3350, Sherman/Fournier - Request for, Studies & ReportsAugust 14, 2014
Project Ref.: GF-2033
Mr. Howard Weinberg, Attorney at Law
Weinberg Law Group
2550 Via Tejon, Suite 2B
Palos Verdes, CA 90274
AUG 1 9 2014
Ci • a R Ili g Hilts
Subject: Geologic Evaluation —Tree Cutting 3y Q
33 Crest Road East -�
Rolling Hills Ca 90274
Dear Mr. Weinberg:
In response to your request, an evaluation of the geologic conditions at the subject site and vicinity has
been performed by this office. Our evaluation consisted of review of available geologic references and
reports and study of available aerial photographs. Our purposes were to evaluate the possible effects of
tree cutting on site and slope stability and to evaluate the opinions expressed in the June 17, 2014
correspondence by E.A. Keller.
Site Description
The subject property and vicinity were described in detail in our May 30, 2014 report and are not
described in detail herein. Rather, only a brief description pertinent to the evaluation of geologic
conditions is provided.
The subject property is located on the southwesterly side of Crest Road, just west of that roadway's
intersection with Southfield Drive. It is situated on a narrow ridge that descends gently from Crest Road.
The flanks of that ridge descend to tributary channels (at both sides of the subject property) that join to
the southwest of the subject property. Both flanks are somewhat gently inclined and are covered with
grasses. Many of the tall trees that would be cut are within these flanks. At the time of our examination,
leaves that had fallen from these trees covered much of the sloping surfaces.
Opinions by E.A. Keller
The correspondence by E.A. Keller indicates that neighbors have requested that about 35 large trees be
removed from the subject property and provides a discussion of the effects of tree removal. It opines that
tree roots provide root strength which help reduce landslide and soil erosion hazards. It points out that
the area in which the subject property is situated is within a geologic environment characterized by a
serious landslide hazard and cites the Portuguese Bend Landslide, the Flying Triangle Landslide and
another landslide that is mapped above Klondike Canyon Creek. The report points out that the subject
property is underlain by the same bedrock (the Altamira Shale) that underlies the Portuguese Bend
Landslide and opines that "...it has been fortunate that larger landslides have not occurred there that
might well damage property..." and that "...it probably is more chance and good luck that other landslides
have not occurred."
Weinberg (33 Crest Road East) August 14, 2014
Project Ref.: GF-2033
Page 2
The report concludes that the planting of trees at the subject property has probably increased slope
stability in a location that has a serious landslide hazard and that if they are removed, the landslide and
erosion hazard at the property will be increased. As discussed below, the subject site is not within an area
in which there is a "serious landslide hazard" (as is indicated in the Keller report) and cutting of these
trees, as is currently sought, will not have an adverse impact on the stability and erosion potential at the
site.
General Discussion
While it is true that a number of landslide have occurred in the slopes of the Palos Verdes Peninsula, it is
not true that all the slopes in the vicinity of the subject property are susceptible to landsliding or that all
the residential properties in the vicinity of the subject property are in an area that can be characterized
as a "serious landslide hazard". In fact, much of the Palos Verdes Peninsula has been developed for
residential and commercial use, and most of the developed properties (including those in the vicinity of
the subject property) have not been impacted by landslides and landslide movement. What is true is that
the subject property is in an area where appropriate geologic and geotechnical investigation is necessary
to evaluate the potential for geologic hazards. When such investigation is performed, reliable conclusions
with respect to the potential geologic hazards can be reached.
Where large landslides do occur in the Palos Verdes Peninsula, they typically develop because of geologic
weaknesses within the underlying bedrock. Where these weaknesses are structurally oriented so that
they are adverse with respect to a given slope or slopes, large and deep seated landslides have developed.
In fact, both the Portuguese Bend Landslide and the Flying Triangle Landslide were extensively
investigated, and it was determined that both of these landslides occurred along adversely oriented
weaknesses (bentonitic clay beds) in the underlying bedrock.
Where landslides have and do occur within the peninsula, there presence is usually evident on aerial
photographs. Topographic land forms evident on aerial photos reveal both the presence of ancient and
active landslides. Hence, whether or not a given area has been affected by landsliding (both past and
present) can be determined from review of aerial photographs.
What aerial photographs do not provide is the information necessary to evaluate surficial slope stability
and potential erosion problems. For that purpose, it is necessary to determine the composition and
constituency of the near surface soils and the heights and gradients of the slopes of concern. When this
information is known, an evaluation of the surficial stability and erosion potential of given slopes can be
performed. Normally, such evaluation is performed by a geotechnical engineer.
Because evaluation of gross, deep seated stability and surficial stability require knowledge of different
information, it is prudent to consider them separately in evaluation of site and slope stability. Accordingly,
we have done so in the sections that follow. For both evaluations, it is also important to note that what is
currently sought is the cutting of trees within the subject property, not the removal of trees as is suggested
in the correspondence by E.A. Keller. As currently sought, trees would be cut down to their base, leaving
remnant trunks just above ground surface.
Tree Cutting and Gross Stability
For practical purposes, tree cutting has little impact upon the gross stability of slopes. It doesn't (have
IOZCJILJF2 DELTA
Weinberg (33 Crest Road East) August 14, 2014
Project Ref.: GF-2033
Page 3
significant impact) because the geologic weaknesses along which deep seated landslides occur are too
deep to be affected by the changes in root structures. Also, because the soils in which the root systems
grow are typically expansive (in the vicinity of the subject property), continuous voids do not develop
where root systems begin to decay. Because root decay occurs slowly, and because expansive soils swell
when they become wet, root decay does not result in continuous conduits through which subsurface
water can migrate. Also, in many instances, tree cutting does not result in killing trees, and so root decay
does not occur.
Review of published geologic references and study of available aerial photographs do not reveal the
presence of active or ancient landslides in the vicinity of the subject property. In fact, topographic maps
and aerial photographs reveal that the subject property is situated on a relatively resistant southwest
trending ridgeline that has, for the most part, proven to be grossly stable. The closest landslides are
situated to the southwest (the Flying Triangle Landslide) and to south within slopes that are not
topographically similar to the ridgeline on which the subject property is situated (see Figures 1 and 2,
portions of a 1962 air photo and portions of the Geologic Map by Thomas W. Dibblee).
Additionally, site specific geologic and geotechnical investigations were performed at the subject property
in conjunction with construction (by Keith W. Ehlert and Coastline Geotechnical Consultants). The geologic
and geotechnical reports of those investigations confirm that no landslides were encountered within the
limits of the subject property and that with respect to gross stability, the subject property was determined
to be grossly stable. Among the statements made in those reports were "It is this consultant's opinion the
proposed improvements can be constructed without hazard of landslide or slippage..." (Ehlert, August 2,
1995, Page 5), "No signs of surficial or deep seated instability were evidenced on the site, or upon
immediately adjacent properties..." (Coastline Geotechnical, August 9, 1995, Page 3) and "It is our opinion
that construction within the building site would not be subjected to geologic hazards from landslides,
slippage..." (Coastline Geotechnical, August 9, 1995, Page 5). Additionally, stability analyses were
performed by Coastline Geotechnical based upon geologic cross sections prepared by Keith Ehlert and
based upon these analyses, it was concluded that "The results of analyses, as shown on Plates 5A through
8A, indicate that the factors of safety are in excess of the normally accepted minimum for stable slopes..."
(Coastline Geotechnical, December 14, 1995, Page 3). These statements indicate that the subject property
is grossly stable and that it is not in an area affected by landslides. Pertinent excerpts from the reports
cited are attached as Figures 3, 4, 5 and 6. Copies of the reports reviewed will be provided in separate
correspondence.
When considered collectively, it is clear that the available data and information (published references,
aerial photographs and reports of geologic and geotechnical investigations of the subject property)
indicate that the subject property is grossly stable and that landslides are not present within and around
it. The subject property is stable because it is not underlain by a landslide and because of prevailing
geologic conditions, which were adequately considered and evaluated by geologic and geotechnical
consultants during construction. Its stability is not the result of chance or good luck, as is suggested by
E.A. Keller. Tree removals will not impact its gross stability.
Tree Cutting and Surficial Stability
For practical purposes, the effects of tree removals on surficial stability and erosion within given slopes
depend upon the composition and constituency of the near surface soils and upon the gradient of given
, � iCa DUP DELT1
Weinberg (33 Crest Road East) August 14, 2014
Project Ref.: GF-2033
Page 4
slopes. Where slopes are steep and composed of noncohesive soils, they are susceptible to problems
associated with surficial stability and erosion. In those instances, tree removals (and consequent root
decay) can result in a loss of effective soil cohesion and have a significant impact on surficial stability and
erosion. Where slopes are not steep and are composed of cohesive soils, they are not susceptible to
problems associated with surficial stability and erosion. In those instances, tree removals do not have a
significant impact on surficial stability and erosion, even with a loss in effective soil cohesion.
The soils at the subject site were derived from weathering of underlying bedrock. Since the underlying
bedrock is clayey in composition, the soils that mantle them are clayey in composition and are generally
cohesive. Because the slopes in which the trees would be removed are not particularly steep and because
the soils in which they grew are cohesive, they are not susceptible to erosion and surficial slumping.
Removal of trees (and consequent root decay) would not have an impact on surficial stability and erosion.
While some root decay could eventually occur after trees are cut, its (root decay) effects would not be
significant because of the cohesive nature of the near surface soils and the prevailing gradients of the
slopes at the subject site.
Conclusions
The slopes at the subject property are grossly and surficially stable. Tree cutting, where trees are cut at
their base, will not have an impact. The subject property is not within an area in which there is a "serious
landslide hazard" as is suggested in the Keller report. That it (the subject property) is not in such an area
is confirmed by the data and information provided in previous geologic and geotechnical reports of
investigations specific to the property and by review of aerial photographs. If tree roots are removed as a
part of tree removal, soils removed during these efforts would need to be locally recompacted when they
are replaced.
We hope the information provided herein satisfies your current needs. If you have any questions, please
feel free to call.
Sincerely,
Group Delta Consultants, Inc.
4 2
Rodney T. Masuda
Principal Geologist, CEG 1230
Attachments: Figures 1 through 6
)''` il®UP DELTA
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ENGINEERING
33 Crest Road East, Rolling Hills
Project Ref.: GF-2033
Figure 1
Portion of 1962 Spence Air Photo
S_E_49-116
33 Crest Road East, Rolling Hills
Project Ref.: GF-2033
Figure 2
Portion of Geologic Map of the Palos Verdes Peninsula and Vicinity
Redondo Beach and San Pedro Quadrangles
by Thomas W. Dibblee, Jr., 1999
Map Sheet DF-70
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August 2, 1995
Keith Ehlert Report on 33 Crest Road
Page 5
Information obtained during this investigation indicates
the site is underlain by bedrock of the Monterey
Formation..:
i No geologic or other maps were reviewed by this
consultant which indicate active faults or landslides
trend through the site.
It is this consultant's opinion the proposed
improvements can be constructed without hazard of
landslide or slippage
Group Delta Consultants
Project Ref.: GF-2033
Figure 4
August 9, 1995
Coastline Geotechnical Report
on 33 Crest Road
Page 3
• No signs of surficial or deep-seated instability were
evidenced on the site, or upon immediately adjacent
properties.
Group Delta Consultants
Project Ref.: GF-2033
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December 14, 1995
Coastline Geotechnical Report
Page 3
The results of the analyses, as shown on Plates 5A
through 8A, indicate the factors of safety are in excess
of the normally accepted minimum for stable slopes.
Group Delta Consultants
Project Ref.: GF-2033
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