7.0 Conclusions and Recommendations 7.0 CONCLUSIONS AND RECOMMENDATIONS
This section summarizes the conclusions and recommendations derived from (1) review of
historical records and previously available geological and geotechnical information; (2) evaluation
of subsurface data collected during this investigation; (3) laboratory tests and geotechnical
engineering analyses of the landslide; and (4) the proposed POC and alternate or supplemental
stabilization measures.
Our evaluation indicates that grading in the northeastern portion of PBL, as proposed in the POC,
will remove the sliding material in an area about 19 acres and will increase the stability of the
eastern half of the landslide by 7% to a factor of safety of approximately 1.07. This will reduce
the rate of movement of the eastern half of PBL. When combined with the proposed dewatering
and surface drainage improvements, it is our best estimate that a factor of safety on the order of
1.14 is achievable for the eastern portion of the landslide shortly after the regrading and
dewatering have taken place. This will probably result in arresting movement of the East-Central
• subslide, which was the goal of the POC. However, our analyses also indicate that continued
movement of the Seaward subslide will occur even after the POC is implemented. This movement
will progressively remove support.from the upslope portions of PBL and ultimately reduce the
effective factor of safety of the eastern PBL, of which East-Central subslide is a portion, to a
value close to unity. The actual calculated factor of safety for this condition is 1.04. We
anticipate that the East-Central subslide will then have an intermittent slow movement and periodic
acceleration following heavy precipitation. The mechanism behind this phenomenon is attributable
to two primary factors: (1)the strength and creep behavior of the slide material, which were tested
during this investigation; and (2) the configuration of PBL as a whole, i.e., its relatively gentle
slope and its steep toe at the shoreline which is subject to erosion.
Due to the potential for continued movement following the POC, two supplemental improvements
have been identified and evaluated. These are (1) enhancement of the shear strength of the
bentonitic clay which is present along the rupture surface; and (2) construction of a revetment
along the toe of the landslide to buttress the Seaward subslide.
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The clay along the rupture surface is known to consist of sodium-rich montmorillonite. The
•
process of cation exchange whereby one element within a clay is chemically replaced by another
has been recognized for many years. Recent simulated in-situ tests by Rao et. al. and past
laboratory tests by Marsi et. al. have indicated that the shear strength of sodium montmorillonite
clay can be increased significantly by exchanging calcium cations for sodium cations within the
clay. Preliminary laboratory tests conducted on PB clay as part of this investigation confirmed
that substantial improvements in shear strength are achievable through a cation exchange process.
The magnitude of strength measured in the laboratory is enough to increase the factor of safety
sufficiently to arrest ground movement. For example, our preliminary analysis has indicated that
strengthening a 400-foot wide strip of sliding surface across the eastern PBL in an area where the
sliding surface dips steeply can increase the factor of safety equal to or exceed that achievable by
regrading alone. This is significant because a 50% increase in strength was assumed in this
example. Laboratory data suggested that the potential strength increase resulting from cation
exchange can be much greater than 50%. However, it is important to note that the tests conducted
• for this report are promising but not extensive, nor is the method of field implementation proven.
Additional laboratory testing is needed to confirm and supplement our preliminary results. Should
further lab testing substantiate these results, a pilot field testing program is the next logical step
towards the ultimate goal of stabilizing PBL. Such a pilot program should be aimed at the
identification and evaluation of the most practical and cost-effective method of introducing the
highly soluble calcium chloride to the in-situ slide material. One of the unique advantages of this
approach is that once the calcium rich solution is introduced, cation exchange of calcium for
sodium 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 solution to other areas of PB clay beyond
the injection point to further enhance long-term stability.
The second supplemental alternative identified above involves the construction of a revetment
along the toe of the landslide. A preliminary geotechnical analysis was made to assess the effect
of construction of a revetment from Inspiration Point on the west to the PBL eastern margin.
Conceptually, this consists of a gravity revetment protected by rip-rap. The estimated volume for
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this revetment is about 200,000 cubic yards. The gravity revetment would provide passive
• resistance to slide movement and reduce the shoreline erosion which has been one of the primary
detrimental factors affecting the stability of the Seaward subslide. The calculated factors of safety
for the Seaward subslide and the entire eastern portion of PBL would both be on the order of 1.12
to 1.14 with this revetment in place. These factors of safety were calculated taking into account
a strength reduction due to slow movement, i.e., this revetment, in combination with the proposed
POC, would likely prevent any further landslide movement. However, any construction in the
vicinity of the existing shoreline will require permits from Federal and State regulating agencies.
Obtaining these permits might be a long and costly process with uncertain outcome.
From a geotechnical engineering point of view, the state of stability of subslides within PBL is
interdependent. This preliminary investigation has focused on the eastern half of PBL and we
have not directly analyzed the interdependence between the western and eastern areas. The
western half of PBL has historically experienced slower movement than the eastern half and there
are technical reasons to suggest that if the eastern half of the slide movement can be stabilized,
• stabilization of PBL is an achievable goal. But stabilization of PBL is a monumental task.
Review and analyses of records accumulated in the last four decades and the current field and
1-aboratory data -represent an initial, but essential, step forward in the development of a
comprehensive stabilization plan. The immediate technical issues which need to be realized are
in the following two areas: (1) evaluation of the behavior of PB clay, particularly with respect to
better definition of the non-linear strength envelope and creep behavior in relation to the ion
exchange characteristics; and(2)completion of a field pilot study involving the in-situ introduction
of lime solution into the PBL rupture surface to assess the overall technical feasibility and cost-
effectiveness of the approach. It is also recommended that the multi-point piezometer (BYA-1)
be incorporated in the City's current routine monitoring program because it is the only available
instrument in PBL where upward hydraulic head across the slide surface can be and has been
measured. Records such as this, together with GPS data and well measurements, will enhance
the understanding of PBL in the years to come.
.410
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Given the conclusions and recommendations summarized above, knowledge gained from this
• investigation and pragmatic experience of the undersigned, we offer the following prognosis
regarding the feasibility of stabilizing PBL:
"Based on the preliminary results obtained during this study, arresting the slow
movements on a long term basis for the East-Central subslide of PBL is feasible.
The most probable means of achieviung this feasible goal is a combination of two
primary methods: regrading and strength enhancement of PB clay. Due to the
variations in sliding plane depth, inclination and the location of groundwater
surface, there should be an optimal combination of these methods which can be
designed based on the findings of the Pilot Field Test. When movements of the
eastern half of PBL are stopped, stabilization of the entire PBL is then an
achievable goal. "
• Prepared by:
-)
jot..
„rA ' -
Perry . Ehlig, Ph.D., 'EG Bing C. Yen, Ph.D., GE
Consu ting Engineerin_ Geologist President
BING YEN & ASSOCIATES, INC.
•
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