Performance Monitoring for a Critical Structure Built Within a Landslide

The Washington Park Station is a key element of the Tri-County Metropolitan Transportation District's (Tri-Met's) Westside Light Rail Transit System in Portland, Oregon. The Station platforms are located 79 meters (260 feet) below the ground surface, with sole access provided through two 9.5 meter (31 foot) diameter concrete-lined vertical shafts. Within theses shafts are high- speed elevators for transferring passengers between the surface and the Station, fixed stairways, ventilation ducts, electric power conduits, signal and communication system conduits, and a fire suppression standpipe system. The lower 55 meters (180 feet) of each of the shafts is constructed in bedrock and the upper portions are within a large ancient landslide. In addition, the drilled pier foundation elements for the headhouse structures extend through the landslide and into the underlying bedrock. The discovery of creep movements within the ancient landslide resulted in the need to incorporate special design features in the shafts. These design features allow for a total of 5 centimeters (2 inches) of horizontal shear movement within the shafts, at the location of the basal failure plane of the landslide. Five centimeters (2 inches) was selected as the design criteria based on an evaluation of the creep movements measured in the inclinometer instrumentation and the extrapolation of these movements over the 50-year design life of the structure. The beneficial impacts of the remediation measures installed during construction of the Station were also considered in the selected 5-centimeter (2-inch) design criteria. Additional design considerations included the guide rail supports for the high-speed elevator: they can tolerate only 0.64 centimeters (0.25 inches) of movement without adjustment to maintain safe operation. With these design criteria, both Tri- Met and the City of Portland Building Department considered it essential to adopt a long-term monitoring program. If performance monitoring showed rates of movement in the landslide that would result in the design criteria being exceeded, then additional landslide remediation measures would need to be implemented. To achieve the long-term performance monitoring objectives, a monitoring system was designed and installed. The system provides real-time monitoring of the elevator shaft movements, and the movements and ground water conditions within the landslide. The monitoring system consists of: 1) instrumentation located at the slip joints within the shaft structures and in the landslide mass near the Station, 2) both manual and automated data acquisition for monitoring the instrumentation, and 3) an integrated warning system and associated response plan if detected movements exceed predetermined threshold levels.