Green Valley Road Landslides
Green Valley Road was built in Watsonville, California in the 1960s on a fill embankment placed over sands and clays of the Pleistocene Aromas Formation. The original ground surface sloped to the east. The four-lane artery performed well until September 1988, when a contractor excavated part of the base of the embankment during grading of lots for a housing subdivision.
Within a day, cracks appeared in the road pavement above the subdivision. Within a few days, it was apparent that a large slump was underway, moving easterly. The landslide geometry was established by: 1) visual observation of the landslide scarp in the road, 2) the location of the toe of the landslide, and 3) analysis of a core sample containing material characteristic of a slip surface.
A tiltmeter monitoring system with three objectives was subsequently specified to:
Define the upslope extent of unstable ground and assure early detection of headward landslide migration.
Provide advance warning of accelerated slide movement.
Establish a quantitative basis for judging the effectiveness of landslide repairs.
Following surface mapping of the landslide, three borehole tiltmeters were installed in two holes drilled in the road (Figure 1). One hole was drilled through the slide mass; Tiltmeter A2 was installed below the slip surface, and Tiltmeter Al was then placed within the slide mass. Tiltmeter B was installed in a second borehole in stable ground, 20 feet to the south of the active slide mass. Figures 1 and 2 show the approximate slide geometry and instrument locations (Egan and Holzhausen, 1991).
The three tiltmeters were connected to a continuously recording, battery powered data logger in order to obtain an uninterrupted record of ground behavior over several months. A field technician retrieved the data biweekly and plotted it on a PC for review and analysis.
Figure 3 shows plots of the Y-direction tilt data (rotation perpendicular to Green Valley Road) collected from Tiltmeters Al and B. Tiltmeter Al in the slide mass detected the greatest movement. The Y-direction plot of Tiltmeter Al shows continuous rotation until late March 1989, when a cast-in-place reinforced concrete caisson wall (Figure 1) was completed. Movement essentially stopped at this time, attesting to the effectiveness of the repair. Accelerated movement in early January 1989 corresponded to a period of heavy rainfall.
The Y-direction plot of Tiltmeter B (in stable ground) shows only about 50 uradians (0.003 degree) of movement over seven months. Little trend is apparent in the data. Tiltmeter B data showed that the ground above the slide remained stable.
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