TABLE OF CONTENTS Oct 2008 - 0 comments

Intake Works at Niagara Tunnel

AWARD OF EXCELLENCE CATEGORY: WATER RESOURCES & ENERGY PRODUCTION; MORRISON HERSHFIELD; Engineers designed a system of huge precast walls built deep under water for the intake to a tunnel for a hydroelectricity station in Ontario.

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2008-10-01

The Niagara River carries water averaging 6,000 cu. m. per second and drops about 100 metres along its route from Lake Erie to Lake Ontario. Since the late 19th century, power plants have been set up on both sides of the Canada/U. S. border to tap into this immense potential.

Ontario Power Generation decided to construct the Niagara Tunnel Project to divert an additional 500 cu. m. of water per second through a 10.4-kilometre tunnel to feed the upgraded Sir Adam Beck hydroelectric station near Queenston. When completed, the project will add 1.6 billion kilowatt-hours of generation.

The work was awarded to STRABAG in August 2005 as a $650-million design-build contract. Morrison Hershfield is the lead consulting engineer for the outside works, including all the tunnel intake works.

Managing ice

The intake works are major river-training structures in the Niagara River to direct water into the new tunnel that is being excavated 50 metres below the river bed.

Each winter the river water freezes and huge masses of ice form in Lake Erie and float down the Niagara River. This situation may create blockages, ice damage, and reduced flow into the plant intakes. Also chunks of ice may enter the intake tunnels unless control measures are taken.

The $30-million intake works include a 360-m river bank training wall called the Approach Wall, and a 530-m long Accelerating Wall situated in the river approximately 150 metres from the bank. The project also required the removal of the existing river training structures.

The walls not only need to withstand the huge forces arising from the water and ice, but also they had to accommodate an uneven river bed and the hydraulic shape and wall alignments. They had to be aesthetically pleasing, and they had to be designed, manufactured and built within 12 months.

Precast modular walls

Morrison Hershfield provided design and engineering support during construction for the intake structures.

For both walls they developed a precast modular concept that can be installed using conventional barges. This type of construction is the first to be done in Canada. Each precast unit weighs approximately 40 tonnes. The final dimensions and weights were worked out carefully with the contractors, suppliers and transportation companies, and were standardized as much as possible. Nearly 500 precast units were manufactured.

Two major innovations were incorporated in the design. The first is a three-dimensional locking system (keys) to ensure the tight fitting of the precast units both vertically and longitudinally. Each key is designed to withstand all forces that may be applied at the joint locations.

The second innovation is an adjustable supporting leg system that allowed workers on the barges to fine-tune the alignments and levels of the submerged bottom units.

Both innovations were developed to avoid problems in the linear installation of the precast units, which is a very important aspect of the design. A slight tilting of one single unit would have resulted in total disarray when the other units started to be placed on top and against it.

Extensive in-water blasting as well as tremie concrete (concrete placed direct in water) were required to form the bed for the unit. The in-situ closures and connections to existing structures were built using permanent steel forms anchored into the river bed and cast using reinforced tremie concrete.

Significant site variations were encountered, including deviations in the river bed elevations, and the previously unknown presence of overburden at the location of some foundations.

Despite the challenges, the intake walls were substantially completed on schedule in December 2006.

Name of project: Niagara Tunnel Facility Project --Intake Works

Award-winning firm (prime consultant): Morrison Hershfield, Toronto (Edward Li, P. Eng., Kevin Pask, P. Eng., Chak Lo, P. Eng., Gurminder Pandher, P. Eng., Joseph Ostrowski, P. Eng.)

Owner: Ontario Power Generation

Client: STRABAG

Other key players: JAN Solution Consulting Services

Suppliers: Dywidag Systems (rock bolts and rock anchors for precast walls)

Photos

The Approach Wall along the river bank, and the Accelerating Wall out in the river channel. The structures are partly to protect the new tunnel's intake from snow and ice.
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Caption: The "Approach Wall" along the river bank, and the "Acce...
Divers help to install precast units. Over 500 units were used.
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Caption: Divers help to install precast units. Over 500 units we...
Adjustable leg for submerged bottom units.
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Caption: Adjustable leg for submerged bottom units.
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