Developing Systems-Level Criteria for Evaluating Performance of Horizontal Drains toward Landslide Mitigation: Case Study of the US20 Pioneer Mountain-Eddyville Realignment Project

Opened to traffic in 2016, the Pioneer Mountain-Eddyville (PME) realignment was a major highway infrastructure project that began in 2005 and created approximately 9 km of new highway on US Route 20 in the Oregon Coast Range. Due to the renewed activity of a series of paleolandslides that traversed the project right-of-way, deliver of the construction project was significantly delayed. Owing to the size and activity of these slope failures, a large monitoring network of piezometers and other instrumentation was deployed as a precursor to the installation of an extensive system of horizontal drain arrays (176 km of drain length) intended as one means of landslide mitigation through control of in situ groundwater. Because there is limited guidance regarding monitoring the long-term performance or design life of horizontal drains, hydrological data in drained areas were monitored as a means of observing performance during and after project completion. This case study describes observations of groundwater behavior prior to—and after—drain installation, thereafter proposing site planning–level metrics for evaluating the continued function of horizontal drain systems. Drain function is considered through groundwater response to precipitation, but also through a simple evaluation of water storage in the catchment of each landslide. The four paleolandslides—Cougar Creek, Crystal Creek, Eddy A/B, and Eddy C/D—demonstrate different responses to drain installation and long-term performance. Using simple metrics based on monitoring, added indices for evaluating long-term horizontal drain performance at the site scale are proposed. Consistent with the Observational Method, these criteria may serve as additional guidance for a monitoring-based framework for evaluating the life cycle of horizontal drains as a landslide mitigation system and a basis for planning drain maintenance or added mitigation measures.