Volume II: Executive Summary s
Table of Contents
Introduction 2
Background 2
General Evaluation 3
Conclusions 4
a) Orientation and seaward extent of active slide plane 5
b) Degree of uncertainty within 400 feet of shoreline . 6
c) Viability of onshore or offshore protective structures6
d) Additional drilling 8
e) Other issues 8
Recommendations 9
References 10
Leighton and Associates Report BA
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1
Introduction
This Appendix presents the Corps of Engineers Geotechnical
Branch evaluation of the geologic conditions along the Rancho
Palos Verdes coastline that pertain to degradation of the
environmental resources of the area , and which are pertinent to
the design of environmental restoration measures . The Appendix
addresses the findings as presented in the Geotechnical Appendix
for the Stabilization of the Portuguese Bend Landslide , City of
Rancho Palos Verdes , California , prepared for Charles Abbott
Associates by Leighton and Associates , Inc . on April 22 , 1997 ,
and is attached as Appendix BA. The Leighton report (Appendix
BA) was contracted to identify significant factors to be
considered in design measures for reduction of shore erosion and
associated sediment and turbidity problems along the Palos Verdes
coastal area , and to address the suitability of the foundation
for construction of a structure to reduce coastal erosion at the
toe of the Portuguese Bend Landslide (PBL) .
The scope of the Corps Rancho Palos Verdes Feasibility Study
does not include stabilization of the landslide , which is
expressly outside the authorized study purpose ; however , the
stability of the landslide as it would impact proposed coastal
structures is an important and relevant issue. Offshore
geophysical surveys and shoreline borings , therefore , were
conducted as part of the local Sponsor ' s in-kind services , and
were intended to determine the near shore and offshore geologic
structure and most importantly the location of the toe of the
landslide and location of a stable foundation for shore
protection structures .
The attached Appendix BA also was intended to function as a
`without project conditions ' statement of the seaward portion of
the PBL . It summarizes previous reports and studies , including
all relevant boring logs . Three subsurface structure maps and
four representative geologic cross-sections as presented
illustrate the PBL geometry .
Background
The most critical portion of the PBL from a geotechnical
construction feasibility standpoint is at the present day
shoreline . The PBL terminates at the Pacific Ocean in a near
vertical sea cliff 80-100 feet high . Composed primarily of
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colluvial slide debris , this prominent feature is bounded by a
411 rocky , cobble beach varying in width of 50-200 feet . At high
tides , depending on the season , waves break immediately seaward
of this soft debris slope , and it is continuously being eroded .
Large storms in the spring of the year may cause more extensive
damage , as huge blocks of material slip down from the upper slope
along seaward-dipping listric faults .
The toe of a landslide is defined as " . . . the margin of
disturbed material furthest from the main scarp" . (NAS-NRC ,
1958) . In published literature (Ehlig , 1982 , 1992) , the base of
this high sea cliff was said to represent the present day toe of
the slide . Interpretation of the active rupture surface , toe
geometry , and slide mechanism is of utmost importance for siting
and foundation design of a protective structure . This area ,
however , due to its transitional location along the strand line
and in the active surf zone , presents the most troublesome and
costly region for geotechnical data acquisition as listed below
and discussed in more detail in the Conclusions .
- Inherent limitations of seismic data acquisition in shallow-
water shoreline areas make it difficult to define the actual PBL
active or ancient slide surfaces by geophysical methods within
411 areas closer than approximately 400 feet of the shore ; and
- Drilling at the shoreline or in shallow water offshore presents
drilling and sampling difficulties in the upper , unconsolidated
sediment portions of borings where the slide planes might exist ;
therefore , slide planes in the toe region are extremely difficult
to locate and define .
As a part of the RPV feasibility study an attempt was made
to determine the toe of the seaward subslide of the PBL , the
seaward extent of sliding, and the subsurface bedrock structure
near the shore . To that end , an offshore geophysical survey to
within 350 to 600 feet (-15 to -20 feet MLLW) of the shoreline
was conducted by Dill and Norall (1995) . This was followed by
three diamond core borings at the shoreline (LC-1 , 2 , and 3) by
Leighton and Associates (1996 , 1996a) (see Figure 11 in Appendix
BA for the limits of the seaward subslide and the location of
these borings) .
General Evaluation
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Geotechnical studies to identify the existing conditions in
the study area are included in Appendix BA, which is attached to
this report . These conditions will not be reiterated here .
Pertinent statements of fact from the Appendix BA with which we
are in agreement are as follows :
1 . The seaward portion of the PBL is currently moving at the
rate of 0 . 25 inches per day, or about 7 . 6 feet per year . Greater
movement , up to 30 feet per year , takes place during wet years
(Appendix BA, page 23 , 24) ;
2 . The toe of the slide is being eroded at the rate of about
208 ,000 cubic yards annually (Appendix BA, page 25) ;
3 . The PBL is subject to a maximum acceleration of 0. 535 g from
the maximum credible earthquake (MCE) on the nearby Palos Verdes
Fault ; the maximum probable earthquake (MPE) would induce a 0 .419
g acceleration (Appendix BA, page 6) ;
4 . Offshore geophysical data are interpreted to suggest that no
sliding occurred beyond about 400 feet from the present day
shoreline (Appendix BA, page 10) ;
5 . The active slide plane is : a) undulatory ; b) found below sea
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level at boring LC-1 but at the base of the shoreline bluff at
boring LC-2 ; c) varies in seaward extent from onshore to some
distance offshore (LC-1) ; d) variable in its stratigraphic
position relative to the Portuguese Tuff member ; and e) may dip
offshore or onshore at the shoreline (Appendix BA, Figures 13-
15) . Inactive slide debris (the landslide toe) is encountered at
an elevation of -40 ft . MLLW in LC-3 and projects offshore .
Conclusions
Although we are in general agreement with the report
information , the Corps of Engineers analysis of Leighton and
Associates geotechnical Appendix BA resulted in different
conclusions about ; a) orientation and seaward extent of the
active slide plane ; b) degree of certainty of structural
inferences in the first 400 feet from the shoreline ; c) the
viability of onshore or offshore protective structures ; and d)
the usefulness of additional drilling.
a) Orientation and seaward extent of active slide plane .
• • Leighton ' s conclusions are that the active as well as
ancient slide planes dip onshore and terminate at or within 100
feet offshore of the present shoreline .
• The Corps Geotechnical Branch believes that the slide
plane(s) near the present shoreline cannot be defined with great
accuracy due to inherent drilling and sampling problems . It is
agreed that at the present shoreline the borings define the slide
surfaces as no deeper than about -40 feet MLLW (LC-3) and as
shallow as the beach surface itself (LC-2) . The present beach
can be considered an active slide surface because the slide mass
can move. across it (Ehlig, 1992) ; therefore , because of varying
rates of slide movement and coastal erosion , the location of the
toe of the slide has been variable over the last few decades and
may or may not be coincident with active or presently inactive
slide surfaces . Although Leighton notes that holes LC-2 and LC-3
were not deflected during drilling operations , these two holes
were not drilled at the same relative position to the toe as LC-
1 , nor did they use the same small surface casing, and LC-2 and
LC-3 were drilled in the fall of the year when movement of the
slide is historically the slowest . LC-3 did penetrate inactive
411 slide debris , indicating that this area possesses an older slide
plane that could be re-activated . The Corps Geotechnical Branch
is of the opinion that all areas of the PBL shoreline are either
now moving or subject to movement at any time .
Of particular concern to the Geotechnical Branch is the Leighton
report interpretation of the slide geometry and offshore
projection of the toe . Although the cross-sections shown on
Appendix BA, Figures 13-15 represent reasonable interpretations
of the very limited data , alternative interpretations regarding
the direction of dip of the geologic structures in LC-1 and the
offshore extent of the toe (Appendix BA, Figures 13 and 14) are
equally valid .
The conclusion is that shoreline borings indicate that the slide
planes could dip offshore as well , inferring that unstable
foundation conditions exist well beyond the 100-foot distance .
The historic maximum southward extent of the shoreline (1870-
1982) was approximately 200 feet seaward (CCSTWS , 1987) . There
is no data to suggest that this represents the maximum possible
seaward toe location .
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b) Degree of uncertainty within 400 feet of shoreline .
• Leighton ' s conclusions are that stable foundation conditions
exist 150 feet offshore , and that locating a structure on bedrock
foundation , stability would be insured .
• Recent drilling north of Palos Verdes Drive South (PVDS) by
the City , has determined that a bentonite bed , several inches
thick , defines the PBL in this area . The undulatory , active
rupture surface occurs approximately 30 feet above the Portuguese
Tuff and is controlled by the bedrock structure . It is important
to emphasize , however , that in the seaward portion of the PBL ,
south of PVDS , the slide surface appears not to be controlled by
this particular layer and in fact cuts across stratigraphic
boundaries ; sometimes occurring within and sometimes above the
Portuguese Tuff . (Appendix BA, figures 13-15) . As such , the
Portuguese Tuff member is not of particular importance other than
to serve as a marker in the stratigraphic sequence . Thin
bentonite layers , susceptible to sliding, occur above , below, and
within the upper portion of the Portuguese Tuff . Slide planes
appear to be dynamic features controlled by the geometry of the
overlying slide mass . This allows new sliding surfaces to
develop at the toe in response to movement of the whole mass . Aft
11,
c) Viability of onshore or offshore protective structures .
• The Leighton and Associates report concluded by recommending
additional borings be drilled offshore to evaluate the foundation
materials and subsurface structure beneath a sponsor proposed
revetted buttress fill . The report implies that the near shore
area is essentially stable and states that the toe of the slide
is near the shoreline .
In another recent , separate , comprehensive engineering study of
the East-Central portion of the landslide for the City of Rancho
Palos Verdes (Ehlig and Yen , 1997) it was concluded that the
slide mass cannot be stabilized by conventional up slope
remediation alone , without either 1) enhancement of the shear
strength of the bentonite along the base of the slide or 2)
construction of a revetted buttress fill at the toe of the
seaward subslide . The study further concluded that the factor of
safety of the seaward subslide without a gravity buttress is less
than 1 . 0 . To be effective , a buttress fill would need to be
constructed adjacent to and partially on the actively moving
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landslide .
111
• The Geotechnical Branch
has concluded that the available
information does not confirm that the near shore area is stable .
Any structure proposed within 400 feet of the existing shoreline
(out to a water depth of about -10 to -20 feet MLLW) could be
subject to displacement , either along an active slide , the
reactivation of inactive slide planes , development of new sliding
surfaces within the south dipping bedrock, or seaward movement of
the landslide mass over the existing sea floor .
It is our opinion that geotechnical data rules out consideration
of certain shore protection alternatives previously proposed as
alternative plans , specifically a revetment , gabions , and gabions
and a submerged breakwater. All these proposed structures would
require construction of a foundation on presently unstable or
potentially unstable surfaces . This would result in displacement
and damage to the structures , requiring frequent repair and
replacement , depending on the rate of continued slide movement
and related impacts on structures . The magnitude of damage is
difficult to predict because of uncertainties inherent in the
stability analysis of the slide . Analyses regarding the
stability of the slide are outside the scope of the Rancho Palos
Verdes Feasibility Study . The Corps Geotechnical Branch can not
recommend , as a shoreline erosion prevention measure , the removal
or redistribution of slide debris landward of the toe , because of
the uncertainty as to whether the removal of material could
reduce a buttressing effect on the existing seaward slide as well
as the potential to thereby negatively impact the overall PBL
equilibrium.
Of the shore protection concepts identified in the Initial
Project Management Plan (IPMP) (RPV IPMP , 1994) , the breakwater
constructed in water depths of approximately -10 to -20 feet MLLW
is the geotechnically preferred option . Having a base width of
about 200 feet , the structure would be constructed approximately
400 - 600 feet offshore . Based on the offshore geophysical
survey by Dill (1995) , the foundation in this area would be
bedrock overlain by about 4 feet of Holocene sediments . Although
there is no analytical technical basis for the location siting,
it is the Corps ' engineering judgment that active or inactive
slide planes are not likely to be present this far from the
existing toe . Furthermore , it is reasonable to assume that
encroachment of a moving slide mass would not overtake the
structures , at least for several decades . If debris did encroach
411 B-7
on the structure , however , it would require removal to avoid
damage and costly repair .
d) Additional drilling.
• Leighton ' s conclusion is that additional drilling is needed
to evaluate foundation materials and determine subsurface
geologic structure (Appendix BA, page 30) .
• The Corps Geotechnical Branch believes that offshore
drilling , as originally proposed in the Rancho Palos Verdes
Feasibility Study (IPMP) in 1994, cannot now be recommended due
to the extremely high cost and very questionable return of useful
geotechnical information . Due to these uncertainties and the
very high cost/benefit ratio for additional offshore
explorations , the actual conditions all along the base of the PBL
may never be known .
Based on the results of the explorations conducted thus far , it
is our opinion that no reasonable amount of additional drilling ,
GPS measurements , seismic , or other geotechnical methods will
pinpoint that theoretical point or zone at which unequivocally
stable foundation conditions exist in the near shore environment .
The difficulties encountered in the recent shoreline drilling and All.
sampling indicate that offshore drilling may not obtain
conclusive data regarding existing slide planes or older inactive
slide planes . Even under ideal conditions where complete core
recovery might indicate a lack of existing slide planes , the
development of new sliding surfaces cannot be precluded as the
location and geometry of the landslide toe changes with time.
Drilling of three to six offshore borings to a depth of 100 feet
in water depths of -10 to -20 feet MLLW (distance from shore : 200
to 700 feet) is estimated to cost between $300, 000 and
$1 , 000,000. (The costs would be high because of the following
major expenses : outfitting/mobilization of jack-up type drill
rig ; need for support vessels ; open-ended 24 hour day rate
contracting; offshore logging expenses ; and other costs such as
environmental impact statements , permits , consultants , etc ..)
e) Other issues .
The probability of a seismic trigger for slide reactivation
is not addressed by the Leighton report . The Corps maintains
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that seismic forces resulting from an earthquake would add to the
II/ existing driving forces acting parallel to the rupture surface .
A potential failure surface close to equilibrium before an
earthquake could rapidly exhibit a factor of safety below 1 . 0
during a seismic event , causing en echelon failures along weaker
beds . Any seismic forces , therefore , no matter how small , will
tend to destabilize the PBL , causing more rapid down slope
movements .
Recommendations
1 . Any proposals for construction at the shoreline or within 400
feet of the existing shoreline should include the cost of
additional O&M for extensive repair and/or replacement of the
structure perhaps several times over the life of the project
due to unstable foundation conditions .
2 . A breakwater structure (offshore dike) is engineeringly
feasible if constructed at or beyond 400 feet from the shoreline .
3 . Based on the results of the explorations conducted thus far ,
it is the opinion of the Corps that no reasonable amount of
411 additional drilling , GPS measurements , seismic , or other
geotechnical methods will pinpoint that theoretical point or zone
at which unequivocally stable foundation conditions exist in the
near shore environment at the PBL .
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References
III.
Charles Abbott Associates , Inc . , 1997 , Analysis of Landslide
Material Loss , Portuguese Bend Shore Protection Feasibility
Study , report dated January , 1997.
Dill and Norall , 1995 , "Offshore Seismic and Geology Survey ,
Portuguese Bend , California" , 30 June 1995 .
Ehlig , 1982 , "The Palos Verdes Peninsula : Its Physiography , Land
Use and Geologic Setting" , in J . D. Cooper , comp. , Volume and
Guidebook: Landslides and Landslide Abatement , Palos Verdes
Peninsula , Southern California : Geol . Soc . Amer . , Cordilleran
Section , 78th Annual Mtg, pp. 3-6 .
Ehlig, 1992 , "Evolution , Mechanics and Mitigation of the
Portuguese Bend Landslide , Palos Verdes Peninsula , California" ,
in B .W. Pipkin and R. J . Proctor , Engineering Geology Practice in
Southern California , Special Publication No. 4, Assoc . Eng. Geol .
Ehlig and Yen , 1997 , "Preliminary Investigation Report -
Feasibility of Stabilizing Portuguese Bend Landslide" , report
dated 3 March 1997 .
Leighton and Associates , 1996 , "Summary of Drilling Operations •
at the Portuguese Bend Club, City of Rancho Palos Verdes ,
California" , 22 May 1996.
Leighton and Associates , 1996a , "Summary for Drilling Operations
of Two Additional Borings at the Portuguese Bend Club , City of
Rancho Palos Verdes , California" , 1 November 1996.
Leighton and Associates , 1997 , "Geotechnical Appendix for the
Stabilization of the Portuguese Bend Landslide , City of Rancho
Palos Verdes , California" , 22 April 1997 .
National Academy of Sciences-National Research Council (NAS-NRC) ,
1958 , "Landslides and Engineering Practice" , Publication 544
Special Report 29 .
US Army Corps of Engineers , Shoreline Movement Investigations
Report-Portuguese Point to Mexican Border , CCSTWS , Volume 10 ,
December 1987 .
US Army Corps of Engineers , Initial Project Management Plan
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(IPMP) for Rancho Palos Verdes , California , Shore Protection
411 Feasibility Study , Los Angeles District , December 1994.
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