A rock slide slid into the upper end of Diablo Lake in eastern Whatcom County. (I love reminding people that Diablo Lake is in Whatcom County). The slide blocks the access road to the Ross Lake Dam power house.
The slide took place in vertically foliated banded biotite schist and is part of Skagit gneiss complex. This formation consists of very highly metamorphosed sandstone that has been heated pressurized during deep burial during the Cretaceous. These rocks were buried as much as 30 km below the surface approximately 75 million years ago. The rock failure appears to be the result of a combination of vertical foliation with cross cutting horizontal joints. And of coarse the slope is a cliff and gravity and some freeze thaw will cause things to fall down. It won't be the last in this area of very steep slopes.
2 comments:
Ross Dam itself is anchored into a wall of Skagit gneiss. The builders "heavily grouted" the fractures in the rock, according the Scott Babcock who co-wrote Hiking Washington's Geology, but y'know... nobody ever seems to discuss the risk of a failure of Ross Dam!
I have been involved in a couple of grout projects. One tends to get nervous about them.
I have been at the early stage of doing research on the dams of Washinton as they are landscape altering features both up stream and down.
I will say that the rock failure took place in banded gneiss that had a sedimentary rock origin whereas the dam is founded on orthogneiss of magmatic origin.
I do have a detailed reference to the grouting, joints and fracture sets at Ross Dam, but am not experienced enough to gage if the grouting at Ross was unusual.
I will say the construction of Gorge Dam used a technique I never have tried: they froze the ground to hold back groundwater during excavation.
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