Glacial Marine Sediments and Quick Clay

The slide shown above was a fatal quick clay slide in the Province of Quebec. Quick clays and quick clay slides are predominantly a feature of glacial marine clay deposits and are a big problem in parts of Norway, Sweden and Quebec. They can take place elsewhere, but are much less common and not well understood.

During the glacial last period, the thick mass of glacial ice that accumulated over Scandinavia and Quebec pushed the local land surface downward hundreds of feet. As the ice began to melt and retreat, the local land surface was covered with sea water and the melting glacial ice deposited thick layers of clay dominated sediment on the sea floor. As the land rebounded the former clay covered sea floor rose above sea level.

When the clay was deposited in the sea, sodium ions derived from the salt water (NaCl - sodium Chloride) within the clay pore water provide weak but effective ionic bond between clay particles giving the stacks of clay minerals some strength. But after the clay rises above sea level, the sodium may be leached out. With the sodium leached out, the ionic bonds are much weaker and clay looses strength and is subject to failure changing the clay from a relatively stiff structure to one that turns into soup in a cascading chain reaction through the clay deposit.

Quick clay failures can be deadly and even very gentle slopes are susceptible to failure.

Northwest Washington State like Quebec and Scandinavia has large tracts of glacial marine clay deposits, but quick clay slides have not plagued our area. Factors may be the types of clay, rate of deposition, underlying geology that provides groundwater that leaches the Na out of the clay and replaces it with Ca with weaker ionic strength and possibly the length of time since glacial rebound. Northwest Washington rebounded very rapidly post glacial ice and areas of common quick clay failures have been uplifted above sea level for much shorter periods of time. Then again, maybe the sodium is still present in our clays.

I've posted this film before. Candy for geotechs. I will say that I have never seen drill cores do what the drill cores shown in the later part of the film did. I do know of one very old slide site in northwest Washington that I suspect might have been partially due to quick clay based on the geometry of the slopes and overall very gentle slope of the slide area, but alas no drilling has taken place to test the idea.