Section 1.0 Introduction 1960050-03
1.0 INTRODUCTION
Purpose and Scope
The purpose of this appendix is to present all the currently available geotechnical data regarding
the Portuguese Bend Landslide. These data can then be used to evaluate the feasibility of
constructing shoreline protection to protect the bluff from wave erosion.
The scope of work for this study includes synthesis of the information contained in previous
reports and maps, compilation of relevant boring logs, and the construction of three
representative cross-sections through the main body of the Portuguese Bend Landslide to
illustrate the active landslide geometry. Both the subsurface geology and geometry of the active
landslide base are illustrated by the structure contour map. Maps included in this report are: the
base of the active landslide, and the top of the unconfined groundwater surface.
Project Descri.tion and Location
This study is part of an on-going U.S. Army Corps of Engineers (USACE) and City of Rancho
Palos Verdes feasibility evaluation of shoreline erosion, seeking mitigation measures to prevent
• coastal erosion of the shoreline.
The study area is located in the City of Rancho Palos Verdes, on the southwestern side of the
Palos Verdes Peninsula in Los Angeles County, California (Figure 1). The study area is a
portion of the ancient inactive Pleistocene-Age landslide, bounded (approximately) by Crest
Road to the north, Schooner Road to the east, Fox Canyon to the west, and the ocean to the
south. The southwestern portion of the ancient landslide was reactivated during road construction
in 1956 (the cross hatched area in Figure 1). The study area is the southern one-half of the active
landslide.
Review of Previous Geologic/Landslide Studies
The first studies in the area consisted of regional studies focused on determining the geology,
geomorphology, and stratigraphy of the Palos Verdes Peninsula. These early studies (Kew, 1926;
Woodring et al, 1946) identified the ancient landslide complex that contains the active
Portuguese Bend Landslide. Reactivation of the current landslide in 1956 precipitated a number
of studies to evaluate the landslide and design stabilization measures (MacKintosh and
MacKintosh, 1957; Merriam, 1960). These studies included not only field work, but also
laboratory testing and engineering analysis. Studies of the landslide and its movement during the
1960s included Emery (1967) and the description of the clay mineralogy of the Portuguese Tuff
by Kerr and Drew(1969).
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With the acceleration of movement during the 1970s, more detailed studies of the Portuguese •
Bend Landslide followed. Vonder Linden's doctoral dissertation (Vonder Linden, 1972)
contained a detailed summary of the earlier subsurface information supplemented by several
bucket auger borings and deep, diamond core borings. A report by Jahns and Vonder Linden
(1973) addressed the timing of landsliding throughout the Palos Verdes area, especially at
Portuguese Bend. Easton (1973) examined the possible causes of the landslide. During the
1980s, studies of mineralization of the bentonite clays (Novak, 1982)were followed by studies of
the marine terrace chronology of the Palos Verdes Peninsula (Bryant. 1982; Rowell. 1982),
which have been used by many geologists to determine the age of sliding for different portions of
the ancient landslide complex. A detailed stratigraphy of the Monterey Formation of the Palos
Verdes Peninsula. including the Portuguese Bend area, was published by Conrad and Ehlig
(1983). A series of-reports for the City of Rancho Palos Verdes describing movement rates,
sliding mechanisms, and plans for stabilization have been prepared by Dr. Perry Ehlig, the city
geologist (Ehlig, 1982; 1985; 1986; 1987a; 1987b; 1988; 1989). A master's thesis by Bailey
(1986) documented a design for a revetment to protect the toe of the active landslide. A master's
thesis by Watry (1992) discussed the inconsistent shear test results that have been published.
Watry evaluated multiple shear test methods and compared their results. His thesis contains a
tabulation of these results and is the most complete published summary of shear test results for
the Portuguese Tuff.
In 1992, the USACE completed a reconnaissance study of the Portuguese Bend area,
summarizing work done to that time. Following the reconnaissance report, three feasibility A
studies were completed, including two offshore studies and an onshore study. These offshore
studies include a seismic survey by Dill and Norall (1995) which was intended to determine the
thickness of the offshore sediments and the structure of the underlying bedrock. Their study was
supplemented by the nearshore sediment survey by Sadd and Davis (1996) in which they
analyzed the sediment particle size and the environmental chemistry of the sediments. Three
borings were drilled at the shoreline by Leighton (1996, 1996a) to determine the extent of sliding
seaward, the subsurface structure beneath it, and to correlate the existing onshore geologic data
with existing offshore data.
This report summarizes these previous studies and incorporates the findings from the three
borings drilled during 1996 by the City of Rancho Palos Verdes. A tabulation of these studies is
included in the reference list,Appendix A.
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