Award of Excellence: Vancouver City Central Transmission
October 1, 2012
By Golder Associates
The Vancouver City Central Transmission (VCCT) Project was constructed to add to BC Hydro’s electricity distribution capacity. It is BC Hydro’s most significant investment in central Vancouver’s electrical grid in almost 30...
The Vancouver City Central Transmission (VCCT) Project was constructed to add to BC Hydro’s electricity distribution capacity. It is BC Hydro’s most significant investment in central Vancouver’s electrical grid in almost 30 years. It ensures a reliable electricity supply into the future since the new installation is resistant to earthquake damage and heat damage.
A vital component of the VCCT project was the installation of three 850-m long 230 kV transmission cables across False Creek inlet. This work required two major breakthroughs in applying existing technologies to new and complex challenges. The breakthroughs were the successful completion of a large diameter bore through glacial till soils using a Horizontal Directional Drill (HDD) method, and the installation of a grouted conduit bundle system and transmission cables which had to conform to very strict seismic and thermal dissipation requirements.
Neither of these technological achievements had been achieved before in Canada. The adopted solution was the most cost effective and timely option and it reduced any adverse environmental effects.
The direct savings to the client in this case of not adopting a more conventional shaft and tunnel option were six months in construction time and $3 million in capital cost. Similar projects in the future will benefit by being able to use this more economical, faster and more reliable methodology.
The project proved the feasibility of completing large diameter bores using the HDD method within dense glacial soils. These ground conditions have historically been considered as high risk with the potential to be “show stopping.”
Using the HDD method to install the conduit resulted in a reduced carbon footprint and is one of the least disruptive pipe installation methods because it does not require trench excavation, seabed dredging or large entry and exit shafts. The HDD entry site on this project was in David Lam Park adjacent to False Creek. The park is much used and located near residential towers, a school and daycare centre. Its condition is now fully restored.
Detailed geotechnical investigations showed that nearly 230 metres of the bore path for the HDD would traverse through glacial soils. Since there was no prior experience in Vancouver of successfully drilling a large diameter HDD bore through glacial soils, the team decided to do a preliminary test by predrilling the full diameter HDD hole through the glacial till and some 25 metres into the bedrock prior to completing the design.
Special grout to replace
the drilling mud
BC Hydro required the three electrical cables to be physically separated by a minimum centre-to-centre distance, and the annular space between the ducts and the HDD bored hole to be filled with a grout with specified thermal resistivity characteristics. It meant designing a grout which was at the same time flowable, dense, stable and capable of replacing the drilling fluid (mud) in the HDD bored hole.
The ability to successfully displace the mud from the 50-m-deep, 850-m-long HDD bore was critical to the success of the project and of major concern since prior industry experience was sparse and in most documented cases results were less than successful.
To solve the issue, an innovative grout mix was designed, which involved testing a number of potential mixes and a large-scale, pre-production field trial. The final result is the first mud replacement grouting on this scale known to have been successfully carried out anywhere in the world.
Although only three cables were to be installed, a relatively simple and efficient multi-pipe conduit bundle design comprising seven pipes was adopted. This approach made field construction easier and provided redundancy in case of damage to one or more of the pipes during pullback.
The multi-pipe bundle also allowed the incorporation of a cable cooling system if necessary. BC Hydro required the new cable system to meet the Institute of Electrical and Electronic Engineers (IEEE) seismic standard and the cables to carry the rated load current without overheating. This significantly added to the complexity of the project.
On the logistics side, the prefabricated 850-m-long, 140,000 kg pre-strapped bundle required transporting over a distance of 1.3 kilometres, including negotiating three 90-degree bends. The bundle had to rest in the streets during the seven-day pull, which meant providing alternative transportation to affected residents and coordinating an effective public communications plan.cce
Name of project: Vancouver City
Central Transmission (VCCT)
Award-winning firm (prime
consultant): Golder Associates (Naresh Koirala, P.Eng., Trevor Fitzell, P.Eng., Tom Bryski, P.Eng., Adrian Hansen, EIT, Don Gamble, Karen McMillan, Doris Zibauer)
Owner/client: BC Hydro
Other key players: Geotherm.
and Constellation Group (designed
the thermal grout design);
Michels Canada (prime contractor).
This was a very well planned and executed project which succeeded in achieving minimal disruption in its challenging urban environment. The innovation of pre-drilling large-diameter horizontally directed bores in difficult glacial soil conditions contributed substantially to the project’s success.