Table of Contents Copy No. 1
A Joint Preliminary Geology and Geotechnical Engineering
Investigation Report:
FEASIBILITY OF STABILIZING
PORTUGUESE BEND LANDSLIDE
•
Perry Ehlig, Consulting Engineering Geologist
South Pasadena, CA
Bing Yen & Associates, Inc.
Irvine, CA
March 3, 1997
CEA BING YEN & ASSOCIATES, INC.
■❑. Geotechnical&Environmental Consultants,Established 1979
•
March 3, 1997
The Honorable Members of the Council
City of Rancho Palos Verdes
through
•
Mr. Charles Abbott
Charles Abbott Associates, Inc.
371 Van Ness Way. Suite 200
Torrance, CA 90501
Subject: Transmittal of a Preliminary Investigation Report:
Feasibility of Stabilizing Portuguese Bend Landslide
• Dear Members of the Board:
Transmitted herewith are ten(10) copies of a preliminary geologic and geotechnical engineering
report to investigate the feasibility of stabilizing Portuguese Bend Landslide. The report is
prepared jointly by Perry Ehlig, consulting engineering geologist, and Bing Yen & Associates,
Inc., consulting geotechnical engineers.
We are pleased to have had the opportunity to conduct this investigation and to present our
fmdings to the citizens of Rancho Palos Verdes.
Respectfully submitted,
•
Perry I . Ehlig, Ph.D., .E.G. Bing . Ye t, Ph.D., G.E.
Consulting Engineering Geologist President, :ing Yen& Associates, Inc.
Enclosures
•
17701 Mitchell North,Irvine,California 92614-6029 Phone(714)757-1941 Fax(714)757-1943 e-mail:bya@earthlink.net
TABLE OF CONTENTS
• Page
EXECUTIVE SUMMARY vi
ACKNOWLEDGMENTS xv
1.0 INTRODUCTION 1
1.1 General 1
1.2 Objective 1
1.3 Project Genesis and Report Outline 1
2.0 LANDSLIDE CONFIGURATION AND CHARACTERISTICS 4
2.1 Configuration 4
2.2 Slide Characteristics: An Overview 5
3.0 LANDSLIDE MOVEMENT AND MITIGATIVE EFFORTS 7
3.1 Past Movement Rates & Mitigative Efforts 7
3.2 Proposed Mitigative Effort: Plan of Control (POC) 13
4.0 GEOLOGIC AND GEOTECHNICAL CHARACTERISTICS OF LANDSLIDE 19
4.1 Geologic Setting and Properties of Bedrock 19
• 4.2 Geologic Structure and Stratigraphy 23
4.3 Subslides: A General Geotechnical Engineering Assessment 29
4.4 Groundwater 32
4.5 Shear Strength and Engineering Properties 40
4.5.1 Overview 40
4.5.2 Previous Testing Results 42
4.5.3 Laboratory Testing Results 44
4.5.4 Comparison of Strength Data of PB Clay with Montmorillonites 53
4.5.5 Back-Calculation From Landslide Behavior 55
5.0 EVALUATION OF PROPOSED STABILIZATION IMPROVEMENTS 60
5.1 Overview 60
5.2 Geotechnical Evaluation of Proposed Improvements 63
5.3 Evaluation of Construction/Implementation Issues 70
6.0 EVALUATION OF POTENTIAL SUPPLEMENTAL STABILIZATION
MEASURES 77
6.1 PB Clay Strength Enhancement 77
6.2 Revetment to Buttress Seaward Subslide 81
6.3 Dewatering 83
6.4 Pilot Field Testing Program 85
• 7.0 CONCLUSIONS AND RECOMMENDATIONS 87
i
TABLE OF CONTENTS
(Continued)
REFERENCES
APPENDICES
A - Historical Borings, Past and On-Going Observation and Pumping Wells
B - Log of Borings of the 1996 Investigations
GLOSSARY
FIGURES
1.1 Site Location Map
1.2 Approximate Boundary of Subslide Illustrated on 1995 Topographic Model
1.3 Portuguese Bend Area Site Plan
2.1 Summary of Key Features and the 1995 Topographic Map of Landslide
2.2 Pre-1956 Landslide Ground Features and Post Landslide 1956-1957 Field Activities
2.3 Topographic and Surface Features of Portuguese Bend Landslide as of 1984
2.4 Dewatering and Phase I Grading Between 1984-86 Shown on Topographic Map of
• 1986
2.5 Topographic and Surface Features of Portuguese Bend Landslide as of 1987
2.6 1988 Topographic Map and Phase II Grading
2.7 Cross Sections A-A', B-B', E-E', and F-F'
2.8 Cross Sections C-C' and D-D'
2.9 Topographic Changes Between 1955 and 1995 Showing the Proposed P.O.C. and
Example of PB Clay Strength Enhancement
3.1 Generalized 1956 Landslide Reactivation Sequence
3.2 Landslide Displacement Rates and Annual Rainfall From Station 44 LACFCD,
1956-1969
3.3 Displacement Rates for Eastern Portion of Landslide and Annual Rainfall, 1956 to
1996
3.4 Displacement Rate for Monitoring Station K and Monthly Rainfall 1990 to 1996
3.5 Topographic Changes Between 1955 and 1980 Due to Slide Movement and Remedial
Grading
3.6 Proposed Plan of Control
4.1 Approximate Elevation Contours for Basal Rupture Surface
4.2 Approximate October 1996 Groundwater Surface Elevation Contours
4.3 Approximate Depth of Submergence of Basal Rupture Surface
4.4 Fluctuation of Groundwater Elevation in Monitoring Well 2D
4.5 Fluctuation of Groundwater Elevation in Monitoring Well W-4E
• 4.6 Fluctuation of Groundwater Elevation in Monitoring Well B88-4
4.7 Fluctuation of Groundwater Elevation in Monitoring Well B88-5
4.8 Installation Schematic for Pneumatic Piezometer
ii
TABLE OF CONTENTS
(Continued)
•
4.9 Statistical Analysis of Palos Verdes Peninsula Bentonites Log - Normal Distribution of
PI
4.10 Statistical Analysis of Palos Verdes Peninsula Bentonites Log - Normal Distribution of
Residual Friction Angle
4.11 X-Ray Defractorgram Results for Sample Taken at PB-2 with Pattern Matching PDF
#29-1499
4.12 SEM of Slickensided PB Clay Taken at PB-1
4.13 Energy Dispersion Spectroscopy of a PB Clay Gliding Material Taken at PB-1
4.14 Schematic of the BYA Ring Shear Testing Device
4.15 SEM of PB Clay after Ring Shear Test
4.16 Typical Ring Shear Testing Results for PB Clay Sample (PB-2)
4.17 Remolded and Preconsolidated PB Clay Peak Strength Sheared in RST at 15-Inches Per
Day Displacement Rate
4.18 Residual Shearing Strength Measured in RST at 15-Inches Per Day Displacement Rate
4.19 Effect of Displacement Rate on Shear Strength of PB Clay
4.20 Comparison of PB Clay Residual Strength Envelope with that of Sodium and Calcium
Montmorillonites by Mesri, 1969
4.21 Effect of Reduction of Overburden Pressure on Shearing Strength of PB Clay
4.22 Stress-Displacement Relationships for Undisturbed Brown London Clay from
• Walthamstow
5.1 Potential Seaward Subslides Evaluated in Slope Stability Analysis
5.2 Enlargement of Cross Sections C-C' and D-D' in Area of Seaward Subslides
5.3 Alignment of Cross Sections Through Localized Seacliff Failures
5.4 Seacliff Cross Section G-G' and Potential Critical Surface Analyzed
5.5 Seacliff Cross Section H-H' and Potential Critical Surface Analyzed
5.6 Seacliff Cross Section I-I' and Potential Critical Surface Analyzed
5.7 Range of Back-Calculated Strength Parameters for Landslide Debris
6.1 Residual Shear Strength of Lime Treated and Untreated PB Clay Tested in RST
6.2 Peak and Residual Shearing Strength of PB Clay Submerged in CaC12 Solution
6.3 Energy Dispersive Spectroscopy of PB Clay Submerged in CaC12 Solution for 2 Days
During RST
6.4 Conceptual Revetment Design Used for Stability Analysis
6.5 Pilot Field Test Site (N4,019,880; E4,178,430)
PLATES (in pocket)
1 Geologic Map
2 Groundwater Table Elevation, Dec. 1996
3 Geologic Cross Sections A-A' through F-F'
•
iii
TABLE OF CONTENTS
(Continued)
•
TABLES
Table 4.2 Geologic Contact Elevations from 1996 Borings
Table 4.4 Estimated Hydraulic Conductivity From Well Production
Table 4.5.2 Statistical Evaluation of Palos Verdes Bentonite Samples from 1957-1992
Table 4.5.3.1 Summary of Geotechnical Parameters for a PB Clay Sample (PB-2) Used for
Analysis
Table 4.5.3.2 Summary of Ring Shear Results for 15 in./day Displacement Rate
Table 4.5.3.3 Summary of Shear Tests to Define Rate Dependency
Table 4.5.3.4 Estimated Hydraulic Conductivity From Well Production Rate
Table 4.5.5.1 Time Frame of the Three Episodes Used for Analysis
Table 4.5.5.2 Calculated Factors of Safety with Rates of Movement Incorporated
Table 5.2.1 Potential Impact of Proposed Grading Improvements
Table 5.2.2 Potential Impact of Proposed Drainage Improvements
Table 5.2.3 Analysis of Potential Seaward Subslides
Table 5.2.4 Landslide Debris Strength Parameters Back-Calculated From Bluff Failures
Table 5.2.5 Potential Impact of Continued Movement of Seaward Subslide
Table 5.3.1 Cases for Temporary Cuts
Table 5.3.2 Preliminary Analysis of Excavation Slopes During Construction
• Table 5.3.3 Estimated Surface Flow onto PBL
Table 6.1.1 Effect of Ca(OH)2 on Shear Strength of PB Clay
Table 6.1.2 Evaluation of Strength Enhancement of a 400-Foot Wide Strip on the Factor of
Safety for Cross Section D-D'
Table 6.2 Potential Impact of Shoreline Revetment on Eastern Half of PBL
Table 6.3 Effect of Shoreline Revetment on Seaward Subslide
Table 6.4 Potential Impact of Dewatering on Landslide
•
iv