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Section 6.0 Landslide Movement 1960050-03 S 6.0 LANDSLIDE MOVEMENT Historic Landslide Movement Rates Since reactiviation in August 1956, the Portuguese Bend Landslide has been continuously moving downslope toward the Pacific Ocean. The initial failure occurred when a cut slope adjacent to the right-of way for the Crenshaw Boulevard extension failed onto the roadway alignment. Approximately 160,000 cubic yards of the landslide debris was removed and placed as compacted fill along the roadway alignment to establish the road grade where Crenshaw would turn southward and then descend to the west onto the landslide. Shortly after the fill was placed, cracks were observed in a masonry culvert at Paintbrush Canyon at the eastern margin of the landslide. The landslide spread outward from the initial failure as shown in Figure 20. By October, 1956 the landslide extended all the way to the beach and was affecting the pier at the Portuguese Bend Club. The pier was adjacent to the east side of Inspiration Point approximately 3000 feet south of the initial failure. By Spring of 1957, the landslide had reached its ultimate boundaries. Displacement of the landslide since the initial failure approximately 40 years ago varies to a maximum of approximately 800 feet(Ehlig and Yen, 1997). After reactivation of the landslide, the Los Angeles County Engineer's Department installed a number of survey monuments on and adjacent to the active landslide. Recently the survey monuments have been supplemented by a series of GPS survey sites along the beach. Since their installation, the monuments have been periodically resurveyed by the County or, after its incorporation in 1974, the City of Rancho Palos Verdes. Some of these survey stations have been destroyed by the landslide movement or by grading activities that were part of the stabilization plan. Survey data were used to calculate average annual rates of displacement for both the East-Central and West-Central portions of the landslide through 1969. These calculations were performed by Karl Vonder Linden (Vonder Linden, 1972) and the results are reproduced in Figure 21 of this report. Figure 21 compares the rates of movement between the two portions of the landslide with the annual rainfall. A comparison of movement rates shows that the East-Central subslide has always moved about twice as fast as the West-Central subslide. The figure demonstrates that periods of high rainfall have increased the rate of movement, generally after a lag of two to three months (Ehlig and Yen, 1997). Survey data have concentrated on the faster moving eastern part of the landslide and more detailed analyses have been performed on this part of the landslide complex. Figure 23 summarizes the data from August 1956 through October 1996 for the East-Central subslide. In addition, data from 1990 through 1996 from Station K, located on the eastern side of the landslide (Figure 11), is presented in Figure 22. These data and figures show that the movement . rate generally increases following increases in the rainfall. Figure 23 illustrates that movement LEIGHTON ANN ASSOCIATES,!NC. 1960050-03 of the eastern portion of the landslide slowed to a long term average of approximately 0.4 inches per year following the initial period of movement. With one exception. this rate of movement generally remained unchanged until 1977. The exception is an increase as a result of the intense rainfall during the winter of 1969. Movement rates since 1977 have been more varied. Initially,the landslide movement was quite rapid(about 3 inches per day). probably as a result of excess pore water pressures along the rupture surface. As is typical for landslides, as pore pressures dissipated, the rate of movement decreased and, after a period of 2 to 3 years, the rate of movement equilibrated at about 0.4 inches per day(Ehlig and Yen, 1997). During the spring and summer of 1957, a group of 23 reinforced concrete caissons were drilled and installed in the_landslide by the County of Los Angeles and the landowner, Palos Verdes Properties. The caissons or "shear pins" consisted of a steel reinforced concrete cylinder embedded in material beneath the active rupture surface. The caissons were four feet in diameter and 20 feet long. Installation locations of the caissons are shown on Figure 11. For a short period after installation of the caissons, the landslide reportedly slowed from 0.8 inches per day to 0.25 inches per day. This slower rate of movement only lasted for five months before the landslide rate again increased to its former rate of 0.8 inch per day. The increased movement rate is assumed to have resulted from failure of the caissons. Several intact shear pins have been moved approximately 700 feet to the beach by landslide movement. The unbroken condition of the caissons indicates that rather than the landslide shearing through them, the mass carried the caissons to the beach. Failure of the caissons resulted from flowage of the slide debris around the caissons, overturning of the caissons, or the landslide mass plucked them from the bedrock. None were sheared (Ehlig, 1992). This failure mode implies that the caissons did not appreciably change the rate of movement. Ehlig (Ehlig and Yen, 1997) attributes the temporary rate reduction to the seasonal effect of dry weather and the resulting change to the groundwater regime. • The winter of 1968-1969 was a season with above average rainfall. While Figure 21 shows that the rainfall for the period of 1946-1996 averages 10.43 inches, Figures 21 and 23 indicate the 1969 rainfall was over 16 inches, about 50% more than normal. The increased rainfall resulted in an increase in the rate of movement, from 0.4 inches per day to 0.7 inches per day in the eastern portion of the landslide (Figure 23). The change in rate of movement lags the change in rainfall by 2 to 3 months. Movement remained nearly constant at 0.4 inch per day until 1977, when an abnormally wet winter again increased the rate of movement. Movement rates increased to 1.1 inches per day (Figure 21). A series of high rainfall years (1979-1980, 1982-1983, and 1985-1986) that followed soon after increased the movement rates to a peak rate of 1.5 inches per day. These higher movement rates continued until 1986. 417. Fi8cpfr.,„! .4 `glINOPPP'` LEIGHTON AND ASSOCIATES,INC. 1960050-03 In 1984 the City of Rancho Palos Verdes began the Portuguese Bend Stabilization Project. This project consisted of dewatering wells, drainage improvements, and grading of portions of the landslide. Initial implementation of the stabilization plan consisted of installation of four dewatering wells within the active landslide. The wells were installed between 1984 and 1986. Production from the four wells was low with an aggregate production of less than 20.000 gallons per day (Ehlig and Yen. 1997). In contrast, individual wells in the adjoining Abalone Cove Landslide have produced up to 90,000 gallons per day. Despite the modest production from the dewatering wells, the effect of the wells and several below average rainfall years contributed to the reduction in movement rates to 0.25 inch per day, as shown in Figure 22. The stabilization plan also included grading to redistribute mass within the active landslide. Three phases of grading were undertaken between 1984 and 1990, and the effect of each was a reduction in the movement rate. Phase I took place from October to December of 1986. It included moving about 560,000 cubic yards of landslide debris. Of this. 200,000 cubic yards was moved from the landward subslide, and 350,000 cubic yards was moved from the northeast part of the east-central subslide. In addition to this, 10,000 cubic yards were moved to create a drainage trough to the ocean (Ehlig, 1992). The material was placed in the southeast part of the east-central subslide to increase the resisting forces. Upon completion of grading, the rate of movement decreased from 0.7 inches per day to 0.35 inches per day. Phase II grading took place from January to March 1988. This phase included moving approximately 500,000 cubic yards from the head of the east-central subslide, and using some of it to reconstruct Palos Verdes Drive South (Ehlig, 1992). This phase was followed by a decrease in movement from 0.25 inches per day to 0.05 inchesper day. The third phase of grading moved about 60,000 cubic yards from the uphill area of the landward subslide to the eastern portion of the active landslide just north of Palos Verdes Drive South (Ehlig, 1992). As a result of this phase of grading, movement of the landward subslide appeared to stop until January 1995. Heavy rains, no maintenance of cracks in the surface of the subslide, secondary slides and alluvial deposition caused the subslide to begin moving. As shown in figures 22 and 23 long term movement of the landslide, as measured on the eastern portion of the landslide at monitoring station K, increased to 0.25 inches per day (7.6 feet per year). The slow increase in the movement rates is considered a result of: increased rainfall that resulted in secondary landslides that loaded the active landslide, movement of the landslide that opened fissures and cracks in the landslide mass with the resultant increased rate of infiltration, loss of support resulting from the continuing rapid movement of the Seaward Subslide, and surcharging of the active landslide by the sediment eroded from Paintbrush Canyon during heavy rainfall periods which is transported and deposited on the active landslide. 410 74,NE% r ar_unfu Ann RMnrrATre Mir 1960050-03 Current Landslide Movement Rates At the present time, the east-central subslide of the Portuguese Bend Landslide is moving at an average rate of 0.25 inches per day (Ehlig and Yen, 1997). Appendix E contains the locations and measurements for 46 GPS monuments in and adjacent to the active landslide. The magnitude and displacement for these monuments are shown on Figure 4. Future Landslide Movement Rates Future movement rates are difficult to predict. Historic movement rates have been influenced by groundwater levels, distribution of mass within the landslide, and loading from external sources such as the secondary landslides and sediment eroded from upslope areas.r For comparison, long- term historical movement rates have varied from near zero (1990-1995) to 1.5 inches per day (Figure 23) in 1983 and 1986. Short-term rates of movement have been as high as 3 inches per day (1956) following reactivation. Assuming no change in the overall current conditions, it can be expected that the rate of slide movement will continue at its current rate of 7.6 feet per year, and then slowly increase. The increase will result from rising ground water levels caused by increased infiltration of surface water due to cracking and fissuring of the landslide mass, and erosion of the seaward subslide. Erosion of the seaward subslide bluffs could be accelerated by heavy wave action due to periodic Pacific storms, such as in the late 1980's. Movement rates could increase to the range of the historically high movement rates of 1.0 inch per day or 30 feet per year(Figure 23). • &R imb BA-24- LEIGNTONAND ASSOCIATES,INC.