Temperature Correction and Strut Loads in Central Artery Excavations

Abstract

Braced excavations for the Central Artery/Tunnel project in Boston, Mass. include several instrumented sections for monitoring excavation and ground response to soil removal and strut installation. Strain in the struts is measured using thermally matched vibrating wire strain gauges (VWSGs). This paper presents issues related to strut load interpretation using VWSG data and the difficulties and errors associated with the use of field data. The paper presents a procedure to separate earth load from thermal load induced in the struts due to temperature variation throughout construction. The procedure is an extension of a method that uses measured temperature and corresponding load change to compute a thermal (temperature change induced) load coefficient. The procedure emphasizes the dependency of the thermal load coefficient on construction stage and support configuration. A numerical analysis is presented to support the proposed procedure. Strut loads and earth pressure diagrams are presented using the proposed procedure. Comparisons are made between measured loads and the design load envelope developed prior to construction.