Shoring Demonstration Offers Regulators Important New Information

A Seattle private family foundation’s headquarters project provided GeoEngineers with the opportunity to advance the understanding of soldier pile and tieback shoring systems through a full-scale demonstration project designed to assess the risk associated with leaving temporary tieback anchors stressed following construction.

Tieback anchors typically include steel strands installed in a drilled hole and then grouted in place to provide lateral support for excavation shoring. The anchors, once grouted, are stressed using a hydraulic jack to resist lateral loads ranging from tens of kips (one kip = a thousand pounds force) to hundreds of kips.

The City of Seattle, like many municipalities, requires that tiebacks installed in the City right-of-way be de-stressed at the conclusion of the project. This requirement is intended to eliminate safety concerns for workers excavating in the future within the right-of-way and potential adverse impacts to existing improvements, such as gas pipelines or underground fiber-optic lines. A project can employ hundreds of soldier piles, and it can cost up to $3,000 to de-stress each one.

For buildings where basement walls are constructed adjacent to temporary shoring walls, de-stressing tiebacks is costly and time-consuming, and often diminishes both the waterproofing and aesthetic characteristics of the basement wall.

These factors led GeoEngineers’ geotechnical engineers to raise the questions: “Is it necessary or good practice to de-stress tiebacks?” and, “What are the risks of not de-stressing tiebacks?”

To answer these questions, GeoEngineers collaborated with Foundation headquarters project partners Seneca Group, and general contractor Sellen Construction on a demonstration project. The project consisted of severing two stressed sacrificial tiebacks under controlled conditions to simulate future excavation work in the right-of-way. The sacrificial tiebacks were severed excavating down to the middle of the tieback and exposing the steel strands of the tieback. One tieback was then severed by the bucket of an excavator. The second anchor was severed by shearing the tieback with a hydraulic shear mounted on an excavator. The response was documented using video cameras and instrumentation.

The demonstration revealed no significant safety concerns or risk of potential adverse impacts to existing improvements or the adjacent basement wall when the tiebacks were pulled free and de-stressed using a standard excavator digging outside the shoring wall.

Matthew W. Smith, PE, geotechnical engineering lead on the project and GeoEngineers principal, joined with others on the team to share the demonstration’s results with the Cities of Seattle and Kirkland, Northwest developers and other design professionals. Matt also presented a paper about the project to geotechnical professionals at Earth Retention 2010, a conference on earth retaining structures sponsored by the ASCE Geo-Institute.

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