TAC BC Talk: Failures and Catastrophic Failures of Tunnels in Sand and Similar Soils
Event Details:
Date: November 3, 2026, 5:30 PM (Doors Open) 6:15 PM (Presentation)
Location:
Steamworks Brewpub – Uber Room
375 Water St., Vancouver, BC
Registration Cost:
$5 for members, $10 for non-members
Presented By:
Nick Shirlaw
Biography:
Nick Shirlaw worked for over 50 years in geotechnical and tunnel engineering, about half of that time employed by Golder Associates. Most of his career was spent in Singapore and Hong Kong. He worked in Ontario for 6 years, including the St Clair River tunnel and TTC expansion projects. In Singapore he was a founding director and office manager of Golder Associates (Singapore) Pte Ltd. For 7 years he helped teach the Underground Space course at the National University of Singapore. He now takes on occasional expert witness appointments and is a Member of the Academy of Experts. He has been a member of TAC for over 30 years.
Synopsis:
Unsupported sand exhibits a range of possible behaviours in a tunnel, depending on factors such as the water pressure, fines content and the nature of those fines. Possible classes of sand behaviour include flowing, running, cohesive running or ravelling. Other soils that exhibit comparable behaviours include gravels, combinations of silt, sand and/or gravel, uniform silt, and some saprolites (weathered rock). Model tests and numerical analysis show that a tunnel failure in sand initially results in a narrow chimney to the ground surface. However, major failures of tunnels in sand have resulted in massive sinkholes with thousands or tens of thousands of cubic metres of ground lost, extending over a wide area. Although the modelling shows the initial failure surface, in practice the behaviour after the initial failure can result in massive expansion of the initial failure. While failure at the face of a pressurized TBM can generally be sealed off quickly to prevent further ground loss, failures behind the TBM or with open face tunnelling methods are very difficult to seal. The initial failure can cascade into a much more widespread and damaging failure, including sequential failure of multiple tunnel lining rings. A study of 13 sequential lining failures shows where and why such failures can be triggered, and the range of ground conditions involved.
