The Fountain Landslide located along I-84, five kilometers east of Cascade Locks, Oregon has moved periodically for over thirty years. Aerial photographs taken prior to recorded movement of the landslide show the headscarp of a large preexisting landslide. In 1952 a cut was made into the toe of the landslide to straighten Highway 30. The recorded movement history begins at this time. Stabilization procedures in the late 1950's focused on dewatering the slide mass. Movement had nearly stopped by 1957. A deeper cut was made into the toe of the landslide in 1966 to widen the highway to the four-laned I-8ON (later renamed I-84). Accelerated movement resulted. The Oregon State Highway Division removed 264,000 cubic meters of material from the head of the movement zone. Accelerated movement continued. The Oregon State Highway Division then began intense research of the landslide. Research included core logs, slope inclinometers, and the ground water data. The western portion of the slide mass was unloaded more extensively in 1970 (1.2 million cubic meters). This later unloading slowed down the movement, but it continues periodically.
The oldest unit found in the area is a volcaniclastic unit. It is found only in core logs in the SW portion of the slide. The basalts of the Columbia River Basalt Group are found intact and as talus in the study area. Quartz diorite intrusives younger than the Columbia River Basalt Group is found at the surf ace and at depth along the entire length of the toe of the landslide. Wind River Lava crossed from Washington, dammed the Columbia River and was deposited within the study area.
The slide mass consists primarily of Columbia River Basalt Group talus and Wind River Lava talus. The slip plane consists primarily of rocky mudstone. The ground water table is elevated over the intrusive at the toe of the landslide and over the volcaniclastic unit at the head. Surface cracks and scarps indicate that the slide mass moves northward, drops at the head and heaves at the toe.
A slope stability analysis of the Fountain Landslide showed that the instability here is the result of elevated groundwater and the removal of material at the toe for highway construction. It also showed that the eastern portion is more stable than the western portion. The differences in the stability result of the addition of fill at the toe and a lower ground water table in the eastern portion. The development of the prehistoric slide resulted when the dam of Wind River Lava was removed and lateral support for the deposit was lost.
This study shows that it is essential to have adequate geologic information prior to construction or remedial design for any preexisting landslide to avoid stability problems.
| Identifer | oai:union.ndltd.org:pdx.edu/oai:pdxscholar.library.pdx.edu:open_access_etds-4265 |
| Date | 01 January 1986 |
| Creators | D'Agnese, Susanne L. |
| Publisher | PDXScholar |
| Source Sets | Portland State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Dissertations and Theses |
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