17 January 2018
The fascinating Rattlesnake Hills landslide in Washington State
In Washington State the Rattlesnake Hills landslide continues to develop under conditions of close scrutiny from the media. The Washington Department of Natural Resources (WA DNR) has a website dedicated to the problem, which includes the following schematic map of the monitoring that is being undertaken and the broad patterns of movement:
The map indicates a highly complex landslide with what I would consider to be an unusual movement pattern, with displacement apparently approximately parallel to the ridgeline. This is probably associated with the underlying geological structure, WA DNR interpret it as sliding on a silty layer between fractured basalts, dipping at about 15° into the quarry. This of course asks questions about how the quarry was permitted in such geological setting (on the face of it, the setting as described by WA DNR appears to be quite prone to instability), but that is a different issue.
WA DNR have extensive monitoring on the slope, including GPS monitoring sites, prisms for total stations, ground-based LIDAR and of course mapping of the slope. This is a good combination, and the authorities have retained the skills of a number of specialists. If find the drone images of the tension cracks very interesting – this video was published on Youtube on 13th January:-
Particularly interesting is the comparison between the deformation on the east and west flanks of the landslide:-
Meanwhile on the west side the deformation is quite different, with limited ground cracking and some bulging:-
This suggests a highly complex movement pattern that is hard to interpret without proper data (which WA DNR are collecting). I can understand why the authorities are being so cautious about predicting the time of final collapse. This one will be interesting to observe over the coming weeks.