No new tension cracks at Site C, BC Hydro says

Site C opponents say a new photograph of the contentious dam site is evidence of continued geotechnical problems and cracks causing headaches for builders of the $9-billion project, but BC Hydro says their concerns are simply remnants of routine excavation and construction.

sitecgeotech
Submitted

An aerial photograph taken Oct. 28 by former independent MLA candidate Bob Fedderly has been circulating widely across social networking sites, marked up to show what opponents say are new tension cracks and erosion problems on the north bank of the Peace River. 

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"A third major tension crack and on-going erosion is evident at the Site C construction site," a caption reads. "Clearly BCUC's concern that the project would exceed even $10 billion due to geological unknowns and instability at the site is valid."

In October, BC Hydro acknowledged to the BC Utilities Commission that two tension cracks on the north bank that emerged earlier this year had pushed back a planned 2019 river diversion to 2020 and added $610 million to the project's cost—adding weight to opposition arguments that the dam site is too unstable to support a dam and will continue to drive the project over its budget. 

Construction crews have been excavating the bank to remove historic landslides in the dam site area, however, slope instability caused a work safety stoppage in July and forced main contractor Peace River Hydro Partners to file for a 435-day schedule delay in August.

But BC Hydro says there are no new tension cracks at the dam site.

"What is indicated as cracks on the photo circulating on Twitter are simply access roads and horizontal cuts visible in the slope that are remnants from excavating the slope with a bulldozer," spokesman Dave Conway said.

"The areas shown as erosion are locations where there has been some localized erosion which is typical during excavation activities of this size and nature."

Conway provided detailed analysis of the concerns highlighted in the photograph (below), and added erosion due to weather, and surgical springs and sand lenses that carry water through the slope is always a possibility during construction. 

"This has been carefully considered and in no way compromises the final design which incorporates extensive measures for the care of water both during and after construction," Conway said.

"These measures included ditches, drainage layers, erosion control matting, revegetation and a central drainage channel designed to carry water off the left bank safely."

A 2009 engineering study of the dam site just outside of Fort St. John notes Site C is being built on shale bedrock, and that the slope of the north bank above the dam would have to be flattened to provide "adequate stability" because of glacial sediments. The design of the slope has undergone several revisions since the initial design in 1982, when it was estimated 15.6 million cubic metres of earth would need to be excavated. 

That material was initially identified in 1981 to be used as impervious material for the main core of the earth-fill dam, however, the 2009 study notes a subsequent reassessment in 1989 found the material to be unsuitable.

"This was due to the fact that 30 per cent of the zone consists of granular material and the fine materials have a wide range of moisture contents," the study reads. "Therefore, suitable sources of impervious materials have to be identified."

BC Hydro has since found that material in 85th Avenue industrial lands just outside the city near the dam site that will be used as a quarry. The material will be excavated and shipped via conveyor belt to the dam site.

In a September 2017 construction review by Deloitte LLP, independent consultants hired by the utilities commission for its economic review, the auditors highlighted continued geotechnical risks at the north bank. Concerns were also about rock conditions where river diversion tunnels are being excavated, where the dam's powerhouse and spillway are being built on the south bank of the Peace River, and on the dam embankment itself.

"The consequences of the dam risks would involve additional time for the construction and potential costs," the Deloitte report reads.

In its submissions to the utilities commission, BC Hydro estimated the costs of further geotechnical problems to run anywhere from $10 million to $100 million.

"The remaining geotechnical risk going forward for surface works has been significantly reduced, as the majority of the work zones at the dam site have been developed and largely excavated. The ground conditions are now known," the utility said.

editor@ahnfsj.ca

BC Hydro Response Re: North Bank Tension Cracks, Site C by mpreprost on Scribd

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