The original photo may have been at this link. Anyway, regardless of source, the engineer has the following questions for @sitecproject and @bchydro, given what we know of the history of geotechnical instability in the Peace River region... #SiteC
@sitecproject @bchydro ..Engineer: "Don't know what to say, looking at this, other than "yikes." That more yellowish material at top could be a layer of what locals in the Peace/Fort St John area have sometimes called 'gumbo', along with some other interbedded material..." 3/x #SiteC 
@sitecproject @bchydro ."I'm wondering about the Old Fort slide near #SiteC. Photos showed that yellow-brown "gumbo" or clay all buckled up in odd mounds. Sitting atop the underlying shale, as in this photo, if the shale started to slide underneath, it'd easily cause those mounds by taking clay w/ it"
@sitecproject @bchydro "No wonder that slide confounded everyone, if it wasn't just the clay layer that slid, but it rode on top of the underlying shale that was sliding, due to rain water percolating into it from the gravel pit..." Peace R. shale is rock-like until wet, when it loses cohesion, -> mud.
@sitecproject @bchydro "Now they've removed the ground cover up to the L in photo, so water can seep down into the shale. On the E. end of the excavation we can see the how the river incised the valley over time by the shale sliding under the clay, & the clay folding over the shale down to the river.."
.."This top layer of clay supports trees & grasses that help keep it together and help stop rain water & into the shale..." but these have largely been removed... #SiteC
"Rebound cracks in the shale [this shale is still expanding after being compressed under ice in the last ice age] let water percolate into the shale as we've seen in other photos, likely allowing mud that forms in the cracks to act like grease -> chunks of shale slide down..."
"Possibly freeze/thaw cycles over the seasons can help this to happen. Over the centuries the valley forms and lowers several hundred feet along the river channel...
.."This clay may absorb water & hold it, rather than letting it percolate through... for a while. Until there is too much water for it to absorb, or cracks from freezing or drying form, & allow rain to get into the shale directly, or humans disturb the clay ground cover.." #SiteC
.."In the failure of the nearby Peace River bridge in 1957, the road ditch channeled rain (storm) water right onto the shale ledge supporting the bridge pier. Maybe with enough velocity it eroded under the pier and start turning the shale to mud, like grease..."
.."That would explain why the gaps started to show up in the road as the edges of the Peace R bridge's pier lost their support and it started to settle and move. Only took a few hours for it to lose enough support that there was insufficient remaining to hold the pier in place"..
.."We usually think of a landslide being the top surface that slides, & it almost always is. But with this shale, the actual slide takes place under the top surface clay & the clay just rides along. Because the cause is not visible, you can't know when or where it will happen..."
All thoughts & questions welcome. Thanks for following along with today's Fun With Mud. #SiteC
PS: this engineer studied under Eng. Prof Thomson, author of a key report on the Peace River Bridge collapse. Thomson brought a sample of the shale from under the collapsed bridge to class. Dry, the sample looked & behaved like solid rock. He put it in a jar of distilled water..
The reason this material slides was explained by another U of A prof, R.M. Hardy: it absorbs fresh water & swells itself apart, loses all its engineering properties, e.g. shear & bearing. It's a chemical or molecular action peculiar to this clay shale's composition of minerals.
With bentonite or other clays, if they get wet, their surface blocks water entering the clay. With this Peace R. shale, it actually attracts water into itself. The swelling is the dangerous thing - it destroys the structure, & clay particles force each other away like magnets
The shale particles repel each other with a v. large force when the water is fresh (eg rain). Water with salts in it doesn't cause as large an effect...the shale still swells but not as forcefully.
The question is this: if reports are available on the atypical behaviour of this shale, with its swelling properties, its locally famous instability with many tales of massive landslides all over the region... why is anyone building anything on it?
