Review of Geologic and Geotechnical Data
for Zone 2
City Manager Evans presented staff report
(of record) of May 20, 2002, and staff’s recommendation to:
A) Accept the conclusions of the Cotton,
Shires & Associates January 14, 2002, Technical Review that:
1.There is insufficient subsurface information
to properly characterize either the depth to the base of landsliding,
strength properties of the landslide materials, or the ground water levels.
Without these data, no accurate slope stability analysis can be undertaken,
no reliable factor of safety can be calculated and no dependable landslide
mitigation scheme can be designed.
2.One cannot quantitatively determine
the factor of safety and, therefore, cannot judge the level of risk of
development in the prehistoric landslide area.
3.The factor of safety of the landslide
mass that underlies Zone 2 is above 1.00, but likely less than the industry’s
standard safety threshold of 1.50.
B)Continue to deny requests for development
permits for new homes in the Zone 2 area of the Portuguese Bend landslide
based on the current lack of evidence that the subject land has a factor
of safety of 1.5 or greater, unless an applicant submits a complete Landslide
Moratorium Exclusion application that is supported by adequate geologic
City Manager Evans provided a brief history
of the landslide areas in the Abalone Cove/Portuguese Bend landslide area;
stated that the landslide was first discovered in 1974; that by 1978,
it became clear that it was a serious problem, which prompted the City
Council to adopt a building moratorium on September 5, 1978, for the Abalone
Cove area. He commented on the installation of wells to remove ground
water and other internal improvements to divert water out of the landslide
area; and noted that there is no evidence of any significant movement
in the area of the Abalone landslide after approximately 1985 and no record
of any appreciable movement in the Zone 2 area of the Abalone Cove landslide.
Highlighting Dr. Ehlig's May 1993 memorandum to the City, he explained
that Dr. Ehlig had divided the Abalone Cove landslide into 8 zones, describing
the geological characteristics of each zone and a scenario under which
the properties in these zones could be developed. He explained that it
wasn’t until February 21, 1995, that the City Council actually approved
the appropriation for the environmental documents to consider a Zone 2
landslide exception project.
City Manager Evans stated that in early
2000, Jack Monks wrote a letter to the City to again address the Zone
2 issues; advised that the City Council had agreed to do so and, therefore,
hired Cotton, Shires & Associates to study the existing data relating
to in and around Zone 2 and to advise the City on various issues, including
whether or not it would be advisable to allow building on the vacant lots
in Zone 2.
Bill Cotton presented the results of his
company's study; advised that he was hired to collect all the geologic/geotechnical
data that pertains to the Zone 2 Abalone Cove and the Zone 1 area immediately
surrounding the Abalone Cove landslide and to evaluate that data and see
if that data was of sufficient technical merit to come to some reasonable
conclusion about the stability of the landslide and the area that underlies
Zone 2. Mr. Cotton stated that the data was collected from a variety of
sources; stated that he researched 20 to 30 different references -- expressing
his belief that he had collected all data that is available in order to
judge how sound the data is. With the aid of slides, he highlighted the
areas that had been addressed. Mr. Cotton stated that he concluded the
existing data is insufficient to make a declarative statement about the
long-term stability of the landslide as far as the standard of care in
the industry to compute a factor of safety that would be recognized as
being safe for development; explained that the level of risk is unknown,
that the subsurface conditions are unknown and, therefore, he could not
arrive at a level of risk.
Mr. Cotton stated that he also concluded
that the building of several homes is not likely to cause enough aggravation
to affect the landslide area; and explained that with a certain amount
of restriction, just by constructing homes on the landslide is probably
not likely to cause enough aggravation on the landslide to cause the landslide
With the aid of a slide presentation,
Mr. Cotton highlighted the various areas of the Abalone Cove area and
Portuguese Bend area; stated that as you get down into the Abalone Cove
landslide, you begin to see a repeated pattern to indicate a small displacement,
about 4/10ths of an inch in movement -- a slow pattern which suggests
that all the bore hole stations are moving towards the ocean, an active
landslide that continues to creep. He stated that in Zone 2, there are
some similar situations where things are moving down at the head of the
landslide, that displacement is heading towards the ocean.
Highlighting the water well data, Mr.
Cotton stated that he found that a lot of the water had been pumped out
of the ground, which is a critical criterion for the long-term stability
of the landslide.
Mr. Cotton stated for Mayor pro tem Stern
that the accepted geotechnical standard is to look for a 1.5 factor of
safety; explained that the lots could be developed without further destabilizing
the large regional landslide, but confirmed that that’s not a standard
that has been adopted by any standard-setting agency.
Mr. Cotton explained for Councilman Gardiner
that the underlying landslide encompasses everything inside the larger
boundary line as shown on the map of the Portuguese Bend Landslide Building
Moratorium Area; stated that he does not know enough about this underlying
landslide to make any comments about the subsurface being segmented or
similar to various parcels within this zone. He confirmed that Zone 2
has one bore hole; that Zone 5 has two bore holes; and that Zone 1 has
one bore hole and that they are deep enough to provide valuable information.
Mr. Cotton reiterated his belief that building a house in this area, under
certain controlled conditions, would not cause an aggravation to the larger
Zone 2 region, would not cause it to be destabilized; and that it’s reasonable
to suggest that the stability in the landslide 2 region isn’t so critical
at that point where a house would destabilize it by itself, but pointed
out once again that the risk is unknown.
Responding to Councilman Clark’s inquiry,
Mr. Cotton stated that at this point, there isn’t enough subsurface data
to determine a building safety factor; and that it’s clear there is no
way to ascertain a building safety factor of 1.5, which is the standard
for building in the gross landslide area.
Because a 1.5 factor cannot be reached
at this point, Councilman Clark noted his concern with the Cotton report
reflecting that it potentially would be okay to build in Zone 2, even
under tight conditions.
Responding to Councilman Clark's comment,
Mr. Cotton explained that it is a policy issue; that the level of risk
is unknown, whether the risk is high or low; and that in his qualitative
judgment, one house will most likely not destabilize the landslide. Mr.
Cotton explained that he knows the factor of safety is above 1.00, that
it is somewhere in between 1.00 and 1.5; and that from what he has seen
in the Abalone Cove landslide itself, he would surmise that it probably
hovers around 1.00. He explained that the land above this area will be
a higher figure or else you wouldn’t see evidence of it moving.
Mr. Cotton explained for Mayor McTaggart
that it would be expensive to investigate and determine what the factor
of safety is in Zone 2; that it would require a great deal of sub-surface
analysis to try to ferret out different parts of the landslide; that a
drilling program would have to be developed; and that ground water and
soil information would need to be obtained.
Mr. Cotton explained for Mayor McTaggart
that exploratory holes could be used for extraction if they were designed
for that purpose; that in determining the depth of the slide plane and
the strength of materials, you’d probably run into water; and that the
ultimate benefit would be to be able to extract water from that exploratory
well or use it for monitoring the water level.
By way of a Power Point presentation,
Mayor pro tem Stern addressed what he believes are appropriate determinations
for this City Council to make based upon what has been heard and presented
in regard to this matter. He highlighted the following facts: that the
accepted geotechnical standard is 1.5; that there’s no recognized standard
"to not further destabilize;" that the conclusions that the
lots could be developed is not based on any recognized geotechnical standard;
that the level of risk is unknown; and that there’s inadequate backup
data from Dr. Ehlig’s 1993 recommendations. He pointed out that what is
known is that some of the work that has gone on was designed to mitigate
the impacts, particularly dewatering, sewers, yet the Cotton report points
out there’s been no quantification of those mitigation measures. Lastly,
he stated that the predictability of a future landslide is unclear.
Mayor pro tem Stern asked the City Council
to make a determination that it find that the accepted geotechnical standard
is a 1.5, noting that this is common in the geotechnical profession; that
no recognized geological agency or standard-setting body has adopted a
standard whether the development of the property "would further destabilize;"
that the conclusion is not based upon the stability of the underlying
geology, a conclusion that it "wouldn’t further destabilize;"
that the level of risk is presently unknown; that the decision is a policy
decision based upon the City’s willingness to accept the unknown level
of risk; that Dr. Ehlig’s report lacks certain data or no quantification
of the mitigation measures; and that there exists a fallacy in the conclusion
that because of the age of this prehistoric landslide, that it is somehow
stable. He noted his conclusion that the City should require that the
development comply with the established standard of 1.5; that the City
should not apply a standard which merely seeks to determine if the proposed
development "will further destabilize;" and that the City should
continue to follow a policy that would protect individuals who would inhabit
structures built in Zone 2, not just be concerned about the impact of
the development upon others.
Joan Kelly, 6 Fruit Tree Road, Zone
2, urged the City to continue its building moratorium in the Zone
2 area; and questioned if earthquakes are taken into consideration when
making a determination.
Mr. Cotton noted for Ms. Kelly that yes,
seismic issues are taken into consideration when determining slope stability.
Tim Kelly, 6 Fruit Tree Road, noted
his objection to building any structure on any land less than a 1.5 stability
Joe Gallagher, 9 Ginger Root Lane,
RPV, asked that his e-mail letter of May 17th the City
Council be entered into the record, noting that he recommends against
the development of new homes in Zone 2. (This e-mail is on file with the
City Clerk’s Office.)
Bob Halderman, 88 Narcissa Drive, RPV,
urged the City to maintain the existing building moratorium until such
time that a determination can be made that the factor of 1.5 has been
John Monks, 107 Aspen Way, RHE,
stated that the geologist and soils engineer he hired to investigate his
3 lots in Zone 2 came to the absolute conclusion that his properties exceed
the 1.5 stability factor; that a determination had been made that building
a house on his properties will have no destabilizing effect on his land
or any land adjacent to it; and commented on his understanding that the
Cotton report indicates that building on his property would not destabilize
the hillside or do any harm to anyone.
Lois Larue, 3136 Barkentine Road, upper
Abalone Cove, noted her concurrence with Mr. Cotton’s conclusion.
Denise who resides at 42 Oceanair Drive,
supported the City maintaining a 1.5 standard for building in a known
Monte Ray, geologist, cautioned
the City that engineers make subjective judgments to be used in their
calculations for the 1.5 factor of safety; explained that there may be
a valid reason to look at ways that the City can go on developing in this
area as it has a need to, taking other considerations into account. He
noted the expense the City will incur if it decides to do this additional
investigation; and expressed his belief that the final study will never
yield a 1.5 factor of safety in this zone. He stated that there might
be ways to do some building in this zone, that there are ways to do it
in a relatively safe manner. He concurred with Mr. Cotton's statement
that the level of risk is unknown in this zone.
In response to Councilwoman Ferraro’s
inquiry, Mr. Ray explained that pumping ground water out of this zone
has slowed the movement over the years; stated that there are structural
design measures that one can use to combat the effects of land sliding
if it were to occur.
Councilwoman Ferraro questioned what should
be concluded from the extensive studies done on Mr. Monk's 3 lots that
indicate stability of 1.5.
Responding to Councilwoman Ferraro's inquiry,
Mr. Cotton explained that there are two different tests: one is for local
stability, stating that that was part of the language -- that if there
were going to be development on individual lots, they had to demonstrate
that there would be local stability of 1.5. He noted that one could do
that if one were on a slope, to come up with values for local geologic
conditions. He pointed out that that situation only applies to building
sites; and stated that that is totally different than what he is addressing
in his report, which is the underlying stability of the larger mass. He
added that one could probably demonstrate that there is local stability
on one of those sites, but that the whole mass is moving; and that it
doesn’t address the deep, underlying landslide mass and how it may come
about in the future.
Mayor pro tem Stern stated that it is
important for the City Council to start setting standards; explained that
he sees no reason to deviate from the accepted standard of 1.5 -- pointing
out that there are valid concerns for the stability of whatever structure
is placed there for the current resident and any future resident; and
advised that he would not support a change at the present time given the
unknown level of risk.
Mayor pro tem Stern moved, seconded by
Councilman Gardiner, to concur with staff's recommendation and moved to
include the 7 additional findings he presented during his Power Point
Councilman Clark echoed Mayor pro tem
Stern' position; and stated that in light of the information presented,
it would be prudent for this City to reject any building construction
in the moratorium area.
City Attorney Lynch suggested that with
respect to Mayor pro tem Stern’s suggestion Nos. 3 and 4, that we add
the proviso in the conclusion noted in Cotton January 2002 report, add
the words "undeveloped lots"; and the same type of change in
No. 4, geologically, the level of risk of development of undeveloped lots
in Zone 2 presently is unknown, the decision to allow new residential
development in that area given the unknown level of risk is a policy decision
which must be made by the City Council.
Councilman Gardiner expressed his belief
that the potential for risk could negatively impact the taxpayers of the
Councilman Clark pointed out that it is
very clear in the Cotton report that Dr. Ehlig did not base his recommendations
on specific geotechnical analysis and that it is his belief that therein
lies the weakness in those recommendations; and stated that some of the
more recent data calls into question some of those assumptions that were
made several years ago.
Hearing no objection to the motion, it
was so ordered by Mayor McTaggart.