Managing Geotechnical Risks in Tight Spaces - Lessons Learned-PPT Presentation

PPT Presentation

Pdf Summary

The presentation examines geotechnical risks on constrained urban sites, where excavation and support-of-excavation (SOE) work can affect neighboring buildings, streets, and underground infrastructure. It emphasizes that models cannot capture every source of movement, particularly impacts from enabling work and slurry-wall installation.<br /><br />In a Boston project involving a 34-story tower on a small site with sensitive neighbors, building movement occurred before mass excavation. The team reassessed the risks, increased bracing preloads, adopted an observational approach, and modified the excavation sequence with subgrade struts. Monitoring and tailored mitigation helped the project proceed successfully.<br /><br />A second case showed how an abutting building’s existing sheeting conflicted with the planned SOE system. The resulting redesign increased costs, delayed the schedule, and reduced below-grade buildable space. The presentation highlights the value of identifying property-line and constructability conflicts early.<br /><br />A third project demonstrated the risks of overly prescriptive specifications for a contractor-designed SOE system. Required sheeting dimensions, embedment, bracing, and soil assumptions did not prevent movement caused by obstructions, vibration during sheeting installation, and water pressure behind the wall. Monitoring detected impacts, and additional bracing, soil berms, and staged excavation helped mitigate them.<br /><br />Key lessons include assessing both primary construction and enabling work; recognizing uncertainty in predictions; monitoring adjacent structures and infrastructure; establishing reasonable lateral and vertical movement criteria; and keeping designs adaptable. Early, ongoing communication among owners, designers, contractors, and other stakeholders is essential. Engineers must balance specification requirements against construction risk, and contractors should carefully evaluate what they agree to build.

Keywords

geotechnical risks

urban excavation

support of excavation

adjacent building movement

slurry wall installation

Boston tower project

observational approach

bracing preloads

constructability conflicts

monitoring and mitigation