The City of Ottawa recently
completed the rehabilitation
on Belcourt Boulevard of a
corrugated steel storm sewer,
a section of which was restored using a
shotcrete lining technique. The sewer
was installed some 30 years ago, but
had been experiencing deformation for
many years. This article describes the
rehabilitation evaluation and design
stages, the key features of the shotcrete
process and some of the issues that
were encountered during construction.
The Belcourt Boulevard storm
sewer services a 250 hectare catchment
area in the Orleans community of
Ottawa. The sewer was installed in the
late 1970s as an 1,800 mm diameter
corrugated steel pipe (CSP), with a
1,981 mm diameter outfall under St
Joseph Boulevard. During pipe installation,
deformation occurred and
wooden struts were installed to restore
the pipe shape. Additional aluminium
struts were installed in 1999 to arrest
continuing deformation.
Under two separate contracts in
2002 and 2003, R.V. Anderson Associates
Limited (RVA) were engaged by
the City of Ottawa to design the opencut
replacement of 560 m of CSP on
Belcourt Boulevard with concrete
pipe. In January 2004, RVA were
retained to develop a solution for the
rehabilitation of the remaining downstream
section of culvert under St
Joseph Boulevard, and this is the subject
of this article.
The 73 m long outfall sewer was
bitumen lined with two 45-degree horizontal
elbows between the outfall
headwall and the upstream maintenance
hole, which was located in a
busy 6-lane traffic intersection (including
a turning lane and a bus lane), with
extensive underground utilities. The
outfall contained 60 aluminium
adjustable struts bearing onto continuous
timber header and footer plates.
Local storm and sanitary sewers, a Bell
fibre optic duct, a gas supply main, 406
mm and 610 mm (high pressure) water
mains and Hydro lines were situated
above the sewer in the road corridor.
The City of Ottawa prioritized this
section of outfall sewer for restoration
because:
The outfall had reached the end of
its useful life and needed to be
renewed;
The struts in the pipe had reduced
the hydraulic capacity of the outfall so
that it was not able to convey the full
development design flow;
The City was incurring costs due
to the regular maintenance of struts in
the pipe; and,
An asphalt overlay on St Joseph
Boulevard was scheduled for 2005.
This dictated that restoration of the outfall
sewer was required immediately.
Rehabilitation options
It was realized that due to the proximity
of extensive street utilities and the
potential traffic impacts, an open-cut
sewer replacement might not be the
most effective rehabilitation method.
Therefore, 11 alternative rehabilitation
methods were considered, including
shotcrete lining, tunnelling, slip lining
(with rigid and flexible pipe), liner plate
lining, cured-in-place lining, high-level
overflow sewer and full sewer relocation,
and combinations thereof.
A two-staged evaluation process
was used involving an initial screening
stage, in which the advantages and disadvantages
of each option were investigated,
followed by a detailed assessment
of the four best options. The
short-listed options were: shotcrete lining,
liner plate lining and open-cut
sewer replacement (along the existing
and new alignments). The detailed
evaluation criteria that were applied
included:
Speed of rehabilitation;
Life expectancy of the completed
system;
Cost;
Impacts on the public;
Constructability;
Impacts on services and utilities;
Environmental impact (most
notably impact to Bilberry Creek).
For each of the criteria, the relative
merit/impact of each option was
assessed and a rating of 3 (favourable),
2 (no significant advantage/disadvantage)
and 1 (unfavourable) were
applied. Double scores were adopted
for the first three criteria, since these
were of greatest importance to the
rehabilitation solution. The final rating
scores and construction cost estimates
of the shortlisted options were as
shown in the table.
It was recognized that the life
expectancy of the shotcrete solution
might be inferior to that of new sewer
pipe and that this would affect the
overall life-cycle costs. However, other
initiatives, such as the City’s asphalt
overlay, Hydro’s facility upgrades on
St Joseph Boulevard and the need to
relocate a watermain prior to open-cut
trenching, dictated that an immediate
solution was preferable and so shotcrete
lining was recommended as the
best method of rehabilitation.
Design of shotcrete lining
Shotcrete sewer lining was used by
the City of Ottawa in 2000 to rehabilitate
a similar CSP culvert. Details of
this previous project were investigated
to see what lessons could be learned
and a site visit was arranged so that the
lining could be inspected. Based on
RVA’s findings, the following items
were identified as being key to the success
of the current project:
Provision of a suitable mix design
and optimum conditions for installation
and curing;
All groundwater ingress through
the CSP wall must be stopped prior to
shotcreting;
Applying the shotcrete in a greater
number of layers to improve durability;
Use of shrinkage compensator
additive and air entrainment in the
concrete mix; and,
Development of a bypassing system
for existing stormwater flows in
the culvert.
If the struts were removed and a
concrete lining formed, the design
flow of 12.0 m3/sec could be conveyed
through a 1,550 mm diameter conduit.
In the most deformed area, the CSP
was 1,764 mm high and so pipe
hydraulics governed that 100 mm
thickness of shotcrete would be
applied to the top and bottom of the
pipe, with 225 mm of reinforced shotcrete
at the spring lines for structural
strength. This application was continuous
throughout the complete length of
the sewer outfall. To avoid culvert collapse
during construction and to provide
continuity of re-bar, the design
allowed for every sixth strut to be removed prior to shotcreting. Once the
lining had cured, all remaining struts,
plus the continuous header and footer
plates, would then be removed and the
spaces grouted.
Access into the culvert was provided from a platform in Bilberry Creek.
Approval for construction work in
the vicinity of the creek was obtained
through the local Conservation
Authority. The City of Ottawa also
obtained temporary easements for construction
access through private land to
the headwall in Bilberry Creek.
Contract specifications and drawings
for the shotcrete rehabilitation
work were prepared by RVA and the
project was tendered. Due to the specialist
nature of this work, only two
tenders were received and the contract
was awarded to Underground Services
(1983) Ltd. (USL) from Toronto for a
price of $281,400. Construction work
commenced early in October 2004.
Construction
Access into the culvert during construction
was provided by a temporary
structure erected in the bed of the creek.
The contract called for the installation
of a temporary flow bypass into
Bilberry Creek at an approved upstream
location. However, USL proposed an
alternative bypass system comprised of
a weir in the upstream maintenance
hole with a series of pipes laid through
the culvert to convey the flow. This was
considered to be an environmentally
preferred alternative and was accepted.
However, only a small proportion of the
culvert’s flow capacity could be conveyed
through the temporary bypass
lines. Therefore, monitoring of weather
conditions during construction and timing
of the key phases of work were critical
to ensure that during peak rainfall
events, the weir could be quickly
removed and the flow conveyed safely
through the culvert without damaging
the infrastructure.
A ‘wet shotcrete’ method was used
and stainless steel threaded rods were
tack welded onto the CSP with
adjustable nuts, to control layer thicknesses.
The shotcrete was applied in
longitudinal sections, with the culvert
invert being shot first, between the 4
o’clock and 8 o’clock positions in two,
50 mm lifts. Layering continued up the
side of the culvert to achieve the
required thickness at the spring lines.
Using the previous lifts for support,
thin layers were then applied over the
top half of the culvert until the complete
circumference was lined. Guide
wires were installed between threaded
rods to control the surface of the finish
coat. The final lift, below the spring
line, was brush finished.
During construction, USL modified
the strut removal process to provide a
more robust permanent reinforced lining.
Instead of shotcreting up to the
timber header and footer plates, USL
were able to cut out the plates between
struts prior to shotcreting and provide
temporary longitudinal strut support
so that full-circumference steel reinforcement
could be installed throughout
the complete length of the culvert.
The project was substantially completed
within six weeks at no extra cost
to the contract price.
Conclusion
This project used an innovative
rehabilitation approach to overcome
some difficult site constraints and provide
the City of Ottawa with a storm
sewer capable of meeting the required
level of service. A key component in
the success of this project was the ability
to learn from the previous culvert
shotcrete project to develop an effective
design. The City realized the following
benefits:
Minimized period for construction
(six weeks), meaning that the asphalt
overlay contract was not delayed;
Construction cost savings of
$100,000, when compared to open-cut
sewer replacement;
Significantly reduced sewer maintenance
needs;
The hydraulic capacity of the system
was restored to meet the required
level of service; and,
Minimal environmental and public
impacts during construction.
Condition monitoring of the concrete
lining over the coming years will
be carried out to verify the durability
of the shotcrete lining.
Jonathan Knoyle, P.Eng., is an
Associate of R.V. Anderson Associates
Limited in Ottawa and was Project
Engineer on the St Joseph Boulevard
Storm Sewer Rehabilitation project.
E-mail: jknoyle@rvanderson.com.
The Project Manager at the City of
Ottawa was Jamie MacDonald, P.Eng.
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