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Section 5.0 Landslide Geometry 1960050-03 5.0 LANDSLIDE GEOMETRY Division into Subslides The active Portuguese Bend Landslide does not move as a single sheet. Rather, it is cut by a pattern of internal fractures and grabens that form divisions between large blocks within the landslide that move at different rates. Based on subsurface exploration and surface mapping, Ehlig (1992) divided the landslide into four semi-independent subslides. These subslides are: the landward, east-central, west-central, and seaward subslides (Figure 12). The landward slide lies in the northern most section of the active Portuguese Bend Landslide. It is a 50-acre crescent shaped piece that is bounded by the west-central subslide on the south. The landward subslide is steeply dipping at the northern boundary, where the rupture surface dips vary from 15 to 25 degrees seaward (cross section A-A', Figure 13). The south section of the subslide flattens out to dips less than 5 degrees due to an anticlinal undulation in the bedrock beneath it. The east-central subslide has an approximate area of 60 acres, in a roughly rectangular shape. The subslide is bounded on the north by a east-west trending graben that is continually created as the slide mass moves southward. To the west, the east-central subslide is separated from the west-central subslide by a north-south tear fault, resulting from differential movement between the two adjacent subslides. This fault is nearly vertical, and trends along Portuguese Canyon. On fl).- the east, the subslide ramps steeply upward into a bedrock monocline that defines the edge of the Portuguese Bend Landslide. To the south, the east-central subslide is bounded by the seaward subslide. The rupture surface of the subslide, on average, dips up to 17 degrees toward the south (cross section B-B', C-C', Figures 14 and 15). Under the east side of the subslide, the rupture surface dips down into a spoon-shaped trough that trends north-south. This trough crosses Palos Verdes Drive South at the neck of the "spoon". The trough is wider to the north, and narrows towards the shoreline. The northern section of the trough dips below sea level before rising back up just above sea level in the southern end (Figure 17). On the western side of the subslide, the rupture surface is flatter,with an average dip of about 7 degrees seaward(Ehlig, 1992). The west-central subslide has the smallest surface area, approximately 40 acres. The subslide is bounded on the east by the tear fault along Portuguese Canyon. On the north and west, it is bounded by the landward subslide that wraps around it. The southern boundary is met by the seaward subslide. The west-central subslide has a more uniform and gently dipping rupture surface than the east-central subslide (cross section A-A', Figure 13). Within the subslide, the rupture surface is folded along an east-west trending axis located just north of Palos Verdes Drive South (Figure 17). These undulations buttress the subslide due to the 30 to 40 foot incline in the rupture surface that the mass must climb to reach the crest of the fold. 411 , "k BA - is LEIGIITONANOASSOCIATES,INC. 1960050-03 The east-central subslide is underlain by the bedrock of the Monterey Formation, while the material beneath the active west-central subslide is landslide debris from the ancient landslide. The inactive debris forms a wedge that thickens to the west, and pinches out to the east. The bedrock beneath the wedge dips into a synclinal trough as the debris thickens. This debris consists of blocks and chaotic bedding that appears similar to the debris of the active Portuguese Bend Landslide (Vonder Linden and Lindvall, 1982). The seaward subslide, a 45 acre parcel, is mostly seaward of Palos Verdes Drive South. The northern edge of the seaward subslide buttresses the east-central and west-central subslides' southern sections. The southern boundary, however, is the shoreline, where constant wave erosion is removing the toe of the active Portuguese Bend Landslide. The eastern edge of the seaward subslide is the edge of the active Portuguese Bend Landslide itself, and is bounded by the Klondike Canyon landslide. To the west, the seaward subslide is bounded by the western subslide and Inspiration Point. The rupture surface of this subslide, in general, dips to the south less than 5 degrees (cross sections- A-A',B-B', C-C',Figures 13, 14, and 15). The seaward subslide is the fastest moving of all the subslides (Ehlig, 1992). This is due mainly to the continual wave erosion of the toe of the active landslide. As the wave erosion removes debris, the resisting force is removed, and the subslide propagates forward. 4111 Mechanism of Sliding The continued movement of the Portuguese Bend Landslide can be attributed to several factors: gravity, the unsupported material at the coastline, the weakness of the bentonite layers forming the rupture surface, the removal of landslide debris from the toe, and high ground water levels. The rupture surface of the landslide, in general, is undulatory, and dipping to the south. The ancient landslide appears to have failed along or near the top of the Portuguese Tuff (Leighton, 1990, 1996; Law/Crandall Associates, 1991). Recent field and subsurface investigations by Bing Yen and Perry Ehlig have defined the failure surface of the active landslide as a bentonite layer approximately 30 feet stratieraphically above the top of the Portuguese Tuff. The .bentonitic clays associated with the active landslide are weak. The bentonite consists of primarily sodium montmorillonite in addition to calcium montmorillonite and illite (Kerr and Drew, 1969; Novak, 1982; Watry, 1990). These clays readily absorb free water into their structure, making them weak. Laboratory tests have demonstrated strength values as low as 5 degrees for the angle of internal friction,as well as 100 pounds per square foot of cohesion. These values were achieved using very slow strain rates and high confining pressures. The shear strength appears to decrease with increasing confining pressures (Ehlig and Yen, 1997). Wave erosion of the shoreline toe is also an important factor to the continued movement of the Portuguese Bend Landslide. The continual erosion of the toe removes the resisting force of the LEIGHTONAND ASSOCIATES,INC. 1960050-03 seaward subslide, and hence. the main landslide mass itself. Once a portion of the resisting force is removed, the landslide continues its seaward movement, and the toe The wave erosion continues, and so does the landslide movement. is built up once more. The role of ground water in the continual propagation of the Portuguese Bend Landslide is of critical importance. Vonder Linden (1972) suggested that the groundwater table and its relative position to the rupture surface is the crucial mechanism that drives the landslide. The Abalone Cove Landslide is often cited as an example of the importance of ground water control to landsliding in the Portuguese Bend area. During the period from 1979 to 1981, dewatering wells were installed throughout the Abalone Cove Landslide (Ehlig and Bean, 1982). This method of stabilization proved to be successful for the Abalone Cove Landslide (Ehlig and Yen. 1997). It is important to stress, however, that lowering of the ground water table alone has not stabilized the Portuguese Bend Landslide. Location of Toe The shoreline at the toe of the ancient landslide complex remained essentially the same until the re-activation of the currently active Portuguese-Bend Landslide(USAGE, 1992). Until 1956, the shoreline was marked by a fairly wide, sandy beach that was the Portuguese Bend Clubhouse and a wooden pier. After reactivation, the Portuguese Bend Landslide destroyed the Clubhouse and pier, and the sandy beach was covered by the advancing landslide mass. Currently, the shoreline of the Portuguese Bend Landslide is a steep bluff(80 - 100 feet high) that descends to a narrow strip of cobble beach. The bluffs continued erosion by wave action removes accumulations of fine grained material from the shoreline. Between 1870 and 1959, the shoreline advanced seaward a maximum of approximately 300 feet (USACE, 1992), with most movement occurring between 1956 and 1959 (Figures 18 and 19). The shoreline has continued to advance, but at a slower rate since 1959. By 1972, the shoreline was a maximum of approximately 200 feet farther south than in 1959. However, this movement took place mostly west of Portuguese Canyon; while east of Portuguese Canyon the shoreline did not migrate. Between 1972 and 1982 the shoreline east of Portuguese Canyon moved southward approximately 200 feet. West of Portuguese Canyon, the shoreline remained unchanged. The only migration of the shoreline west of Portuguese Canyon took place in the area 1300 feet east of Inspiration Point. The location of the toe is presently near the shoreline. The location of the active toe was confirmed by the three recent borings drilled along the shoreline (Figure 11, Borings LC-1 through LC-3) and measurements of eight GPS monuments placed at the beach (Figure 4). Data from the GPS monuments are included in Appendix E. These data include the measurements at each monument and the total resultant movement. The direction and magnitude of displacement are shown on Figure 4. As noted in Appendix D, station B-7 is the only station with movement. aL r. • LEIGNTONANDASSOCIATES,INC.