Award of Excellence Niagara Tunnel

The Niagara Tunnel is the largest hydroelectric project in Ontario in 50 years and will provide renewable power over the next 100 years. The tunnel is 10.2 kilometres long, with a bored diameter of 14.4 metres. At its deepest point, it is 140 metres below the city of Niagara Falls, Ontario.

It was built by Ontario Power Generation to divert water from the Niagara River, and carry it downstream to the Sir Adam Beck Generating Stations. Gravity alone will propel the water at an incredible 500 cubic metres (17,660 cubic feet) per second.

As the owner’s representative, Hatch Mott MacDonald (HMM) and Hatch developed the concept design and set out the owner’s mandatory requirements to ensure a 90-year service life without any outages.

Put into service on March 9, 2013, the tunnel enables the production of 1.5 TWh per year of clean renewable energy for the next 100 years or more. That is enough energy to power a whole city the size of Kingston or Barrie in Ontario.

After selecting a two-pass design-build method, HMM/Hatch administered the contract, audited the contractor’s detail design and after required revisions, accepted its designs of mission critical systems.

Worldwide firsts

The sheer size of the tunnel and the geological conditions along its route required important innovations in design and construction techniques. These involved several worldwide firsts.

The largest hard rock tunnel boring machine at the time was used, along with the largest pre-stressed non-reinforced 600-mm thick concrete liner. Also, a full surface electrically testable polyolefin liner was developed specifically for the project to protect the concrete liner from corrosive water in the host rock and to prevent the transfer of freshwater/chlorine ions to the rock (which was prone to swelling). The project also used full-surface scanning (no targets) revolving lasers to measure concrete deflection in real time during high pressure (20 bar), pre-stress accurate to +/- 0.5 mm.

The diversion tunnel was excavated using a two-pass tunnelling system. The 14.44-m diameter Robbins open-gripper TBM known as “Big Becky” was was the height of a four-storey building and weighed 4,000 tonnes. The machine excavated approximately 1.7 million cubic metres of rock, which was transported through the tunnel by conveyor belt and stored on Ontario Power Generation property.

The bored tunnel was initially supported by rock bolts, steel channels, steel mesh and shotcrete, the latter applied from the TBM trailers. The final lining consists of an impermeable membrane and a 600-mm thick unreinforced concrete liner that is pre-stressed with interface grout.

Innovative tunnel lining

The tunnel has a waterproof membrane between the initial and final lining that was fully testable in-place prior to pouring the concrete liner. The waterproofing membrane system combines a layered polyolefin membrane with a layer of geotextile fleece that provides protection from damage by contact with the shotcrete. The fleece is backed by a thin plastic membrane that facilitates flow of the interface grout. The outer layer of polyolefin and the fleece are conductive. By applying a high voltage (low amperage) potential across them, any flaw in the main membrane is exposed by an electrical short. The resultant singeing is visible and is recorded by heat cameras.

The interface between the inside of the membrane and the cast-in-place concrete lining was contact grouted over the full circumference of the tunnel. Low-pressure cement grout filled any voids and imperfections within the concrete lining. A second stage of interface grouting, with pressures of up to 20 bar, was carried out through a system of grout-hose rings installed at regular intervals between the initial shotcrete lining and the waterproofing membrane. Grout blocking rings were provided around the circumference to control the flow of grout along the tunnel during the high-pressure grouting operation. These details ensured uniform grout distribution through the geotextile fleece and facilitated the filling of joints and cracks in the initial lining and the surrounding rock.

Single entry point

The environmental assessment meantthat all the underground work had to be accessed from a single entrance at the outlet end of the tunnel on property owned by OPG. In order to allow concurrent activities, all tunnel operational equipment had to be designed to allow traffic to pass to and from the TBM. To limit traffic in the tunnel, all rock removal was done using a conveyor that ran at the end of the excavation for approximately 11 km to the rock disposal site. cce

Project name: Niagara Tunnel Project, Niagara Falls, Ontario

Award-winning firm (owner’s

representative): Hatch Mott MacDonald with Hatch (Harry Charalambu, P.Eng., John Tait, Paul Moorhouse, P.Eng., Kevin Child)

Owner: Ontario Power Generation

Other key players: Strabag (contractor); ILF Beratende Ingenieure (L Design), Morrison Hershfield (surface works design); Dufferin Construction (subcontractor).