Wednesday, February 10, 2021 from 5:30 PM to 7:00 PM PST
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Ashraf Islam (House and Hospitality Co-Chair) 
ASCE Seattle Section 

Joint ASCE Seattle Section / ASCE Geo-Institute Seattle Section Group Meeting 

Accommodating Ancient Landslide Movement with a Compressible Inclusion

by Tom Westover


17:30 - Welcome
17:40 - Networking - Break Out Rooms
17:50 - Sections Announcements
18:00 - Networking - Break Out Rooms
18:10 - Presentation
18:55 - Closing
19:00 - Adjourn


The Portland Water Bureau, until recently, had operated two open drinking water reservoirs (Reservoir 3 and Reservoir 4) at Washington Park, located in Portland, Oregon. The open reservoirs were part of a gravity-fed water system constructed during the 1890’s. During original construction at the Washington Park site, a large ancient landslide was re-activated and movement rates were measured up to several inches per week. The landslide is approximately 1,700 feet long and 1,100 feet wide with an average depth of about 100 feet.

A project to improve the reliability and protection for the City of Portland’s water supply is currently in progress, where Reservoir 3 will be replaced with a gravity-fed buried 12.4-million gallon concrete tank. Reservoir 4 has been disconnected from the water supply and will be used to process on-site stormwater runoff and process/cleaning water. When completed, the project will provide a seismically resilient supply of drinking water to 360,000 people, including all of downtown Portland.

Structural options to stabilize the landslide were not cost effective, as seismic displacements were difficult to tolerate with rigid solutions. Instead, the new reservoir is located just outside of the actively moving ground, adjacent to the landslide toe. The reservoir will be isolated from continued landslide movements using an innovative compressible inclusion. The compressible inclusion will deform in response to continued landslide movement, and in turn will impart relatively small, predictable loads on the reservoir walls. During a seismic event, the compressible inclusion will absorb landslide displacement as well as dampen tank displacements, reducing the demands on drilled shaft foundations and reservoir walls. A comprehensive model was developed to evaluate the behavior of the landslide, compressible inclusion, foundations, and reservoir structure under complex seismic and hydrodynamic loading.

This presentation will discuss the history of the landslide, the current improvement project, the foundation design and soil-structure interaction model, and the performance of the landslide during construction. The design philosophy and mechanics of the compressible inclusion will be described, along with an overview of the seismic response and soil-structure interaction. The construction of appurtenant structures affected by the landslide, including mechanically stabilized earth walls, soldier-pile/tieback walls, bridge slabs, and a reflecting pool will be discussed.


Tom Westover is a Senior Associate Engineer with Cornforth Consultants in Portland, Oregon. Mr. Westover’s geotechnical engineering experience in the Pacific Northwest includes high-capacity ground anchors, soil-structure interactions, landslide and rockfall analysis, transportation projects, and dam safety evaluation and modifications. His main interest involves using 3D modeling, data visualization, custom numerical models, and creativity to develop novel, efficient, risk-based solutions for complex geotechnical problems.