Ontario Power Generation Inc. decided to construct the Niagara Tunnel Project to divert an additional 500m3 of water per second to feed the upgraded Sir Adam Beck hydroelectric stations located near Queenston. When the project is completed, 1.6 billion kilowatt-hours will be added, enough electricity for 160,000 homes.
The work was awarded to STRABAG Inc. Morrison Hershfield is the Lead Consulting Engineer for the outside works including all tunnel intake works. The intake works are major river training structures in the Niagara River that direct water into the new tunnel being excavated 50m below the river bed.
Each winter, apart from freezing of the river water, huge masses of ice form in Lake Erie and float down the Niagara River. This situation may create blockages, ice damages, or reduction of flow into hydro-electric power plant intakes. Chunks of ice may even enter intake tunnels causing potentially serious damages to the facilities, unless control measures are taken.
The $30 million intake works control measures include a 360m river bank training wall called the “Approach Wall” and a 530m long “Accelerating Wall” situated in the river approximately 150m from the bank. The walls need to withstand the huge force of water and ice; be flexible to accommodate the uneven river bed and the hydraulic shape and wall alignments; and be designed, manufactured and quickly built within 12 months. The walls need to be aesthetically pleasing when completed. The project also includes the removal of existing river training structures.
Morrison Hershfield provided design and engineering support during construction for the intake structures. A precast modular concept was developed for both walls that can be installed using conventional barges and spud barges. This type of construction is the first to be done in Canada. The final dimensions and weights of the 400 ton precast units were worked out carefully with the contractors, suppliers and transportation companies to optimize production, storage, delivery and installation, as well as effecting economy. Close to 500 precast units were fabricated and placed in the Niagara River for the intake works.
Two major innovations were developed in the process of design. The first is a three dimensional locking system (keys) to ensure tight fitting of the units both vertically (stacking) and longitudinally along the wall. 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 which allowed fine-tuning of levels of the submerged bottom units by workers working on the barges or boats.
Both innovations were developed to avoid potential problems in the linear installation of precast units, which is a very important aspect of the design. Otherwise, a slight tilting of one single unit would result in total disarray when the units start to be placed on top and against it. For a 500m long wall, errors would quickly add up.
Extensive in-water blasting as well as tremie concrete (concrete placed direct in water) paddings were required to form the bed where the unit will sit. In-situ closure sections and end nosing as well as connections to existing structures were constructed using permanent steel forms anchored into the river bed and cast using reinforced tremie concrete.
Fine details were not overlooked, including aesthetic details, maintenance walkways, fall arrest protection system and navigational lights and signs. Heavy silt screens were installed in the river to minimize contamination of the water from construction activities. A section of the approach wall was also designed as a temporary dock for the unloading of units onto barges.
As in most in-water works, significant site variations were encountered including deviation in river bed elevations and the presence of overburden at foundation locations not previously known. Problems associated with construction were also encountered such as out-of-place installations, as well as construction errors in placement and manufacturing. Morrison Hershfield engineers were able to tackle each issue with prompt solutions to address the changed site conditions as well as construction problems.
Our environmental team used their expertise in environmental regulatory compliance to ensure that the project and its components met environmental standards.
The intake walls were substantially completed on schedule and on budget. The walls provide a smooth and uniform appearance. They have well performed their ice management function throughout two winters.