McMillen Jacobs Associates performed the exploration, design, preparation of bid documents, and resident engineering services for the stabilization of Rock Shed #5 near Oakridge, Oregon. The rock shed, built in the 1930s, protected the track from frequent and large rockslides along a section of sidehill cut. The exposed slope extends up to 200 feet above the 20- to 30-foot wide bench where the shed is located. Below the bench is a downhill slope that falls over 300 feet to the valley below.
The structure consists of vertical piers on the outside, a continuous wall on the inside (field side), and a sloping horizontal roof slab. The horizontal loading from the buildup of debris, and likely high impact loads from slides, caused the structure to rotate outward. This overt loading caused the sinking or failure of several piers on the bench shoulder.
The exploration program included drilling exploration borings along the bench on the downhill side near the sinking piers and into the back wall of the reinforced concrete structure. The design included:
- Removing up to 10 feet of the slide debris overburden
- Installing 30- to 40-foot-long tiebacks through the slide debris behind the wall and anchored in sound volcanic tuff and/or lava
- Anchoring thebackwallto sound foundation with minipiles
- Construction of a concrete wall over the piles that tie into the existing piers.
The latter was anchored to the minipiles to create a sound foundation for the sinking piers. The work was performed during four-hour live-track windows over a period of five months.