Prevention of additional building settlement due to adjacent construction at St. Francis Hospital and Medical Center in Hartford, CT

During excavation for the foundation of the new John T. O’Connell North Tower at St. Francis Hospital and Medical Center (SFHMC), in Hartford, Connecticut, in 2008, Building 3, an immediately-adjacent five-story brick masonry building suddenly settled up to 1.5 in. and moved laterally about 5/8-in., resulting in severe cracking of interior and exterior bearing walls and floor slabs. The excavation was immediately backfilled within 30 feet of Building 3 and all construction activities were stopped until the condition of Building 3, which provided critical laboratory, pharmacy, and administrative support functions to the hospital, was evaluated and the cause of movement determined. SFHMC retained Simpson Gumpertz & Heger Inc. (SGH) to perform an investigation of the cause(s) of the building settlement and lateral movement, and to perform a condition assessment of the affected building. SGH determined that the building settlement was due to bearing failure of the Connecticut Valley Varved Clay (CVVC) stratum underlying the soil nailed excavation support wall installed below the perimeter soil bearing foundation of Building 3. In addition, it was concluded that Building 3 would not be able to sustain additional settlement, and alternate construction processes were needed to allow construction of the North Tower foundations to resume. SFHMC retained SGH to design the alternate construction processes needed. This paper presents the results of the forensic investigation and condition assessment of the distressed building as background for a detailed discussion of the alternate construction processes implemented to control further movement. This paper will present the design, monitoring, and construction considerations for new jacked pile underpinning of a portion of Building 3, a new cross lot braced soldier pile and lagging excavation support system adjacent to Building 3, and new micropiles to replace driven H piles specified for the North Tower foundation in the immediate vicinity of Building 3, all selected to minimize additional damage to the adjacent building. We will also discuss the challenges faced by Moretrench, the subcontractor engaged to implement the work, during construction of these systems and how these challenges were successfully overcome.

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