Digging for Electricity – The Niagara Tunnel Project

By 2009, the Niagara Tunnel will deliver an additional 500 cubic meters per second of water to the Sir Adam Beck Generating Complex. The tunnel's intake is being constructed beneath an existing water control structure (INCW) while its outlet will connect to a series of canals immediately upstream of the intakes to the Sir Adam Beck 1 and 2 generating stations.

The tunnel has various technical challenges, not the least of which is that it is situated in a geologic phenomenon known as the Queenston Formation, a mudstone that swells when exposed to fresh water. The potential for swelling has been a major challenge throughout development of this project.

The tunnel is being built on a design/build basis by Austrian construction firm Strabag AG. Hatch Energy, associated with Hatch Mott MacDonald, is acting as the Owner’s Representative for OPG. The current project started in mid-2004 with pre-qualification of potential contractors and preparation of proposal documents followed by a call for proposals in late 2004. In 2005, three international proponents submitted proposals. After evaluation and negotiations, the contract was signed in mid-August 2005, with the start of construction in September of that year. Actual tunneling commenced a year later after procurement, fabrication and erection of the tunnel boring machine (TBM) within the constructed outlet canal. 

Hatch Energy’s involvement with this project dates back to 1989 with Phase 1 definition studies of the Niagara River Hydroelectric Development (NRHD) for development of additional hydroelectric facilities at the Sir Adam Beck (SAB) Generating Complex. In the early 1990’s, Hatch Energy (then Acres) participated in Phase 2 definition studies including construction of an underground test excavation to determine the constructability of underground excavations in the Queenston rock formation. The NRHD project consisted of twin 10.4-km diversion tunnels, an underground powerhouse, and intake/outlet structures near the existing Sir Adam Beck 1 and 2 powerhouses. Detailed geotechnical investigations were performed as well as an extensive testing program to determine swelling and other characteristics of the Queenston Formation. Hatch Energy also participated in preparation of the Environmental Assessment that was approved by Ontario’s Ministry of Environment in October 1998. The approved project included construction of two additional diversion tunnels and an underground generating station north of the existing SAB generating stations.

Overcoming challenges in the Queenston Formation

Rock swelling in the Queenston Formation is caused by a combination of reduced in-situ stresses in the rock surrounding the tunnel and availability of a source of fresh water from within the tunnel. The porewater of the Queenston Formation is highly saline and, through a process of pore-water dilution and ion diffusion, the clay minerals in the rock expand and absorb water resulting in swelling of the rock. This process would impart large pressures on the tunnel linings if these conditions are allowed to occur.

Extensive modeling of the swelling process and its effects on the load build-up on tunnel linings were studied throughout the 1990s. Modeling included development of a 'swelling law' to characterize the relationship between swelling and in-situ stresses. In this material, swelling decreases in proportion to in-situ stresses and ceases altogether at a confining stress between 4 and 5 MPa. The entire process was modeled using specially developed 'Fish' routines for the 2D FLAC geotechnical analysis program. Various options for dealing with the stress build-up were considered including very high-strength liners and compressible grouts.

Tunnel design

The tunnel is being constructed in two passes with rock dowels, steel ribs, mesh and shotcrete, followed by a polyolefin membrane and unreinforced 600-mm thick cast-in-place concrete lining. The lining will be prestressed to resist internal water pressure using a high-pressure 'interface' grout applied between the shotcrete and the final lining. The combination of the membrane and prestressed lining system will prevent water from entering the rock and resultant swelling. Two layers of membrane are being applied to the shotcreted rock enabling the space between the membranes to be vacuum tested after installation in order to ensure membrane integrity. The membrane will also protect the concrete lining from the aggressive groundwater conditions found in the Queenston Formation.

Project status

The TBM is currently about 850 m into the tunnel with excavation starting at the outlet portal. Installation of the membrane and final liner is expected to begin in 2008. Preparatory works are ongoing at the intake area, including demolition and construction of an ice-accelerating wall, construction of the intake cofferdam and an extensive grout curtain to allow construction of the intake portal within the river. The project is scheduled for completion in autumn 2009.

Published with permission of Ontario Power Generation.

For further information, please contact:

Michael Hughes
Engineering Manager, OPG Niagara Tunnel Project
Niagara Falls, ON
905-374-0701 ext 5270
mthughes@hatchenergy.com