All tunnels are not created (or located geologically) equally. Here's the 3 high risk factors for Seattle Bertha, from: https://www.bizjournals.com/seattle/new

All tunnels are not created (or located geologically) equally. Here's the 3 high risk factors for Seattle Bertha, from: https://www.bizjournals.com/seattle/news/2014/12/12/mcginn-era-tunnel-study-proving-true-bertha.html https://www.bizjournals.com/seattle/news/2014/12/12/mcginn-e... Technically, he said the machine's diameter, as the largest in the world, warranted a "high" level of risk, a five out of five. Also rating a high level of risk – a five – was the underground material. Neff wrote what he called the "geologic setting" was "Highly complex, sticky clays, flowing sands, abrasive materials, large boulders." And third, the high water pressure at the tunnel depths, which seems to be proving a problem now, rated "above average" risk at a four. ---------------------- This is not to try and take something away from TBC - they may be making very good choices (2 smaller bore tunnels than 1 big one? Better locations?) who knows, but it's just not apples to apples.