RPVCCA_CC_SR_2012_07_31_02_Landslide_WorkshopCITY OF
MEMORANDUM
RANCHO PALOS VERDES
TO:
FROM:
DATE:
SUBJECT:
REVIEWED:
Project Manager:
HONORABLE MAYOR &CITY COUNCIL MEMBERS
JIM HENDRICKSON,INTERIM DIR.PUBLIC WORKS
JULY 31',20-'2
LANDSLIDE WORKSHOP (Supports City Council Goal
-Public Infrastructure)
CAROLYN LEHR,CITY MANAGER 09--.
Ron Dragoo,Senior Engineer
RECOMMENDATIONS
1.Receive and file.
2.Provide direction to Staff on possible future remediation projects as presented during
the workshop.
BACKGROUND/DISCUSSION
At its March 20,2012 meeting,the City Council adopted the Goals and Priorities list
which includes an item to address Public Infrastructure,specifically an in-depth review
of the Portuguese Bend Landslide.Based on this desire to better understand the
Landslide,Staff has assembled a team of 3 geologists who will present information and
answer questions,interpret data and offer opinions during the Workshop.The
Geologists working with the City on this project are:
Robert Douglas -graduate of UCLA Geology (Ph.D.),Professor Emeritus,University
of Southern California,member of Geological Society of America,American
Geophysical Union,The Oceanography Society,American Association for the
Advancement of Science,Society of Economic Paleontologists and Mineralogists
(SEPM),Union Geofisca Mexicana,and long term resident of the Portuguese Bend
community;
Scott Kerwin -graduate of UCLA Geology (BS),Professional Geologist and Certified
Engineering Geologist,member of Association of Engineering Geologists and American
Society of Civil Engineers,currently serving as the Portuguese Bend Club geologist;
James Lancaster -graduate of UCLA Geology (BS),Professional Geologist and Certified
Engineering Geologist,member of Association of Engineering Geologists, International
Code Council Los Angeles Basin Chapter,International Code Council Orange Empire
2-1
Engineering Geologist,member of Association of Engineering Geologists,International
Code Council Los Angeles Basin Chapter,International Code Council Orange Empire
Chapter,serves as City Geologist.
These experts and Staff will focus on explaining the complex movement within the
landslide,mitigation efforts that have been installed and possible projects that could
provide additional land movement mitigation.
Staff has attached a paper authored by Robert Douglas,"Notes on the Portuguese
Bend Landslide"(Attachment A),which provides a concise summary of this very
technical topic.Also attached is a map which shows the location of the landslides
(Attachment B).
Attachments:A.
B.
Notes on the Portuguese Bend Landslide
Landslide Map
2-2
ATTACHMENT A
2-3
ACLAD Misc.Rept.8/2011
NOTES ON THE PORTUGUESE BEND LANDSLIDE (PBL)
~Robert Douglas,
Abalone cove Landslide Abatement District (ACLAD)
Prologue
While the Portuguese Bend Landslide has achieved the level of a local "rock star",
known to nearly everyone in the community and to many beyond,and been the subject of
numerous studies conducted by college students to professional geoengineering companies,
information about the landslide is largely confined to technical reports written by and for geo-
specialists and most ofthese are not easily obtained.Paradoxically,one of California's most
famous geological features is little covered in books and articles written for the general public.
Reference to the internet produces a list of entries,most of which are not about the landslide
per se and those that are contain about as much misinformation as facts.So with this in mind,
the follOWing notes try to review what we know and don't know about the landslide,its origins,
its geological setting and history.While these notes were prepared by a geologist who works in
the PB area,they are intended for the non-professional and I have tried to keep the technical
jargon to a minimum.
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ACLAD Misc.Rept.8/2011
Background
The Portuguese Bend Landslide (PBL)is the largest,continuously active landslide in the
United States,possibly North America.One of its key characteristics is its slow,steady rate of
movement over time.Another is that the landslide rupture surface is typically in bentonite
clay.While the south flank of the Palos Verdes Peninsula contains many active and inactive
landslides,PBL is the most conspicuous and well known (fig.1).It covers about 250 acres and
has a maximum width of 3600 feet and a maximum head-to-toe length of about 4200 feet.The
PBL,together with the Abalone Cove and Klondike Canyon landslides,are actually reactivated
parts of the large Ancient Altamira Landslide complex which dates back to the Ice Ages.
Calif.Stale Geol Survey
Landsrlde Map 2001
\:
,'
I
,.',
\.
.r
...'
.l
"";o,t\-··
,u·_:'
..$.5"rr."5--t ,...',~
:::( I .......
<.~
,j'-R.II!~inl.t Hill,;
.....f-~1:rt.+
Fig.1 Map ofthe landslides located on the south flank of the Palos Verdes Peninsula (active
=red;inactive=yellow;dormant=orange).The largest active slide is the Portuguese Bend landslide.
(Calif.State Geological Survey,2007)
Reactivation occurred with residential development in the area and human activities
played an important role.Also in the same area is the active Flying Triangle landslide and
2
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ACLAO Mise.Rept.8/2011
although it is generally considered to be separate,an argument can be made that it is part of
the same complex.
Since its reactivation in the 1956,the PB landslide has moved in a slow,southward
direction with the greatest movement (700-800+feet)in the central-eastern portions and the
beach area and the least in the west and inland areas(ca.500 feet).In the past 55 years,the
movement has varied and sometimes slowed but the landslide has never stopped moving.
Originally described by geologists as a "block-glide"type landslide in which a thick slab of strata
fails and slides along a bedding plane,the continued movement and deformation within the
mass over the past half century has reduced the central,eastern and seaward parts to a jumble
of blocks and debris with little or no internal structural or stratigraphic continuity.Basically,
most of the landslide is moving by plastic flow in the bentonites which compose the rupture
surface.Secondary slumping in the head area indicates the landslide is slowly propagating---
upslope.
Fig 2.Map of the Portuguese
Bend area in 1956-57 shoWing the
then existing roads,houses,the
PB Beach club and pier and
proposed Crenshaw Extension.
Also shown is the location of the
caissons (e)embedded just north
of Yacht Harbor Road in 1956-57
by LA County and Palos Verdes
Properties as the first attempt to
halt the landslide.(modified after
Ehlig,1997)
3DlJ "'FIIT
3
2-6
ACLAD Misc.Rept.8/2011
In the 19505 the Portuguese Bend was a popular suburban community with a spectacular view
of the Pacific Ocean and Catalina Island.More than 150 homes covered the area,mostly build
in the post-war boom of the late 19405 and early 1950s.On the beach was located the popular
Portuguese Bend Beach Club and pier,a private recreational club.Ultimately,all of the homes
adjacent to the beach (69),the Beach Club and the pier were so severely damaged they had to
be razed (a few were moved).All of the inland houses sustained major damage and
Fig.3 (top)Aerial photo of the Portuguese Bend area in early 1956.Crenshaw extension is under
construction but has not crossed Portuguese Canyon.The light spot below the switch-back in the
center-right of the photo is the approximate location where the PBl movement began.(below)A view of
the Portuguese Bend Beach Club and pier showing the scarp located behind the clubhouse in 1947.
4
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ACLAD Misc.Rept.8/2011
many were also razed;a few were repaired.Following the outcome of the lawsuit in 1961,LA
County basically abandoned the area and allowed condemn houses to stand.Of the original
homes,about 30 remain,including several that are not inhabited.The area has become well
known for the novel engineering approaches used to stabilize and level houses in the moving
landslide.One of the current methods is to use three steel cargo shipping containers arranged
in a triangle as a foundation and to place the house on top of them.The containers are
periodically re-Ieveled at the corners of the triangle.Living in a moving landslide is challenging
as the residences will tell you.All services (water,gas)must be above ground and electric poles
require alignment as the ground shifts.The area relies on septic tanks,most of which are old
and in need of repair and the storm drains are in need of constant repair.The roads along with
the topography have changed significantly over the years (compared fig.2 to fig.4)and locating
some of the original addresses can b~challenging.·
The landslide generated a major lawsuit between the County of los Angeles and the
property owners who lost homes.In 1961 the County of los Angeles was held responsible for
starting the landslide and the plaintiffs received a settlement of 9.5 million dollars.The Court
ruled that the County had failed to inform the property owners that the area was within a
documented landslide (based on geological studies conducted in the 1920s and 30s).
Construction of the Crenshaw extension caused the reactivation of the landslide by loading the
head of the landslide.largely ignored during the trial was the fact that the houses in the area
were entirely on septic tanks and without storm drains,both of which had significantly added
to groundwater build-up.Conserve engineers hired by LA County correctly pointed out that by
the time the landslide began pore-fluid pressure caused by elevated ground water levels was a
more significant causal factor than the loading effect of 160,000 cubic yards of dirt.The PBl
lawsuit was a landmark case that led to the development of modern geoengineering practices
and building codes that require geotechnical studies as part of the evaluation for a building
permit.
Subslides and Movement
The first indications of the slope failure that evolved into the PBl occurred in 1956.The
County of los Angeles was in the process of extending Crenshaw Blvd.to connect Crest Road
with Palos Verdes Drive South and as the grading and canyon fill proceeded down the south
flank into the Portuguese Bend area,a small landslide developed NE of the current landslide
area (a small triangle-shaped landslide labeled 1956 in fig.5).Subsequently about 160,000
cubic yards of this slide material was used to establish a suitable grade into the PB area.On
August 17,1956,after placement ofthis fill,road engineers observed cracks in a recently
completed road culvert and additional ground cracking quickly developed in the area and
progressed southward.By September,a crack and four inch offset were observed in Palos
Verdes Drive South along the eastern margin of the reactivating landslide and by October
deformation was evident in the pier of the Portuguese Bend Club.The rapidity with which the
landslide expanded strongly suggests that it was just barely stable at the time and the build-up
of pore-fluid pressure following the heavy rains of the early 1950s and the domestic water from
5
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ACLAD Misc.Rept.8/2011
the newly constructed homes had brought it to a near-failure level.The sequence of
development of the landslide is shown in fig.4.
---:}:0c~4~
'..':;~'l ~.~
Fig.4 Approximate sequence in which the PBL devel~~d
following the initial movement (marked by the X).A-Aug
15 to 25,1956;B-Aug.to Sept.1956;C-Oct.,1956;D-Dec.,
1956;E-Spring,1957.(modified after Ehlig,1997)
Fig.5.Map ofthe landslides in
the Portuguese Bend area,
including the PBL,Abalone
cove,Klondike Canyon,the
Flying Triangle and the Ancient
Altamira landslides.The
subslides of the PBL (seaward,
eastern,central inland and
western)are indicated.
6
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ACLAD Misc.Rept.8/2011
Based on variation in the rate of movement and amount of deformation,Perry Ehlig
(then RPV City geologist)divided the landslide into several subslides,here simplified as the (1)
eastern,(2)central,(3)seaward,and (4)western and inland subslides (fig 5).From the
beginning,the eastern,central and seaward portions of the landslide have been more active
with higher rates of deformation than the inland and western portions and only the small areas
immediately inland from the two points,Portuguese Bend and Inspiration Points,have
remained stable.This pattern has continued for the past 55 years.
Rates of movement over the years generally have ranged from less than 0.1 to 0.8
inch/day but in the beginning and during rainy years,movement has been more than an inch
per day with the highest rates in the eastem'and seaward subslides.Until recently rates in tile
western-inland subslide were less than 0.5 inch/day.Generally the landslide accelerates during
years of higher than normal rainfall.
Although 1956 is usually given as the beginning (reactivation)of the PBL,Ehlig
discovered evidence in early aerial photographs of movement prior to 1948 in the area directly
behind the Portuguese Bend Club.Fresh scarps in this area are not seen in photos taken in the
1930s.Evidence was also found that the PB club pier experienced damage in 1946,in the same
place that it failed in the 1956 landslide.It seems clear that movement began prior to 1956 and
the real question is how long has the area been unstable and prone to landslide movement?
Judging from aerial photographs taken in the early 20 th century,movement had been occu,rring
for centuries.A more precise answer requires understanding the origins of the larger,ancient
Altamira slide from which the PBL formed.
The rolling,hummocky topography of PB was recognized by Kew in 1926 as a landslide
terrain and Woodring,Bramlette and Kew in their classic U.S.Geological Survey Professional
Paper 207,based on geological mapping done in the 1920s and 30s,carefully documented the
large,ancient Altamira landslide,reactivated portions of which now form the PB,AC and
Klondike Canyon landslides.They concluded that it was old,extending back into the
Pleistocene (Ice Ages).Pond sediments exposed in a road cut along PVDS (now destroyed by
grading)contained fossil charcoal material that was radiocarbon dated by Emery (1967)as
4,800 +/-180 years old.The depression which held the pond,much like "Lake Ischibashi"which
formed in the same area in recent years,was created by landslide movement.The date
prOVides evidence that movement in the PBL has been occurring for at least millennia.
Two geologists from Stanford University,Richard Jahns and his student,Karl Vonder
Linden (who investigated the PBL for his doctoral dissertation in the 1960s)wrote a paper in
1973 in which they suggested that the ancient landslide complex represented a series of semi-
independent slides that formed in three separate time intervals during the late Pleistocene and
Holocene (roughly the last million years).The up slope slides are the oldest and the slides next
to the coast are the youngest.PBL was one of the youngest landslide masses.To date the
landslides,correlation was made with the marine terraces described by Woodring,et al..In this
way,Jahns and Vonder Linden concluded the oldest landslides were above terrace 7 and older
7
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ACLAD Misc.Rept.8/2011
than 250,000 years.(Today,terrace 7 is correlated with Marine Isotope Stage 13 and dated at
about 500,000 years old.)They concluded that the youngest landslides were late Holocene (the
time since the end ofthe Ice Ages,the last 10,000 years)based in part on the 4800 year old
charcoal found in the PBL.Thus,they saw the area composed of a complex series of landslides
that had developed separately over a long span of time (ca.500,000 years)and that extends
right up to the present day.
After studying the PBl,ACl and ancient landslide complex for many years,Ehlig came to
a very different conclusion.He suggested that the whole complex is a megaslide that started
moving as a unit but fragmented as movement progressed (fig.6).He dated the original
movement based on where the seaward edge of the ancient slide is overlapped by terrace 2 on
Portuguese and Inspiration Points.Here,the upturned edge of the old slide is overlain by
marine terrace deposits that are 120;000 years old (correlated with Marine Isotope Stage 5-:-51.
Further,he could not document any of Woodring's,et al.terraces within the ancient landslide
which supported his belief that the megaslide initially moved as a unit and destroyed any
terraces which might have existed within it.Thus,he concluded that the megaslide could not
be much older than the marine terrace deposits and at most no more than 200,000 years old.
'--~-.-.........J
Fe':l
I4lKl
1200
•1(0)
flOO
f\OO
400 .rorlu~ue'{'
~-.-c',:.:.-/::;~;'~:~,,,,.,.~-~':....~-'-------..:":,;-~-~~"'='
~":""'Jp",,:,,:.,.;;':'i:=-~..-.--'--.--
._---
pn=slidc rrlluml surfa!:('
I mile
,,/'...
/'pr.:slidc
...'
Tuff
V~lk)'
Vi-.:w h~~l
Gtabcn I-'f"
I"
Fig.6.The original Pleistocene megaslide as envisioned by Ehlig which initially moved as a unit,but
with subsequent movement,broke into subslides.
So,to answer the original question,the historic PBl slide reactivated in 1956 but the
landslide is part of a much older feature,the ancient Altamira landslide that dates back at least
200,000 years and possibly 500,000 years.Based on the hummocky topography,the area was
recognized as a landslide terrain in the 1920s and evidence suggests that movement has been
occurring off and on for millennia and well before the residential development of the 1940s and
50s.
Geology and the role of the bentonites
The south flank of the Palos Verdes Peninsula is underlain by south dipping,thinly
bedded sedimentary rocks called the Altamira Shale (a member ofthe Monterey Formation)
and by intrusive volcanic rocks (basalt sills).In the PBl area,the Altamira Shale (Tuffaceous
Lithofacies)is predominately siliceous and tuffaceous shale interbedded with a few beds of
porcelaneous shale and chert and silty sandstone and intrusive basalt sills.
8
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ACLAO Misc.Rept.8/2011
The younger (upper)beds of the Altamira Shale are largely of biogenic origin
(diatomites,diatom-rich shales,and phosphatic-rich mudstones)and have been removed by
erosion from the southern flank of the peninsula.
Fig.6 Typical view of weathered
tuffaceous beds of the lower Altamira
Shale.The thicker beds at the notebook
are thinly bedded porcelaneous and
cherty shales.
.5 Me
mm
ppp
71
.99
l.-Phosphatic ]
QJ
.0 lithofacies V1
c:E 'E
QJ Cherty lith.~E Q.
QJ xxxxxxxxx Iii
m..c.\
Vl Tuffaceousmlithofaciesl.-
E xxxxxxxxx p
m.....
<d::
15
Fig.7 Schematic stratigraphic column of the sedimentary rocks that occur in the
Portuguese Bend landslide area.The small /l xxxxx"represent the stratigraphic
occurrence of two units composed of tuff (volcanic ash)and bentonite.The
upper is the Miraleste Tuff (mmm)and the lower the Portuguese Tuff (pppp).
The rupture surface (slide plane)ofthe PBL is typically within or above the upper
surface of the Portuguese Tuff.
The lower Altamira shale contains varying amounts of volcanic ash
(called tuff)from 5-10%up to beds that are nearly pure ash.The material was derived from
volcanic eruptions (roughly 12 to 15 rna ago)that occurred at the surface and underwater.
Thick intrusive basalt sills are exposed in Inspiration and Portuguese Points and the former
Livingston quarry.Several basalt sills are present in the subsurface and two major ones can be
traced across most of the PB area.Pillow basalts,indicative of lava extruded on or just under
the sea floor are exposed in the original 1956 landslide located just west of Paint Brush Canyon
-..-and north of the Crenshaw extension (fig.8).
'..'~'..,.,.,.-
Fig.8 Pillow basalts exposed in the small
landslide located west of Paint Brush Canyon and
north of the Crenshaw extension.
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ACLAD Misc.Rept.8/2011
The tuff beds include both very fine-grained pyroclastic (airborne)particles and lapilli (small
shards of lava thrown through the air while still molten)and ash that was either generated by
volcanic explosions on the seafloor or was transported in the ocean by bottom currents.The
hot,molten basalt intrusions caused a hummocky seafloor with low mounds and swells.As the
ash settled through the water to the seafloor,bottom currents tended to swept it in to the low
areas and as a result the ash beds vary in thickness and continuity.The Portuguese Tuff (Pt),
one of the two main volcanic ash-rich deposits in the Altamira Shale,typically varies from 20 to
over 60 feet in thickness.However,a borehole located outside the PBL cored more than 80
feet of Pt.The ash has a high silica content (some beds are nearly pure silica)and is unstable
over geologic time,especially when it comes into contact with groundwater and heat.As the
ash breaks down it turns into atype of montmorillite clay called bentonite.When fresh,the---
bentonite clay is blue,blue-grey,blue-green,reddish-orange,white or yellow with a distinctive
waxy texture:Its distinct properties have contributed significantly to the generation of
landslides in the area.
Fig.10 Fresh bentonite (blue-grey beds)
exposed in the sea cliff at the toe of the
PBL in Inspiration Cove.Bentonite varies
in color,depending upon its
geochemistry from grey to blue-grey,
blue-green,white,orange,reddish brown
and red.
Fig.9 Typical exposure of volcanic tuff and bentonit~
which weathers to a light grey color with yellow-brown
streaks.This outcrop was exhumed by erosion at the end
of the culvert leaving Portuguese Canyon.
10
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ACLAD Mise.Rept.8/2011
Bentonite clay has a crystal structure in which the molecules are arranged in sheets held
together by positively charged ions of sodium or calcium.The clay absorbs water molecules
between the sheets,causing it to expand when moist and shrink when it loses water.This is
the reason for the "expansive"(adobe)soils found in the area.Bentonite clay is weak,with
little internal cohesion,and under pressure (stress)becomes plastic and flows.Experiments
have shown that when bentonite is repeatedly stressed,it becomes even weaker and will
undergo plastic flow on slopes of only a few degrees.When bentonite is exposed to
groundwater it swells and forms impermeable layers (called aguicludes)that prevent or impede
the upward or downward flow of water.As groundwater builds up under bentonite layers,it
increases the pore-fluid pressure and when the water is confined or trapped,the pore-fluid
pressure can increase to the point wrrere it equals the weight (pressure)of the overburden.-ln
such cases,the overlying rocks are actually being lifted,reducing the friction along the rupture
surface.The result is slope failure and movement.However,the numerous small cracks or
fissures in the siliceous and tuffaceous shale allow some groundwater flow,albeit slowly,and
the failure condition tends to occur only when there is an increase in the water table.In the
case of the PBl (and the other landslides in the area resting on tuff and bentonite),it appears
that pore-fluid pressure is just enough to allow for plastic flow in the bentonites and steady,
slow movement (creep)of tenths of inch/day (mm/day ).In years of high rainfall,the additional
infiltration of storm water to the groundwater,largely through the bottom of the major
canyons and surface fractures,causes pore-fluid pressure to increase and landslide movement
to accelerate.
Boreholes drilled through the Portuguese Bend landslide have help reveal several small
geological structures in the bedrock which playa significant role in the landslide's movement.
These structures confine the landslide to a shallow depression that prevents its lateral
expansion.On the eastern margin there is a monocline with very steeply dipping shale beds
that forms an eastern barrier;to the north is a small E-W trending anticline with steeply dipping
beds in the southern limb that forms a northern barrier and in the subsurface,located
approximately underneath Portuguese Canyon in the middle of the landslide is a small north-
south trending fault that offsets the bedrock and basal beds of the landslide (Fig.4).The fault
forms the boundary between the central and western subslides and tends to confine higher
rates of movement to the east.Collectively these structures form a shallow trough or
depression along which the landslide is moving.What may also be important in the western
area of the landslide is the distribution of the major basalt sills in the subsurface.Also in the
subsurface are two small flexures (fig.11)that,like the anticline in the north,trend basically
west to east.As the landslide moves over these flexures they cause vertical fractures in the
landslide and impede movement.The northern flexure creates the boundary between the
eastern and inland subslides.According to Ehlig,the landslide rupture surface (slide plane)in
the north is contained in a thin bentonite bed interbedded with volcanic ash located near the
top of the Portuguese Tuff whereas near the shoreline it is at the base of the Tuff.
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ACLAD Mise.Rept.8/2011
Cren~haw
Extension 'n'
..------....s.oc--->7-:
__/.,;r"__.or ~..~.
~-----'SeCOndary l_
!(]ndslido
Inland
subslideone
PVDS
./-r-----------I ..' _.
••),)-"..__.__-:-:._.~-_-.-~.'''~.~JI -
::..-.-.:-:c..-_~~.9!l2__
1':oQ 'Seaward
subslide
Eastern
subslide
Fig.11 Cross-section of the PBL through the eastern margin of the landslide.Note the two small
flexures in the bedrock below the slide surface over which the landslide must move.These flexures tend
to divide the landslide into the subslides.(modified from Ehlig,1986;1992)
Elsewhere,the basal rupture can occur in a Variety of lithologies,generally above the Tuff.
While the bentonites exert an important influence on the landslide,it does not always control
the position of the slide plane.
Remediation Efforts
Numerous efforts have been made over the years to halt the land movement,including
regrading and shifting landslide materials,gabions and caissons,installation of culvert drains,
dewatering wells,and altering the geochemistry of the bentonites.Initially,the LA County
road engineers believed the landslide would be short-lived and stop by itself.
When it became clear that movement was continuing,the County of los Angeles a~d
Palos Verdes Properties (the major landowner at the time)attempted to "pin"the landslide by
insertion of shear pins;4 ft.diameter,20 feet long,steel reinforced caissons that were
embedded ten feet into the bedrock below the "failure surface"(rupture plane).Twenty-three
pins were emplaced,mostly located south of PVDS in the seaward subslide (see fig.4).The
landslide slowed from 0.8 to 0.25 inch/day for about five months but in early 1958 abruptly
returned to its earlier rate of movement.The shear pins had failed and several intact caissons
ultimately were displaced to the shoreline by landslide movement.In 1993 a caisson appeared
on the eastern edge of the landslide that had traveled more than 700 feet from the point where
it was installed in 1956.Pieces of the caissons can still be found on the beach at the toe of the
landslide.
Following a series of rainy years in the late 1970s and early 80s which increased
movement,the City of RPV in 1984 initiated the first of a series of steps to halt the landslide.
Funded by a 2 million dollar grant from the City's Redevelopment Agency (RDA),these efforts
were directed by Perry Ehlig and continued for the next fifteen years (until his unexpected
death in 1999).They involved several projects:(1)Extensive grading and shifting of landslide
materials from the interior of the eastern and central subslides to the toe,(2)installation of
dewatering wells,(3)installation of gabions along the beach to reduce wave erosion,(4)
building a culvert system to convey storm water from Portuguese Canyon,across the landslide
to the ocean and (5)imitating an experiment of injecting fluids to facilitate a sodium to calcium
cation exchange in the bentonite clay to increase its strength.
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ACLAD Mise.Rept.8/2011
During the three phases of regrading to shift weight from the middle of the landslide to
the shoreline (thereby increasing the friction at the toe)more than 10 million cubic yards of
material were moved.In the mid 1990s,the mound of soil and debris created a 30-50 foot high
sea cliff at the shoreline (fig.12).Ultimately wave erosion over the next fifteen years removed
nearly all of this material.Today,there is little evidence of the mound that once existed at the
toe.
Fig.12 View looking north from Inspiration Point at the sea cliff at the toe of the PBL in 2006.The cliff
was 30-50 feet above the beach as material graded from the central and eastern subslide was placed on
the toe to increase its weight and friction.As can be seen in the photo,the material was rapidly being
eroded by wave erosion.
Based on the success of dewatering wells in lowering the groundwater table in the
Abalone Cove landslide,about twenty five dewatering wells were drilled,located mostly across
the upper part of the landslide.The wells extracted groundwater at rates of up to 20,000
gallons/day from within and below the landslide in an effort to reduce pore pressure and
increase stability.Unfortunately,the wells were drilled in the mid 1980s following years of high
rainfall and when landslide movement was high.Well pipes were deformed and ultimately
sheared in the majority of these wells,typically within a matter of a few years after their
installation.Today,only three wells located in the NE corner of the inland subslide remain
operational.
Following intense wave erosion of the landslide toe by EI Nino generated storms in the
1980s,gabions,steel-mesh,rock-filled baskets,were placed at the base of the sea cliff in an
attempt to reduce cliff erosion and protect the materials which had been shifted to the toe.
The gabions had limited success and eventually were destroyed by wave action.Pieces of them
can be found at the shoreline.
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One of the most controversial remediation measures is the 24 inch diameter steel
culvert storm drain that crosses the landslide.The culvert originally linked the storm outlet in
Portuguese Canyon,where the Crenshaw extension crosses the canyon,to the ocean.Much of
the steel culvert is a half-round to allow easy access for debris removal (fig.13).The culvert
crosses the lower part of the eastern and central subslides and as movement has occurred,it
has changed the grade,disrupting flow in the pipe and separated the pipe ..It was expensive to
install but had a very limited life and was in disrepair nearly from the beginning.Also,by 2004,
Fig.13 The steel culvert ("half round")which was
designed to convey storm water from Portuguese Canyon,
across the landslide to the ocean.Due to landslide
movement the pipe separated and was ineffective.The
culvert is now separated from the outlet of Portuguese
Canyon and allows storm water from the canyon to flow
directly into the head scarp of PBL.
the culvert had separated at the junction with the outflow from Portuguese Canyon and storm
water from the canyon now flows directly into the head scarp of PBl.
Low shear strength is the major reason why bentonite or bentonitic ashes are typically
the materials which fail and form the basal rupture surface in the PBL.Thus,Ehlig reasoned
that if it were possible to increase the strength of the bentonite clay then it would be possible
to prevent the bentonites from failing or at least to increase the friction at the slide plane and
reduce the rate of movement.Bentonite is a type of montmorillonite clay which can be sodium
or calcium rich,the sodium rich variety being the most common in the area and also much
weaker than the calcium variety.Engineers have long known that it is possible to increase the
shear strength of clay by the addition of lime (calcium ion).In theory,exchanging calcium for
sodium within the bentonite has the potential for increasing the shear strength and improving
the stability of the landslide.Laboratory tests showed that the addition of CaCh to bentonite
from the area could increase its strength by 70%so Ehlig proposed a field experiment in which
calcium-rich fluids were injected at the rupture surface in order to facilitate a cation exchange
(Ca++for Na++).The experiment was conducted at the margin ofthe eastern subslide where the
slide surface is a 3 inch thick bentonite bed at shallow depth.A 6 inch diameter PVC pipe was
inserted through the landslide,down to the rupture surface and calcium chloride solution
injected.The experiment was conducted for about a year and discontinued shortly after Ehlig's
death in 1999.While the cation exchange to strengthen the bentonite was a clever idea,the
results from the short experiment were inconclusive.However,there are good reasons to
doubt that the method would have worked,primarily because 1)the impermeable nature of
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the bentonite that prevents fluid flow within the clay and 2)whether enough bentonite could
have been strengthen to really improve the stability of the landslide.
In sum,while some of the remediation efforts caused temporary reductions in the rate
of movement,by 2005,the landslide movement had returned to pre-1984 rates and it is safe to
say that none of the remediation efforts have had a long-term effect of halting movement of
the landslide.The City of RPV's RDA eventually invested more than 3 million dollars in the
attempts to halt and stabilize the PBL.Despite the setbacks,Ehlig remained confident that the
landslide could be halted.It is interesting to speculate what might have occurred if Perry Ehlig
had lived and been able to continue the remediation efforts.The outcome might have been
different.
The Landslide in 2011-55 years later
It is now more than fifty years since the PBl reactivated and there have been many
changes,some resulting from the continuous deformation and movement and some from the
efforts to stabilize and halt the land movement.When the landslide began,two roads crossed
the area,one near the beach that provided access to the beach homes and the Beach Club and
PVDS,and several small residential lanes.Only PVDS remains,after having been relocated
twice to accommodate the constant shifting and it remains in need of constant repair.The
seaward road (Yacht Harbor Drive)is long gone and many of the small lanes have had to be
rerouted.In the 1950s,PVDS was bordered by a road-cut 20 feet higher than the road on the
inland (north)side ofthe road.This area is now a depression (lithe sand box")that floods \
during major rain storms.The material was removed as part of the remediation effort and
relocated to the toe of the slide (fig 12).Over the past fifteen years the mound at the sea cliff
has been eroded away by wave action and the seaward subslide now tilts seaward,further
jeopardizing PVDS.
In 1956 two canyons cut across the area.Portuguese Canyon (see fig 3,top)was a deep
ravine that bisected the area and conveyed rain water to the ocean.The ravine is now
abandoned,cut-off from its head-waters by the growing depression (graben)at the head of the
landslide.Storm water from the upper canyon now flows directly into the head of the
landslide.Paint Brush Canyon was located along the eastern margin of the landslide.Grading
destroyed the canyon and storm waters from upper Paint Brush Canyon now dump into the
head scarp.The rate of movement in 2010 remains unchanged (tenths of inch/year)from
historic averages;in that year the central and eastern subslides moved over 2 feet while the
seaward subslide moved more than 3 feet.Of the many remediation projects completed in the
1980s and 90s,only three dewatering wells,located in the NW corner,close to Peppertree lane,
are still operational.All the others have failed or are no longer operational.And the landslide
keeps moving.
Acknowledgements
In the past seventy-five years,a small army of engineers and geologists,from students to
professionals,have studied,probed and analyzed the landslide complex of which the PBL is the most
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conspicuous part.They have produced hundreds of reports,from dissertations to company files that
describe their results.While some of them are published in the scientific literature,the vast majority
are internal company reports and files,including the logs of literally hundreds of boreholes and auger
holes prepared by geoscientists for the City of RPV or consulting companies hired to investigate some
aspect of the landslides.Regrettably,many of these reports,like the scientists who prepared them,are
now gone and a treasure load of information has been lost with them.
For these notes I have particularly relied upon the reports by:Karl Vonder linden,whose
Stanford University doctoral dissertation (1972)was the first comprehensive investigation of the PBl;
Richard Merriam (1960)whose early ideas shaped many of those that followed;Richard Jahns and Karl
Vonder linden,1973;Perry Ehlig (1984;1986;1992,1997)who spent a professional lifetime
investigating the ACl and PBl,often with his students and directed most of the efforts to remediate
them;and,the many unpublished company reports and files of leighton and Associates,Robert Stone
and Associates,Moore and Taper and law Cranaall.To all of them my thanks and appreciation;
however,any errors in reporting or interpreting the results described herein are mine.
Key references for interested readers:
1.Merriam,R.,1960,Portuguese Bend landslide,Palos Verdes Hills,Calif.Journal of Geology,v.68,pp.
140-153.
2.Jahns,Richard and Vonder linden,Karl,1973,Space-time relationships of landsliding on the southerly
side of Palos Verdes Hills,in,Moran,D.E.,Siossen,J.E.,Stone,R.D.,and Yelverton,C.A.,(eds.),
Geology,seismicity and environmental impact:Assoc.Engineering Geologists,Special Pub.,pp.123-138.
3.Ehlig,Perry,1986,The Portuguese Bend landslide:Its mechanism and a plan for its stabilization;in
Ehlig,P.(compiler),landslides and landslide Mitigation in Southern California:Geological Society of
America,Cordilleran Section,82nd Annual Meeting,p.181-190.
4.Ehlig,Perry,1992,Evolution,mechanics and mitigation of the Portuguese Bend landslide,Palos
Verdes Peninsula,Calif.,in Pipkin,B.W.,and Proctor,R.1,(eds.)Engineering Geology Practice in
Southern California:Special Publication 4,Association of Engineering Geologists,Southern California,
Belmont Publish.Co.,pp.531-553.
5.Hill,CA,Douglas,R.G.,and Hammond,D.E.,2007,A hydrological assessment of groundwater sources
in Portuguese Bend and Abalone Cove landslide areas,California:Implications for landslide movement,
in,Brown,A.R.,Shlemon,r.J.and Cooper,J.D.(eds.),Geology and Paleontology of the Palos Verdes
Hills,California:A 60th Anniversary Revisit to Commemorate the 1946 Publication of the U.S.Geological
Survey Professional Paper 207:Pacific Section,Society for Sedimentary Geology (SEPM)Book,PP 271-
292.
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