3.0 Landslide Movement and Mitigative Efforts 3.0 LANDSLIDE MOVEMENT AND MITIGATIVE EFFORTS
One of the objectives of this investigation is to evaluate alternative measures for minimising or
arresting movement of the PBL. The rate at which the landslide has deformed over the last 40
years, and the effect of factors such as fluctuations in the groundwater level, topographic changes,
and grading in the mid-1980s have had on the movement rate provide valuable insight into the
behavior of PBL. A preliminary evaluation of historical landslide displacement rates and past
mitigative efforts has been completed as part of this investigation and is discussed in this section.
This discussion will be followed by a summary of the proposed Plan of Control (POC) and an.
outline of the field exploration and testing program which has recently been completed during this
investigation.
3.1 Past Movement Rates & Mitigative Efforts
• In 1955, the County of Los Angeles was in the process of constructing Crenshaw
Boulevard extension down the southern flank of the Palos Verdes Hills to a proposed
intersection with Palos Verdes Drive South. Available records show that during the
summer of 1956, a failure occurred northeast of the current landslide area which deposited
nearly 200,000 cubic yards of debris within, or adjacent to, the Crenshaw Boulevard right-
of-way. Approximately 160,000 cubic yards of this material was subsequently used as fill
to establish a suitable grade where the road descended onto the then inactive Portuguese
Bend Landslide. On August 17, 1956, shortly after the placement of this fill, cracks were
noted in a masonry storm culvert which had recently been constructed across what is now
the eastern landslide margin in the area of the road fill. Additional ground cracking
quickly developed in this area and subsequently progressed southward. On September 4,
a four-inch offset was observed in the pavement at Palos Verdes Drive South along the
eastern margin of the reactivating landslide and, by October 4, 1956 deformation was
evident in the Portuguese Bend Club pier in the southwestern portion of the slide. A
generalized sequence of this reactivation is shown in Figure 3.1.
7
Following the initial distress, the former Los Angeles County Engineer's Department
• ill established a series of survey monuments on or adjacent to the landslide in 1956. The
County and then later the City of Rancho Palos Verdes have maintained and periodically
surveyed a network of monuments in the landslide area through the present time. Based
upon these data, the average annual displacement rates for the East-Central and West-
Central portions of the failure through 1969 are presented in Figure 3.2 (yonder Linden,
1972). The annual and average rainfall (14.4 inches) during this time period is added to
Yonder Linden's data to this figure for comparison. As shown, the average annual
displacement rate measured from survey monuments located for the East-Central portion
of the landslide during this time period was approximately twice that of the West-Central
portion of the failure. The rate of movement for both portions accelerated during years
of heavy rainfall.
Since the emphasis of the current investigation is on the historically faster-moving eastern
portion of the landslide, a more detailed analysis has been conducted for this area. A
• summary of the available data regarding the average daily rate of movement for the East-
Central portion of the landslide, between the time of its reactivation in August of 1956 and
October of 1996, is presented in Figure 3.3. More detailed movement data for a point
(Station K) established in the eastern portion of the landslide is presented in Figure 3.4.
The location of this survey point is shown in Figure 2.1 and detailed movement records
are included in Appendix A. As can been seen from Figures 3.3 and 3.4, high rates of
slide movement occurred with a two to three months delay following the rainfall.
However, as shown in Figure 3.3, during the first two years (1957-1958) following PBL's
reactivation, this portion of the landslide gradually slowed to a displacement rate of
approximately 0.4 inch per day. This average displacement was maintained for nearly two
decades with slightly higher movement rates associated with wet years. Since the
reactivation of the landslide, variations in annual precipitation and various grading or
construction activities within the limits of the failure appear to represent the primary
factors which have influenced the rate of movement of the landslide. The results of
• analysis and comments are summarized chronologically as follows:
8
Road Fill (1956): The initial high displacement rate of the failure (i.e.,
• approximately 3 inches/day) following its reactivation is most likely
attributable to the excess pore pressures generated along the rupture surface
by the =160,000 cubic yards of fill material placed near the head of the
landslide in conjunction with the Crenshaw Boulevard extension. From a
geotechnical engineering viewpoint, this episode of initial landslide
movement is a classical case of undrained shear failure caused by surcharge
load and induced excess pore pressure. The excess pore pressures appear
to have gradually dissipated over the following 24 to 36 months resulting
in the deceleration of the failure.
Shear Pins (1957): Between March and August of 1957, The County of
Los Angeles and Palos Verdes Properties installed a group of 22 reinforced
concrete caisson "shear pins" across the active failure surface in an effort
to stabilize the landslide (23 to be exact, a lone "pin" was at the northwest
• corner of Peppertree and PVDS which is outside of PBL as discussed in
this report). The approximate locations of these shear pins are shown in
Figure 2.2. Each of these caissons was four feet in diameter, twenty feet
in length, and embedded ten feet into the material underlying the "failure
surface" as it was understood at that time. The landslide reportedly slowed
by approximately 65% (from 0.8 to 0.25 inch per day) following the
installation of these shear pins. This may be partially attributable to the
seasonal (dry) adjustment of the slide movement rate. This reduced rate of
movement was only maintained for approximately five months. In early
1958, the landslide abruptly returned to its pre-shear pin displacement rate
of nearly 0.8 inch per day. This increase in movement was assumed at that
time to be an indication that the shear pins had failed. Several intact shear
pins have since been displaced to, and deposited on, the shoreline by
subsequent landslide movement and wave action (Figure 2.2). The most
• recent appearance of such a caisson was in 1993 near the eastern edge of
9
the landslide. This caisson emerged from the toe of the landslide at a
• location approximately 700 feet seaward of the point where it was installed
in 1956.
Rainfall (1968-69): The rainfall during the winter of 1968-69 was about
50% greater than the long-term average for this area. During January of
1969, from nine to fifteen inches of precipitation was recorded at three
monitoring stations in the vicinity of the landslide. As shown in Figures
3.2 and 3.3, this rainfall significantly increased the rate of landslide
movement.
Rainfall (1977-1986): Approximately 24-inches of precipitation fell in the
vicinity of PBL during the winter of 1977-78. This rainfall was the
heaviest since the reactivation of the landslide and more than double the
long-term average of 10.4 inches per year according to rain station LAFCD
• 44. As a result of this heavy precipitation, the landslide movement rate
accelerated from approximately 0.4 to 1.1 inches per day, as shown in
Figure 3.3. Above average precipitation during the subsequent winters of
1979-80, 1982-83, and 1985-86 was sufficient to sustain this higher rate of
movement through 1985. Maintenance cost of the Palos Verdes Drive
South was reported to account for about 20% of the City's total road
maintenance budget. Effort to develop some form of Plan of Control was
initiated in 1983 by the City Council and implemented regrading and
dewatering as described below.
Portuguese Bend Stabilization Project(1984 to 1988): In 1984, the City of
Rancho Palos Verdes funded a Plan of Control (POC) which included
dewatering and regrading activities under the direction of Ehlig. These
activities and their impact on the rate of movement are described below.
•
10
• Four dewatering wells (Figure 2.4) were installed within the limits
• of the landslide between 1984 and 1986 in an attempt to lower
groundwater levels. However, the groundwater production from
the wells was relatively low with a combined total of less than
20,000 gallons per day (gpd). Nonetheless, the combination of
dewatering and drought conditions during the mid to late 1980s
contributed to localized reductions in groundwater elevations and a
gradual decrease in the rate of landslide movement, as shown in
Figure 3.3.
• Grading (Phase I) was completed within the boundaries of the
landslide between October and December of 1986. The purpose of
this grading was to (1) reduce driving forces by relocating slide
debris from areas where the slide plane was inclined relatively
steeply seaward to areas where that surface was inclined either
• landward or less steeply seaward; and (2) to reduce infiltration by
improving surface drainage, closing open cracks and fissures, and
filling a large (5 to 10 acre) depression in the eastern portion of the
landslide (Lake Ishibashi) which perennially filled with run-off
water. A total of approximately 560,000 cubic yards of slide debris
was relocated as part of this grading.. The majority of this slide
debris was placed in the south-east portion of the East-Central
subslide. The approximate lateral extent of the cut and fill areas
associated with this grading are shown in Figure 2.4. The post-
graded ground surface profile is illustrated in Figure 3.5, cross
section D-D'. The rate of movement of the East-Central subslide
decreased from approximately 0.7 inch per day at the start of
grading in October 1986 to approximately 0.35 inch per day at the
completion of grading in January of 1987.
•
11
• Grading (Phase II) was performed between January and March of
• 1988. The approximate lateral extent of the cut and fill areas
associated with this grading are shown in Figure 2.6. This phase
of grading involved the removal of approximately another 500,000
cubic yards of earth from the east-central portion of the landslide
where the sliding surface dips most steeply seaward. The majority
of this soil was used as roadbed fill in conjunction with the
concurrent realignment of Palos Verdes Drive South. The
remainder of this material was used to construct a berm along the
shoreline where the toe of the slide ramps upwards in the southwest
portion of the landslide. However, a substantial portion of this
material was eroded by storms during the winter and spring of
1988. Nonetheless, the rate of movement of the East-Central
subslide decreased from approximately 0.25 inch per day at the start
of grading in January of 1988 to less than 0.05 inch per day at the
• completion of grading in March of that year. The topography
following the Phase II grading of PBL is illustrated in Figures 2.6
and 3.5. The approximate cut and fill zones in the East-Central
area of PBL is shown in cross section D-D' of Figure 3.5.
• Following a period of severe wave erosion and shoreline regression
in early 1988, rock filled wire baskets (gabions) were installed
along the western shoreline of the landslide in 1988 in an attempt
to reduce the rate of wave erosion. Although this temporarily
abated the erosion, the gabions were essentially destroyed within an
18 to 24 month period by the combination of wave action, corrosion
of the wire baskets, and landslide deformation.
• Grading (Phase III) was completed during August and September of
• 1990. This phase of grading involved the relocation of
12
approximately 60,000 cubic yards of soil from the central uphill
• margin of the landslide to the eastern portion of the failure
immediately upslope of Palos Verdes Drive South. Following this
unloading, perceptible movement of the Landward Zone appears to
have stopped until the heavy rainfall of January 1995.
• A sixth dewatering well (referred to as Peppertree) and three
groundwater sumps were installed in 1990.to intercept groundwater
from the uphill portion of the landslide(Figure 2.1 shown as WPT).
Although the Peppertree well has since been a modest producer
(approximately 3,000 gpd), available data indicate that this well and
other wells in the general area have generally not had any
perceptible effect on the groundwater levels or flow patterns within
the landslide. Data regarding detailed well production rates up to
October 18, 1996 is compiled in Appendix A.
•
Between the completion of the 1990 Phase III grading and 1995, the rate of landslide
movement gradually increased to approximately 0.25 inch per day, as shown in Figures
3.3 and 3.4. This acceleration is attributable to several factors including: (1) locally
intense and/or above average rainfall during the winters of 1991-92, 1992-93, and 1994-
95; (2) the opening of cracks and fissures within and around the periphery of the landslide
with an associated increase in the rate of infiltration; (3)the loss of support associated with
the continued relatively rapid movement of the Seaward subslide; and (4) the surcharge
imposed by sediment which was transported down Paintbrush Canyon during periods of
rainfall and subsequently deposited within the East-Central subslide of PBL.
3.2 Proposed Mitigative Effort: Plan of Control (POC)
In 1993, Ehlig proposed a conceptual POC for stabilizing the northeastern portion of the
411 active failure. A major component of this POC involves the regrading of about 19 acres
13
to remove PB clay slide material from the relatively unstable northeastern portion of the
• landslide. Upon disposal of the PB clay, the rest of the excavated soil is planned to be
used as compacted fill at the same location or relocated to more stable portions of PBL.
The approximate extent of the base for removal and replacement are shown in Figure 3.6.
Questions regarding the feasibility and potential impact of the proposed regrading arose
in light of the variable success of the remedial grading activities in the 1980s. These
questions include the following:
1. Is the position and orientation of the landslide rupture surface accurately
defined in the areas of the planned removal and recompaction? Prior to
this investigation, considerable uncertainty existed because not all data are
consistent and because many previous boring locations were obtained from
topographic maps that were no longer valid as a result of landslide
movement.
• 2. What is the stratigraphy and geologic structure beneath the landslide,
especially in the area where the slide will be removed and replaced by
compacted fill? Are other planes of weakness or potential rupture surfaces
present beneath the active failure surface? This information is needed to
evaluate rock strength beneath the slide base and to determine if the POC
is consistent with the geologic structure.
3. What are the groundwater conditions in PBL and especially in the vicinity
of the proposed area of removal and recompaction? Is groundwater
restricted to material above the primary slide plane or base or is there
groundwater beneath the slide base? Does groundwater "leak" out of the
landslide into the underlying deposits or do those deposits convey
groundwater upwards towards the primary slide plane or base? This
information is needed to calculate the effect of groundwater on landslide
stability, to design and implement an effective groundwater control plan,
14
and to evaluate the stability of the temporary slopes of the excavation which
• will exist during construction.
4. What are the geologic conditions that control the secondary landslide
located between the uphill edge of the Portuguese Bend landslide and
Crenshaw Boulevard extension? Fill removal is planned within this area,
however there have been no previous subsurface investigations in this
portion of the landslide.
5. What is the effective shear strength of the clay along the sliding surface?
To what degree does this strength vary with the depth of the surface and the
elevation of the groundwater table (i.e., soil strength as a function of
overburden pressure)? How should the slow but varying rate of movement
of the slide be considered in the calculation of the site stability safety
factor? The shear strength of soil is conventionally defined in a simplified
manner using two parameters cohesion C and residual friction angle (4r)
Based on a comprehensive compilation of past testing results (Watry and
Ehlig, 1995), the variation of cohesion and residual friction angle are too
wide to be useful for stability analysis. For example, the soil cohesion
ranges from less than 40 pounds per square foot(psi)to more than 520 psf;
the residual friction angle ranges from less than 5° to more than 21° (see
Table 4.5.2 in Section 4 of this report) Reliable projections of the effects
of mitigative actions, such as grading and dewatering, cannot be made
without a better understanding of the strength characteristics and better
determination of the C and 4) values of the bentonite clay which comprise
the sliding surface.
In order to address these issues, a field exploration and a limited laboratory testing
program was developed. This program included:
•
15
• A. Field Drilling
• The drilling and down-hole logging of eighteen 30-inch diameter
bucket auger borings ranging in depth from 14 to 118 feet. The
combined footage of all eighteen borings is 1,154 feet.
• The drilling and logging (cuttings) of eight air rotary borings
ranging from 112 to 212 feet in depth. Each of these borings was
extended to the base of the Portuguese Tuff. The total footage of
these eight borings is 1,360 feet.
• The drilling and logging of four combination air rotary / core holes
ranging from 135.6 to 216.5 feet in depth. Air rotary method was
used to drill from the surface to just above the slide base at which
• point coring was conducted to below the base of the underlying
Portuguese Tuff. The combined footage of these four holes is 741
feet.
These tasks are designed as a preliminary effort, to the extent possible
under budget constraints to address the questions numbered 1, 2, 3 and 4.
B. Field Monitoring
• The installation of 13 monitoring wells in air rotary, bucket auger
or core holes. The casing used in these monitoring wells extended
below the base of the landslide. The depth at which each casing
became deformed was used to verify the depth of the rupture
surface.
•
16
• The installation of three multi-stage pneumatic piezometers within
14-inch diameter hollow-stem auger borings.
• The monitoring of groundwater levels in borings and in new and
pre-existing wells, standpipes and piezometers.
These tasks are designed to provide supplemental information for question
number 3.
C. The collection and evaluation of samples with emphasis on the sliding
Portuguese Bend clay (PB clay). The nature and results of this testing will
be discussed in Section 4.5. This task is specifically aimed, to the extent
possible, to answer question number 5.
• The locations of the borings, monitoring wells, and standpipes which have been installed
as part of the current investigation are shown in Figure 3.6. The drilling activities
associated with this investigation were directed by Ehlig, while Richard Crook, CEG, and
Dana Cole assisted with geologic logging of the excavations. Geologic map cross
sections, including geologic drilling logs and well monitoring logs, have been completed
by Ehlig and included in Appendix B and in the pocket at the back of this report. Sections
on geology and bedrock properties were written by Ehlig and he also reviewed BYA's
analysis sections of this report.
BYA's role in this investigation included: (1) assisting with the field sampling and
instrumentation; (2) a limited laboratory testing program to document the composition and
strength characteristics of a PB clay sample which is believed by Ehlig and Yen to be
representative of the slide material; (3)periodic monitoring and interpretation of the multi-
staged pneumatic piezometers installed by BYA at three locations of the landslide for spot
checking the upward gradient of the groundwater across the basal ruptural surface; (4)
compilation and evaluation of geotechnical and hydrologic records; (5) development of a
17
computer-based analytical model based upon the available geotechnical data and the
observed behavior of the failure to date; (6) evaluation of the geotechnical feasibility of
the POC, and its potential effects on the landslide's rate of movement; (7) assessment of
specific construction issues associated with implementation of POC; (8) identification and
evaluation of mitigative alternatives; and (9) preparation of this joint report in
collaboration with Ehlig.
•
IP
18
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LEGEND
TOPOGRAPHIC CONTOUR SEQUENCE OF SLIDE ACTIVATION
IN FEET(MS0 A=EARLY SLIDE TO
E=LATER suDE
--- -- — DIRT ROAD
-- "-- ROADS
UPI BUILDING
0 300 600 FEET
NOTE: There ore evidences indicating the general area C may have moved
slowly between 1946 to 1956. The boundary of the earlier movement is NNE BI NG YEN & ASSOCIATES, INC.
visible from the photo in the cover sheet of this report even though the
western boundary of the earlier movement was farther inland. b❑❑d Geotechnical&Environmental Consultants,Established 1979 GENERALIZED 1956 LANDSLIDE
Modified from sequence of slides originally by yonder Lindon 1972 REACTIVATION SEQUENCE
PROJECT NAME: PORTUGUESE BEND LANDSLIDE
based on additional data obtained during this s.tudy
(Bose Map, 1955) PROJECT NO:G-940989 I DATE: MARCH. 1997 FIGURE 3.1
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Cross Section C-C' DISTANCE IN FEET
D D'
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Cross Section D-D'
NV 41111 :u: BING YEN & ASSOCIATES, INC.
Geotechnical&Environmental Consultants,Established 1979 TOPOGRAPHIC CHANGES BETWEEN
1955 AND 1988 DUE TO SLIDE MOVEMENT
a PROJECT NAME: PORTUGUESE BEND LANDSLIDE AND REMEDIAL GRADING
Key Map PROJECT NO: G-940989 DATE:MARCH 1997 FIGURE 3.5
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LEGEND ,
_80_-- TOPOGRAPHIC CONTOURS BYA-1 0 PIEZOMETER INSTALLED BY BYA
IN FEET(MSL) B-96-18• 1996 SOIL BORING >`
11:11,INN.
..'"•... ...0 APPROXIMATE SLIDE BOUNDARY pB-1 • BULK SOIL SAMPLE
�l' TOE OF SLIDE PBS-2• BORING DRILLED IN 1996 ‘.
DIRT ROAD W4E• WELLS MONITORED DURING 1984-96
- --. ROAD 96W-1 O 1996 BORING 0 300 600 FEET N
UPI BUILDING
O■1r BING YEN & ASSOCIATES, INC.
BOUNDARY OF PROPOSEDWO
❑
GRADING. ▪ i GeoleGmloel8 Environmental Consultants,Established 1979 PROPOSED PLAN
MINIMUM TEMPORARY
3.1:1 EXCAVATION SLOPE RATIO PROJECT NAME: PORTUGUESE BEND LANDSLIDE OF CONTROL
(Base Map, 1995) PROJECT NO:G-940989 I DATE: MARCH 1997 FIGURE 3.6