Modeling to Monitoring: Evaluating Adjacent Structures Feet from a Boston Excavation-Paper

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The paper describes a deep excavation for a 30-story Boston tower, with excavation depths of approximately 27–43 feet beside a transit tunnel and two sensitive masonry buildings. The project used a 3–4-foot-thick reinforced-concrete diaphragm (slurry) wall, internal bracing, and other site-specific support systems. PLAXIS 2D models of 20 representative sections predicted soil, wall, and adjacent-structure movements during staged construction.<br /><br />Monitoring began before construction and tracked settlement, lateral movement, tilt, cracking, and vibration. The models did not include enabling work or slurry-wall installation. During these early activities—before mass excavation—the theater and brick building moved more than anticipated, with the brick building recording up to 0.7 inches of settlement and 1.4 inches of horizontal movement by cap-beam completion. The team responded by increasing brace preloads and preparing additional measures.<br /><br />During mass excavation, adjacent structures experienced little further movement. As excavation reached final subgrade, the team addressed concerns about wall creep and a delay before the lowest slab was installed by modifying the excavation sequence, placing slabs in smaller sections, and installing concrete subgrade struts. At the end of below-grade construction, measured wall deflections were generally below PLAXIS predictions, and measured structure movements remained within project tolerances.<br /><br />The case study highlights that excavation models are estimates and may omit important movements caused by enabling works, wall installation, and construction methods. It emphasizes the value of comprehensive monitoring, early stakeholder coordination, and flexible, observation-led mitigation in protecting neighboring structures during complex urban excavations.

Keywords

deep excavation

Boston tower

diaphragm wall

slurry wall

PLAXIS 2D

settlement monitoring

lateral movement

internal bracing

adjacent structures

urban excavation