Anchored and Internally-Braced
Soldier Pile Walls
EIGHTH & VIRGINIA PROJECT
The 8th & Virginia site is located at the north end of downtown Seattle. The excavation for the project is approximately 120 feet by 240 feet in plan and varies in depth from about 63 feet along the north side of the site to about 73 feet along the south side of the site. The site is located in a congested urban environment. City streets with a 65-foot-wide right-of-way are located along the north and west sides of the site. A pile-supported commercial building is located approximately 60 feet away from the south side of the site. The conditions along the east side are the most complex. A high-rise condominium tower is located across an alley with a separation distance of 16 feet. The condominium tower has a below-grade level and is supported on deep foundations consisting of both piles and drilled shafts.
The shoring system for the project consisted of temporary soldier pile and tieback walls. Several innovative design and construction methods, such as a truncated no-load zone for the tieback anchors and a two-phase shoring system for the east side of the excavation, were employed to successfully complete the excavation.
Tieback anchors were used in the design of the north, south and west walls. The anchors along the north and west sides could not extend beyond the 65-foot right-of-way. With excavation depths of 63 to 73 feet, a conventional no-load zone was not practical. A truncated no-load zone was therefore used and this allowed the upper rows of tiebacks to have about 10 to 15 feet of additional bond length. For the south shoring wall, the tiebacks had to be threaded through deep foundations supporting the commercial building 60 feet to the south.
The excavation on the east side of the site was completed adjacent to a 16-foot-wide alley, and three separate shoring design conditions had to be evaluated. The middle portion of the east wall was designed as noted previously for the north and west walls. At the south end, the 9th & Stewart (9S) project was under construction directly across the alley. The base of the excavation for the 9S project was approximately 20 feet higher than the base of the excavation for the 8th & Virginia project. Horizontal tierods were installed from the 9S project site below the alley to allow direct connection of the shoring systems during construction. Below the base of the 9S excavation, the shoring was designed using apparent earth pressures and included the alley wedge surcharge and the adjacent 9S building surcharge. A typical cross section is presented on Figure 1.
Several design challenges were associated with north end of the east wall. The adjacent condominium tower across the alley is supported on a foundation system consisting of closely-spaced piles and drilled shafts that extend 40 to 50 feet below the alley surface. Easements could not be obtained from the adjacent property owner because the existing foundation and shoring systems could not reasonably accommodate installation of tieback anchors from the 8th & Virginia site.
The design had to be completed for two phases of construction because of the constraints described above. Phase I of the shoring system design consisted of two soldier pile walls connected with a raker system. The rakers were designed to connect the outer wall with the top of the inner soldier pile wall, located approximately 35 feet inside the property line and 28 feet lower in elevation. Figure 2 presents the design cross section for Phase I excavation for the east walls and Figure 3 shows a photograph of the east wall at the end of the Phase 1 construction.
Phase II of the design consisted of removal of the inner east wall, connecting raker system and soil and utilizing the recently constructed building core to provide lateral support for the exterior soldier pile wall. A key component for Phase II was designing the outer soldier pile wall beams for long-term unsupported lengths of 18 feet (two building floor levels). The design also had to consider temporary unsupported lengths of up to 25 feet. The formwork system designed to brace the outer east shoring wall with the building floor levels consisted of Peri Formwork, which is typically used to support concrete floors during pouring and curing. Figure 4 presents a photograph of upper level of Peri Formwork bracing.
The project was successfully completed and performed in accordance with design predictions. The project was awarded a 2010 ACEC Gold Award: Best in State for Original or Innovative Application of New or Existing Techniques.