An expert in this shale, Prof Hardy of U of A, has "warned about rebound (expansion) cracks in this shale from the weight of the ice-age glaciers being removed. Rebound is typical of all glacial deposits across the prairies, like a foam pillow expanding when weight is removed.."
.."But in contrast to the Peace River (clay) Shale, most clays show some cohesion & stick together when they rebound. The microscopic clay particles that make up Peace River shale do not stick together at all (cohesionless)... #SiteC
.."If you take some clay & roll it between your hands & it sticks together & forms a ribbon, that's cohesion & is the exact opposite of Peace River shale. Peace shale also attracts instead of repels water so its lack of cohesion also helps make it weak when it gets wet & swells.
.."Peace River Shale has a double-barrel rebound...cracks develop because it is cohesionless, and if water gets into these cracks it swells and develops large forces to make these cracks larger, and the water causes it to disintegrate into particles that don't stick together.."
.."We know cracks (fissures) are present in the shale at #SiteC from early photos of construction showing ice falls (frozen springs) in numerous places just downstream of the dam's powerhouse area..."
"With these rebound cracks in the shale under the dam, reservoir water pressure will be continually trying to force water into them, causing swelling, breakdown of shale into cohesionless particles, & offer no resistance to lg. sliding forces on the dam by the reservoir pressure"
"If this happens, the dam has nothing holding it in place. Since the water pressure from the reservoir is relentless, it may keep forcing water into these natural cracks that are present. There's no way to tell how far or deep that water can penetrate..."
"As water moves into cracks in the shale, the foundation under the dam gets weaker & the dam loses its ability to hold back the reservoir. The impervious core they add when building an earthfill dam like #SiteC has to prevent water seeping through the dam..." @EngineeringFai4
@EngineeringFai4 .."The impervious core of the dam also has to be deep enough below the dam to stop water from percolating under the dam and coming out on the downstream side, which could erode and undermine it, possibly causing failure..."
.."To calculate the req'd depth of an impervious core under the #SiteC dam, the ease of water moving through the original shale (permeability) must be known & used in the calculations. With cracks in the shale that can allow water to move thru it, this value will never be known."
I had relayed to this engineer info from another source about the taking of core samples from #SiteC in the 70s: "The samples had to be frozen or isolated from the air in order to make it to the lab for testing. Some were packed in C02 or wrapped in wax to slow the degradation.."
And "The techs at that time were all in agreement that the material wasn’t suitable for any kind of construction, let alone base for a dam." That was in the late 70s, and is one reason why this dam kept getting rejected... until Gordon Campbell. Back to comments from engineer:
.."These are my opinions, unproven, but we don't want #SiteC to be a research project. We have to remember it is the first large structure to be build on this shale, after smaller structures including the Taylor bridge failed in 1957 from water causing swelling & shale failure.."
Engineer ends by saying "many lives will be at risk downstream if this dam were to fail in the future, so there can be no uncertainties as to its safety." But as he has said, there can never be certainties as to the safety of #SiteC. Not on material that lacks any shear strength.
So. Peace River shale can feel like solid rock..until it gets wet, whereupon it transforms into the kind of thick jelly that can easily swallow your gumboot, as any resident of the Peace River region will confirm.
Thanks for tuning in to this episode of What I Learned at Work!
Thanks for tuning in to this episode of What I Learned at Work!
PS This thread hasn't even dealt with the seismic risk to Peace River dams, which not only lie on a crazy quilt of natural faults, but are surrounded by rapidly expanding fracking which is causing an uptick in tremors & quakes, cracking the shale & injecting fluid into it...
I feel sometimes as if this fiasco of building an earthfill dam on cohensionless mud & then putting fracking right next next to it & 2 other pieces of major concrete infrastructure - it's like a Picture of Dorian Gray for BC, the true self portrait that we keep in the attic.
..We wouldn't have to speculate about the reason for #SiteC's shale content & properties or its odd straight line design if @bchydro, @sitecproject & Min. of Energy weren't so opaque. Can we see shale test results, full explanation of the design, & seismic readings at all 3 dams?
