Construction of the three-level be low-grade space for a new 61-story tower in Boston required excavation to depths of up to 14.3 m (47 ft) and up to 7.6 m (25 ft) below the foundations of an adjacent structure. The excavation removed the stiff clay crust that provides increased lateral wall support and resistance to basal instability. Historically, excavating to this depth for building construction in the Back Bay area of Boston was avoided due to excessive ground movements that tend to occur when the stiff clay crust is removed. To address this problem, concrete lateral support elements (subgrade elements) were installed below the bottom of the excavation prior to mass excavation. During excavation, the subgrade elements limited wall movements, controlled basal heave, and prevented excessive settlement of the adjacent structure. The successful implementation of the subgrade elements was the result of a collaborative effort between the design team, contractor, and the contractor's geotechnical engineer.
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