By Shari Faye Dell (published in the West Marin Citizen October 6, 2011)
West Marin Report on KWMR http://kwmr.org/blog/show/749
At a Bolinas Community Public Utility District (BCPUD)
meeting in June, the county unveiled plans to reinforce and rebuild the bluff
on upper Terrace Avenue, a road that has been closed since January 2010.
Now entering the third week of excavation, on average, 160
truckloads of material are moved daily to a temporary storage site on the
soccer field at Mesa Park.
Standing on the rim of the massive pit, spectators
speculate, “See those trees over there? They are about 120 feet high, if you
stood one of those on the bottom, I bet we would be level with the crown,” says
one bystander.
However, appearances can be misleading, according to Marin
Department of Public Works Principal Civil Engineer Ernest Klock, the
construction crew has nearly reached the target of 135 feet above sea level, or
60 feet below the surface. The goal of this bench excavation, to reach bedrock,
will allow access to dig a keyway into the bedrock. This keyway, five to ten
feet deep, is the foundational base for layering compacted soil, or soil lifts,
between Geogrid fabric, a plastic mesh with very high tensile strength. The
layering process continues until ground level is reached, allowing the whole of
the repair to act as a unit founded on bedrock. “Advantages to using this kind
of repair is that it is very earth friendly,” Klock explained during the June
meeting. “We will be using all the existing on-site-materials, no concrete
hauling and no mining in southern California for lime to make a concrete
retaining wall. The finished surface can be replanted so that it looks exactly
like it looked before the slide happened.”
The June estimate of 50,000 cubic yards of material was
readjusted just before construction began. Now approaching the adjusted figure
of 20,000 cubic yards, the enormous mounds at Mesa Park appear to be at
capacity. If the rains let up, excavation goals could be reached with several
more days of digging.
Landslide
Sand, not soil, comprises all but the topsoil removed from
the dig. It is because of the sand that the cave in occurred. Standing before a
diagram, Klock explains, “The underlying bedrock, this upper sandy mass, is
basically sliding on the bedrock. What happens is: rain falls and permeates
through that sand, just like on the beach–there is no water build up on the
beach, water goes through the sand, hits the bedrock and moves horizontally.
So, when all this sandy mass is saturated it tends to slide on the harder
material.”
Funding a landslide
Severe erosion at the site drew county attention as early as
2006. Unable to obtain federal funding at that time, Marin County Department of
Public Works eventually closed the road to vehicular traffic after storms
caused severe slumping in 2010. Additional closures were enforced in April of
this year. A cyclone fence was installed around the section that became a
deeper, broader pit and signage instructed bicyclists and pedestrians to use an
alternate route.
Many locals had grown fond of the auto free zone through the
sacred Eucalyptus grove, an annual migration layover to Monarch Butterflies,
and the iconic Surfer’s Overlook with the sweeping view of Bolinas’ southern
exposed beach, Mount Tamalpais, San Francisco and beyond. While alternative
transportation is popular in Bolinas, alternative routes are not: A cliff-side
trail allowed locals to sidestep the barricade.
“Funding was always an issue, so until the board allocated
funding to fix it, our hands were tied. Being an engineer here, working for the
county, I would love to fix everything but money drives all the repairs,” Klock
explains. “Basically, after a storm occurs, the engineering division of public
works goes out and assesses the damage. Typically, in larger disasters–state of
federally recognized disasters–the county receives state or federal aid, but,
in this case, we had not. We approached the board, asking for help and they
allocated money to fix these high priority slides.”
Motivation
In conjunction with the County, BCPUD hired a geotechnical
engineering firm to explore options to save Surfer’s Overlook from erosion and
the immanent failure if left unattended. “Having the known failure at [upper]
Terrace and the at risk failure of Surfer’s Overlook threatened accessibility
to 41 homes between those two slides,” says Klock. The fact that upper Terrace
is an easier, cheaper fix, requiring fewer studies and permits led the county
to move forward with repairs.
Bolinians gets a new road and a new field
Where to store one acre of soil ten feet high? That was the
question in June but local entities–BCPUD, Bolinas Fire Protection District,
and Mesa Park–were able to map out a plan. An agreement was signed, in exchange
for resurfacing and reseeding the soccer field at Mesa Park, otherwise known as
the Firehouse Park, use of the field was granted through December.
Mixed feelings
Bolinas Fire Chief Anita Brown describes safety concerns
that can soon be dismissed.
“I am very happy that the counthy has undertaken this
project being that the majority of our residents live on the mesa and being
that mesa road which is now the only road on and off the Mesa, has been close
twice this year. That means no one can get downtown and because the firehouse
is on the Mesa, we can’t get fire engines downtown. We can’t get an ambulance
up to residents on the Mesa, so the fact that the county is doing this is a
great project. We need both access roads.
Gavi Emunah, a seventh grader at Bolinas-Stinson School
says, “Terrace is a safe place to ride bikes and skateboards and it is nice to
walk down a road with a view and without cars.”
“I feel that could have built a short road through the sewer
ponds or the quail preserve with eminent domain, routing traffic from Overlook
to Marin Way,” says Albert Hollander, a Bolinas resident since 1973. “Then
Terrace could be a walking street. The main reason they are doing the repair is
so people on Marin Way can have access if Surfer’s Overlook fails. I, however,
have a home at the foot of terrace. I have enjoyed the street being closed. It
has reduced traffic and fumes that I experience from the constant flow of
traffic. Other people also express their joy at walking unencumbered by
automobiles. The road is dangerous–narrow and there is no soft shoulder.”
On the edge
Early last week the house on the southeast corner of
Overlook and Terrace avenue still had a picket fence and yard. The house is now
perched five feet from the precipice after loosing twenty feet of yard to fall
in around the dig.
“The contractor is responsible for that damage. We had no
intention of going onto private property with the repair, that was the result
of the sands being loose in that area and the contractors operation caused
that,” says DPW’s Ernest Klock. “So, Maggiora & Ghilotti is on the hook for
that. They are in contact with the property owner and working with them.”
In briefest form, this story can be he heard at
kwmr.org/news
B shows the time course for eight hypothetical neurons. Note how some remain activated for longer than others and that they overlap frequently. Resurfacing Concrete
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