Arizona Inn Landslide Drainage Shaft Replacement: Innovative Engineering Solutions to Address Geohazards

The Arizona Inn Landslide is located on US Highway 101, between the coastal communities of Port Orford and Gold Beach, Oregon. The landslide has experienced persistent movements since the initial road construction in the 1930s, which periodically closes the road. When this portion of the road is closed, the detour is multiple hours out of the way, so keeping the route open is imperative. In the late 1990s, a vertical drainage gallery was installed in the landslide to reduce groundwater pressures. The vertical gallery consisted of an approximately 200-foot-deep (61 m) shaft with arrays of horizontal drains installed at three different elevations. This system successfully reduced pore water pressures and controlled slide movement for more than 20 years.

However, in January 2023, the lower portion of this slide moved over 20 feet (6 m) downslope in response to weeks of heavy precipitation. This movement completely sheared the shaft at approximately 140 feet (42.7 m) below ground surface. Subsequent to the failure, a subsurface investigation was conducted to reestablish the groundwater and movement monitoring network. Based on the results of the subsurface investigation and past performance of the drainage system, it was decided that the most appropriate approach was to reestablish the drainage system with a new design developed by Delve Underground.

Delve Underground was retained by the Oregon Department of Transportation (ODOT) in October 2023 to develop a fast-tracked design for the replacement drainage system, which included a new collector shaft and horizontal drain arrays in the lower portion of the landslide. As part of the investigation phase, a LiDAR scan of the existing shaft was completed, which provided a detailed record of the displacement and loading caused by the landslide movement. Through an alternatives analysis that included the performance of the original shaft and the observed geotechnical conditions, the team decided that the best option was to relocate the shaft outside of the landslide boundaries.

The new 20-foot-diameter (6 m), 200-foot-deep (61 m) shaft was designed to be constructed immediately south of the landslide, through complex mélange terrain using ring beams and shotcrete. The horizontal drain arrays were designed to lower the groundwater levels within the lower portion of the slide by a minimum of 20 feet (6 m). This will be achieved with approximately 65,000 feet (19,812 m) of horizontal drains installed in arrays from the new shaft. The shaft will collect the groundwater and discharge it in an ocean outfall. This design option provided ODOT with a more resilient system than the original, as future movements will not impact the primary structure for the system as it was located beyond the landslide limits. If horizontal drains are sheared, they can be reestablished from within the shaft.

The project was issued for bid in fall 2024. Construction began in April 2025, and underground excavation began in May 2025. Shaft construction is advancing well and is anticipated to be completed in the first quarter of 2026. Delve Underground is currently providing Owner’s engineering support during construction, which includes documentation of the shaft ground conditions during excavation. Following completion of the shaft, horizontal drain construction will begin. Work is scheduled to be completed in late 2026.