The Washington State Department of Transportation (WSDOT) said a sinkhole developed within contractor Seattle Tunnel Partners’ work zone near South Main Street, about 35 feet north of the access pit, at approximately 9pm Tuesday night. It is located more than 100 feet south of the cutterhead’s current location, in ground that crews mined through last week. STP filled the sinkhole overnight with 250 cubic yards of concrete.
"This section of the tunnel drive is protected by an underground wall built by STP before tunneling." WSDOT said. "The wall 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 surveying and monitoring the ground, viaduct, utilities and other structures. The cause of the sinkhole is still under investigation. STP is analyzing the portion of the tunnel that crews have excavated since mining resumed. There is no indication that any other locations have experienced ground loss.
The protocols STP outlined to enhance monitoring were used in the first 1,000ft of tunneling and WSDOT said it is "disappointed they were not used when STP restarted tunnelling 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.
In a separate incident, on Tuesday, January 12, STP’s barge filling operation resulted in a barge listing beyond STP’s control. As a result the barge was either let go or broke free from the pier, spilling tunnel spoils into Elliott Bay, and drifted out of control damaging both Terminal 46 and Pier 48.
STP resumed tunnelling that same evening, using trucks to remove excavated material while they continued working to resolve the barge issues. The contractor’s inspection of pilings at Terminal 46 is ongoing. The terminal is structurally sound, but crews noted some damage to pilings. STP is preparing to install fenders along the pier that will allow them to continue safely barging from this location.
Work to move material from the damaged barge to a second barge is expected to be completed this week. Once complete, STP will be able to determine the amount of clean material that spilled into Elliott Bay. Repairs will be needed before the damaged barge can be used again in the tunnelling operation.
In light of both incidents this week, WSDOT issued a letter to the contractor on Thursday stating the suspension and requiring the contractor complete a detailed analysis and modify tunnelling operations to ensure appropriate ground control. "STP will not be allowed to resume tunnelling until their analysis and work plans meet the satisfaction of the design-build contract and WSDOT’s experts."
The tunnel contract contains a few different mechanisms for stopping work. In this case, WSDOT suspended work "for cause" per section 14.2 of the contract, which says the state can suspend work without liability to WSDOT under a number of conditions, including the contractor’s failure to "correct conditions unsafe for the project personnel or general public."
This suspension for cause only addresses tunneling operations involving the tunneling machine, including loading of barges at the site. It does not affect any of the other work under the design-build contract with STP or in any other contracts being managed by the viaduct program.
