Video: Caltrans geologist explains Devil’s Slide

Posted by on Wed, July 25, 2007

Darin Boville

Darin Boville continues his one-year anniversary celebration of the re-opening of Devil’s Slide with an explanation of the geology of the Slide.

 width= Geologist Explains the Slide | Quicktime | Flash  |

When I first moved here three years ago I heard all of the stories about the closure of Devil’s Slide in 1995. But I didn’t really understand what was happening in terms of what was causing the Slide to move. As far as I know there had never been a proper explanation made to the public about the geologic issues.

That changed shortly after the 2006 closure when Grant Wilcox, the geologist with Caltrans assigned to lead a geologic team at the Slide, came to the coast. His unexpected presentation at a Midcoast Community Council meeting offered an outstanding layman’s education on the technical issues involved in getting the Slide open.

Before this video was published on Coastsider there were all sorts of demands by frustrated coastsiders, wondering why a bridge could not be built, wondering why a permanent solution could not be found, wondering out loud that maybe Caltrans wanted the Slide to stay out to teach us a lesson in return for frustrating their plans to build a highway behind Montara into Moss Beach.

After this video was published all of that talk evaporated. People now had the information they needed. The questions didn’t stop—but the questions got a lot better.

So here, once again, is Grant Wilcox explaining the geology of Devil’s Slide.

I love this video.  It sure does explain everything.  Thanks for the repost.

Darin et al.,

Some may not have been paying attention, and the old HMB crowd refused to pay attention, but the geology of the major landslide on Devil’s Slide was extensively studied and then explained almost as extensively for more than two decades. Local geological engineer Bill Bechtell of Montara did a beautiful job of explaining landslide dynamics and relating them to Devil’s slide in public meeting after public meeting. He drew on both the large geologic field of landslide studies and engineering knowledge of how landslides are handled when people want to do something about them.

Beyond Bill’s synthesis, extensive studies and debates were conducted on the makeup of Devil’s Slide. The groups opposing the bypass hired several geologists over a period of years to analyze and report on the situation, and yet more local geological expertise was drawn in from the USGS. Environmental studies for the proposed bypass and its possible alternatives added yet more. Much of this information poured out in public meetings and in several rounds of draft and final environmental impact statements for the bypass project. The HMB library’s reference section carried multiple copies of much of it for years.

During the multiple rounds of the bypass debate, the last people anyone went to for objective geologic information on Devil’s Slide was Caltrans, where the in-house geologists during the years the bypass was debated always skewed their reports to suit their bureaucratic bosses.

I quite realize that coastsiders of perhaps the past ten years have not availed themselves of all the existing information and that they are wet behind the ears in dealing with Caltrans and its PR apparatus. In fact, I speculate some in the more recent environmental crowd rolled over for the interstate-sized and unnecessary underground freeway of the twin tunnels out of ignorance of both the landslide and of Caltrans’s bureaucratic culture.

The demon in Devil’s Slide is, and has been for decades, a mythical construct of Caltrans abetted by overdevelopment advocates and whatever others they can con into going along with their massive agency—a state agency with a larger budget than the *entire* individual budgets of well over half the states in the U.S. The cursory dismissal of dewatering and reinforcement of the roadbed after Measure T passed put the final “win” clearly on the side of the big spenders at Caltrans and those wanting a multi-lane highway onto the midcoast from the north. The geology of the landslide, the basics of which have been known for several decades now, has never been the main issue.

Carl May

Cal Trans had an agenda in 2006 and our local government representatives were all to willing to be spoon fed information from CalTrans.  Weeks into the crisis, we get Mr. Wilcox’s hand waving dog and pony show at the MCC.  In the following weeks CalTrans did not share their drilling, slippage or water level data with the public, until they had their $7M solution. Nor did CalTrans ever answer the question asked in the MCC meeting, about whether their lack of maintenance of swales and drains contributed to the roadway failure in April 2006.  Many of us continued to dog them with why not put in a Bailey bridge(truss span used to span breaks in emergencies and permanently) or just fill it in and maintain it?  No, their Engineers were working on it, in six weeks their analysis of the water levels, rates of slippage and design would be complete. Mr. Wilcox at a later meeting admitted that some slides in Marin County on Highway 1 had twenty eight feet of vertical asphalt fill on them.

After their design was complete, they were willing to talk about their geo-technical assessment based on the data they collected and kept under wraps. Six weeks into the closure is not the time to have a design review!  At that point in time, we were all calling for a solution, any solution, even theirs.  Our local politicians were being played by CalTrans and squabbling about the timing of traffic signals which CalTrans controlled. CalTrans effectively held this community hostage to their desire to do a big project and set themselves up to be the big heroes.

Did CalTrans really know what was going on at the slide?  While they dilled, cabled and tensioned, the slide continued to move as it dried from the rains. By their own admission they didn’t know what they were anchoring to. Some drilling went faster than expected and they pumped a lot more mortar into the anchor holes than they expected to.  CalTrans and the contractors changed their design as they proceeded with the work.

It’s not clear at this point who was right or whether the $7M solution will even last, until the tunnel is completed.  I certainly hope so.

Politically there were some lessons learned in 2006:
-If the slide roadway fails again, this community had better be a lot more aggressive early on getting information and demanding a say in the solution.
-We all got a taste of what it will be like on the Coastside when the number of vehicles(not homes) doubles.

I don’t want to set myself up here as the defender of Caltrans but what I’m seeing described here doesn’t match what I saw at the time.

You’d get the impression from some posts that Caltrans is some monolithic organization with a central brain guiding its minions to deceive the public and accomplish its nefarious goals.

I’ve worked in large government organizations and once-upon-a-time studied government organizations and let me tell you, heads of agencies would *love* to have anything approaching a monolithic organization with a central brain!

Of course, politics plays a role. Of course, organizations act in their own self-interest.

But that doesn’t make them evil, incompetent, or lazy.

As for the Caltrans employees that visited the coast (including the geologist) I had a chance to talk with them and one of their dominant reactions to coastsiders was confusion and frustration. They were hit from all sides from people putting pressure on them to do this or that with no group clearly speaking for the community. Then they tried to do things that they thought were shining examples of good government—cut through the red tape, solve problems (the light in HMB comes to mind)—and to their utter confusion they were attacked for it (not realizing that the light caused controversy in the micro-politics of the coast).

I think to be effective with Caltrans next time we need to have solved some of our internal problems here on the coast—at least to the point where we can speak to them with one (or maybe two) clear voices rather than the babble that happened last time.

We can’t blame Caltrans for everything when our own house is clearly not in order.



Carl May makes some really good points.  If you go back and read the history and all the studies you will come to many of the same conclusion he has.  Taking a hard look at the documents it’s obvious CalTrans has never wanted to admit that Dr. Hovland or others had a viable option in dewatering.  Dewatering is not in CalTrans’s solution space.  CalTrans is not an evil empire.  CalTrans had/has a technical bias against anything that touches on dewatering the slide, because they were attempting to fund the tunnel. That technical bias at CalTrans was in play in April 2006 when there were heavy cumulative rains on the slide.

I agree completely we do not have our act together on the Coastside in terms of governance.  This was a regional problem.

I missed the MCC meeting, but saw your excellent recording.  I attended the Pacifica City Council and HMB City Council meeting with CalTrans.  At Pacifica, many Coastsiders showed up and gave CalTrans and earful.  When the Pacifica Council members asked direct questions, CalTrans was not forthcoming with answers.  I got the sense the Pacifica City Council was uncomfortable with what they were hearing from CalTrans.  Five weeks into the closure, at HMB Council meeting, the Council was clearly in bed with CalTrans.  What actually happened?

I never heard that the Pacifica City Council, Supervisor Gordon, the MCC and the HMB City council ever all sat down together with CalTrans.  If you followed CalTrans from meeting to meeting and saw the same slick CalTrans presentation evolve and saw the same direct questions deflected again and again, I think you would have had a different opinion of CalTrans.  CalTrans was slick and Mr. Wilcox was part of the campaign for the “big project.”  To be fair, most of the citizens effected were busy coping with the closure. There were only a handfull of us that made all the meetings. CalTrans shuttled from stake holder to stake holder and sold their “big project”.  If CalTrans had their eyes open, they would have realized it was fairly easy to exploit devisive issues like the right turn signal on N.B. Highway 1 and signal timing.  Our local officials were out manuevered, abdicated responsibility and squabbled over traffic signals. Five weeks into the closure it was pretty clear CalTrans was in control of the political situation.

Next time, I’d suggest a public REQUIREMENTS meeting days after the incident, after an initial assessment is made.  The future meeting should be held on a Saturday in a large venue on the Coastside or Pacifica.  It should include Pacifica City Council, HMB City Council, MCC, two SMC Supervisors and CalTrans to represent the interests of statewide stake holders. Let the public comment go on for hours, let everyone have a say.  Then, let our regional officials and CalTrans close public debate and deliberate IN PUBLIC to provide a consensus set of requirements to CalTrans, before anyone goes home.

CalTrans’ feeble attempt to explain “why can’t a bridge be built?” falls flat, contrary to Darin’s implication:

Before this video was published on Coastsider there were all sorts of demands by frustrated coastsiders, wondering why a bridge could not be built,

I submit that we have 100-150 feet of engineering that proves that a bridge could have been and still could be built.  Just drive the road past the repair and note how short the repair is.  Now notice how—outside of the very short slide zone—the roadway remains solid and much higher than the now-depressed slide zone which has gotten to the point of being like an amusement park roller-coaster the way that CalTrans just lets it keep sinking.  Apparently, many people think that the slide area is the whole mountain.  It’s not—it’s basically just the part where they bolted the concrete barrier to the side of the mountain with long bolts/cables into the solid part of the mountain.

Now visualize a bridge about 3 times as long as the bolted concrete mini-fortress that they built, going straight from peak-to-peak, anchored on solid ground beyond the slide zone, with the bolted concrete underneath eventually sliding into the ocean without disturbing the bridge.  Easy.  And inexcusable for CalTrans to dismiss it out of hand with no engineering basis.  As bridges go, this one would be pretty small.  It could be prefabbed offsite and put up relatively quickly.


I have one minor technical correction. You wrote:

“It’s not—it’s basically just the part where they bolted the concrete barrier to the side of the mountain with long bolts/cables into the solid part of the mountain.”

Not all the tension anchors are embedded in bedrock.  It may look like a fortress. But, it’s basically retaining walls above and below the roadway with anchors buried in slide debris.

Have a look:

The red dotted line is the diagram is not a boundary between debris and bedrock. It is a theoretical boundary between two layers of debris moving relative to each other.  Right after the roadway failure, CalTrans reported data from their two vertical bore hole slippage meters below the roadway that the slippage plane had a depth of about 80 feet.  Historical theory has it that there were multiple slides that covered previous slides. One was theorized to have occurred during the 1906 earthquake. The cumulative slide debris is a couple hundred feet “thick” measured horizontally east at the roadway.  The horizontal drains CalTrans installed(concession that Dr. Hovland may have been right?) below the roadway run 300 feet eastward.

Most cables are just drilled into the debris of the slide and anchored with mortar pumped in. On the south end the debris is more like large blocks that have bulk movement.  I’d speculate some of the tension anchors on the north end penetrate the debris completely and are in the solid rock of the north east side of the fault.  CalTrans reported while drilling that it went faster than anticipated and they needed to pump more mortar than anticipated to fill the voids for anchors.  So, it would appear their extrapolation of their two vertical bores to drilling horizontally(red dotted line) had some inconsistencies.  In addition their bore hole slip meters continued to show slippage as they drilled and tensioned.  If you assume a certain slide density and estimate the weight of something like 70,000 cubic yards of debris being held back by the whaler walls and the tensile strength of the cables, it’s all rather puny despite it’s impressive looks to the taxpayers whizzing by.  Another consideration is if there really is a distinct slip plane between layers of debris, the vertical shear strength of horizontal cables may be the first failure point.  But, it only has to last four more years.  Hopefully the weather will cooperate and we won’t have a heavy cumulative rains like we had in March and April 2006.

The overall landslide is longer at road level that where they bolted the retaining walls to rods pushed into the hillside. One point is that this and other kinds of roadbed stabilization *together* with removal of the excess storm water that makes the slide slide might well have saved hundreds of millions of dollars in twin tunnel construction, hundreds of thousands to millions per year in operations and upkeep of the tunnels, acres of environmental disruption, years of time, etc.—to say nothing of conforming much better to what flimsy environmental laws we have, including the Coastal Act.

But the fact of the tunnels makes discussion of alternatives moot except to keep the learning experience alive for those able to study, think, and maintain principle over politics. I’m glad the coastside is not of one voice, because when HMB was of one voice for so many years after its incorporation, it was the ugliest, greediest, most destructive voice imaginable for a town that once had a chance to be so much better. Playing to politicians and bureaucrats by gathering communities into one front in order to be more effective with them has it exactly backwards—those bozos are supposed to be serving us!

Carl May

Carl May wrote:

“The overall landslide is longer at road level that where they bolted the retaining walls to rods pushed into the hillside.”

Technically it may be true that the section south of the whaler wall is part of the slide. But the historic movement of this section is a lot less than the northern part where the whaler wall is. Look at the photo’s of previous failures.  The major rifts were all at the northern end of the current whaler wall.  These rifts follows the fault line between the north east solid rock of fault and the south west loose debris on the other side.  South of the whaler wall the composition of this section is large blocks interlocked.  The section of the whaler wall could almost be thought of as a chute. To the north of the whaler is the north east solid rock. To the south of the whaler wall is the more stable large blocks of the south west side of the fault.

I agree with Leonard’s contention that both the north section and this southern section can provide suitable anchorages for a steel truss bridge to span the troublesome chute section with the high rate of slippage.  Providing an anchor for a light steel truss bridge is a simpler problem than trying to tie all the debris in place and hold it there. Steel plates can be uses to form expansion ramps to accommodate slippage. It’s relatively easy to jack a steel span, if there is movement. CalTrans waved their arms and said they couldn’t provide a suitable anchorage for a steel bridge span.  They were asked this numerous times. They never provided any information to back up their assertion that a light steel bridge was not possible.  There were never any public design reviews of their design.  CalTrans had no problem using the both the north and south intact section of roadway for multiple large cranes, mortar tanks, cement trucks and other equipment, before they tensioned their cables. A steel truss bridge has a relatively low static load and CalTrans has already provided a worst case dynamic load to the potential anchorages.

The issue for me is what to do, if CalTrans current repairs fail in the next four years.  We (us and our local governments) should insist a temporary light steel span be seriously considered.

Hey guys:
I messaged a couple of you privately through Coastsider’s messaging feature to take the discussion offline, however, I don’t know if you never received my message, or did, but don’t want to respond.  :)

I messaged you to say that I’m writing about Highway One and would hate to regurge the same old story as mentioned several times above.  I will go to the HMB Library, as Carl did say they have an extensive collection of reports made over the decades that speak towards a dewatering and reinforcement solution.

From what I’ve studied (and I’m merely a “gentleman geologist”), with a professor at SFSU who has over thirty years of experience in the field, a PhD in Geology, and who has written several articles on Devil’s Slide, this major landslide complex has no clear solution, that no bridge, roadway, nor even the tunnel, will provide permanent transport.

Also, I did a ride-along with CalTrans of the Devil’s Slide repair site and tunnel project and the two CalTrans hosts did vent a little about the frustration CalTrans faces in being blasted no matter which way they provide information.  Do we have a road builder alternative?

I would like to meet face-to-face if any of you would be amenable to that encounter.  Although I write, I do hold a day job and can happily buy you a cup of joe while we talk.  :)  You don’t have to go on record with whatever I write, mainly I want to meet to learn more about what you are speaking toward.  If you are interested, please write to me by using the messaging feature that Coastsider provides.  I’ll then provide my personal contact info.  :)

Thanks a bunch.