Smithsonian National Zoo GBS Stabilization Winner of 2017 Exposing Excellence in Concrete Award by NCC ACI

SNZ GBS Finished Wall and Slope Overview

In May 2017, the National Capital Chapter of the American Concrete Institute (NCC ACI) has named the Smithsonian National Zoo – General Services Building Stabilization project as one of their 2017 “Exposing Excellence in Concrete” award winners. It was further commended by the NCC ACI that receiving one of this year’s awards was an accomplishment in light of their having received a total of 19 different project entries competing for recognition.

Project Background

The 100,000 sf General Services Building (GSB), built into a steep hillside, provides maintenance and back-of-house support to the Smithsonian National Zoological Park (SNZP) as well as visitor parking on its roof. The Zoo is on 163 wooded acres east of Connecticut Avenue, NW Washington, DC bounded by Rock Creek Park. The Smithsonian initially commissioned structural engineers McMullan & Associates, Inc. to investigate the cause of large cracks in concrete columns and indications of apparent movement of the structure down the hillside.

The post-tensioned slab and concrete frame was originally constructed in 1975. Tiebacks anchored in rock were used to tie the building back into the slope. Wood lagging, steel soldier piles and tiebacks were used to stabilize the hillside above the parking deck up to the east-west road through the zoo. The lagging and exposed tiebacks were in very poor condition and severe cracking in the road indicated ongoing slope movement. There was also constant leaking through the existing basement wall into the occupied spaces below the parking deck.

The SNZP’s objectives were to permanently stop the lateral movement and water leaks, stabilize the hillside and road, ensure that the existing building was structurally adequate to support additional levels of parking, and comply with the current IBC code including seismic requirements. The SNZP also needed the building to be renovated to improve functionality and efficiency.

A comprehensive geotechnical investigation determined the rock profile through a combination of boreholes, ground penetrating radar and seismic refraction surveys. The investigation also provided allowable soil pressures and the capacity of rock anchors. Limited testing of the tiebacks indicated likely corrosion of the buried anchors, and a detailed survey determined the amount of lateral movement of the top deck relative to the foundations. The survey also led to the remarkable discovery that the building was not only moving down the hillside but rotating around the west end of the structure where the building was anchored to large shear walls with more substantial tiebacks.

When the building was constructed in 1975, the SNZP had anticipated the need to add extra floors over part of the footprint and the structural engineer had designed the affected columns and footings to carry the extra load. However, the current situation required the design team to rethink the entire approach. McMullan & Associates, Inc. developed a scheme that solved several of the problems in one multi-layered approach.

The earth pressures were first reduced by cutting into the hillside to remove overburden on the soil immediately adjacent to the GSB. A soil nailed, reinforced shotcrete retaining wall provided stability of the terraced excavation. The soil nailed wall also enabled the simultaneous stabilization and widening of the access road at the top of the slope and improved drainage to draw water away from the GSB.

The construction inside the GSB followed the progress of the earth removal and included the installation of a new interior post-installed retaining wall braced by mezzanine levels held in place by concrete shear walls and rock anchors. The new retaining wall doubled as an internal waterproofing system. The mezzanine levels greatly expanded the operational space inside the building and enabled efficient column strengthening by attachment to the existing column midpoints, reducing the column effective lengths.

The total approach provided a full stabilization system independent of the existing tiebacks, that were abandoned in place. This also provided maximum flexibility for placement of future foundations for the planned addition.