Bertha drive forced to another halt 13 Jan 2016

The trouble-hit Alaskan Way viaduct replacement tunnel drive in Seattle is suffering a series of new problems just days after newly-repaired TBM Bertha broke out of the recovery shaft in early January.

At 9pm local time on Tuesday evening (January 12) a sinkhole measuring 35ft x 15ft opened up 100ft south of the current location of the cutterhead in ground that the TBM mined through during the previous week. The machine is being operated by project contractor Seattle Tunnel Partners (STP).

The sinkhole is located just yards away from the foundations of a section of the Alaskan Way viaduct that is still currently in use, although the TBM is in a drive section protected by an underground wall that was constructed to isolate ground loss away from the viaduct. The sinkhole has since been backfilled with 250yd³ of concrete.

Project owner WSDOT expressed its disappointment concerning the circumstances behind the latest problem, promising that “STP’s operational protocols will undergo an additional review by an expert to assure public safety”. The machine is heading to Safe Haven 3 where it will undergo a programme of maintenance and mechanical review.

A WSDOT spokeswoman said: “The protocols STP outlined to enhance monitoring were used in the first 1,000ft of tunnelling, and WSDOT is disappointed they were not used when STP restarted tunneling in December 2015.”

She added: “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 its daily operations as a result of this incident and will immediately enhance monitoring protocols by requiring crews to manually verify the amount of soil removed during excavation of each ring.”

STP is scheduled to mine under the viaduct itself, and towards Downtown Seattle, in March. WSDOT has confirmed it will close the elevated structure to all traffic for a period of two weeks during advance of the TBM beneath it.

Since backfilling of the recovery shaft in December, Bertha has advanced another 190ft and installed 30 more 6ft wide rings of segmental lining, bringing the total distance mined to 1,280ft and installation of 188 lining rings. The machine is now 220ft short of the planned stop at Safe Haven 3.

The latest problem, which has resulted in another shutdown of mining operations, follows on from an incident earlier in the same day during which a barge that was being loaded with tunnel spoil started to list and had to be cut free from Pier 46 to prevent damage to Bertha’s conveyor system. An unspecified volume of material described as ‘clean’ spilled into Elliott Bay as a result of the incident.

Tunneling activities were halted as marine surveyors inspected damage to Pier 46, and as a barge-mounted clamshell was engaged to remove spoil from the listing vessel and on to another barge.

The survey found that Pier 46 was structurally sound but noted some damage to pilings. The WSDOT spokeswoman said that repairs would be needed before the damaged barge could be used again in the tunneling operation, and that as a temporary measure, STP plans to resume tunnelling using trucks to haul away excavated material.

TunnelTalk also understands that additional problems have arisen with the conveyor mucking out system as a result of the high volume of water that is being used to condition the soil, reduce torque during rotation of the huge 17.48m cutterhead, and to maintain effective EPB operation of the excavation system. This has reportedly resulted in a much sloppier consistency of the excavated material than is considered ideal, and has caused spillage of material from the continuous conveyor system along the tunnel, through the working shaft and overhead to the barge discharge.