Four Sandvik roadheaders were used to open up diversion tunnels on the banks of the Peace River, in western Canada, to take the waters around the planned dam construction site for Site C hydropower project in rural British Columbia.
Site C Clean Energy Project is being developed by BC Hydro, a key power utility in the province and also a major player in both the Canadian and North American electricity markets. The project is the utility’s third large hydropower project on Peace River and is located near Fort St John. It is a large hydropower scheme in its own right and its surface powerhouse is planned to have an installed capacity of 1.1GW.
Electricity will be generated by the flow of impounded water, released from behind the major dam and spillway built across the river valley. But to build the hydraulic structures the site had to be dry and that called for the river to temporarily bypass the site. With banks rising to the sides of the river, the bypass structures had to be tunnels, cut through the hillside.
The contractor on Site C project is Acciona Infrastructures Canada and Samsung C&T Canada, working together in a 50/50 joint venture. Acciona is leader of the JV, which was awarded the contract in early 2016 for a 96-month civils construction period.
The choice of underground structures for the bypass were two large, if relatively short, parallel diversion tunnels.
With full lengths of 700m and 792m, respectively, and excavated widths of 12m (14m in spots), the tunnels were sized with sufficient capacity to pass flows safely and so keep the dam site dry and safe, over the duration of the construction phase, as river levels vary with seasons.
A further role for the vital structures would also be after the completion of the dam, when they would help to regulate river flows during reservoir filling. The impounding would take a while as the reservoir will be about 85km long. With the river was impounded as a reservoir, the gates that were installed across the portals of the tunnel would then be fully closed, allowing the tunnels to be decommissioned, their tasks done.
However, there was the initial challenge of excavating the diversion tunnels.
The tunnels, with centres 37m apart, run through the rock of north bank of the river (that is the left bank when looking downstream). On the opposite bank are located the surface powerplant and spillway structures.
With their inverts more than 20m below river level, the tunnels themselves needed preparatory works to begin and also to protect the excavations as they proceeded. Cofferdams were installed into the river bed near the ends of the tunnels to establish their construction zones. Once completed, the tunnels would be open to deep flows from the river but before then the waters had to be held back from both the inlets and outlets.
While the JV contractor designed a number of M&E elements as well as some temporary works for the diversion tunnels and the final design of the cofferdam, BC Hydro was responsible for final design of the diversion tunnels, including the inlet and outlet. The final size of the tunnels was 10.8m-wide with thick concrete lining, cast by pouring with circular slip forms.
To construct the twin diversion tunnels, the JV used 4 x Sandvik roadheaders to excavate through bedrock geology consisting of interbedded siltstone, sandstone and shale of the Shaftesbury Formation. Exploratory adits gave good site data to support the design. The shales are Cretaceous origin, generally dark grey and massive, and they have 70 degree dip relaxation joints close to slopes and sub-horizontal bedding planes. The stratigraphy is similar on both banks of the river.
The bedrock is overlaid by fine-grained glaciolacustrine deposits and morainic silty and clayey diamictons, arising from the extended glacial history of the area
With the Site C project in planning for some decades, and with more intensive ground investigation then having taken place over 2009-2015, the project owner had performed a notable degree of site investigation. The work helped to establish that the diversion tunnels would be located entirely in bedrock, generally with cover of about 80m, except near the portals.
While the glacial period is long gone, the region experiences very low temperatures at times and conditions were below -40°C during the construction works at times, adding to the tunnelling challenge for the JV.
The roadheaders used on the job were 3 x MT520/023 (2 x new, 1 x refurbished) and 1 x MT720/084 (refurbished).
The JV used the roadheaders to excavate downstream to take out approximately 180 000m3 of bedrock in total, opening up the tunnel profiles in head and bench steps. The top headings were opened up to 7m-height (60m2 face area), and the bottom benches excavated to the same face area. Each tunnel had a 6m radius profile and flat bottom.
Excavation support was generally shotcrete layers with rockbolts (4.5m long x 22mm dia), except for near the portals where steel ribs were also used. A further challenge was the demanding silica exposure controls.
For the final lining of each diversion tunnel, slip forms (12m-long, self-launching) were used to cast concrete in 600mm thick walls to create a circular finish. A total of 80 000m3 of concrete was poured from the telescopic distribution hose pipe. Contact grouting was then undertaken to fill any voids remaining behind the permanent concrete walls.
The JV undertook design and production of all mix designs for grout, shotcrete and concrete.
The two diversion tunnels were completed during the start of the covid pandemic and the river was diverted in Fall 2020, completing Phase 1 of project development. Phase 2 is underway for construction of the dam and power plant, and then impounding of the river.
While significant, the diversion tunnels though were not the only tunnelling works on the project. Other tunnels were excavated on the opposite river bank for groundwater drainage.
