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Section 8.0 Foundation Analysis 1960050-03 . 8.0 FOUNDATION ANALYSIS Although stabilization of the landslide is not an authorized purpose for this feasibility study, protection of the toe of the landslide against continued erosion is an authorized purpose. The stabilization concept for the active landslide by the City of Rancho Palos Verdes includes construction of a gravity buttress or toe berm at the present shoreline. A stone revetment in shallow water would protect the toe berm from wave erosion (Figure 27). The gravity buttress would consist of a mass of earth placed where the landslide rupture surface is inclined to the north or is nearly horizontal. This mass of earth would act as a counterweight and increase the forces resisting landslide movement. Construction of the gravity buttress would require the placement of a large yolume of compacted fill. The fill would be derived from the redistribution of landslide debris from within the active landslide. This method has the potential benefit of reducing driving forces within the landward subslides and thereby increasing their stability by removing debris inland at the same time as the resisting forces in the seaward subslide are increased by placement of the buttress fill. As shown in the cross sections (Figures 13, 14, and 15), the geometry of the landslide rupture surface requires that such a gravity buttress would be placed offshore of the existing shoreline. The specific location of the toe berm would be determined at a later date, following site specific studies. However, it is assumed that, for ease of construction, the buttress would be constructed in the shallowest water possible. As shown on Figure 27, the toe berm or buttress would be located very near the shoreline. The berm would be approximately 150 feet offshore at its farthest point from the beach, located about 450 to 1,000 feet east of Inspiration Point. At this area the buttress would be founded on the seafloor bottom between -4 and -20 feet. A rock revetment would be used to protect the earthen buttress from wave erosion. Conceptual designs for the rock revetment have been discussed by Bailey (1986), USACE (1992), and Ehlig and Yen (1997). These designs generally consist of rock placed on the seaward facing side of the buttress to dissipate the wave energy. Preliminary designs for Portuguese Bend indicate that the revetment would be founded on bedrock about 4 feet below MLLW. The revetment would extend approximately 2,000 feet along the shoreline, and be located about 150 feet offshore. Existing Foundation Sites for Shore Protection The proposed revetment would be located approximately 150 feet offshore and extends from Inspiration Point 2,000 feet to the east. Placement of the revetment in this area allows the toe berm to be located seaward of the toe of the landslide and, in the area west of Portuguese Canyon, seaward of the inactive ancient landslide debris. Within this area water depths vary to a maximum of about 25 feet (Dill and Norall, 1995). Within the area proposed for the revetment foundation, unconsolidated Holocene age sediment appears to vary from approximately 5 to 410 about 8 feet in thickness (Dill and Norall, 1995). SA LEIGHTONANOASSOCIATES,INC. 1960050-03 The proposed location for the revetment is seaward of the toe of the active landslide, on the11, bedrock seafloor bottom. Based on the three recent drill holes at the beach, the revetment would be located on bedrock dipping seaward at 10 to 15 degrees. The seafloor bottom slopes to the south at approximately 2 to 3 degrees in this area. Therefore, the bedrock should dip more steeply than the seafloor topography and a daylighted bedding. condition should be avoided. Although a basalt bedrock foundation would be the best alternative for location of the revetment, a continuous basalt outcrop may not extend eastward from Inspiration Point to the beach at the east side of the study area. Basalt has been mapped on the seafloor adjacent to Inspiration Point and at the beach approximately 2,700 feet east of the point. Additional drilling, in the offshore environment, would be necessary to demonstrate continuity beneath the Holocene sediments. The sedimentary bedrock of the Monterey Formation, however, should be suitable as a foundation material for the revetment. • • • -1-?'.411-44111111411144.1 LEIGHTON AND ASSOCIATES,INC.