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Trenching operations were difficult for three of the Eljen drains at the upper site because of cave-ins caused by saturated ground conditions and groundwater pooling in the bottom of the trench. Therefore, shorter sections of trench were opened, and the Eljen drains were installed immediately and then backfilled. Extremely wet ground conditions and cave-ins from the old slide debris were also encountered during the excavation for an Eljen drain located directly below the headscarp of the slide. McKean and Inouye (December 2000) discussed similar cave-in problems at wet locations at their evaluation sites and recommended Eljen-type drains so they could be preassembled and installed quickly after excavation. This proved to be correct advice for this project, with one additional observation. Cave-in of the trench wall at saturated locations was almost always on the uphill side of the trench (the direction of ground water flow). To maintain drain alignment, it was necessary to abandon the manufacturer's recommendation and place the drain panels on the downhill side of the trench and backfill on the uphill side over the cave-in debris (figure 7). In deeper trench sections, laborers tied off Eljen panels with bailing twine and lowered them into the trench to maintain their position and grade during backfilling.

Figure 7. Excavation for Eljen drain
located directly below headscarp.
Note trench cave-in onto Eljen drain,
mid-photo.

Middle Site

The middle site is located approximately 600 to 1,000 feet downhill of the headscarp below the section of toppled trees and undergrowth (figure 1). This site was selected for additional drains because it still has local unstable slidemass, apparently affected by a different aquifer than the one at the upper site.

Lower Site

the lower site included drainage systems between the middle site and the upper road (figure 1). In addition to Eljen drains extending the entire width of the landslide, lined ditches were installed at closer intervals across the width of the slide to ensure that sufficient surface water is diverted off of the landslide before it reaches the steeper slope below the upper road. After a Parshall flume was located along the upper road, the original work was completed on October 10, 2001.

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