LANDSLIDES ON FAULTED AND TILTED TYEE FORMATION REACT TO CUTS, FILLS AND GROUNDWATER, CENTRAL COAST RANGE, LINCOLN COUNTY, OREGON

Ancient landslides in Oregon’s central Coast Range react to cuts, fills and groundwater at a highway realignment project. Monitoring has evolved at the site since 2007 and shows that ancient landslide terrain is reacting to seasonal and storm-induced groundwater levels, removal of lateral support, and loads place at various points of a landslide’s geometry. Some of the movements have been sudden, such as rockslides, while other deep-seated landslides have been very slow, taking years to discern. Specific movements are being mitigated, and some movements are questioned: is it landslide or induced creep.

The site geology consists of colluvial landslide debris overlying interbedded siltstone and sandstone (turbidite) of the Tyee Formation that is gently tilted, folded, and faulted with high-angle and bedding-parallel shears. Over geologic time, differential erosion has resulted in large translational slides, the characteristics of which range widely from massive slide blocks to highly fractured rock to completely disintegrated rock (clayey silt/sand soil).

Monitoring with manual slope inclinometers and in-place inclinometers with data loggers reveals landslide translation, nested shear zones, settlement of fill and underlying slide materials, lateral compression in buttresses and shear keys, one-time small movements, rates of seasonal and induced ground deformation that vary from 0.25 to 115 mm/year (0.01-4.5-inch/year), and movement triggers. Monitoring with vibrating-wire piezometers reveals the hydrogeologic systems that are at the heart of the landslides, which include perched, confined, and fractured rock aquifers, and seasonal and storm-induced groundwater levels that vary from less than 300 mm (<1-foot) to over 15 m (50 feet). Precipitation is also being recorded.

This presentation overviews various Tyee Formation landslide geometries; characteristics of the subsurface materials and groundwater conditions; and the reaction of the landslides and underlying rock formation to excavation, fill load, buttressing, horizontal drains, and seasonal and storm-induced groundwater.