Orchestrating a firm foundation for new Seattle Symphony Hall
By CAROLE L. B. MITCHELL
Geotechnical consultants faced complex challenges in preparing the site for the new Seattle Symphony: an underground railroad tunnel, bus tunnel and bus station; existing buildings, city streets and utility lines and serious surface settlement due to previous subterranean construction.
Shannon & Wilson served as geotechnical consultant on the project, providing design analysis and instrumentation monitoring during construction to confirm the foundations and shoring wall design. Other team members included Benaroya Hall Project Management; Skilling Ward Magnusson Barkshire as structural engineer; Loschky, Marquardt, & Nesholm as architect; and Dr. Cyril Harris as acoustical consultant.
Building above a railroad tunnel
Running diagonally beneath the Benaroya Hall site is an active railroad tunnel belonging to the Burlington Northern Santa Fe Railroad. When the railroad tunnel was hand-excavated in the early 1900s, it caused considerable settlement of the buildings and streets above. In the vicinity of Benaroya Hall, this surface settlement was reported to be as much as two inches. In addition, tunneling disturbance and subsequent rotting of temporary timber supports that were used to facilitate construction of the tunnel had resulted in many voids in the earth.
To alleviate these less-than-favorable building conditions, the new symphony hall's foundation scheme included the use of spread and continuous footings, as well as a combination of drilled shafts and a foundation mat. The foundations required that the railroad tunnel be isolated from the symphony hall building loads.
Excavation for Benaroya Hall removed up to 55 feet of soil overburden, and came to within 12 feet of the tunnel crown. Shannon & Wilson conducted a finite difference (soil-structure interaction) study of the proposed concert hall excavation. The analysis indicated that approximately one-quarter of an inch of movement would occur in the underlying railroad tunnel, due to the building excavation.
To minimize foundation loads on the tunnel (and also to reduce vibrations from train traffic in the concert hall), Shannon & Wilson recommended that the structure in the vicinity of the tunnel be supported on drilled shafts connected to a foundation mat. The shafts extended below the tunnel base, and ranged in size from four to six feet in diameter. A 6.5-foot-thick, reinforced concrete foundation mat was built connecting the shafts on either side of the railroad tunnel, forming a "bridge" to span loads over the top of the tunnel. Spread footing foundations were located in areas that would be beyond the influence of the tunnel.
During drilled shaft installation, obstructions such as boulders had to be cored through. Tiebacks from previous construction had to be cut out and removed. Voids and disturbed, sloughing or caving soils were encountered in many of the shafts.
A series of extensometers and optical surveys within the tunnel confirmed the success of the combination foundation system. Generally, tunnel movements were recorded at 0.03 inch or less, with a maximum measurement of 0.05 inch.
Shaping up the shoring
Complex loading and geometry conditions controlled the design of the shoring wall system at Benaroya Hall. The excavation for the basement of the concert hall abuts the twin transit tunnels and the University Street Station of the Downtown Seattle Transit Project, as well as four heavily-used streets. A combined system, including soldier pile/tieback walls and a soil-nail wall, was used to shore the 15-to 50-foot-deep excavation.
A 25-foot-wide block of soil remained between the east shoring wall and the existing underground bus station. Geotechnical engineers performed a finite difference analysis to determine potential movements of the University Street Station attributable to the excavation and the proposed soil-nail shoring method.
Another important consideration was the maintenance and protection of the station, bus tunnels, street traffic, utilities and surrounding buildings during excavation and construction. This was complicated by the fact that the excavation varied in height from 16 to 34 feet along the east side. In addition, the configuration had to account for an existing building basement and the station's present (and proposed) pedestrian access tunnels.
The geotechnical analyses factored in the influence of rakers, grade beams, footings, and cut slopes at the top of the excavation. A third finite difference analysis was performed for the new pedestrian access into the transit tunnel station.
Drilling for the soldier pile and tieback walls uncovered even more obstructions. These included buried footings, concrete walls and hidden tiebacks from the transit station construction.
Performance of all the shoring walls was monitored during construction through the use of crack gauges in existing buildings, video in the station, an inclinometer behind the soil-nail wall, and optical surveys of the soldier piles and angle irons attached to the top of the soil nail wall. Deflection estimates from the finite difference analysis closely approximated actual field measurements. The soldier pile/tieback walls moved, on average, a mere 0.2 inch, with a maximum movement of 0.5 inch. The soil-nail wall averaged just 0.1 inch.
Dealing with existing underground facilities such as railroad and bus tunnels as well as adjacent buildings and utilities is becoming more crucial in the design and construction of projects in the urban areas of the Puget Sound region. Innovative geotechnical design and analyses are essential when constructing facilities that will integrate with their surroundings and minimize structural risk.
It was through a combination of leading-edge analytical tools and creative design solutions that Shannon & Wilson was able to provide a practical yet cost-effective foundation and shoring wall solution -- that helped contribute to the timely (and safe) construction of our region's new landmark concert hall.
Carole Mitchell is a senior engineer at Shannon & Wilson, a geotechnical and environmental engineering firm headquartered in Seattle.
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