TBMs today are capable of excavating in a startling array of conditions. They are driven by state-of-the-art computers and can be equipped with monitors and data acquisition programs to track nearly every tunneling parameter.
As a result, tunneling today is certainly much safer and more precise than in decades past, and it is getting more efficient all the time. But there is room for improvement.
We would venture a guess that a large percentage of incidents in tunnel excavation projects that result in significant project downtime - whether that means a stuck TBM, a sinkhole, or other occurrences - are rarely due to the TBM or equipment operations at all. Human error has been, and continues to be, the greatest source of in-tunnel incidents in our industry.