Preparations are under way in Seattle this week to establish the filter cake ahead of the start of a programme of hyperbaric interventions behind the cutterhead of TBM Bertha.

The 17.48m diameter machine stopped mining on Friday (June 23) at 7am for the second of four planned maintenance stops along the 2.82km alignment. A total distance of 3,144ft (947m, 467 rings) has been excavated since launch in 2013. Bertha has now completed the steepest 4.6% downgrade section of the drive, and currently rests at a location under Spring Street in downtown Seattle at a depth of 120ft (36.5m) below the tunnel crown.

“We are now at the lowest point of the drive from a vertical alignment point of view,” STP Project Manager Chris Dixon told TunnelTalk from Seattle on Friday. “When we resume mining, following this maintenance stop, we will move to a 1.6% upgrade, but the topography of Seattle rises much faster than that so we still have some distance to go, maybe 2,000ft (601m), to get to the point of highest overburden, some 200ft (61m) below the surface.”

Prior to last week’s maintenance stop the TBM had been operating under a working face pressure of just over 3 bar, considerably less than the 7 bar design specification of the Hitachi Zosen-manufactured machine.

Since the drive out of Safe Haven 3 began on April 29, Bertha has advanced 1,516ft (462m, 234 rings), including a safe undercrossing of the Alaskan Way Viaduct during which time crews from Seattle Tunnel Partners moved to a 24/7 operation. Progress during May was recorded at 780ft (238m, 120 rings) over 21 work days for an average of 37.5ft/day (11.5m, 5.7 rings). During June, until the stoppage last week, 721ft was mined (220m, 111 rings) for an average progress of 45ft/day (13.7m, 6.9 rings). While the machine is advancing STP crews work 24 hours a day over two 12-hour shifts, Monday-Friday, with the machine stopping over weekends.

The best production rate since leaving Safe Haven 3 has been 10 rings installed (65ft, 20m) – on two occasions.

Hyperbaric work

“While we prepare the machine for the hyperbaric interventions we will be moving to two 10-hour shifts per day over six days of the week,” explained Dixon. This week STP crews have been bringing in compressors and preparing the filter cake ahead of those interventions, which are scheduled to begin after the July 4 holiday weekend.

This involves the removal of approximately one third of the excavated material that is currently trapped behind the cutterhead, to leave a working chamber with a height of about 20ft at the top of the machine. This space will then be injected, under pressure, with bentonite, which will penetrate the soil ahead of the cutterhead to create the impervious membrane that prevents soil and water from flowing back into the working chamber, and prevents the compressed air – which will be pumped into the chamber as the bentonite is removed – from escaping.

The cutterhead has eight spokes in total, each of which is fitted with cutting tools and scrapers that need to be checked for wear and replaced as necessary. The design of the machine allows for tools and bits that are located close to the openings through which excavated material passes into the screw conveyor – which lie towards the centre of the cutterhead – to be replaced in free air from inside the machine; but those set towards the outer half of the circumference of the face require hyperbaric intervention for inspection, cleaning and replacement.

“Some 6-9mm of wear has been recorded since leaving Safe Haven 3, but different tools have different parameters according to their location on the cutterhead before they need to be changed,” explained Dixon. “Some of the tools are fitted with wear detectors but these figures need to be physically verified by an actual inspection and the taking of precise measurements.”

During the last six-week maintenance stop at Safe Haven 3, during which 125 hours of hyperbaric interventions were carried out, 11 tools were replaced.

“Once the latest round of interventions start we have seven crews that we will be sending in to work in the chamber,” explained Dixon.

“Those crews comprise five people each – three STP workers and two employees of Ballard Diving, who are providing hyperbaric services for the project. That crew will go in and go through compression, enter the plenum behind the cutterhead, and then work in a compressed air environment before returning to decompress where they are held under observation for a few hours.

“That whole cycle takes between 6-8 hours and in that timeframe they can work behind the cutterhead for between one and one-and-a-half hours. With seven crews working in rotation we can get 7-10.5 hours of work done each day.”

Each spoke of the cutterhead will now be examined in turn, with divers from Ballard Diving carrying out the necessary cleaning, maintenance and tool replacement while hanging from platforms that are taken into the chamber and attached to the bulkhead behind the cutterhead. Once one spoke is completed the platforms are removed, the cutterhead is turned through approximately 45 degrees, and then work resumes on the next one until all eight are completed.

“Last time we stopped for six weeks, and the length of this stop will be dependent upon the amount of work that needs to be done under hyperbaric conditions,” said Dixon. “We won’t know the duration of this stop until we get in there and inspect the conditions but one spoke might need more work than another. It will take as long as it needs to take, but we are anticipating no longer than the six weeks we had at Safe Haven 3.”

Two more maintenance stops are scheduled for the balance of the tunnel drive, but the locations of these have not yet been determined and may, to some extent, depend on ground conditions. Currently the machine is located in almost a full face of clay, which offers optimal conditions in which to stop for maintenance. Once the TBM resumes mining it will enter a potentially more challenging section of alluvial material comprising a higher mix of loose sands and gravels.

Machine performance

Machine performance since replacement of the main bearing and the seal assembly is reported as good. “We’ve been monitoring seal gaps, temperatures, thrust forces and a variety of other things and all of those are within tolerance,” said Dixon. “We have extensive geotechnical instrumentation in the ground, including sensors 5ft (1.5m), 10ft (3m) and 30ft (9m) above the top of the TBM, and we have noticed negligible movement as tunneling has proceeded.”

Surface settlement point instrumentation mounted on a number of buildings along the drive path, and most of which is automated, has not recorded movement of any structures above the ground or any utilities below it.

“We review all the operating parameters on the TBM for each ring that was completed the day before; we go through the volume and weight reconciliation of the excavated material to verify that there is no overmining or overexcavation or ground loss occurring; before going on to sign a daily TBM parameter log,"said Dixon.

“This log specifies what parameters the TBM is going to operate at for that day; we specify what the total cutterhead thrust is going to be, what the torque is going to be, what the penetration rate will be; we go through the working pressures, we look at the grout pressures, and establish limits for those; also for the gaps we decide what the soil conditioning regime is going to be for that day, and what is going to be used – so if we are using foam, for example, we establish the foam expansion ratio and the injection ratio that is going to be used for that day.”

At the end of the daily meeting the document is signed off by STP’s Tunnel Supervisor, Construction Manager and Geotechnical Manager, and then on a shift-by-shift basis in the tunnel by the Tunnel QC Engineer, the TBM Operator, the Tunnel Foreman as well as the Geotechnical Supervisor.

“Their signatures basically acknowledges receipt of the document and how the TBM is going to be operated both during day shift and night shift,” explained Dixon. “We do have the ability to make some adjustments to the TBM parameters if conditions change but again there a protocol process that we go through if its deemed necessary to change any particular parameter during the shift.”

In addition to this, a team from machine manufacturer Hitachi Zosen is permanently on site and will remain with the project until final breakthrough. Their main responsibility is to operate the PLC (programmable logic controller) of the TBM.

Schedule

STP is currently working to prepare a revised schedule, which will be announced closer to the date that maintenance is completed. Dixon explained: “We have 962 rings to go, so if we average 5 rings/day that’s 9 months, 6/day would take 7.5 months, and 7/day, 6 months. This is just for the tunneling though, and is based on 22 working days/month.”

To this needs to be added the time for the current maintenance stop and the two that remain after that. “We are hoping this stop will be quicker than the last one, but even of we average 7 rings/day from here, and add two months for maintenance that would take us to February or March next year (2017).

“We are in the process of re-evaluating this formally but we probably won’t have a better idea until we see how long this stop actually takes.”