Rock Cut Slope Instrumentation within Variable and Potentially Unstable Sedimentary Strata: Fid-Bau Portal

Rock Cut Slope Instrumentation within Variable and Potentially Unstable Sedimentary Strata

Kutschke, Walter G. / Petersen, William / Meyers, John R. / Zorn, Erich V.

Site characterization for a new 8.7 km (5.4 mile) railroad alignment traversing mountainous terrain suggested the presence of ancient landslides that presumably occurred during Pleistocene time. Excavation of the proposed alignment would extend to depths up to 46 m (150 ft) with the potential for either reactivating ancient landslides or triggering new ones. Excavation would expose sedimentary rock strata that exhibit considerable horizontal and vertical variation in strength and deformability but with a general tendency for vertical repetition of behavioral characteristics. Right-of-way restrictions limited rock cut slope angles to those steeper than traditionally used for these slide prone and highly erodible geologic strata. Cut slopes were designed to fit within the proposed right-of-way by developing countermeasures to minimize weak rock degradation, installing sub-horizontal drains to lower groundwater levels and developing an instrumentation program consisting of inclinometers and piezometers to monitor slope movement and water levels. Contingency plans to arrest global movement were to be employed if slope movements occurred beyond tolerable values. Instrumentation was monitored during and post construction. The inclinometer data indicated movement along distinct shear planes. However, slope movement generally ceased upon completion of blasting and excavation operations, although some inclinometers have shown several cycles of minor movement and subsequent stabilization attributed to perched water. Countermeasures to arrest slope movement were never employed. The use of an instrumentation program for this project allowed the owner to construct a safe and cost effective new rail line.