Stadium Place Phase One Development

GeoEngineers provided geotechnical design and construction-monitoring services for Stadium Place Phase One, a mixed-use, high-rise residential development located in Seattle’s historic Pioneer Square district. Stadium Place Investors, LLC is developing the project, which features a four-story concrete podium containing nearly 400 parking stalls and ground-level retail. The podium supports three residential towers, and extends as high as 240 feet.

Unique Seismic, Subsurface and Neighborhood Considerations

The Pioneer Square project site is located in the former tide flats where the Duwamish River entered Elliott Bay. During the late 1800s and early 1900s, Seattle developers placed non-engineered fill in the vicinity to raise the grade above tide levels. Groundwater in the area is shallow, typically less than eight feet below street grades, and the site soils are corrosive.

A variety of successive pile-supported structures have occupied the site since it was filled, and abandoned piles and a significant amount of wood and concrete debris are present below grade. As a result, the near-surface soils are both compressible, potentially liquefiable during earthquake shaking and contain significant obstructions for installation of deep foundations. Furthermore, the site is located within one mile of the Seattle Fault. Thus, the site has a relatively high seismic hazard.

These seismic considerations, subsurface conditions and the site’s proximity to a densely developed neighborhood with many nearby vibration-sensitive structures and utilities presented GeoEngineers and the project team with significant geotechnical design and construction challenges.

Approach

Evaluation and Testing

GeoEngineers began the project with a comprehensive subsurface evaluation of the site. Then the team completed site-specific, seismic-response analyses to assess the response of the site during earthquake shaking and to establish the appropriate/optimal seismic loading for design of the building. GeoEngineers also worked with the project team to complete two test-pile installation programs in order to identify the preferred foundation system for this major development.

The first pile-load test program GeoEngineers undertook investigated both vibrated and driven steel pipe piles. The team monitored the driving resistance and vibrations induced as a function of distance from the vibratory and impact pile hammers and reviewed the results of dynamic pile load test measurements. The results of the first pile-load test program demonstrated that driven-steel pipe piles could be successfully installed; however, the project team determined that the cost of corrosion protection for the steel pipe piles made driven-concrete piles more cost effective.

The team completed a second pile-load test program to assess the capacity, driveability, and vibrations for driven 18- and 24-inch octagonal pre-cast concrete piles. The results of the second pile-load test program demonstrated that the driven pre-cast concrete piles could be successfully and safely installed at the site and could provide the pile capacities required to support the Stadium Place development.

Foundation Design

Using the results of the pile-load tests and the subsurface explorations completed across the site, GeoEngineers assisted the contractors by designing a foundation system that optimized the pre-cast concrete pile order. The optimization accounted for the variability in the depth to the bearing surface and the variability in the bottom of pile-cap elevation, anticipated pile embedment in bearing soils, and accommodated the required development length of sleeves cast into the piles for rebar connection to the pile caps.

Vibration Monitoring

During construction, GeoEngineers monitored pile driving full time, confirming that the piles met the required pile driving criteria and confirming that the vibration thresholds established to protect adjacent buildings and a sewer main located in close proximity to the development were not exceeded. The pile-driving criteria were complex due to the required embedment for lateral, compression, and uplift pile capacity; the requirement that the piles have a minimum development length of the sleeves cast in the piles for rebar connection to the pile cap after cutting off excess pile; and the desire to minimize vibrations to the extent possible.

The team completed vibration monitoring using a series of real-time vibration-monitoring devices located in adjacent buildings and near sensitive utilities. They conveyed data from the vibration-monitoring program to the project team in real time so that pile driving could be modified if pre-determined alarm thresholds were exceeded.

Results

The Stadium Place development presented unique and significant geotechnical challenges. Through GeoEngineers’ expertise in pile driving, seismic and foundation design and vibration monitoring, the project’s 464 24-inch octagonal pre-cast piles were successfully installed and adjacent improvements protected from pile-driving vibrations.