Following previous issues relating to soil barge removal, and the more recent and serious development of a sinkhole some 100ft south of Bertha’s current cutterhead position on Seattle’s SR 99 Tunnel Project, client, WSDOT has notified Seattle Tunnel Partners (STP) that it must “suspend for cause” tunnelling operations involving the TBM and the loading of barges.STP must 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 tunnelling operations involving the tunnelling 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. For example, the Aurora Avenue North lane closures will occur as scheduled next week.
Seattle Tunnel Partners had resumed mining on Tuesday evening using trucks to remove excavated material while they continued working to resolve the issue with a soil-removal barge.
Approximately two hours later, a sinkhole developed within STP’s work zone near South Main Street, about 35 feet north of the access pit and 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 tunnelling. 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.
STP is reviewing their daily operations as a result of this incident. Immediately they will enhance 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 tunnelling and WSDOT has stated it is disappointed these were not used when STP restarted tunnelling in December 2015. STP has several hundred feet of mining before reaching 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.