LANDSLIDE MONITORING AT THE WASHINGTON PARK LIGHT RAIL STATION AND OREGON ZOO, PORTLAND, OREGON

Monitoring systems for landslides are employed to meet a variety of objectives. Variable rates of ground movement and tolerance of infrastructure or facilities pose different needs with respect to protection of the public and critical infrastructure. Two adjacent public facilities, the Washington Park light rail station (WPS) and the Oregon Zoo, are located within a massive ancient landslide. Long-term monitoring systems have been utilized for both facilities to evaluate landslide movements. Numerous stability-enhancing improvements have been made over time while monitoring allows evaluation of these various improvements.

The Washington Park Station (WPS) is an underground light rail which provides public access to the Oregon Zoo and adjacent public attractions. The WPS platforms are located 79 meters (260 feet) below the ground surface, with access provided through two 9.5 meter (31 foot) diameter concrete-lined vertical elevator shafts. The lower 55 meters (180 feet) of each of the shafts is constructed in bedrock, the upper 20 to 25 meters (60 to 80 feet) are built within a large ancient landslide, which has experienced recent movements.

Active creep movements were disclosed during WPS construction and required incorporation of shear joints in the shafts. Because of the potential for long term impacts due to slide movements, Tri-Met and the City of Portland Building Department considered it essential to adopt a long-term monitoring program to compare actual movements against the key criteria for elevator / shaft performance. The monitoring system consists of: 1) automated instruments located adjacent to and near the shafts, 2) manual ground movement and water level instruments across the landslide area, 3) automated performance tracking of the drainage improvements, and 4) an integrated warning system and associated response plan if detected movements exceed predetermined threshold levels.

Located just downslope of the WPS, and encompassing the historically active toe of the Highlands / Zoo landslide lies the Oregon Zoo. Fifteen years of ongoing monitoring has disclosed creep movements in the landslide mass, and recorded the impact of construction activities. Together with the upslope WPS monitoring system, these two programs portray how these public facilities coexist with landslide activity.