The $CDN200 million ($US210 million) Vancouver City Central Transmission (VCCT) project will be installing the new electricity system under False Creek, from David Lam Park in the Yaletown neighbourhood of Vancouver to Laurel Street and West 7th Avenue in the Fairview Slopes neighbourhood of Vancouver.

The crossing

Michels Canada Co was contracted by BC Hydro for the horizontal directional drilling (HDD) crossing, among other trenchless works on the project. Michels constructed the HDD crossing with a minimum diameter of 44 inches, within which was pulled a 34 inch conduit bundle consisting of 7x10 inch, 5x4 inch and 1x5 inch HDPE conduits.

In addition to this, Michels also had to grout the annular space between the borehole walls and the bundle, displacing the drill mud using the 4 and 5 inch conduit as grout pipes. Once complete, Michels also constructed a transition on either end of the crossing and HDPE to PVC duct system terminating in cable vaults installed by others.

Michels also was contracted to construct some minor electrical works and restore all worksites to their pre-existing conditions.

Why Trenchless Technology

Trenchless Technology was employed for the VCCT project for a number of reasons, including the minimal environmental and public impacts of constructing the crossing, and to minimise the duration of the construction impacts.

VCCT Project Manager Marcel Reghelini said, “Current engineering standards require that the new power line remain serviceable following a design earthquake event. The depth of installation required to meet this serviceability requirement was such that installation by dredging would not have been a practical proposition. The installation would therefore be possible only with a trenchless method.”

HDD was chosen as the preferred method after considering both conventional tunnelling and HDD.

“After analysis, it was concluded that the design of cables with post-seismic stability was significantly more complicated and construction more expensive for cables installed inside tunnels than for cables inside HDPE conduit pipes installed in a HDD bored hole. Also, both the cost and schedule duration of tunnel construction was significantly more than to complete a HDD bore,” said Mr Reghelini.

Challenges

A number of challenges were overcome during the HDD crossing, especially as work was carried out close to the environmentally-sensitive water body of False Creek. Part of the HDD bore path traversed through glacial till soils that contained gravels, cobbles and boulders. There was limited previous experience of extensive drilling through such material, except for an exploratory boring carried out approximately two years ago by BC Hydro.

Some length of the hole, drilled during the exploration, was used in the final product hole, but some new length of the hole was also drilled through till.

During drilling, a number of underground utilities that were not known about at the design stage were encountered. The construction plan had to be amended to address problems presented by these utilities.

The design required that a minimum of 90 per cent of the drilling fluid volume in the annulus between the bored hole and the bundle be replaced by a purpose designed, low thermal resistivity grout. At its deepest point, the HDD bore path was approximately 40 m below the bed of False Creek.

Tom Bryski of Golder Associates Ltd (Project Geotechnical Engineer) said, “To our knowledge, nothing like this scale had been tried anywhere in the world before. There were a number of challenges in designing the grout that had to possess suitable flowability characteristics to traverse the 425 m distance to the lowest point of the HDD borehole, and adequate density to replace the drilling fluid by tremie method and an elevation difference of some 50 to 60 m.

“At the same time the grout also had to comply with the thermal resistivity requirements established to ensure adequate heat dissipation to maintain the transmission cable design performance.”

The installation of the grout, 50 to 60 m below ground level, and simultaneous replacement of the drilling fluid presented further challenges.

In addition, the work was carried out in city streets in a very dense urban area, under high public visibility and with lots of vehicle, bicycle and pedestrian traffic. The pipe bundle pull route was also along ten city blocks and with two 90 degree bends.

As crews began to pull the pipe bundle into the crossing under False Creek, the bundle started to twist, and progress was stopped one-third of the way into the bore hole.

“Crews had to remove the temporary bindings, untwist the pipe bundle using fork lifts and excavators, and lay the pipes directly onto the street to prevent further twisting,” Mr Reghelini said.

Careful design and planning

To overcome these challenges, BC Hydro used careful design and planning to ensure the successful completion of the crossing.

“Readiness to change the construction strategy when problems occurred – for example, when unexpected utilities were encountered and it was evident that if the boring had continued as originally planned, frac-outs could be a serious problem, it was decided to use a pilot hole drilling intercept method to overcome the potential problem. A conductor casing was also installed at the exit end,” Mr Reghelini said.

The project also conducted an extensive construction communications program. The project team kept the public regularly informed regarding the construction, and used various methods including:

• 24 hour construction phone line during eight day pipe bundle pull and installation
• Local neighbourhood advisory group meetings
• Hand delivered notices (sometimes on a daily basis)
• Email updates (sometimes on a daily basis)
• Website updates.

The team also used ‘silent’ equipment, in that the equipment used was as silent as possible and sound curtains were employed where practical to mitigate noise.

Mr Reghelini also said that the great teamwork involved also helped to overcome some of these challenges. “The project team – owner, contractor and consulting engineer – worked collaboratively through the project. When construction challenges occurred, the focus of the discussion amongst the project team was always in solving the problem.”

Environmental and cultural considerations

The HDD entry-pit was located in David Lam Park adjacent to False Creek, a high public-use area located near condominium towers, a school and a day-care centre. The area was identified as an area with a potentially high risk of encountering contaminated soil and water.

An Environmental Protection Plan was required of Michels Canada to prevent release of sediment into nearby catch basins and drains which outfall to False Creek, and to prevent and address inadvertent spills of substances used during construction like fuel, hydraulic fluid and drill mud. Measures were required for the proper disposal and treatment of any contaminated water or soil encountered during the course of construction.

The HDD entry and exit points were located within fills overlying the historical shoreline of False Creek. The historical shoreline of False Creek has some known archaeological sites associated with former settlements of First Nation communities.

Although the location of the entry point was selected to avoid known archaeological sites, a Chance Find Recovery procedure was employed by the contractor in case a previously unidentified site was encountered.

Representatives from the Tsleil-Waututh Nation had archaeologist monitors on site, funded by the project, during excavations in areas not previously excavated with archaeologist monitors present.

Mr Reghelini said that some key environmental benefits from using HDD for the VCCT project are:

• Minimum disruption to the environment, both temporary and permanent
• Reduced carbon footprint compared to dredging or tunnelling.

The trenchless crossing component amounted to less than 10 per cent of the total project cost.

The VCCT project is expected to be commissioned in the Spring of 2013.