Lessons learned while replacing aging watermain

Esemagh.com
June 16, 2020

With a population of about 335,000, the City of Vaughan, Ontario is one of the fastest-growing communities in Canada. It is home to more than 12,000 businesses that employ more than 222,000 people. By 2031, its population is expected to reach 416,000.

The city retained the services of Chisholm, Fleming and Associates (CFA) to complete the design for replacement of an existing watermain with a new PVC one. The goal of the project was to replace an approximately two-kilometre section of badly deteriorated watermain along a busy arterial roadway in a densely developed, mixed-use urban area, with minimal disruptions to traffic, residents and the natural environment.

The 2,040 m length of 300 -- 350 mm cast iron and ductile iron watermain, parts of which were over 70 years old, stretched between Jane Street and Keele Street along Major Mackenzie Drive West. It was experiencing a very high frequency of breaks, that caused severe disruptions to traffic as well as service to businesses and residences.

The $4.2 million project also included replacing 19 valve chambers, as well as hydrants and water-service connections. The design was completed between April 2017 and March 2018. Construction took place between May and December 2018.

A challenging project site

The project site was inherently challenging. Major Mackenzie Drive West is a four-lane urban arterial road, travelled by an average of over 40,000 vehicles daily. There are particularly high levels of traffic during morning and afternoon rush hours. There are two large shopping plazas on either side of the street at the west end of the project site and commercial development on the north side at the east end.

The majority of the roadway is fully built out, except where the roadway crosses the West Don River, which is one of two major tributaries to the Don River, and in a sensitive, protected natural area.

In addition, above and below ground, the corridor is congested with utilities. There is overhead and buried electrical and telecommunications infrastructure, as well as buried gas mains on both sides of the street and sanitary and storm sewers.

Project drivers
The project was completed as part of the city’s state-of-good-repair program, but three significant factors contributed to the urgency of the replacement.

One was the increasing service demands created by development. Between 1996 and 2006, Vaughan was the fastest-growing municipality in Canada, with an 80% increase in its population during that period. A larger and more reliable water supply was needed to meet the already heavier domestic and commercial water demand and to allow for further development.

A second factor was reliability of the water supply, in particular for Canada’s Wonderland. It is a premier entertainment attraction and the largest theme park in Canada, with approximately 3.8 million visitors each year. It is the city’s largest customer in terms of annual water consumption.

The final factor was the planned construction of the Mackenzie Vaughan Hospital. This $1.6 billion, 350-bed facility, which is currently on track for completion in 2020, will be the city’s first hospital. A reliable, high-volume water supply to the 1.2 million square foot facility, expandable to 550 beds, is critical.

Finding a fast, low-impact and cost-effective solution
The preferred alignment, agreed upon by the city and the Regional Municipality of York (which owns the road), included a new, upsized 400 mm PVC watermain running primarily within the eastbound curb lane between Jane and Keele Streets. It was determined that installation within the boulevards was not feasible because of the high degree of utility congestion. Consequently, it was agreed that the watermain would be installed by conventional open-cut construction, except for the crossing at Jane Street, which would be installed by horizontal directional drilling (HDD).

Because of concerns about the impact of construction on an already congested traffic route, the city asked that CFA further explore the feasibility and benefits of using HDD across the entire limits of the project. The only design stipulations were that all entry and exit pits had to be located outside of the roadway and that the pipe for the watermain had to be cast-iron-outside-diameter (CIOD) compliant, for compatibility with waterworks valves, appurtenances, and restraints and so that future repairs and service connections could be made without special fittings.

CIOD compliance eliminated high-density polyethylene (HDPE) pipe, as its diameter is measured as outside diameter, rather than as inside diameter, as is the case for both cast iron and PVC pipes. Also, the fittings for HDPE pipe are different. As lockable PVC pipe is only available to a maximum diameter of 300 mm, the only pipe material that would meet the city’s requirements was 400 mm fusible PVC pipe (Class 235, DR18), which was supplied by IPEX.

The project site was investigated to determine the feasibility of HDD construction through a geotechnical investigation, as well as a review of available laydown areas. The geotechnical investigation found very dense clayey silt till and sandy silt till, which are suitable for HDD and present relatively low risks of inadvertent fluid loss through hydraulic fracture (frac-out).

A review of the laydown areas confirmed that HDD was feasible for approximately half of the proposed watermain length, so the city authorized CFA to proceed with the detailed design.

The preferred alignment primarily within the roadway remained unchanged. Careful planning identified specific locations where entry pits and exit shafts could be located within the boulevard, without impact on existing utilities, or within auxiliary turning lanes to minimize traffic disruption.

Open-cut installation was used as originally planned in the eastern half of the site. Here, HDD was not feasible given the number of houses and businesses fronting the street and the difficulties of laying out and fusing the pipe lengths without blocking driveways and entrances.

Location of the entry and exit pits took into consideration the minimum bending radius of 110 m of the 400 mm fusible PVC, so that the pipe could be laid out, fused and pulled within the boulevard. To avoid as much of the utility congestion as possible, the watermain was designed to sweep out into the roadway along the preferred alignment and then sweep back into the boulevard between each successive HDD section.

Maximum pull length of each HDD section was a function of the available area in the boulevard to fuse and layout the pipe and the typical thrust/pull strengths of HDD drill rigs. The average length of each HDD section was approximately 300 m. This pull length meant smaller equipment could be used. Installation was completed using a Vermeer D 36×50 (II) with 36,000 lbs thrust/pull (16,330 kN).

Use of fusible rather than lockable PVC for the replacement meant a number of field conditions had to be taken into consideration, including the weather. The fusing process needed to be protected from rain, wind and dust and temperatures had to be above 4°C. Connections of the fusible pipe had to be completed by a certified licensee of the pipe manufacturer.

One of the most significant challenges of the design was finding laydown areas for the pipe. Additional effort was made during the design to minimize impacts on traffic, while ensuring constructability. Recommended entry and exit pits were identified on the contract drawings, even though the location of the pits is sometimes left to the contractor to determine in the field.

A complete subsurface utility engineering investigation was a priority, given the number of underground utilities and the age of the existing watermain. However, despite thorough information about the location of the watermain and other infrastructure, a break in the existing pipe occurred during drilling of a pilot bore, even with 3 m of clearance. An investigation into the break found that the ductile iron pipe was in very poor condition.

A construction schedule with extended working hours and alternating daytime and nighttime shifts shortened construction of the watermain installation by almost three months (a temporary noise by-law amendment allowed for this). The HDD work took place mostly during the day, given its limited impacts on the site. The open-cut sections were generally completed during the night shift, and the following day the watermain trench along the previously installed watermain was restored with asphalt. This meant the roadway could be reopened to traffic to minimize disruption. In addition, the asphalt was sawcut and utilities were daylighted by the day crews in advance of the night crew shift.

Outcomes

By seeking to maximize the use of trenchless construction throughout the project site, the city and CFA were able to realize significant additional project benefits.

To start, there was a major reduction in the overall impact of the project on residents, businesses and road-users. No property-access or service-disruption complaints were logged from any of the commercial or residential properties along sections of the replacement completed through HDD. Because the HDD construction work had no, or only minimal, impacts on traffic, it could take place during the day, eliminating complaints such as those about nighttime construction that arose along the open-cut sections.

As well, the HDD sections of the watermain were installed approximately 1.75 times faster than the open-cut sections. One 244 m section took only eight days to install using HDD, versus 14 days for a similar length through open-cut construction.

In addition, extensive use of HDD allowed the city to protect its investment in its existing infrastructure and preserve valuable natural features. There were no utility impacts or damage within the entire HDD section of the project and only minor disruption to sidewalks, the roadway and streetscaping. Because HDD allowed for the installation of the new watermain directly over the existing one, space was conserved for additional infrastructure within the corridor. As well, there were no tree removals in either built-up or natural areas.

Since fusible PVC has no joints, the new watermain passed the hydrostatic pressure test with zero leakage on the first round of testing. Leaks that sometimes occur in a newly installed watermain that uses many joints and mechanical bends can be difficult to locate and repair. Also, they can significantly impact the construction schedule and delay the infrastructure being put into service.

Use of HDD also lowered the carbon footprint of the project in several ways. Keeping construction out of the roadway as much as possible greatly minimized traffic delays, which in turn minimized traffic idling. What’s more, the carbon emissions and nonrenewable resources for the HDD section of the project were insignificant. In addition, by eliminating the need for large amounts of new material, HDD significantly reduced the embodied-energy costs of the project.

All of these benefits were realized with little extra cost. The HDD sections were completed at a cost of $1,080/m as compared to $960/m through open-cut.

Project lessons
The project produced valuable benefits for the team, in the form of greater insight into how complex infrastructure-replacement projects with multiple stakeholders in highly developed project sites can be successfully realized.

Some of the takeaways of the project may be valuable for other utility owners and consultants. Extra upfront effort during the design to more accurately define the alignment and to locate entry and exit pits ensured the constructability of the design and reduced traffic impacts to zero for much of the HDD section.

A more-detailed-than-normal geotechnical investigation gave more information to the contractors for bidding, which helped reduce the cost of the construction. The initial plan for nine boreholes at 200 -- 300 m spacing was augmented to 17 boreholes to tighten the spacing to 100 m and give a more thorough understanding of soil conditions.

All of the large, 150 -- 200 mm service connections to businesses along the roadway were also completed through HDD to avoid the major traffic disruptions that would have resulted if the service crossings were installed by conventional open-cut construction. As a result of proactive and continuous communication with businesses, the contractor was able to use private property as the laydown and equipment area for each connection, with full cooperation from the property owners.

Consultation with HDD contractors during the design phase and with Direct Drilling Ltd., who completed the HDD installation, after construction gave valuable input and feedback to the team for this project and the next one.