Engineers Australia

The performance of steel struts in a braced excavation in Perth

Natusha L. Zaremba and Barry M. Lehane

Abstract

A deep and wide braced excavation constructed for the new Esplanade Station in the Perth CBD provided a useful opportunity to assess appropriate parameters for retaining wall design by backanalysis of its performance. The section of the wall under consideration involved a staged excavation adjacent to sheet piles supported by three levels of props at the final excavation depth of ~13 m. Two adjacent sheet piles were strain gauged to allow assessment of the wall’s bending moment, while the lateral soil movements adjacent to the wall were monitored using inclinometers. The struts were also instrumented with strain gauges and the output from these gauges provides the main focus of this paper. The effects of temperature on the inferred strut loads are examined and it is shown that the data recorded during excavation pause periods could only be explained if the operational axial strut stiffness was about a quarter of its theoretical stiffness. Finite Element backanalyses of the measurements obtained during the excavation phases also show that a best fit between measured and predicted strut loads is achieved for a similar operational stiffness. It is concluded that for large braced excavations of this nature, imperfections/curvatures in long struts lead to strut loads that are significantly lower than expected.