Permanent Soil Nail Walls


The Microsoft Block C Garage project, located in Redmond, Washington was begun in 2007 and completed in 2009 with GLY Construction as Prime Contractor and Malcolm Drilling as Shoring Subcontractor. The 45-ft deep below-grade parking garage excavation was one of the largest of its kind in overall extent, with plan dimensions covering roughly 1100 feet in length by 400 feet in width (Figure 1).

Figure 1. Aerial View of Microsoft Block C Garage under Construction.

The excavation for the garage required roughly 113,000 square feet of vertically shored wall face, supported using soil nailing. Because the garage was located interior to the overall property limits, all of the soil nails were designed to provide permanent earth support. This feature reduced the cost of the permanent structure since there were essentially no lateral loads to be supported by the interior decks and shear walls.

Because of the length of the garage, there were several special design considerations. The decks required that significant shrinkage and temperature gaps be placed in the slabs, which reduced the ability to transfer lateral loads through decks and across the gaps. Rigorous finite element analyses were performed of the soil nailed support system to estimate lateral movements during the design seismic event. The analysis results were used in turn to design the gaps between the decks and the perimeter walls so that seismic loads would not be inadvertently transferred to the structure.

Ground Support PLLC worked closely with the project structural engineer, ABKJ, and the project geotechnical engineer, GeoEngineers, to coordinate the designs between the support system and the garage structure to accomplish the seismic design goals. The result was a comprehensive permanent below-grade garage structure with extensive seismic performance considerations as well as significant cost savings to the permanent lateral support system.

Figure 2. View of Staggered Soil Nail Walls at Garage Access Ramp.