5.0 Evaluation of Proposed Stabilization Improvements 5.0 EVALUATION OF PROPOSED STABILIZATION IMPROVEMENTS
•
The primary engineering objectives of this investigation have been to evaluate the geotechnical
feasibility of Ehlig's POC and to evaluate other potential alternatives of arresting the landslide
movement. Assessing the feasibility of halting perceptible movement within all, or a portion of,
the failure represents one aspect of this investigation. The response of the landslide to the past
grading related mitigative efforts of the 1980s represents a valuable source of information which
has been utilized in our analyses to assess the feasibility of decreasing the rate of movement. The
pertinent engineering characteristics of the landslide and the preliminary analyses which have been
completed are discussed in the following sections.
5.1 Overview
The POC proposed by Ehlig consists of regrading, dewatering, and improvement of
surface drainage. Namely:
•
1. Removal of about 2.3 million cubic yards of the landslide over an area of about 19
acres in the northeast part of the slide. Except for 100,000± cubic yards of
bentonite which is anticipated to be used either as blanket fill to retard surface
water infiltration or to be exported from the site, the material would be restored
as compacted fill. The five acre secondary slide that moved onto the uphill edge
of the PBL would also be removed and replaced by compacted fill as part of this
project. The extent and configuration of this conceptual grading is shown on the
site plan in Figure 3.6 and in cross section D-D' in Figure 2.9.
2. Installation of a subdrain system along the northern, western and southern edges
of the removal area described in Item 1.
3. Removal of about 350,000 cubic yards of earth from the downhill part of the ridge
• east of Portuguese Canyon and the flat area further east where the slide base dips
60
most steeply. Conceptual grading is shown in Figure 3.6. The earth would be
• placed as fill in the area where the landslide was removed under Item 1, above.
4. Establishment of impervious drainage channels along Portuguese and Paintbrush
Canyons within the landslide area.
5. Installation of about six new dewatering wells. Two wells would intercept
groundwater that enters the northwest corner of the slide and the others would
deplete groundwater in the central and southern parts of the slide's east side.
6. Re-establishment of surface drainage within the developed part of the slide west of
Portuguese Canyon.
This section discusses details of the POC and analyses thereof.
• POC Item 1: The area where the landslide would be removed is shown in Figure 3.6,
with preliminary proposed cut slopes. The evaluation of construction stability and
precautions are discussed in Section 5.2. The approximate depth of removal is illustrated
in Figure 2.8. There are several reasons why this area was selected for slide removal:
1. The base of the slide and underlying bedrock is relatively flat within a band about
600 feet wide that trends westward across this area. By replacing the slide with
compacted fill, this area can potentially provide resisting force to sliding. Slide
removal will extend into areas on either side of the flat dips in order\to reduce
driving force. This is particularly important on the downhill side.
2. The slide is thinner (average thickness is about 60 feet) in the area of proposed
removal than in most other areas, see Figure 2.8.
•
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3. The compacted fill, with possibly a clay cap constructed from a portion of removed
slide plane material, would form a relatively impervious barrier. The clay-capped
compacted fill should reduce groundwater recharge in the Landward portion of the
landslide.
Based on experience gained during the Phase I grading in 1986 in which failure occurred
when bentonite beds were undercut, slot cutting will be necessary in areas where bedding
dips more than a few degrees out of the cut. We also recommend that the cut surface be
scarified by down-dip ripping and that 6% lime, by weight, be added to the first few lifts
of fill in order to increase the strength and the bond between the cut and fill.
POC Item 2: A subdrain is proposed to be constructed along the base of the fill removal
area. The primary purpose of this subdrain is to intercept groundwater that infiltrates or
migrates into the removal area before it can reach the rupture surface south of the
proposed removal area. Since the zone of removal will actually extend several feet below
the existing groundwater table, it is anticipated that this subdrain will also drain
groundwater from the northern portion of the remaining landslide deposits and thereby
lower the groundwater level in that area. Because of the depth at which the subdrain is
to be constructed on the downhill slide of the removal area, it will be necessary to pump
the water which is collected into a drainage pipe or channel located at the ground surface.
The steady state flow rate associated with this subdrain (or subdrains which will be
decided in the design phase of the POC) is anticipated to be low.
POC Item 3: The removal of about 350,000 cubic yards of earthen material included in
this control measure is needed to reduce the force causing this part of the slide to move.
Because the base of the slide dips seaward at angles ranging from 8 to 16 degrees beneath
this area, the earth generates about twice as much driving force as is balanced by friction
based on the behavior of PB clay at the corresponding overburden pressure range as
discussed in Section 4.5.
S
62
- The excavated material would be placed as fill in the area of gently dipping bedrock where
slide material has been removed. At this location, it will increase resistance to sliding or
to serve as the core material in the seashore revetment stabilization option to be discussed
later.
POC Item 4: Permanent impervious drainage channels are needed along Portuguese and
Paintbrush Canyons. However, permanent channels should not be constructed before all
ground movement stops in the vicinity of the channels. After completion of the first three
control measures, Portuguese Canyon might be re-routed along the west side of the fill
area and hence to its former position along the east side of the existing developed area.
A high density polyethylene(HDPE)or other suitable liner system should be installed with
erosion protection to reduce the rate of infiltration from this and other gullies which
transect the landslide area. In this regard, a debris basin with impermeable base in
Paintbrush Canyon uphill from Crenshaw Extension should be incorporated in the POC
design.
•
5.2 Geotechnical Evaluation of Proposed Improvements
The slope stability model which was discussed in Section 4.5.5 has been used to estimate
the potential effects of the proposed POC. In the first part of this analysis, the potential
effects of the proposed grading on the East-Central Subslide were evaluated. Cross
sections C-C' and D-D' were modified to account for the proposed topographic changes
(Figure 2.8) and for the removal of the rupture surface in the Landward portion of the
landslide. These cross sections were then analyzed with the same 1996 groundwater levels
and shear strength parameters which were utilized in the initial analysis. The results of
this analysis are summarized below.
•
63
Table 5.2.1. Potential Impact of Proposed Grading Improvements
Shear Estimated
'Displacement Strength Weight Combined
Cross Factor of Relative
Condition Rate Rate Section Safe (Tons per Mass factor of
(Inches/Day) Factor tY Foot of Safety
f(Rate) Width)
1995-1996 C-C' 0.87 15,450 49.8%
(As-Is) 0.15 to 0.2 94.2% 1
D-D' 1.12 16,200 50.2%
With C-C' 0.93 13,800 49.3%
Proposed "0 93.0% 1.07
Grading D-D' 1.21 14,200 50.7%
'Used to calculate Shear Strength Rate Factor
The results for the "As-Is" condition are also presented in this table for comparison. As
shown, the proposed grading yields an estimated increase in the factor of safety of 7%
along cross section C-C', 8% along cross section D-D', and approximately 7% for the
East-Central subslide as a whole based upon a mass-weighted average. The resulting
calculated factor of safety for this subslide after the proposed grading improvements is
• 1.07 . The fact that this value is slightly above unity suggests that synchronous movement
of this portion of the landslide should not occur after the proposed grading. In accordance
with this postulation, "zero rate" shear strengths (i.e., at 93% of the bench mark 15 inch
per day shear strength) were utilized in the analysis. It should be noted that the 15-inch
per day rate of movement was chosen arbitrarily, during the laboratory investigation. It
has been assumed that this "benchmark" reference rate of movement should not be
sensitive to the rate factors which were used to adjust the strength. Another "benchmark"
reference rate, for example at 1.5-inches per day or slower, should be used to test the
assumption and to confirm the conclusion that resulted from the analysis still holds true.
The second part of this analysis involved assessing the potential effects of the proposed
surface drainage improvements, subdrains, and dewatering wells. In this regard, two
separate cases have been considered. In the first and more conservative case, it was
assumed that the proposed subdrain installation would lower and maintain the groundwater
• level to the base of the proposed fill within the immediate area of that installation. This
corresponds to an average reduction of approximately 25 feet in the groundwater level
64
along the southern edge of the regraded area. For this case, the subdrain system and other
4114 improvements were assumed to have negligible impact on the groundwater levels within
the central and southern portions of the landslide. For the second case, it was assumed
that the proposed improvements would reduce groundwater levels to the base of the
proposed fill in the regraded area and uniformly lower the groundwater level within the
remainder of the landslide by ten feet. The results of the associated stability analysis are
summarized in the following table:
Table 5.2.2. Potential Impact of Proposed Drainage Improvements
Estimated
Weight Combined
Cross Factor of Relative
Assumed Conditions Section Safety (Tons per Mass factor of
Foot of Safety
Width)
Proposed grading only C-C' 0.93 13,800 49.3%
without groundwater - 1.07
level reduction D-D' 1.21 14,200 50.7%
• Proposed Grading + C-C' 0.96 13,750 49.3%
=25' Reduction in
Groundwater Level in 1.12
Area of Subdrain D-D' 1.27 14,150 50.7%
Installation
Proposed Grading +
=25' Reduction in C-C' 1 13,650 49.2%
Groundwater Level in
1.16
Subdrain Area + 10'
Reduction south of the D-D' 1.31 14,100 50.8%
regraded area
As shown, 5% and 9% improvements in the factor of safety are calculated for the first and
second cases, respectively. The overall factor of safety estimated from this analysis for
the combined grading and drainage improvements ranges from 1.12 to 1.16.
In the analyses of the potential effects of the proposed grading and drainage improvements
which are described in the previous paragraphs, synchronous or uniform mass movement
Ask
of the eastern portion of PBL (i.e., approximately east of Portuguese Canyon) was
IIP
considered. However, the potential exists for the seaward portion of the failure to move
65
independently of the landslide as a whole. This type of movement has occurred in the past
- an example of which is the Seaward subslide. Continued movement of a Seaward
subslide could progressively remove lateral support and potentially perpetuate movement
of PBL. In order to consider the potential for this type of progressive failure, three
potential Seaward subslide configurations were analyzed. The configurations of these
Subslides are illustrated in Figures 5.1 and 5.2 while the results of this analysis are
summarized in the following table:
Table 5.2.3. Analysis of Potential Seaward Subslides
Distance From Estimated
Shoreline to Weight Combined
Head of Potential Cross Factor of Relative
Section Safety (Tons per Mass factor of
Subslide Foot of Safety
Width)
C-C' 0.88 3,000 54.5%
700 feet 1.19
D-D' 1.28 2,500 55.5%
• 900 feet C-C' 0.95 4,200 45.2%
0.97
D-D' 0.98 5,100 54.8%
C-C' 0.91 9,850 41.0%
2,000 feet 1.09
D-D' 1.21 14,200 59.0%
Each of the analyses represented above was completed using the estimated existing
groundwater levels as they are represented in Figure 4.2 and Appendix A. The results of
these analyses indicate that movement of a subslide extending from the area just south of
the current alignment of Palos Verdes Drive South to the shore line is likely to continue
after the proposed grading repairs if the current groundwater levels persist. If the
groundwater level in this area of this subslide is lowered ten feet, then the calculated gross
factor of safety for this subslide is increased to 1.02.
The shear strength parameters for the portions of the failure surfaces which extend
downward through slide debris from the head of the potential Seaward Subslides (Figure
5.2) to the basal rupture surface were back-calculated from ongoing failures which are
66
occurring in this material in the seacliff along the toe of the landslide. This seacliff has
been over steepened by landslide movement and wave erosion. As a result, relatively
localized failures along the bluff involving several thousand cubic yards of slide debris are
common. For example, during an inspection of the site on July 2, 1996, at least five
recent failures ranging from an estimated 2,000 to more than 10,000 cubic yards could be
counted along the shoreline. Inspection of these failures revealed that each had either
occurred, or been covered with a layer of fresh debris, since the previous high tide. Cross
sections were subsequently prepared through the general area of three of the larger of these
bluff failures for analysis. These cross sections are presented in Figures 5.4 to 5.6, and
their locations are shown in Figure 5.3. For each of these cross sections, several
combinations of cohesion c and friction angle (4)) which yielded a factor of safety of 1.0
were back-calculated. For each c-4) combination, the most critical surface was identified
through a computerized search procedure and utilized in the back-calculation. Typical
critical surfaces are shown on the cross sections in Figures 5.4 through 5.6. The c-4)
combinations which were calculated for the three cross sections are presented in the
• following table and shown graphically in Figure 5.7. It should be noted that Figures 5.4
to 5.6 assumed a rotational failure mode in our attempt to calculate the c-4) strength
combination of the bluff material which consists of slide debris or dilated bedrock. While
it is desirable to have additional subsurface investigation to better define the stratigraphy
in the bluff and beach front area of the Seaward subslide, the rotational failure mode
implies that the slide debris is grossly homogeneous. The limitation of this assumption
should be recognized.
Table 5.2.4. Landslide Debris Strength Parameters
Back-Calculated From Bluff Failures
Bluff Section Cohesion Friction Angle
(Psf) (4))
I 38°
II 250 32°
III
31°
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• Bluff Section Cohesion Friction Angle
(Psi (4))
I 25°
II 500 24°
III
23°
I 19°
II 800 16°
III 16°
I 15°
II 1,000 12°
III 13°
Based upon this analysis, values of c= 600 psf and 4) = 20° were adopted to be generally
representative of the strength of the landslide debris. These values.were utilized for the
slide debris in the Seaward subslide analysis which was discussed previously (Table
5.2.3).
•
The analysis discussed above indicated that movement of a Seaward subslide is likely to
continue after grading, until and unless groundwater levels are lowered and/or other
stabilization measures are implemented. Therefore, the potential impact of this movement
on the East-Central subslide was evaluated. In this analysis, it was assumed that an active
failure "wedge" would develop at the head of the Seaward subslide if movement of that
subslide continued and movement of the upslope portion of the failure is at least
temporarily stopped as a result of the proposed POC grading. It was also anticipated that
continued movement of the Seaward subslide would progressively remove lateral support
from the upslope portion of the landslide. Assuming the level of support for the upslope
portion of the landslide due to the moving Seaward subslide is equivalent to an "active"
state of soil pressure of the Seaward subslide, the active pressure was estimated to be on
the order of 300,000 and 350,000 pounds per foot of width for cross sections C-C' and
D-D', respectively, see Figure 5.2. A stability analysis was subsequently completed in
•
68
which the static resistance of the Seaward subslide was removed and replaced with these
• effective active resistances. The results of this analysis are summarized below:
Table 5.2.5. Potential Impact of Continued Movement of Seaward Subslide
Cross Estimated Combined
Condition Section Factor of Weight Relative factor of
Safety (Tons per Foot Mass
Figure 5.3 of Width) Safety
After Proposed C-C' 0.93 13,800 49.3%
Grading Without _ 1.07
Movement of
Seaward Subslide D-D' 1.21 14,200 50.7%
After Proposed C-C' 0.92 9,600 51.3%
Grading With
Continued 0.99
Movement of D-D' 1.06 9,100 48.7%
Seaward Subslide
After Proposed C-C' 0.97 9,450 51.4%
Grading and 10
Foot Reduction in
• Groundwater Level 1.04
With Continued D-D' 1.09 9,000 49.6%
Movement of
Seaward Subslide
As shown, continued movement of the Seaward subslide reduces the post-grading factor
of safety of the East-Central subslide from 1.07 to 0.99. This lower factor of safety
suggests that creep movement of the East-Central Subslide would be likely under these
conditions.
The stability analysis discussed above indicates that the proposed POC consisting of a
combination of regrading and dewatering combination will increase the stability of the
eastern portion of the PBL and decrease its rate of movement. This analysis also suggests
that the proposed grading combined with moderate reductions in groundwater levels will
need to be supplemented with other means in order to stop this portion of the failure.
411
69
The next section presents the geotechnical engineering design considerations for the
• proposed POC.
5.3 Evaluation of Construction/Implementation Issues
At Ehlig's request, BYA has completed a preliminary evaluation to address several
specific geotechnical issues pertaining to the feasibility of design and construction of the
- proposed POC improvements. It is important to note that this is not a design report and
our comments regarding construction issues are necessarily preliminary. The issues which
we have been asked to address include the following:
1. What is the best way to attach the proposed fill to bedrock in areas where
bedding is dipping into the excavation at angles up to 200?
2. What is the most effective subdrain configuration and how should water
• from the subdrain system be collected and conveyed offsite?
3. How steep can the temporary back-cuts be around the perimeter of the
proposed excavation?
4. What is the best way to handle the surface drainage which enters the
landslide area from Paintbrush and Portuguese Canyons?
The first two of these issues will be addressed concurrently. In this regard, we envision
depressions or swales can be excavated in the surface of the bedrock in the removal area.
These swales could be spaced approximately 100 feet on center with their inverts generally
orientated in a north-south direction perpendicular to the strike of the bedrock. The
surface of the bedrock could be excavated to slope towards the invert of each swale at a
gradient at about 4%. Six-inch diameter, schedule 80 PVC collector pipes should be
• installed along the invert of each swale and connected to a common header. The upper
70
270° of the collector pipes would have continuous factory cut 0.020" inch slots. The
• collector pipes and the invert of the swales should be backfilled to a depth of at least two
feet with gravel which meets Caltrans specifications for Class 2 permeable material. The
remaining exposed bedrock surfaces should then be scarified. Suitably processed native
soil backfill may then be placed and compacted directly on the gravel backfill and scarified
bedrock surface. Given the gentle slope (i.e., =7°) which the majority of the base of the
excavation is expected to have, it is anticipated that the presence of the gravel backfill
combined with proper scarification of the exposed bedrock surface will generally provide
adequate fill bonding.
It is also recommended that a bench be excavated along the southern portion of the
proposed excavation where the rupture surface dips more steeply towards the south. This
bench would provide a means to "key" fill more securely to the relatively steeply dipping
bedrock surface in this area and provide a surface upon which to install an interceptor
drain immediately upslope of the remaining rupture surface. Slot cutting will be necessary
• for the construction of the bench. Hydraugers can be installed to further facilitate
drainage. Localized failures are anticipated during construction. Details for the proposed
regrading and their effects during construction on areas further north are beyond the scope
of this study but should be addressed in the design phase.
With respect to construction of the temporary excavation side slopes, we anticipate that
the following five general cases will be encountered at the site:
410
71
Table 5.3.1. Cases for Temporary Cuts
•
Rupture Estimated Estimated
Surface Orientation Maximum Piezometric
Cases Considered Relative to Depth of Level
Face of Excavation Excavation Above Base
of Excavation
1. South Facing Cuts =5° to 20° out of 30 feet none
slope
2. West Facing Cuts No pre-existing 50 5 feet
rupture surface
3. North Facing Cuts =15° into slope 50 feet 10 feet
4. East Facing Cuts
(Central Portion of Relatively flat 80 feet 25 feet
Excavation)
5. East Facing Cuts
(Northern Portion Relatively flat 30 feet 25 feet
of Excavation)
Available data indicate that the basal rupture surface will dip out of slope by approximately
5 to 20 degrees in the south facing portion of the excavation(Case 1). It is anticipated that
these slopes will not stand unsupported at any cut-back angle. Accordingly, slot-cutting
to take advantage of soil arching action along the northern margin of the removal area
would appear to represent the only viable alternative for excavating in this area. Even so,
construction failures should be anticipated.
Itis BYA's understanding that bedding along the eastern margin of the landslide in the
area of the proposed excavation dips steeply to the west-southwest and that the basal
rupture surface daylights above the material which will be exposed in the west facing cuts.
It is also our understanding that the underlying bedrock in this area is reasonably
competent and that bentonite-rich potential failure surfaces are not present within. The
available data indicate that the base of the excavation should extend no more than five feet
beneath the groundwater table along this portion of the removal area. Based upon these
conditions, it is believed that the excavations along the eastern margin of the landslide
• (Case 2) should be made no steeper than 1:1 up to a maximum height of 25 feet. For the
72
anticipated maximum height of 50 feet, additional study is needed and the cut slope should
• be flatter than 1-1/4 to 1.
Along the southern margin of the removal area (Case 3), the available data indicate that
the bentonitic rupture surface will be inclined into the face of the proposed excavation at
an average angle of approximately 15°. This data also indicate that the base of the
excavation will extend no more than ten feet below the groundwater table in this area.
Along the western margin of the removal area the available data indicate that the
orientation of the bentonitic rupture surface will be nearly horizontal perpendicular to the
face of the excavation. In the southern portion of the western margin (Case 4) the
maximum depth of the excavation is projected to be on the order of 80 feet with the lower
25 feet of this excavation below the groundwater table. In the northern portion of the
western margin (Case 5) the maximum depth of the excavation is projected to be on the
order of 30 feet with the lower 25 feet of this excavation below the groundwater table.
•
Preliminary stability analyses have been conducted for the Case 3, 4, and 5 slopes in order
to estimate the maximum gradients at which these slopes will stand during construction.
Separate analyses were completed for both "drained" and "undrained" conditions. For
the more conservative undrained stability analysis, it was assumed that the excavation
would be made quickly relative to the permeability of the saturated landslide debris. For
. this condition, the groundwater pressures within the soil were assumed not to dissipate as
a result of the groundwater level being lowered within the limits of the excavation. For
the less conservative drained analysis, it was assumed that soil pore pressures would
dissipate relatively quickly as the excavation was made. For both analyses, strength
parameters for the landslide debris and PB clay rupture surface were identical to those
used as in the previous analyses. A reduction rate factor of 0.94, corresponding to a
displacement rate of 0.1 inches per day, was used in calculating the effective shear
strength of the PB clay rupture surface. A "temporary construction" factor of safety of
• 1.10 was adopted for these analyses and the slope gradient for each case was adjusted until
73
this value was achieved. The results of these stability analyses are summarized in the
following table:
Table 5.3.2. Preliminary Analysis of Excavation Slopes During Construction
Maximum Depth Maximum Level of Maximum Gradient
Case of Excavation Groundwater at Base 'Condition for Temporary 1.10
(ft) of Excavation(ft) Factor of Safety(H:V)
3. North Facing Drained' 1.5:1
50 10
Undrained 1.6+:1
4. East Facing Drained' 3.0:1
•
Cuts
(Central 80 25
Portion of Undrained 3.1+:1
Excavation)
5. East Facing Drained' 2.2:1
Cuts
(Northern 30 30
Portion of Undrained Unstable at any angle
Excavation)
' Fully drained condition is improbable to achieve. They are listed herein to convey the following
messages: (1) construction dewatering is not a cost-effective option because of the relatively
impermeability of the material, and (2) to identify potential problems in the adjacent area to the
proposed grading.
The gradients listed above apply only to temporary slopes which are intended to exist for
a short period of time (i.e., a few days). These slope angles represent preliminary
estimates based upon the conditions and assumptions which are outlined in the previous
paragraphs and Table 5.3.1. Confirmation of the consistency of the actual field
conditions, and of the validity of the analytical assumptions, will be necessary for design
as well as indicating these uncertainties in bidding documents.
The forth issue identified previously deals with effective means of intercepting surface
runoff. The unabated infiltration of rainfall and runoff from upslope areas into the
landslide has the potential to decrease the benefits which are achieved by regrading.
Accordingly, limiting groundwater recharge in the vicinity of the landslide is a necessary
Scomponent of any effective POC. As discussed previously, surface water flows onto the
74
landslide primarily from three tributary canyons: Portuguese Canyon on the west;
• Paintbrush Canyon on the East; and Ishibashi Canyon intermediate between the two. As
a result of the generally steep topography in the areas drained by these canyons, as well
as the presence of the active Flying Triangle Landslide along the eastern wall of
Paintbrush Canyon, the associated runoff carries with it relatively large quantities of
sediment and debris (Figure 1.3). Any runoff collection plan for the site must be capable
of coping with these conditions.
A preliminary analysis of the area which is drained by each of the three tributary canyons,
and the approximate peak flow which is anticipated for each, are summarized below:
Table 5.3.3. Estimated Surface Flow onto PBL
Approximate Approximate
Canyon Drainage Peak Flow
from 1 inch per
Area
Hour Storm
• Portuguese 135 acres 100 cfs
Middle 85 acres 65 cfs
Paintbrush 115 acres 115 cfs
The peak flows presented in this table are based upon the "Rational Formula" using a 0.75
run-off coefficient. The one-inch per hour rainfall intensity corresponds to a return period
of approximately 10 years. These values would be approximately 50% higher for a 100
year return period.
The suggested approach for dealing with this run-off involves conveying the flow from the
three canyons to a central debris collection basin constructed in the proposed landslide
removal area. It is anticipated that the channels leading from the canyon outlets to this
basin could be lined earth channels. The debris basin itself should be sized to handle the
anticipated flows and sediment loading, and constructed with a high density polyethylene
• (HDPE) liner to reduce infiltration. Excavated slide material (estimated at 100,000 yards)
75
may be spread in areas of tension zones to reduce infiltration. Infiltration originating from
• the channels which lead into the debris basin should be intercepted by a subdrain system
which is to be installed along the base of the fill. It is suggested that consideration be
given to conveying outflow from the debris basin across the landslide to the ocean through
welded polyethylene piping. This type of piping is relatively economical, flexible, and
UV resistant. Our preliminary calculations indicate that a 36-inch diameter pipe will
handle the combined flow associated with a 10-year storm, while a 48-inch diameter pipe
would be required.for a 100-year storm. These estimates have been made to demonstrate
the feasibility of the POC and are not intended to represent final design.
•
76
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LEGEND
---80— TOPOGRAPHIC CONTOURS au BUILDING D' ',
IN FEET(MSL)
.."'"*........• APPROXIMATE SLIDE BOUNDARY —"`'1+ SUBSLIDE BOUNDARY
�'‘"TOE OF SLIDE
0 300 600 FEET N
------sROAD
• -^ DIRT ROAD
9NEE BING YEN & ASSOCIATES, INC. POTENTIAL SEAWARD SUBSLIDEs
Geotechnkel&Environmental Consultants,Established 1979 EVALUATED IN SLOPE
PROJECT NAME: PORTUGUESE BEND LANDSLIDE STABILITY ANALYSIS
(Base Map, 1995) PROJECT NO:G-940989 I DATE: MARCH 1997 FIGURE 5.1
•
ELEVATION IN FEET FROM MEAN SEA LEVEL ELEVATION IN FEET FROM MEAN SEA LEVEL
O
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— -A(Proj- ted 30'E)
-p D-1 (Projected 10'E)
m
A-A(Pr..'ected 180W)
D-2(Projected 10'W)
c A- - ojected 160W) •
A-2(Projected 140W) E-3(Projected 110'E)
Q 0 E-4(Projected 80'E)
gN , B-A(Pr.lected 100'E1 r7
— B-1 (Projected 80'E) y o`
B-2(Projected 60'E) Z 8
17 G&7(P ojected 230E)
m
Z
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LEGEND
—80–— TOPOGRAPHIC CONTOURS BYA-1 Q PIEZOMETER INSTALLED BY BYA IINP'`
IN FEET(MSL)
0-96-18• 1996 SOIL BORING D'
..•-".... ...• APPROXIMATE SUDE BOUNDARY PB-1• BULK SOIL SAMPLE
e....„411Jr TOE OF SLIDE PBS-2• BORING DRILLED IN 1996
DIRT ROAD 96W-10 1996 BORING
ROAD WN• WELLS MONITORED FROM 0 300 600 FEET
1984 TO 1996 1 N
IN BUILDING
k❑AminBING YEN & ASSOCIATES, INC. ALIGNMENT OF CROSS
❑a Geororhnlcal8 Environmental Consultants,Established 1979
SECTIONS THROUGH LOCALIZED
PROJECT NAME: PORTUGUESE BEND LANDSLIDE SEACLIFF FAILURES
(Base Map, 1995) PROJECT NO: G-940989 I DATE: MARCH 1997 FIGURE 5.3
Go
100-
•
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60
w
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40-
20-
0 I I I I I I I
0 20 40 60 80 100 120 140 1(0
DISTANCE(FEET)
nun BING YEN & ASSOCIATES, INC
ESE Geotechnical& Environmental Consultants SEACLIFF CROSS SECTION G-G' AND
PROJECT NAME:PORTUGUESE BEND LANDSLIDE POTENTIAL CRITICAL SURFACE ANALYZED
PROJECT NO. G94-0989 DATE:MARCH 1996 FIGURE 5.4
H H'
100-
•
80-
•
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w
U-
O
W
J
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40-
r�
r
20-0
r
0 20 40 60 80 100 120 1/10 16i
DISTANCE(FEET)
nor BING YEN & ASSOCIATES, INC
mom Geotechnical& Environmental Consultants SEACLIFF CROSS SECTION H-H' AND
POTENTIAL CRITICAL SURFACE ANALYZED
PROJECT NAME:PORTUGUESE BEND LANDSLIDE
PROJECT NO.G94-0989 DATE:MARCH 1997 FIGURE 5.5
I
100-
80-
60-
w
z ;
0
Q :
u,
W
40-
20-
it 20 40 60 80 100 120 140 160
DISTANCE(FEET)
2.0 BING YEN & ASSOCIATES, INC
sou Geotechnical& Environmental Consultants SEACLIFF CROSS SECTION I-I' AND
POTENTIAL CRITICAL SURFACE ANALYZED
PROJECT NAME:PORTUGUESE BEND LANDSLIDE
PROJECT NO.G94-0989 DATE:MARCH 1997 FIGURE 5.6
I.
40 I I I I I I I 1
Q –
•
30 — —
20 — —
c
o
b - �
u_ ........2....2
10 — —}-- Cross-section G-G' -
-O – Cross-section H-H'
– — Cross-section I-I' –
0 I I I I I I 1 I I
0 200 400 600 800 1000
Cohesion (psf)
Doo BING YEN & ASSOCIATES, INC
mom Geotechnical& Environmental Consultants RANGE OF BACK-CALCULATED STRENGTH
PROJECT NAME:PORTUGUESE BEND LANDSLIDE PARAMETERS FOR LANDSLIDE DEBRIS
PROJECT NO.G94-0989 DATE:MARCH 1997 FIGURE 5.7