Lennar Town Hall, Seattle, WA (2020)

Shoring System: Anchored Soldier Piles, Internal Bracing, Soil Nailing

Client: JTM Construction

General Contractor: JTM Construction

Shoring Contractor: Malcolm Drilling Company Inc.

Shoring Face Area: 64,500 Square Feet

Wall Depth Range: 72 to 102 feet

Status: Completed 2020

The development included two thirty-story residential towers and up to eight levels of below-grade parking. The site was particularly challenging for the following reasons: (1) soil conditions included 10-ft to 40-ft of fill (deeper on west next to I-5 CIP retaining wall), and the lower glacial soils included dense caving sands and fractured hard silts; (2) the depths up to 102 feet meant that movements could be a real problem and loads were going to be much higher than are normally dealt with in practice; (3) the existing historic Town Hall building was within 8 feet of the north wall and we would have to have cantilevers up to 19-ft high before stressing anchors; (4) and the west wall was too close to the existing I-5 retaining wall and ROW to allow for anchors on that wall, so that internal bracing would be required. In the end, a design-build solution involving Ground Support, JTM Construction, and the geotechnical engineer PanGEO Inc. would combine soil nailing, anchored piles, and internal bracing, with a higher design load for movement reduction, and result in a relatively open site that functioned beautifully with walls that moved less than an inch on all sides.