Dear Sir
The excellent report on the discussion of the Nicoll Highway cut and cover excavation collapse at the ICE in the February 2007 edition of T&TI captured many of the key factors involved. An important matter, however, which got lost in translation was in reply to a question by Terry Crabb where it was reported that I replied that ‘this had been the only place where large movements had occurred’. The reply was, in fact, that this was NOT the only place where large movements had occurred and this had important implications on the attitude adopted by those involved at the time.
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Excavations in deep deposits of soft clay which exceed a ‘critical depth’ frequently result in large ground movements below the level of the excavation. In principle this is because the ‘active’ earth pressures below the excavation level exceed the ‘passive’ earth pressures and the retaining wall has to span from struts above the excavation level to firmer strata at depth. Elsewhere on Contract 824, ground movements of 200mm-400mm below the excavation level had been observed but it was still possible to complete the works without collapse.
Similar movements were observed at the Nicoll Highway section prior to collapse and it is possible that a ‘plastic hinge’ had developed in the wall. However, this in itself would not have resulted in the collapse. It was only when the strut-waler connection failed at the 9th level that total failure of the excavation was initiated. There was no early warning of this from the instrumentation since, taken at face value, strut load measurements showed the load in the struts was well within the capacity of the strutting system. It was not until the 11am reading of the strut loads on the day of the collapse, when a marked drop in load at the 9th level struts and an increase in load in the 8th level struts was observed, that the instrumentation indicated that something was wrong. This was about two hours after ‘sway’ had first been observed at the strut-waler connection at a strut located about 12m east of the instrumented section.
It is of interest to note that at an ‘all experts’ meeting it was agreed by all that nothing could have practically been done at 9.15am (when ‘sway’ of the strut-waler connection was first observed) to avert the collapse and, without knowledge that it would collapse so rapidly, it was reasonable to attempt to save the works.
Yours faithfully
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