MH Goldsworthy, MA, MSc, DIC, CEng, MICE. This paper was first published in GE’s October 1986 edition.
A sheet pile cofferdam has been excavated to over 20m below ground level in order to reinstate a collapsed canal tunnel. The ground at the site is firm clayey till becoming stiff with depth, with horizons of stiff lacustrine clay.
During excavation strut loads were monitored using vibrating wire gauges and displacements of the ground adjacent to the sheet piles were observed using an inclinometer.
The loads developed in the lower supports during the later stages of the excavation were considerably less than those which would be calculated using the traditional approaches. It is suggested that consideration of the horizontal stress field existing prior to excavation leads to an explanation of the bottom strut observations.