Case Study: Slurry Walls for Flagship Wharf, Boston, MA

Flagship Wharf, Boston, MA.

The Flagship Wharf project was an addition to a pre-existing building in Charlestown, and was constructed in land reclaimed from the sea. It consists of two eleven story residential/retail towers and a low-rise (3 story) plaza/retail terrace structure built above a four level below-grade parking. The plan area of the excavation for this project is 255 ft. (E-W) by 107 ft. (N-S)).

Flagship Wharf Building, Chicago, plan view, cut section and measured displacements

A 60-ft deep, 2.5-ft thick slurry wall provided temporary and permanent lateral earth support for the excavation. The excavation is approximately 50-ft deep, and thus the slurry wall was embedded 10ft into glacial till (Fig. 4.29). The slurry wall was constructed using the patented "ICOS-FLEX" wall system. This wall differs from traditional slurry walls in that the wall is reinforced with post-tensioned, high strength strands, similar to those used for tieback anchors.

The soils found in the site and the surrounding area comprised of miscellaneous fill, organic deposits, glaciomarine, and glacial till deposits. The stratigraphy can be classified as profile C according to Johnson [1989].

The concrete diaphragm wall serves as both the temporary excavation support wall and permanent foundation wall. The slurry wall was designed to resist temporary and permanent static lateral pressures from soil, water, and lateral surcharge loads from building #197 to the north, and other adjacent surface loadings.
Cross-lot bracing was used for the lateral support of the foundation walls of this project. Three levels of temporary prestressed (jacked) steel struts and corner braces supported the slurry wall. The struts were hollow cylinder steel pipes with a diameter ranging from 24" to 34" and 3/8" to 0.5" wall thickness. Steel HP sections were in the connections between struts supporting the north and south wall (Fig. 4.29). The horizontal forces jacked on the struts were transferred on the wall by means of walers.

Most wall movements at the Flagship Wharf project were moderate but along the northern slurry they were large reaching dW=2". The caissons of building #197 added significantly more surcharge on the northern slurry wall, and as a result, the southern wall was pushed back into the retained soil by as much as 1" at the top. Wall movements at opposite panels, braced by common struts, showed opposing trends (0). That is when a panel moved towards the excavation the opposite panel moved back into the soil. The concave bending seen in the slurry walls of this project is indicative of the cross-lot bracing construction sequence.

Settlement control was important in the north side of the project due to the proximity of building #197 that was supported by a series of caissons. Settlements up to dV=1.8" occurred along the section containing the caissons but the settlements were not transmitted to the exterior walls or the floors of the building (1). Wall movements and settlements tied very well together.

A cave-in occurred during trenching of a panel, at a small depth in the fill layer during excavation without slurry. Clearly if trenching was carried out under slurry this cave-in might have been avoided. The cave-in caused additional problems with the vertical construction of the trench. Construction of this panel took a longer time since numerous underground obstructions were encountered.

Data for strut loads as measured from strain gages showed that a quick 0.5" movement at I-2 occurred as a result of a jacking box failure that resulted in the sharp load loss of the L3 strut during April 1989 (2). The structural factor of safety of the struts was only Fs=1.2 and as a result 50% of the total wall movement occurred after the excavation base was reached (indicating some creep movement in the bracing). The upper and lower level struts picked up only 70% of the design loads, that were in the order of 500 tons for the 3rd level (2). The load at second level struts picked up as the excavation progressed to the third level (El -38ft).More frequent readings indicated that daily variations in temperature caused expansion and contraction of the struts that showed up as variations in the strain gage readings.

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