Temporary Anchored CSM Shoring Wall
and Groundwater Cutoff
505 FIRST AVENUE SOUTH
The Sodo district in Seattle is comprised of commercial and industrial properties constructed on reclaimed ground which was previously covered by the waters of Elliot Bay. The 505 1st Avenue site is two blocks from the current water front on approximately 30 ft of fill placed in the early 1900's. Glacially consolidated till underlies the fill soils, and was encountered at depths of 30 to 35 feet below ground surface. Glacial outwash soils consisting of clean gravelly sands were encountered at depths in excess of 65 feet. The presence of a high water table (depth of 7 feet at the site) and permeable fill, which is susceptible to consolidation from dewatering, have previously limited the depths of excavations in this area to less than one level below grade. The 505 1st Ave building is the first structure in the Sodo District of Seattle to tackle the challenges associated with a deep excavation below the water table for the construction of multiple levels of below-grade parking.
The building site is a triangular parcel located between two city streets and two existing buildings. The depth of excavation was 43 feet, with a requirement that the cut-off wall extend an additional 25 feet below the base of the excavation. The ground water regime at the site consists of an upper and lower aquifer separated by the till aquitard, with a slight upward gradient between the two aquifers. To prevent base instability during construction, temporary de-pressurization of the lower aquifer was required. However, the shoring was required to support full hydrostatic water loads, in addition to the design earth pressures, in order to maintain groundwater levels in the upper aquifer.
In order to address the above challenges, Condon-Johnson provided a design-build excavation support system which relied on pre-trenching through the wood debris to remove known obstructions in the fill before installing a 68-ft deep soil mixed cutoff with three rows of tiebacks to support the excavation. Ground Support provided the design of the embedded soldier piles within the soil mix panels, of the tieback anchors and their connections to the soldier piles, as well as the strength design of the CSM panels that acted as lagging between the anchor-supported soldier piles. The deep soil mix wall system was installed using Cutter Soil Mixing (CSM) technology. The CSM shoring system eliminated impacts to the shoring installation process by removing the known debris in the fill. In addition, the real-time verticality controls of the CSM equipment provided an assurance of panel overlap and a continuous water cutoff which neither soil freezing nor secant piles were able to provide.
The shoring design had to determine the pile sizes and spacing, tieback locations, and compressive strength of the soil-cement. The CSM unit used for this project created a panel with plan dimensions of 2.6 feet x 9.2 feet. By utilizing a primary-secondary sequence of panel installation and inserting one pile in the primary panels and two piles in the secondary panels, the resulting pile spacing was 5.5 feet on center.
Because of the dominance of water pressure loading versus earth pressure loading, the individual tieback design loads were controlled by the intermediate stages of construction, not the completed excavation. The total anchor design loads were approximately 25 percent higher than would be inferred from the design earth pressure diagram. The anchors were installed at steep 30 to 40 angles to develop their bond in the underlying till, resulting in axial loads of over 290 kips on each soldier pile. Punching shear stability of the pile within the soil-cement panel required consideration of skin friction around the pile perimeter both above and below the base of excavation and end bearing of the pile within the soil-cement panel. The soil-cement panel acted as watertight lagging and transferred the water and lateral earth pressure loads to both flanges of the soldier piles by arching. The calculated compressive arch loading within the soil-cement panel was 60 psi; the specified design strength of 200 psi represented a factor of safety greater than 3.
A typical design cross-section for the shoring is shown on Figure 1 and a photo of the completed site shown on Figure 2. The shoring performed in accordance with design requirements for both settlements of adjacent streets and groundwater inflow to the excavation.