Bertha, the huge tunneling machine currently working on Seattle's SR 99 project, stopped again Tuesday as a 35-foot wide sinkhole opened up near South Main Street, about 35 feet north of the access pit. Chris Dixon, project manager for Seattle Tunnel Partners, said the 15-foot deep sinkhole was "an unfortunate event, but isn't something that is unique or something that wasn't anticipated." after explaining that walls built in the ground to isolate the tunnel from the viaduct during the shallow portion of the dig protected the viaduct from any issues in the tunnel.
The cause of the sinkhole is still under investigation. This section of the tunnel drive is protected by an underground wall built by Seattle Tunnel Partners (STP) before tunneling and was designed to isolate ground movement and protect the nearby Alaskan Way Viaduct. A manual survey of the viaduct conducted after the sinkhole developed found no movement. WSDOT and STP will continue monitoring the ground, viaduct, utilities and other structures. STP is reviewing their daily operations as a result of this incident and are immediately enhancing monitoring protocols by requiring crews to manually verify the amount of soil removed during excavation of each ring.
The protocols STP outlined to enhance monitoring were used in the first 1,000 feet of tunneling and WSDOT is disappointed they were not used when STP restarted tunneling in December 2015. STP has several hundred feet of mining before they reach the next planned maintenance stop. Before leaving the maintenance stop, STP’s operational protocols will undergo an additional review by an expert to assure public safety.
Watch live video of Bertha and the SR 99 Alaskan Way Viaduct project at WSDOT.wa.gov.