Landslide Reconnaissance Following the December 3, 2007 Storm - Thurston County
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Landslide Reconnaissance Following the December 3, 2007 Storm - Thurston County 
 

Highway 101 and Highway 8

On December 3rd , two landslides failed along the median and bank of Highway 8, moving onto Highway 101. These failures closed Highway 101 and Highway 8 on December 3rd, until debris was removed and engineers had a chance to check the stability and safety of Highway 101.

 

Map 1: The location of the two landslides. Click on the map to view a larger image.

Northwest Landslide
The northwest landslide failed from the Highway 8 median and descended down onto Highway 101. The landslide moved a soil mat approximately 2 to 4 feet high by 15 feet wide by 30 feet long. The slide material appeared to be highway fill that had rested on top of glacial till, which appeared to act as an impermeable substrate. 

 

Figure 1: The northwest landslide from Highway 101. Note the till acting as an impermeable substrate beneath loose highway fill. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

 

Figure 2: A view of the remains of a debris-clogged culvert that used to feed into a cement cistern that collected runoff from Highway 8. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

 

Figure 3: The clogged culvert may have helped initiate the slide: careful viewing of the grass in the lower left of the image suggests that water had flowed across the intake to the lower left of the image. A clogged intake would have caused the water to spill across the median and onto the embankment where it helped saturate and mobilize the loose highway fill. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

A sizable pebbly sand deposit on Highway 101 at first sight seems to suggest a debris slide or debris flow in this location. Closer examination of the seemingly unaffected drainpipe in the path of the event suggests the possibility that the pebbly sand may have been deposited by floodwaters - perhaps hyperconcentrated? – instead of a landslide. Two small scarps, both beneath a root ball of a tree, appear to mark the source area for much of the sediment. These two scarps were no more than 4 feet wide by 4 feet high. 

 

Figure 4: Moss coating remaining on the drainpipe after the event – one might have expected a debris flow to scrape off much of this moss. The washed pebbles that can be seen trapped in the void spaces between the rip-rap would also seem consistent with floodwaters moving through the hollow instead of or following a landslide. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

 

Figure 5: Shallow landslides that were located above the hyperconcentrated flow. The shallow slides occurred under the root ball of the tree. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

 

Figure 6: A closer look at the shallow landslide; note the edge of the roots near the top of the photo. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

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 Contacts

Stephen Slaughter
Geology & Earth Resources Division, Hazards Geologist
Washington State Department of Natural Resources
360-902-1498
Fax 360-902-1785
stephen.slaughter@dnr.wa.gov

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