1st Presbyterian Church, Portland, OR (2006)

Shoring System: Temporary Prestressed Soil Nails with Vertical Elements and Top-Down Wall

Client: Malcolm Drilling Company Inc.

General Contractor: Hoffman Construction Company

Shoring Contractor: Malcolm Drilling Company Inc.

Shoring Face Area: 21,435 Square Feet

Wall Depth Range: 35 to 45 feet

Status: Completed 2006

This project involved the construction of a new parking garage for the historic First Presbyterian Church that was north of the site; and the site was currently occupied by the Danmore Hotel. Because the existing church walls were immediately adjacent to the new garage walls, and because the existing Danmore Hotel basement walls occupied the same location as the new basement walls, an innovative design-build approach was adopted that allowed for underpinning of the church, construction of the new basement from the top-down, and open-cut removal and backfilling of the demolished basement walls while the shoring and excavation progressed below. The prestressing of the soil nails reduced wall movements to less than a half inch on all sides. Underpinning and prestressing of the system for the church resulted in no perceptible movement in the church to the north. Vertical elements provided vertical load transfer, reduced face ground loss, reduced movements, allowed for the backfilling of walls against the vertical elements while shoring and excavation proceeded below, and allowed for nails to be eliminated or adjusted to deal with utilities.