Disclaimer: I’m not affiliated with the project, and the info in this post is based on my notes taken during Mr. Wilcox’s presentation. I don’t claim any of the information is completely accurate, use it with caution.
The Devil’s slide area is along California State Route 1 (the Pacific Coast Highway) in San Mateo County between the town of Montara to the south and the city of Pacifica to the north. The road sits about halfway up a steep 900-ft bluff/cut with the Pacific Ocean below. It isn’t one landslide that they are dealing with, but a landslide complex consisting of at least 4 large slides. (Photo below by Caltrans)
History of Failures
In 1980 there was a slide event that took out an entire lane of the highway. In 1982 a portion of the highway dropped about 4 to 6-ft and was repaired with dowels to pin the failure. Then in 1995, there was a large slide and rockfall event that closed the highway for 5 months and dropped the road by around 3 to 4-ft.
Then in 2006, there was another failure where one of the “smaller” slides at about 1,300-ft long moved and some 6 or so 4-ton boulders came down the slope onto the roadway and over the roadway from a large talus pile. This was the most recent major fix to the slide area. At that time, Caltrans utilized some blasting and then scaling by some remarkable Spider excavators from AIS Construction to handle the rockfall problem so the could get in and stabilize the slide. That work was completed in about 1-week and included erosion control mats and a Richie ditch. [Updated Thursday May 29, 2009 – 1:00 PM PDT] (Photo of spider excavator, from AIS Construction) [/Update]
Over the course of 2 weeks, a fix for the slide was designed that would remediate a 300-ft section of the slide at the roadway. It included tiebacks in the upper part of the highway to lengths of 100 to 150-ft and rock dowels or soil nails on the down-slope side of the road (see photo at the top of the page). A micropile wall was installed used to prevent headward movement. The contractors for this work were Condon Johnson and DrillTech. The entire remediation was completed 1-month ahead of schedule, in around 4-months total(?).
I posted about some info on the tunnel in my original post on the project. Mr. Wilcox indicated that 17 borings were performed for the roughly 4,000-ft twin tunnels tunnels ranging in depth from 3-m to 350-m at varying orientations. But even so, that doesn’t seem to be much coverage so they utilized a geotechnical baseline report to group the rock into various categories. The contractor, Kiewit Pacific, has adopted the NATM. To determine pay quantites for the different categories of rock excavated, the rock face is exposed and Kiewit and Caltrans personnel have 10-minutes to map it to both party’s agreement before they coat the face with shotcrete. Mr. Wilcox indicated that they have progressed to almost 50% of the total tunnel length (around 650-m out of 1400-m) and expect to open it in 2012. (Rendering by Caltrans)
[Updated Thursday May 29, 2009 – 1:00 PM PDT] I forgot to mention some interesting photos of the disposal site for the material excavated from the tunnels. The Coastsider has some comparison photos showing the area of the disposal embankment or whatever they are calling it in 2006 and then in 2009.
You can also check out a great high res photo of the same area and from about the same perspective taken by Karen Ziv (jklm250 on Flickr) [used with permission] taken at the end of April of 2009.