Site C instability

BC Hydro admits that the massive dam foundation structure it has poured on the south bank of the dam, a roller-compacted concrete buttress, is showing signs of serious foundation issues. BC Hydro’s talk of attempting to stabilize it with various forms of anchors and new drainage methods indicates that the buttress is sinking and/or shifting, as well as experiencing water pressure. This buttress supports key parts of the dam: the spillways and the generating station/turbine hall. BC Hydro admits it does not know how to fix the problem, nor how much trying to fix it might cost. It describes the situation as code red. See BC Hydro’s reports or read commentary on them by Vaughn Palmer in the Vancouver Sun: Site C Dam has a huge problem and it’s not Covid-19.

There is no bedrock at Site C, only an unstable shale that is several hundred feet deep at the dam site and along the valley. This shale is deceptively hard when dry, but soft, swelling and cohesionless when exposed to water. This shale is the cause of the region’s famous landslides. It is also the reason many experts and engineers urged the BC gov’t not to proceed with the dam.

Read a superb summary of Site C’s geotechnical problems, including interviews with engineers, by investigative journalist Ben Parfitt: “Site C’s Radical, Risky Makeover – BC Hydro says it has just discovered new, costly problems for the megaproject. In fact, engineers have known about them for decades.”

The Long Answer

In December of 2019 BC Hydro discovered serious stability problems with the Site C dam. Hydro only disclosed this fact to the public in two long-overdue Site C quarterly reports that it finally released on July 31, 2020 —the Friday of a summer long weekend. In the reports, BC Hydro admits there are serious foundation problems with the roller-compacted concrete buttress that is the foundation of the dam’s spillways and generating station (turbine hall etc).

BC Hydro admits that it does not know how to solve the problems but it talks about some form of stabilization: anchors, shear keys (like a wedge into underlying material to brake movement), and extra drainage which suggests that they need to solve water ingress and/or relieve hydrostatic pressure building up in the shale geology under and behind the buttress.

Most of these geotechnical problems were long predicted by engineers as well as locals in the Peace region familiar with the area’s instability.

The dominant underlying geology of the Peace River Valley is a unique dark grey shale that some engineers just call Peace River shale. It’s a compacted marine Cretaceous shale that is deceptively hard when dry—hard enough that it must often be jack-hammered—but when exposed to water it becomes soft, swells, loses cohesion and separates. The swelling and lack of cohesion are the reason for the Peace Valley’s frequent landslides also the disintegrating roads. This shale is many hundreds of feet deep at Site C. When BC Hydro speaks of “bedrock” at Site C, it is this shale they are speaking of.

A confusion over the meaning of “rock” partly arises because geologists often refer to shale as “bedrock,” even though it only consists of tiny rock flakes, but engineers generally do not—or at least should not.

A further problem with this shale is that it is currently in a state of “rebound” after being released from the compression of the weight of glaciers thousands of feet deep during the last ice age. If you place millions of tons of concrete on rebounding shale, the shale will be likely to compact again, producing settling. This may be what we are seeing under the buttress at Site C.

That this shale has extremely low compression strength was actually admitted in an October 2016 paper by several engineers working for the main engineering contractors for the dam itself, Klohn Crippen Berger (KCB). (Note: one of its authors is former lead engineer of the Site C dam design team, John Nunn. He is a former KCB engineer who is now on the BC Hydro board as well as the Project Assurance Board that the BC NDP government formed to oversee Site C. He is thus effectively checking his own work and questions have been raised about conflict of interest.)
The 2016 paper was published in the Canadian Dam Association Quarterly magazine in summer 2017.

Site C L-shaped dam design

As the dam’s engineers described in the paper above, Site C’s unique L-shaped dam design—a design that has never been tried with an earthfill dam before—was devised in late 2010/early 2011 to avoid attaching the dam to known weak layers of geology on Site C’s south (right) bank. Instead of affixing to the bank, the dam takes a dogleg and situates the spillways and generating station on that short side of the L, all sitting on a concrete buttress. Yet that buttress is now slipping, even though this foundation area was meant to be more stable than the side of the south bank.

Many engineers see no way to solve the problems of the shale’s low compression strength and the risk of water penetration. BC Hydro says it’s looking into 3 solutions: anchors (to what?), a shear key (something to stop 1.2m cubic metres of concrete from moving? Jam the key into what?) and extra drainage (clearly water is building up behind the buttress and creating pressure against it). BC Hydro needs to be more transparent about the exact signs of trouble and how much the fix would cost—but they admit they don’t know how to fix it or how much that would cost.

Slide history:
• A heavy rain undermined the footing of the Peace River Bridge at Taylor in 1957 and collapsed it. It had recently been built by the US Army Corps of Engineers for the Alaska Highway. A review of that bridge failure by U. of Alberta engineering professor R.M. Hardy analyzed the geological cause.

• Slides near Site C have on occasion been large enough to block off the whole river, impressive given that the river is often called the “Mighty Peace.” Near Site C, one such slide occurred at Bear Flat around 1890 or 1900, and another was the 1973 Attachie slide at the confluence of the Halfway and Peace Rivers (would be submerged in the future Site C reservoir). That slide is shown here, shortly after the river managed to carve a channel through the slide.

Attachie Slide, Peace River, 1973

Veteran BC hydro engineer Vern Ruskin, who worked on the construction of the early Peace and Columbia dams and is one of the few engineers in BC with actual BC dam experience, is on record as saying that Site C can’t be guaranteed to be safe. His main worry wasn’t just that there was no bedrock under the dam and it could fail, but also that a slide into the Site C reservoir could create an overtopping wave that would collapse the dam (common cause of dam failure especially with earthfill dams). His view was that filling the reservoir would force water into the shale and likely cause numerous slides along its banks. See Vern Ruskin’s official submission to the BCUC Site C review in 2017.

For a useful 2013 compilation of information about Peace Valley instability and the dangers of Site C, see this letter from Arthur Hadland, a former director of the Peace River Regional District.

Also see a superb summary by investigative journalist Ben Parfitt of Site C’s geotechnical problems with interviews with engineers: “Site C’s Radical, Risky Makeover – BC Hydro says it has just discovered new, costly problems for the megaproject. In fact, engineers have known about them for decades.”