In mid-October, 2001, I e-mailed you with my “Zone 2 Questions” submitted to the City consulting geologist, Cotton Shires & Associates. This related to the question of whether or not we allow building on the vacant subdivided lots in Zone 2 of the Portuguese Bend Landslide Moratorium zone. I have received Cotton Shires answers. This topic will shortly be agendized for city council consideration. As always, your thoughts on this topic are appreciated.
From: Douglas W. Stern, City Council Member
To: Cotton, Shires & Associates
Re: Questions on September 12, 2001 Zone 2 Report
1. ACCEPTED GEOTECHNICAL INDUSTRY STANDARD-OF-CARE
You have set forth a description of the "GEOTECHNICAL INDUSTRY STANDARD-OF-CARE." This is described in the Report as "a ‘standard-of-care technical procedure’ in the geotechnical industry." You then describe the Factor of Safety (FS) of 1.5 as the accepted standard. Your Report states:
"Without considerable surface and subsurface information to define the landslide dimensions, groundwater levels, basal rupture surface geometry, and soil parameters, the geotechnical conditions of the ACL and the Zone 2 region cannot be characterized. Unless the landslide conditions are properly characterized, no reliable slope stability analysis can be performed, nor dependable landslide remediation offered."In addition, in your cover letter accompanying the Report, you observe:"At the present time, the landslide mass that underlies Zone 2 is stable, but we cannot quantitatively judge the degree of that stability. We think it is reasonable to conclude that the factor of safety is somewhat above 1.00, but likely well below the geotechnical standard of 1.50 that is typically considered to be a threshold for safe hillside development."QUESTION
A. Do I interpret your Report correctly in reading it to state that the accepted geotechnical standard in the geotechnical community is a Factor of Safety of 1.5?
ANSWER
The accepted geotechnical standard for residential construction is a Factor of Safety of 1.5 for slope stability. This is a common standard among geotechnical professionals and to our knowledge has been adopted by most, if not all, municipalities in Southern California. The required factor of safety is referenced in the County of Los Angeles Department of Public Works, Manual for Preparation of Geotechnical Reports dated February 2000 (revised May 8, 2001). Part II, Section G.1.b.4 under the heading Slope Stability Analysis-Static and Seismic Stability Analyses states: The minimum factor of safety for shear strength is 1.5 for static loads and 1.1 for pseudostatic loads. The factor of safety for strength is defined as the ratio of the shearing resistance force to the actual driving force acting along the potential failure surface.
QUESTION
B. If that is not the accepted standard recognized in the geotechnical community, what other accepted standard is recognized as the proper Factor of Safety (or other standard) in the geotechnical community, and can you provide a citation to the source of such standard, and a copy of such standard.
ANSWER
There is no other “accepted standard”.
2. STANDARD OF CARE BASED UPON "NOT FURTHER DESTABILIZING"
You state in your cover letter and in the Report that:
"With regard to the issue of allowing development of the 41 remaining vacant lots in Zone 2, we believe that, given adherence to prudent policies, the lots could be developed without further destabilizing the large regional landslide."QUESTION
A. Has any recognized geotechnical standard setting agency adopted the "standard" described above as the accepted test for allowing building habitable structures on vacant lots located in regions like Palos Verdes? Your Report only mentioned the FS 1.5, leading me to believe that a standard asking whether development of new building "would further destabilize" is not a recognized standard by the geotechnical industry. Is its, or is it not?
ANSWER
A. You are correct. There is not a standard with regard to “further destabilizing”.
QUESTION
B. If such a standard has been accepted, please provide us with the proper citation and a copy of the standard, and any information you can provide to evaluate how widely accepted such a standard is in the industry.
ANSWER
No such standard has been accepted.
You do observe and comment in the Report that:
"In our judgment, the additional development in Zone 2 will be exposed to the same level of unknown potential risk to which existing residents are exposed."QUESTION
C. Do you regard the decision to allow vacant lot owners to build under the condition noted above -- that they are subject to the same level of unknown risk as existing home owners in that area -- as a geologically based conclusion justifying allowing such building, or is such a conclusion to allow building under those circumstances politically based?
ANSWER
D. Geologically, the level of risk is unknown. To allow or deny building on the vacant lots is a policy decision, and must be based on a willingness to accept the unknown risk. It is important to note that newly built homes will be subject to the same unknown risk as existing ones. CSA is not making a recommendation that building be allowed in Zone 2, nor justifying building within Zone 2. The statement that “lots could be developed without further destabilizing the large regional landslide” is based upon the fact that it is feasible to construct homes without 1) significant redistribution of earth mass (i.e., minimize grading) or 2) contributing to groundwater rise (i.e., capture surface water and irrigation water before it enters the landslide mass). These two factors are the primary causes for destabilization of an existing landslide mass.
3. CONCLUSION OF THE ADEQUACY OF THE DATA
In your cover letter you state:
"We completed our technical review of the geologic and geotechnical documents describing the Abalone Cove landslide and the Zone 2 area. The goal of our review was to determine if the existing data are of sufficient extent and quality to allow new residential development in the Zone 2 region.In the Report itself you note:
Our conclusion is that there are insufficient data necessary to judge the state of stability of the large landslide mass that presently underlies Zone 2. We are forced to conclude, therefore, that the level of risk of future landslide activity is presently unknown."
"As a consequence of the lack of geologic data to properly characterize the landslide, an accurate slope stability analysis cannot be undertaken. If the landslide is presently in a static condition, then the factor of safety is greater than 1.00. It is reasonable to assume, however, that the FS is less than the geotechnical industry standard of 1.50. There is no way to calculate the most likely FS of the Zone 2 landslide mass given the existing data base. Consequently, the risk of future landsliding in the Zone 2 area remains unknown."QUESTION
A. Am I correctly understanding that based upon the data available, and given the known situation of the Factor of Safety, the risk is "Unknown," but we know that the area is not up to the recognized Factor of Safety of 1.5?
ANSWER
A. Page 12 of our January 14, 2002 report states “It is clear that 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”. We do not have a reliable estimate of the factor of safety of the landslide due to lack of available data, as discussed in our report. The factor of safety of the larger dormant landslide must be above 1.0, since the landslide is not moving. Inasmuch as the area is underlain by a weak sheared bentonite layer that is inclined toward the ocean, and no mitigative measures have been implemented, we conclude that it is likely that the existing factor of safety is less than 1.5, but we do not “know” it.
B. You obverse that assessing the risk of future landslides is:
"difficult because very large landslides are inherently much more challenging than smaller slope features. Without considerable surface and subsurface information to define the landslide dimensions, groundwater levels, basal rupture surface geometry, and soil parameters, the geotechnical conditions of the ACL and the Zone 2 region cannot be characterized. Unless the landslide conditions are properly characterized, no reliable slope stability analysis can be performed, nor dependable landslide remediation offered.QUESTION
How are you able to conclude that it is possible to allow building in Zone 2 without "further destabilizing" the area, given your statement about the difficulty of evaluating the risk, the need for more data, and your inability to provide dependable remediation?
ANSWER
To be “not further destabilizing” does not mean that the environment is risk free. It means that the level of “unknown” risk would probably not be exacerbated. This is an unquantified geologic judgment that derives from geologic experience.
4. SCOPE OF THE ANALYSIS
Your Report notes that it is limited by the engagement:
"The scope of work was . . . 1) To evaluate the existing geologic and geotechnical database so as to determine if it is sufficient to estimate the state of stability of the large, regional landslide that underlies all of Zone 2 ; and 2) To evaluate the geotechnical feasibility of allowing residential construction on the existing 41 undeveloped parcels in Zone 2. Our evaluation was restricted to these two issues, and the conclusions and recommendations presented in this Report pertain only to the ACL and the existing subdivided lands in the Zone 2 area. Geologic and geotechnical data from the landslide terrane of Zone 1 and Zone 3, which adjoin Zone 2, were evaluated, but the conclusions and recommendations presented here do not directly address the feasibility or safety of residential development in those two areas."QUESTION
A. Have you evaluated Zone 1 to determine if there is any geologically based differentiation which can be made as compared to Zone 2.
ANSWER
No, we did not evaluate Zone 1, but we believe that both Zones 1 and 2 are insufficiently characterized to allow quantification of risk.
QUESTION
B. Has your analysis evaluated any adverse impacts that might take place should the City either voluntarily or involuntarily (court order) allow building in Zone 1?
ANSWER
We did not evaluate adverse impacts of development in Zone 1.
QUESTION
C. Can you give us any insights into the impact that building in Zone 1 might have as a result of the cumulative impact of allowing building in both Zone 2 and Zone 1?
ANSWER
Inasmuch as we do not currently possess sufficient data with which to characterize either zone, the quantitative impact of building in them is unassessable.
QUESTION
D. Is there, in your view, a geologically sound basis, based upon the existing data, to draw a valid geological distinction between Zone 2 and Zone 1 insofar as it is relevant to building habitable structures in either zone?
ANSWER
There are insufficient data to make a distinction between Zones 1 and 2 regarding their appropriateness for building habitable structures.
5. DR. EHLIG'S BUILDING GUIDELINES
Your Report comments regarding Dr. Ehlig’s prior proposal for allowing building on the Zone 2 vacant lots that:
"No detailed geotechnical analysis was completed by Dr. Ehlig prior to developing the guidelines and, therefore, he was relying on his long experience with the Portuguese Bend landslide region and his knowledge of the Abalone Cove landslide in particular. Consequently, his recommendations were based solely on his geologic judgment of the future behavior of the ACL and the Zone 2 region.QUESTION
A. Please explain what you mean by this. Were you not able to find any back-up analysis of some sort to demonstrate how Dr. Ehlig reached his conclusion?
ANSWER
We have reviewed Dr. Ehlig’s complete personal library of Portuguese Bend and Abalone Cove data that was turned over to the City by Dr. Ehlig’s widow shortly after Dr. Ehlig passed away. We uncovered no geotechnical slope stability analyses of the Zone 2 area in Dr. Ehlig’s files. His conclusion was apparently a subjective assessment, based on his long experience and intuition.
6. LACK OF ANALYSIS OF IMPACT OF MITIGATION MEASURES
You note that the City has undertaken a number of "mitigation efforts"
"In our opinion, the remedial measures, which include sanitary sewer lines, collection of surface drainage, lining portions of Altamira Canyon stream channel, and water well extraction to lower the groundwater, are all very positive activities that help to maintain the stability of the ACL and the Z2L area. However, to our knowledge there has not been any quantitative geotechnical analysis to determine the effectiveness of these measure.QUESTION
A. Are you aware that the Altamira Canyon lining has not been done?
ANSWER
It is our understanding that Altamira Canyon will be lined next year.
QUESTION
B. Would you opinion be any different if you assumed that the Altamira Canyon lining is not done?
ANSWER
If Altamira Canyon lining is not done, remedial mitigation efforts will be diminished, but we cannot quantitatively evaluate this impact.
QUESTION
C. You note that there has been no quantitative geotechnical analysis to determine the effectiveness of these measures. Why is that important?
ANSWER
Quantitative geotechnical analysis is the best way to evaluate the effectiveness of mitigation efforts. There are a number of ways to calculate the “sensitivity” of different geotechnical conditions by completing a well-designed slope stability analysis (SSA). The SSA cannot be done without very detailed subsurface data. Such data are not available.
QUESTION
D. What types of geotechnical quantification could or should be undertaken. (I note that Councilman John McTaggart rejected any notion of evaluating the effectiveness of such mitigation efforts, suggesting that no such measurements can be made. Our council would benefit greatly from having experts advise us on methods of evaluating such mitigation efforts.)
Ultimately, sufficient characterization to allow Factor of Safety determination is the only satisfactory criterion to evaluate stability.
7. FALLACY OF PREDICTING STABILITY FROM THE "AGE" OF THE LANDSLIDES
It is often repeated by lay persons seeking to develop the vacant lots that the City Council should allow such building, as Zone 2 has been stable for thousands of years. Your Report addresses this argument directly.
"There is some discussion in the technical documents regarding the geologic age of the Z2L mass. References have been made to the estimated age of the landslide as being 100,000 years old. However, there is no clear evidence to document this age of the landslide. The mistaken conclusion, expressed by some, is that somehow the old age of the landslide deposit equates to a long period of stability or even portends how the landslide will behave in the future. Predicting the long-term stability of landslides in advance of detailed surface and subsurface information is founded on ill-advised technical reasoning. There is simply no technical basis to determine the future stability by estimates of a landslide age or prehistoric history.QUESTION
A. Please explain why this argument is "founded on ill-advised technical reasoning."
ANSWER
Conditions (e.g., ground water levels, earthquakes, etc.) may alter quasi-stable conditions in an old landslide. Therefore, only adequate characterization can yield a meaningful stability estimate. Indeed, reasoning based on “age” may not even be a valid “technical” criterion. For reasons of scale, a landslide that appears to be “old”, may actually have undergone very recent, but small, movements. Stability judgments based only on age are “ill-advised” because one should not reason to conclusions without having sufficient data.
You also advise of the Big Rock landslide in Malibu and the consequences of the that problem, in a situation somewhat similar to that in this case:
"One example of the fallacy of judging ancient, prehistoric landslides to be incapable of reactivation is strengthened by the Big Rock Mesa landslide, located on the Malibu coast. The Big Rock Mesa landslide is an immense, ancient landslide that was reactivated in 1983 due to elevated groundwater. Prior to 1983, the landslide was recognized as an ancient feature that likely formed late in the Pleistocene Epoch (i.e., 11,000 to 100,000 years ago). The amount of movement in 1983 was relatively small, with total displacements ranging between 3 to 4 feet, and therefore left no permanent geomorphic features to document the failure event. While the displacement was small, and the damage to public and private property was relatively minor, it did trigger extended lawsuits, high settlement costs (i.e., nearly $100 million), extensive geotechnical investigations, and expensive remedial measures to control the landslide. The Big Rock Mesa landslide case is a clear example of how difficult it is to judge the future behavior of large landslides. Prior to its reactivation in 1983, stability of the Big rock Mesa landslide was not well defined, and was thus characterized by the same uncertainty as is inherent for Zone 2. Inasmuch as the factor of safety of the Zone 2 landslide is likely between 1.00 and the industry standard of safety of 1.50, the most prudent approach is to judge its stability as unknown, and the level of risk as uncertain.
8. ERRONEOUS UNDERSTANDING OF THE GEOLOGY BY PRIOR OBSERVERS
Your Report points to a prior study of the Abalone Cove Landslide by Robert Stone and Associates in 1979. You note that:
"the geologic map and cross sections were then used to prepare a structural contour map of the subsurface configuration of the landslide failure surface (e.g., basal rupture or slip surface). These documents . . . constitute the primary graphic exhibits that have been used to describe the dimensions of the ACL. They were used to help establish the ACLAD and by the Peer Review Group to aid in their deliberations for the Redevelopment Agency (RDA).You then note:
"The results of the inclinometer monitoring clearly documents that in certain areas of the ACL, the depth of landsliding is much deeper than originally interpreted by RSA. The active landsliding (between 1988 and 1993) along the critical geologic section drawn through the center of the ACL is deeper by 70 to 98 feet. Where the original RSA cross sections show the base of landsliding to be 50 feet to 70 feet, the slope inclinometer data document sliding to be as deep as 165 feet to 170 feet. Thus, the physical dimensions of the landslide are quite different than what geologists had thought.QUESTION
A. Please explain the implications of the fact that the RDA Peer Review Panel that made the recommendations for the guidelines for allowing building in Zone 2 may have not appreciated the depth of the landslide.
ANSWER
It appears that the Peer Review Panel could have had knowledge of the greater depth of active landsliding in the early 1990’s. The inclinometers measured active sliding between 1988 – 1993 and the RDA Peer Group was active until at least 1996.
QUESTION
B. Do you believe that the fact that the landslide is now understood to be deeper impacts the validity of the prior analyses? How.
ANSWER
Even if the landslide is now believed to be deeper, without accurately characterizing it, we still do not know its stability in a quantitative sense.
You also note:
"The documented depth of 165 to 170 feet places the base of the ACL very near the base of the prehistoric landslide that is interpreted to underlie the Zone 2 area."I would appreciate answers to these questions. To the extent that you believe that I have failed to raise questions that you think might be helpful to our understanding of this matter, given the questions I have raised, I would appreciate you adding such comments to your discussion.