Three Capes Scenic Loop Landslide
Three Capes Landslide, Click Photo for Mitigation

A landslide severed the Three Capes Scenic Loop Highway in Tillamook County, Oregon, on January 29, 1999 resulting in a 25-mile inland detour. Landslide Technology was retained by Oregon Department of Transportation to investigate and evaluate stability for emergency bypass. Permanent mitigation was also needed to restore the county road.

Objectives for remediation included: emergency access across the slide for summer; high winter groundwater and difficulties for earthmoving equipment; rapid beach erosion threatening to undermine the slide; limited uphill right-of-way; and aesthetic constraints of a State Park.

Landslide Technology investigated the slide with reconnaissance, borings and instruments, and developed a remedial solution combining groundwater dewatering with a rockfill shear key to buttress the landslide. The first stage was performed immediately to establish the emergency bypass, and involved horizontal drains to lower groundwater. This also allowed for safety during the second stage deep excavation and rockfill shear key. Discharging groundwater through a trench drain beneath the beach sand, and covering the rockfill with soil and re-vegetation maintained the scenic area.

Following a successful restoration of the county road over the stabilized slide, Tillamook County wrote: "Landslide Technology engineers were among the first people on-site [and] were critical players. "Communications between Consultants, lead contractor, and the County were excellent. The County was able to furnish the best information possible to the public… [The team] did an excellent job of coordinating and resolving issues unique to a project in a rural county."