an internal drop, the concern about
cracks in the concrete drop structure was alleviated and the
cost of construction reduced dramatically.
Construction of the maintenance shaft and drop structure
took two weeks as all components were precast concrete
maintenance hole components. Only two pieces per day
were permitted to be installed since the space between the
precast concrete structure and the metal-lined wall of the
shaft was filled with non-shrink fill, as work progressed.
Engineers required proper time for the fill to set.
The entire shaft is made accessible by a combination of
aluminum safety platforms and steps built into the risers at
the Hanson plant. Design engineer for this structure was
Dave Grahlman of Gamsby and Mannerow Limited.
When work reached the elevation where the inlet pipe
from the pond entered the vertical drop shaft, a special transition
structure comprised of a weir and benching was specially
designed by the Hanson engineering team. The challenge
was to provide benching that did not cause turbulence
of the stormwater as it entered the structure, and reduced the
entrant coefficient for the flow transitioning from the 975
mm horizontal pipe to the 900 mm vertical pipe. Hanson
engineers used a unit of elliptical pipe with convex benching
at the base as a reducer to direct the flow into the drop shaft.
The reducer was placed between the outlet of the 975 mm
concrete pipe extending from the stormwater management
pond and a 2400 mm x 1200 mm riser constructed with a
200 mm wide weir (designed for a 100-year storm event)
that prevents normal flows from the pond from entering the
maintenance access shaft that parallels the length of the drop
structure.
Once the transition structure including reducer and weir
were in place, the contractor was able to continue with the
construction of a standard 1200 mm diameter maintenance
hole to the surface.
Jacking the outfall pipe from the structure at the bottom
of the shaft, and construction of the headwall took 50 working
days. The process was described by the contractor as a
traditional jacking job through silty clay till. Since the cylinders
of the jacking machine required more space than was
available in the working shaft, the contractor built a 3-metre
“tail tunnel” that extended beyond the circumference of the
working shaft. The tail tunnel accommodated the cylinders
of the jacking equipment. Once the pipe jacking was completed,
the metal lined tunnel was completely filled and covered
by steel wall plates. The jacking pipe was 1200 mm
diameter pipe supplied by Hanson.
Niran Construction Limited of Toronto, under the supervision
of David Schaeffer Engineering Limited (consulting
engineer for Mattamy Development Corporation), constructed
the drop structure, inlet, and outfall to the Rouge River in
approximately four months.
The precast concrete drop structure is believed to be the
deepest in Canada for completing a stormwater management
system for a residential development. Precast concrete manhole
components and standard concrete drainage and jacking
pipe proved to be an easily installed, economical, long-term
solution.
Albert Wissa, P.Eng., is with
Niran Construction
Kent Campbell is with
Hanson Pipe & Products Canada Inc.
Contact: kent.campbell@hansonamerica.com
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