Executive Summary EXECUTIVE SUMMARY
A. INTRODUCTION
This executive summary highlights a preliminary geologic and geotechnical engineering report
prepared jointly by Perry Ehlig(Ehlig)and Bing Yen&Associates,Inc. (BYA)for the Portuguese
Bend Landslide (PBL) area. This report is presented to the Council of the City of Rancho Palos
Verdes. The project is administered by Charles Abbott Associates. All statements contained in this
summary are subject to the contents of the entire report. This summary presents a concise but
simplified overview of the report. For a full understanding of any subjects presented in this
summary, one should refer to the full report.
B. OBJECTIVE AND SCOPE OF INVESTIGATION
The objective of this investigation is to evaluate the feasibility of a revised Plan of Control (POC)
-0 which evolved from an earlier mitigation effort by Ehlig and was proposed to the City in October
1995. The POC is intended primarily to stabilize the eastern portion of PBL north of Palos Verdes
Drive South (PVDS), known as the East-Central subslide. Since the eastern half of PBL has
historically moved more rapidly than the western half, the proposed POC and the impact of its
implementation may shed some light on the feasibility of stabilizing PBL. It is this ultimate goal
of stabilizing PBL which prompted this preliminary phase of investigation. From a geotechnical
engineering perspective, stabilizing PBL is a monumental undertaking. Although this project
represents the most recent and rigorous geotechnical investigation within the boundary of PBL since
the slide occurred four decades ago,the laboratory and analytical work performed in connection with
this study is only an initial feasibility assessment. The following tasks have been completed in the
course of performing this assessment:
Task 1: Collect, compile, and evaluate available historical surface and
subsurface information including topographic,geologic,geotechnical,
groundwater, drainage,and slide movements records;
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Task 2: Conduct additional subsurface investigation, surface mapping and
• sampling (commensurate with budget constraints) to enhance the
understanding of PBL to the level of formulating a preliminary
geotechnical model for engineering analysis;
Task 3: Construct monitoring wells,and conduct laboratory testing to better
characterize groundwater and the slide material behavior;
Task 4: Utilize the slide model(s) developed from Task 2 and the material
properties evaluated from Task 3 to back-calculate the observed
historical behavior of the slide (Task 1) for calibration and
confirmation of the technical approach and the geotechnical model(s);
Task 5: Using the same model, evaluate the effectiveness of the proposed
POC in mitigating the slide movement and assess the adequacy of the
411 POC to stabilize the eastern portion of PBL;
Task 6: Identify and evaluate supplemental measures for PBL stabilization;
Task 7: Perform preliminary analysis on the feasibility and limitations of the
POC and the supplemental measures in order to formulate
conclusions,recommendations; and
Task 8: Prepare this report.
To carry out the above,the following subsurface and laboratory investigations have been completed:
• Excavation of eighteen(18)30-inch diameter bucket auger borings,eight(8)rotary-
air borings,four(4)combination air-rotary/core holes with a combined footage of
3,255 feet.
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• Installation of thirteen (13) monitoring wells and three (3) multi-stage pneumatic
• piezometers.
• Completion of a suite of standard soil index tests on selected representative samples
from the slide surface as well as specialized testing including: Ring Shear.Tests, X-
Ray Diffraction(XRD), Energy Dispersive X-Ray Spectroscopy(EDS), Scanning
Electron Microscopy(SEM), and cation exchange capacity.
C. BACKGROUND
PBL is a portion of a reactivated ancient landslide complex which encompasses approximately 200
acres east of Inspiration Point on the southern margin of the Palos Verdes Peninsula in Southern
California. Since its reactivation in 1956, movements have been continuous. In some locations,
total displacement of the slide is in the range of 500 and 800 feet. However, the flattened
topography of the slide mass after the slide reactiviation in 1956 and the relatively dry period
• between the 1960s and the mid 1970s resulted in slower rate of movement than that between the late
1970s and 1980s when generally wetter seasons prevailed. The cost of maintenance of infrastructure
within the landslide boundary is affected by the rate of slide movements. There are two major public
infrastructure items within PBL, i.e.,Palos Verdes Drive South(PVDS) and mainline sewer. The
cost of maintenance for these items was borne by the County of Los Angeles. When the City of
Rancho Palos Verdes was incorporated in 1973,the City took over the maintenance of PVDS. The
heavy rains in 1978-79 and the relatively wet 1980s have had an impact upon the City budget. It was
reported that approximately twenty percent of the City budget for street maintenance was spent for
the 0.8±mile of PVDS through the landslide. The City Council decided in 1983 to establish a panel
of geologists and engineers to:
• determine the consequences of a"do nothing approach"to landslide mitigation; and
• to explore the possibility and develop a plan, if affordable,of stabilizing PBL or at
• least reducing the rate of movement to a manageable degree.
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Once convened,the Panel was informed that the City had no intention of ever allowing residential
• or commercial development within the active PBL. In 1984,after being informed by the Panel that
the chance of further expansion of the existing landslide was likely if a"do nothing approach"was
taken. The City Council began to implement a Plan of Control, recommended by the Panel, and
directed by Ehlig,to mitigate the movement of PBL. Dewatering and two phases of grading were
implemented between 1984 and 1988. Generally speaking,the rate of slide movement responded
positively to dewatering,regrading,and surface drainage improvements,but these have not been able
to stop the slow movement. In fact,the rate of movement has increased in recent years as earlier
work deteriorated.
A global positioning system(GPS) satellite survey network established in recent years has shown
that the eastern portion of the slide is moving about twice as fast as the western portion. The rate
accelerates when groundwater rises and/or when the Landward(northern)portion of the slide exerts
additional driving forces due to local slope failures or debris accumulations. Erosion of the toe of
the slide along the shore exacerbates the instability of the Seaward portion of the slide.
•
D. SUMMARY OF FINDINGS
D.1 Strength of Portuguese Bend Clay
The slide plane material, known as Portuguese Bend (PB) bentonite clay, is rich in sodium
montmorillonite and is an extremely weak geological material. The current investigation has shown
that PB clay is characterized by a non-linear strength envelope and its viscous behavior is one of the
primary reasons for the continued movement of PBL. The strength of the PB clay depends on the
effective overburden pressure(i.e.,the depth of the sliding surface under the ground surface and the
location of the groundwater table). However, this dependency cannot be adequately defined by
conventional simplified formulas in which the strength is assumed to be linearly proportional to the
effective overburden pressure. Defining this non-linear relationship provides the opportunity to
realistically and quantitatively evaluate the effects of re-grading and groundwater control in
• assessing the state of stability.
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• PB clay strength is dependent upon the rate of displacement, i.e., the slower the rate of shear
displacement, the lower the apparent clay shearing strength resisting the displacement. This
characteristic accounts for the viscous behavior of the clay and the continued slow movement.
Defining the relationship of strength reduction as a function of reduced displacement rate affords the
opportunity to quantitatively assess the rate of slide movement. The relationship between shear
strength reduction at low rate of displacement also demonstrates that there is a diminishing return
in trying to"stop"the movement by regrading and groundwater control which amount to decreasing
stress only. Improving the clay by increasing its strength to resist slow movement is an area which
has been explored in this study.
A limited amount of laboratory testing has been carried out on a representative sample of PB clay
to evaluate its strength as a function of effective overburden pressure and the reduction of strength
as a function of lowered rate of shear displacement. The results of the laboratory testing have been
applied to the geotechnical engineering analyses using historical topographic changes,groundwater
fluctuations and the surveyed rates of slide movement. The same approach is used to evaluate the
• effectiveness of Ehlig's proposed POC in terms of achievable factors of safety. The results of these
analyses are summarized in the following section.
D.2 Evaluation of Proposed Plan of Control(POC)
The proposed POC involves removal of about 100,000 cubic yards of PB clay from the northeastern
portion of PBL and excavation of a total of approximately 2.3 million cubic yards of landslide
material within an area of about 19.3 acres in the northeastern part of PBL. The excavated material
will be recompacted in place as buttress fill. In addition, about 350,000 cubic yards will be removed
from an area of about 5 acres in the south facing slope on the ridge east of Portuguese Canyon,
where the PBL basal plane dips most steeply. The material removed from the south-facing slope
will be added to the aforementioned buttress fill and subdrains will be installed in the excavated area
receiving buttress fill. Geotechnical analysis shows that these remedial measures, when
implemented,can result in an increase of about 7%in the calculated factor-of-safety for the eastern
111 portion of PBL, i.e.,the factor of safety will be increased to about 1.07 based on the factor of safety
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being 1.0 initially. This calculation takes into account the nonlinear and viscous nature of PB clay
• and uses the 1995-96 average rate of displacement of the eastern portion of PBL(about 0.2 inch per
day). The Ehlig POC also includes installation of subdrains around the perimeter of the buttress,the
addition of four to six additional dewatering wells,and surface water drainage improvements. These
actions,when implemented,can have a positive impact to groundwater control and improve the local
stability in the regraded area where slide material is removed.
There are several construction-related concerns associated with implementation of the proposed POC
in the northeastern area. These include consideration of the allowable maximum depth of
excavation,the allowable steepness of the excavated slope,and the strike of the excavated slope with
respect to the known dip of the bedding or slide plane. A preliminary analysis has been made in this
regard including recommendations and precautions associated with removal and replacement of the
buttress fill to minimize construction failure potential and comments on subdrains.
Depending on the combined effectiveness of subdrains, dewatering wells, and surface drainage
improvements in lowering the groundwater level,the calculated factor of safety can be expected to
• be in the range of 1.12 to 1.16,with the higher number associated with lowering groundwater level
in the subdrained area within the entire eastern portion of the PBL to 10 feet below its 1996 level.
An average factor of safety of approximately 1.14 is estimated based on the assumption that the
entire eastern PBL, from the Landward subslide to Seaward subslide,will react as a unit after the
proposed POC is completed.
However,engineering analysis also revealed that the Seaward subslide,exacerbated by its steep and
dilated bluff and erosion at its toe, will have a lower factor of safety than the regraded northeast
PBL. Hence,the Seaward subslide may move first and, consequently,pose the risk that the East-
Central subslide may lose its lateral support towards the ocean. Engineering analysis shows further
that the reduction of lateral support will reduce the factor of safety of the East-Central subslide to
1.04. This means that, while it appears to be theoretically feasible that the proposed POC can
improve the current.state of stability in eastern PBL, the margin of safety for the East-Central
subslide (at a factor of safety of 1.04) is too small and the East-Central subslide will have an
• intermittent slow movement and periodic acceleration following heavy precipitation. These
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intermittent movements can result in tension cracks south of the regraded area. The mechanism
• behind this phenomenon is attributable to two primary factors: (1)the behavior of strength reduction
at a slow movement rate of the slide material,which was tested during this investigation;and(2)the
configuration of PBL as a whole,i.e.,its relatively gentle slope and its steep toe at shoreline which
is subject to erosion.
E. SUPPLEMENTAL REMEDIATION ALTERNATIVES
A number of supplemental remediation measures were investigated in the current analysis. The most
promising alternatives are described below:
E.1 Strengthening PB Bentonite Clay
It is well known that the strength of sodium montmorillonite can be significantly increased by a
chemical process known as cation exchange.•Cation exchange involves replacement of the sodium
• ions by calcium ions. This process is normally accomplished by mixing lime into sodium rich clay,
a process which is impractical for the PBL because on the average the PB clay is about 100 feet
under the existing ground surface. Pressure injected lime,a special form of grouting,has been used
since the 1970s for stabilization of shallow foundations in stiff fissured clays;but the migration of
lime into the relatively dry and fissured clay was reported to be slow.
However,recent research(Rao and Rajasekaran, 1996),investigating the injection of a lime solution
into saturated marine clay(which has similar properties to the PB clay)has shown that fast cation
exchange rate and dramatic (6 to 8 times) increase in strength can be achieved. For the current
investigation, ring shear tests that were run on PB clay samples submerged in a calcium chloride
solution attempted to model two different slide scenarios: (1)portions of the slide which are moving,
(2) portions of the slide which are not. For scenario 1, the submergence in a calcium ion rich
solution resulted in a 15%increase in residual strength of the sliding clay. For scenario 2,the test
results indicated a 70%increase in peak shearing resistance when slide movement is re-initiated.
•
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The practical implications of the above discussions are: (1) introducing calcium ions to PB clay
where there is no current slide movement is likely to increase peak strength to a level that can resist
slide reactivation;and(2)introducing calcium ions to PB clay where shear displacement is ongoing
may increase residual strength enough to slow down the rate of displacement and potentially(with
continuous presence of the calcium ions) stop the shear displacement. The limited scope tests
performed for this study offer a preliminary indication that it may be feasible to achieve an increase
in strength potentially capable of raising the factor of safety much higher than that achieved by
regrading alone. These results are consistent with research conducted earlier(Mesri, 1969, 1970)
on pure calcium montmorillonite and sodium montmorillonite, which the strength of the former
ranges from 2.5 to 4 times higher than the latter depending on normal stresses, i.e., depth below
ground surface.
Given that the stability analyses indicate a limited increase in the factor of safety by regrading, an
increase of PB clay strength represents a new and significant element in order to arrest the rate of
movement and potentially stabilize PBL. For example, preliminary analysis indicated that an
411 approximately 300 feet wide strip of sliding surface strengthened by calcium chloride across the
eastern PBL in an area where sliding surface dips steeply can have a net effect on the increase in
factor of safety equal or exceed that by POC. This is significant because a 50%increase in strength
was assumed in this example. Laboratory data suggested that the potential strength increase can be
greater than 50% due to cation exchange. However, it is important to note that the test results
presented here are by no means complete or conclusive. Additional laboratory testing is needed to
confirm and supplement these results and a Pilot Field Testing Program is the next logical step
towards the ultimate goal of stabilizing PBL. A pilot program should be aimed at the identification
and evaluation of the most practical and cost-effective method of introducing highly soluble calcium
chloride in the vicinity and within the well-defined PBL base. One of the unique advantages of this
approach is that once the calcium chloride solution is introduced, cation exchange from sodium to
calcium is a proven process and its effect will increase with time. Infiltration of surface water
through cracks and fissures which normally carry water to the relatively impermeable slide plane can
further transport and spread the calcium chloride solutions to other areas of PB clay beyond the
injection point to further enhance the strength.
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E.2 Revetment to Buttress the Toe of Seaward Subslide
•
A preliminary geotechnical analysis was made to assess the effect of construction of a revetment
from Inspiration Point on the west to the PBL's margin to the east along the shoreline.
Conceptually, a rip-rap protected gravity revetment with 2.5:1 (horizontal to vertical) slope will
provide passive resistance to slide movement and reduce the shoreline erosion which has been one
of the factors exacerbating the instability of the Seaward Subslide. Installation of hydraugers
through the revetment into the Seaward Subslide to lower groundwater can further ameliorate the
effectiveness of the design.
Upon implementation of this scheme, the calculated factor of safety for the Seaward Subslide
increases to 1.12 whereas the eastern PBL is projected to have an overall factor of safety at 1.14.
These two calculated factors of safety (1.12 and 1.14) are sufficiently close as to indicate that
revetment is an effective design and that the Subslides will act (and react) synchronously. The
estimated volume for this revetment is about 200,000 cubic yards. However,any construction that
• extends to and beyond the existing shoreline will require permits from Federal and State regulating
agencies and can be a long,tedious and costly process.
E.3 Dewatering
Since the high groundwater levels,particularly in the eastern PBL,contribute substantially to the on-
going movement, subdrains and dewatering wells associated with the POCs are necessary but may
not be sufficient to lower the groundwater to the desired level. During the current analysis, a
parametric conceptual analysis was carried out to assess the effect of lowering water level uniformly
in the general area ranging from 10 feet to 50 feet from the present (1996) level. This analysis
assumed the POC regrading was in effect. This analysis was performed using non-linear PB clay
strength and reduction factors to account for creep. In other words,the arrest of slide movement has
been theoretically incorporated into the calculations. For example, the results of this analysis
indicated that, with the proposed grading and 20 foot uniform lowering 'of groundwater, the
calculated factor of safety is 1.19. If the water table is lowered 50 feet, the calculated factor of
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safety is 1.33. As has been experienced in the past dewatering efforts,much of the material above
• the slide plane has a relatively low permeability. Dewatering uniformly to between 20 to 50 feet
below the existing water level may not be achievable no matter how rigorous or aggressive the
dewatering effort-which can be expensive and maintenance intensive.
F. CLOSING REMARKS
This report represents a preliminary but comprehensive effort,to the extent possible within budget
constraints,that integrates the results of engineering geology and geotechnical engineering analyses
to explore the feasibility of a proposed POC and to identify supplemental remediation methods
necessary to stabilize the eastern portion of the PBL north of PVDS, known as East-Central
subslide. Historical records on rate of displacement between various subslides,geologic setting,and
sliding plane configurations suggest that stabilization of the eastern portion of PBL is the first step
towards stabilizing the eastern half of PBL. This study has shown that there are technical reasons
to believe that stabilization of PBL can be an achievable goal if the proposed Plan of Control is
• combined with strength enhancement of PB clay. The current investigation represents an initial step
taken in the development and implementation of a comprehensive PBL stabilization plan. However,
there are remaining technical issues which will need to be addressed before such a plan can be
realized. For example,the current investigation did not address the western portion of PBL even
though it has historically been a slow moving portion of PBL. Subslides within PBL are
interdependent from a geotechnical engineering point of view. Additional work is imperative in the
following two areas: (1) evaluate the behavior of the PB clay, particularly with respect to better
definition of the non-linear strength envelope and creep behavior,and(2)carry out a Pilot Field Test
to strengthen PB clay in-situ to ascertain the feasibility and the cost-effectiveness of the approach.
S
Note: A glossary of terms has been prepared and attached to the back of this report.
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ACKNOWLEDGMENTS
•
A technical review panel was retained by the City of Rancho Palos Verdes to review the joint
report. Members of the review panel are Mr. Glenn Brown, Mr. Arthur Keene, Dr. John Mann,
Mr. Monte Ray, and Mr. Patrick Shires. The comments and input by the members of the panel
are gratefully acknowledged. The project was administered by Mr. Charles Abbott of Charles
Abbott Associates on behalf of the City. Representing the City are Les Evans and Paul Bussey,
Director of Public Works and City Manager, respectively. Their support to facilitate this
investigation are appreciated.
Messrs. Richard Crook and Dana Cole assisted Perry Ehlig in the field work. Staff members
assisting Bing Yen included Glenn Tofani, Christian Mackin, Mi Bastani, Matt Hunter, and Da
Wu. All illustrations for this report were prepared by Berge Basmajian and Pei Yen of BYA's
Graphic Service Group. Acknowledgment is extended to these colleagues who have ably
• contributed to this joint report.
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