Niagara Tunnel Project

The completion of the Niagara Tunnel project in 2013 will allow the delivery of an additional 500 m3/s of water to the Sir Adam Beck hydroelectric stations at Niagara Falls, Ontario. The Niagara Tunnel Facility Project will allow the owner, Ontario Power Generation, to increase the stations’ average annual energy production by 14%, or 1,600 GWh. The project comprises the design and construction of one 10.2-km long water diversion tunnel from

the Niagara River above the Horseshoe Falls to the existing Sir Adam Beck hydro stations, along with the associated intake and outlet structures. The tunnel follows the alignment of the Niagara River and is 140 metres below ground level at its deepest point.

The project is being constructed following the design-build project delivery model, with Strabag AG from Austria as the main contractor and local sub-contractors including Dufferin Construction and Dufferin Concrete.

ILF Beratende Ing. from Austria is the principal designer, assisted by Morrison Hershfield of Toronto. Hatch Mott MacDonald in association with Hatch is acting as owner’s representative for OPG.

The tunneling began in September 2006 and was completed in March 2011. The scale of the tunnel and the geological conditions required innovations and careful engineering design, of which just a few highlights are outlined below.

Big Becky – Largest Hard

Rock TBM in the World

The diversion tunnel is constructed using a two-pass tunneling system from the outlet canal to the intake excavation. The first pass of the two pass system is excavation with the TBM and installation of initial rock support. The second pass is the installation of a cast-in-place concrete liner.

The tunnel boring machine (TBM), named “Big Becky” by children from Port Weller Public School in a local competition, is a 14.44-m diameter Robbins open gripper TBM. It is the largest hard-rock TBM in the world to date.

An initial rock support consisting of swellex rock bolts, steel channels, welded wire mesh and shotcrete was installed from the TBM and trailing gear, followed by the cast-in-place concrete lining. The approximately 1.7 million cubic metres of excavated spoil material was transported from the TBM by conveyor belt and stored on OPG property between the two existing power canals.

Unreinforced

Cast-in-Place Liner

Some of the design criteria that led to the adoption of the two pass system and the choice of an unreinforced cast-in-place concrete final liner were:

• The final liner must be completely watertight to prevent fresh water from entering the rock mass. A characteristic of the Queenston shale formation through which the lower section of the tunnel is constructed is the potential of rock swelling in the presence of fresh water. This potential swelling is due to a migration of chloride ions from the chloride-rich pore water.

• The ambient ground water in the host rock formations is very corrosive, which led the designers to minimize the use of steel reinforcement in the final liner.

• The tunnel needs to be as hydraulically efficient as possible to reduce head losses due to friction. Any such losses would directly affect the generation capacity at the Sir Adam Beck hydro stations.

Taking these criteria into consideration and following a thorough review of the design’s constructability, the design-build team of Strabag and ILF adopted two novel concepts for this size of tunnel:

• The tunnel has a waterproof membrane between the initial and final lining that is fully testable in-place.

• The final lining is unreinforced cast-in-place concrete which is externally pre-stressed by high pressure grouting between the waterproof membrane and the initial shotcrete support.

Innovative Waterproof Membrane

The waterproofing membrane system consists of:

• a geotextile fleece fixed to the shotcrete with nails and Velcro discs;

• a vacuum testable dual-layer polyolefin (FPO) membrane system (2 mm thick layer plus a 1.5-mm thick dimple layer) used in the tunnel invert in rock formations with swelling potential;

• a prototype electrically testable 3 mm thick laminated FPO membrane used in the tunnel arch.

The geotextile fleece protects the waterproofing membrane from damage by contact with the shotcrete. The fleece is backed by a thin plastic membrane which facilitates the flow of interface grout.

The polyolefin membrane systems were developed to meet the design criteria of being 100% tested.

Pre-stress Grouting

The interface between the inside of the membrane and the cast-in-place concrete lining is contact grouted over the full circumference using low pressure cement grout to fill any voids and imperfections within the concrete lining. A second stage of interface grouting, with pressures of up to 20 bar, is carried out through a system of grout hose rings installed at regular intervals between the initial lining and the waterproofing membrane.

Grout blocking rings are provided around the circumference to control the flow of grout along the tunnel during the high pressure grouting operation. These details ensure the uniform grout distribution through the geotextile fleece and facilitate the filling of joints and cracks in the initial lining and the surrounding rock.

Currently the success of the interface grouting is being carefully monitored by precise deformation measurements of the final lining around the full circumference of the tunnel. Fixed monitoring sections are surveyed before and after interface grouting, while mobile monitoring sections, supported by gantry mounted laser scanners, measure deformations to ± 0.5mm during the interface grouting process. The grouting pumping pressures defined by structural analysis are thus kept within the allowable limits and the pumps are automatically shut off as soon as the threshold values are reached.

Interface grouting is the final step in ensuring that the tunnel has the 90-year design life that the owner mandated in the contract.cce

Owner: Ontario Power Generation
Design-Builder: Strabag AG with Dufferin Construction, Dufferin Concrete, COH
Principal engineering design: ILF Beratende, with Morrison Hershfield
Owner’s engineer: Hatch Mott MacDonald with Hatch (John Tait, Paul Moorhouse, P.Eng.)

https://www.canadianconsultingengineer.com/issues/de.aspx