Instrumentation of Underpinning Piles in a 94-ft Deep Excavation

Abstract

Deep foundation construction in urban settings poses the unique challenge of working with deep excavations in a relatively confined area while preserving the integrity of nearby structures. A 94-ft-deep vertical excavation for a 31-story residential tower adjacent to an existing building in downtown Seattle, Washington, presented an opportunity to monitor shoring system performance in real time. Additionally, the analysis of monitoring data allowed for validation of the original design parameters. The shoring system consisted of a soldier pile and tieback walls. Underpinning piles were designed to support the adjacent building while minimizing deflections. The shoring system was designed to limit total lateral deflection adjacent to the existing structure to less than 1/2-inch to minimize the potential for damage. An instrumentation program was designed to monitor loads and deflections in the shoring system. The instrumentation consisted of strain gages, load cells, vertical inclinometers, and optical survey monitoring. Where the adjacent building prevented the use of an inclinometer, two underpinning piles were instrumented with vibrating wire strain gage pairs. Strain gage pairs were installed to: 1) monitor total axial load, 2) monitor the axial load in pile versus depth, and 3) monitor combined axial loading and bending in the pile. Load cells were installed on tiebacks on instrumented underpinning piles to monitor changes in the load during the excavation. The underpinning pile data was monitored remotely. This paper provides a discussion of the design and construction of the shoring system and an analysis of monitoring data collected from the instrumentation system.