Building A Bypass On Shifting Sands

by Mark Scaramella, April 24, 2010

"Little Lake Valley (town site of Willits) is an intermontane basin within the northern California Coast Ranges that contains a record of basin sedimentation and deformation during the Pleistocene. Little Lake Valley is remarkable because it is a basin containing a thick section of Quaternary to upper Neogene (?) sediment situated at a major drainage divide between the north-flowing Eel River and the south-flowing Russian River. Well logs indicate that Quaternary and upper Neogene sediment fills the valley to depths greater than 140 meters. On the basis of well logs as well as fault-line-scarp morphology, the eastern margin of the valley is bounded by an inactive steep fault, which has at least 50 meters of displacement up to the east. In contrast, the actively creeping dextral Maacama fault trends north-northwest through the southwest corner of the valley. Although valley fill may be as old as Pliocene, tephra layers in the sediment fill at or near the surface have Pleistocene ages: Thermal Canyon tephra, the Rockland tephra and an unnamed tephra correlative to tephra in the Clear Lake basin. Valley fill mainly consists of fine grained lacustrine and overbank sediment and coarser grained gravel alluvium with minor amounts of colluvium on the east margin. Facies relationships inferred from well logs indicate that streams aggrading the valley were through-going streams traveling along the valley axis and not fan delta systems aggrading from steep valley sides. Valley fill strata tilt uniformly to the north with a mean dip of 8° and a range of 5° to 25°. Locally, sediments dip steeply in the Maacama fault zone. Paleo flow direction, as determined from clast imbrication and channel margin trends, is to the south; in contrast, surface drainage on the valley floor is presently to the north. We infer that the northward tilt of basin strata is part of a regional deformational response to northward passage of the Mendocino triple junction. A byproduct of the regional northward tilt is Little Lake Valley's reversal in drainage direction to the north in the late Pleistocene. We infer that basin northward tilt predates the northward propagation of the Maacama fault zone into the Little Lake Valley."

This is a geological description of Little Lake Valley by Humboldt State Geologist Adam C. Woolace. The small town of Willits sits on the west side of Little Lake Valley.

Buried in the geo-techno language here is the phrase "Quaternary and upper Neogene sediment fills the valley to depths greater than 140 meters." And, "Valley fill mainly consists of fine grained lacustrine and overbank sediment and coarser grained gravel alluvium…"

That's several hundred feet or more of, basically, sediment. So Little Lake Valley is full of hundreds of feet of the geological equivalent of fine and course silt and relatively loose materials that have accumulated in the basin over the eons.

None of this will come as news to anyone who has lived in Willits for any length of time. Nor should it be news to Caltrans because, among other things, Caltrans has engaged in almost continuous level of highway repair and reconstruction on the Willits Grade south of Willits off and (mostly) on for years where slides and slipouts caused by the area's unstable geology are common.

In fact, Caltrans' own assessment of the Little Lake Valley geology admits that, "Overall, [the selected route of the Willits Bypass project] appears to have the fewest geotechnical challenges."

Meaning, of course, that all the routes they considered had serious "geological challenges," but they picked the one with the fewest, without defining what those remaining "challenges" are.  Caltrans goes on to say the selected route of the Willits Bypass will "avoid the Holocene Deposits with the highest liquefaction potential and avoid major road cuts and embankments in the Plio-Pleistocene Non-Marine Sedimentary Deposits and the Franciscan Melange that are prone to landsliding."

That's not as comforting as I'd like to see. They will "avoid" the areas with the "highest liquefaction potential" and the areas most prone to landsliding?

This is all very weasel-worded, wouldn't you say? Especially when combined with the techno-geological language used to describe the area that very few ordinary people would be able to decode.

Remember, the Willits Bypass project will consist of "seven mainline structures and four ramp structures. The major structure in this group would be the Floodway Viaduct that would span the Little Lake Valley floodway. This viaduct would be designed to convey the base flood without substantially increasing the 100-year water surface elevation."

In other words, a big segment of the Bypass will be an elevated roadway that Caltrans says will look like this almost cartoon-like sketch:

If I'm reading this correctly, Caltrans is proposing to construct an elevated highway several miles long which will be carried above ground level to avoid flooding by being constructed on pilings placed in ...

LOOSE SEDIMENT!

And this Bypass is supposed to carry heavy semi-18 wheeler trucks which will vibrate and bounce along at highway speeds 24/7!

Caltrans seems to acknowledge this problem, albeit vaguely, when they say, "Because all of the proposed alternatives cross over questionable compressible deposits, it is anticipated that their embankments will experience settlement. Mitigation Measure GEO-4 will reduce settlement impacts."

And what is "Mitigation Measure GEO-4"?

"GEO-4: To minimize or prevent settlement, Caltrans will incorporate foundation treatments or long-term settlement periods into the design and construction of the project."

Nowhere in any Caltrans planning document does Caltrans say what those "foundation treatments" will be. Nor is the statement that Caltrans will incorporate "long-term settlement periods into the design and construction of the project" explained. (We can guess that Caltrans is saying they'll put the pilings in and let them sit there for the "long-term" (whatever that may be) before building the actual elevated roadway. Somehow we don't think that'll really solve the settlement problem in an area as uniquely unstable and sedimentary as Little Lake Valley.)

A casual glance at a terrain map of Little Lake Valley (and a very rough approximation of where the Bypass is supposed to go sketched over it) shows that the Bypass is further out and east into the deeper "sedimentary basin" than the existing Highway 101.

You'd think that with all the "geological challenges" that the Willits Bypass presents, Caltrans would pay special attention to them in their planning documents.

You'd be wrong. All there is is these few brief, vague -- and entirely unsatisfactory -- claims that the "foundation treatments" and "long-term settlement periods" will "minimize or prevent settlement."

Prediction: The project will get underway sometime in the next few years and during that construction period, the Construction Company Caltrans hires will find that things are sinking and cracking and slipping and sliding. When they discover this the project will have to be put on hold while Caltrans tries to decides what to do. Cost and schedule estimates will increase dramatically. There won't be enough money to finish the project. And Willits will be left out in the cold, having suffered through a big part of the Bypass impact (construction vehicles, increased traffic, dust, noise, etc.) with nothing but a partially built and useless Bypass project that nobody will know how to finish to show for it.

Willits better hope I'm very -- VERY -- wrong.

2 Responses to Building A Bypass On Shifting Sands

  1. never go to willits anyway Reply

    May 18, 2010 at 10:52 am

    I live on the coast and I drive through the AV on the way to Ukey. Willits is a place I just never need to go to, so if Caltrans isolates them by messing up their bypass – it makes me no never mind!

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