Voids Open as a Result of Tunneling Under Seattle’s Beacon Hill


As boring proceeded, there were times when more soil than normal passed through the conveyors. Contractors attributed the spikes to a measurement glitch, but in hindsight they turned out to be sand flows, said (Transit Spokesman Bruce) Gray. Sound Transit monitors buildings on the hilltop for settlement, but none occurred, he said.

Interestingly enough, I found reference to these sands in an August 2006 article on the Beacon Hill tunnel at SeattlePi.com that I linked in a previous post. In retrospect, this sand formation may have been related to the additional $1 million geotechnical investigation that was needed by project geotechnical engineers, Shannon & Wilson back in the spring of 2007. Apparently the sand pockets, which were not encountered during the original design phase, were bad enough that they needed to shift the alignment by 88-ft. The additional geotechnical investigation and filling of the voids has cost approximately $1 million so far.