Drop shaft

Melas said that district engineers had located Deep Tunnel dropshafts that were near a portion of the freight tunnels and that they would drill, cut or blast through to the flooded areas "as soon as possible."

The water flows from a large suitably fed tank into the dropshaft through a sharp-edged orifice, whose diameter (Do) is 100mm large.

The downpour led to some dramatic images of stranded motorists and geysers bursting through manhole covers. The spouts are a rare effect which occurs when too much water rushes into a dropshaft and traps air that’s normally vented.

The gigantic drill that began digging the deep-bore tunnel from Ballard to Wallingford in 2021 passed its halfway mark last month, chewing through soil that gets hauled away to a Ballard drop shaft (a vertical hole that reaches down to the tunneling site from street level) in 25-ton train carloads.