Nile Failure Plane

November 19, 2009

One of the things we like to try and figure out is at what depth is the landslide moving and what is it sliding on. Usually, landslides like to slide on beds of weakness, in Eastern Washington, this can be interbedded silts and sands between basalt flows. In Western Washington, the Lawton Clay in the Seattle area perches water of the Esperance Sand, creating a perfect failure plane for landslides in that area. The Nile landslide is a little different, since we are talking about an area that has experienced at least three major sliding events. The subsurface could be torn apart and we would never really be able to figure out a good clear place where it is sliding. However, we may just be in luck. On the area known as the Woodshed Restraint a recent well log might have given us a giant leap forward on what the sliding plane might be:

Well Log on Woodshed Restraint

One word of caution, well logs can be misleading and sometimes downright falsified. However, this log does seem to fit quite well to what we know of the area. So, jump down to the Black and Brown Broken Clay. Although we could jump to the conclusion that is was broken due to previous sliding, there would be some other arguments coming into that as well as to what could have broken it up. This clay could very well be an old paleosol (old soil layer). The groundwater looks like it is right in that area as well, so clay + water = great sliding plane. What we think has happened in this area of the Woodshed Restraint is that this block was pushed over various sliding activities into the valley. I think this well log helps to support that claim, since below the soil is gravel and boulders, most likely fluvial deposits within the valley floor. Basically, we are thinking this is the historic sliding plane and the plane we are expecting it to continue to use. So, before we make any final conclusions, WSDOT has a lot of drill holes and we are studying the other well logs as well, so we need to try and correlate these findings with other data points in the area. Stay tuned…


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