What Happened to Big Bertha?

Anyone who is involved in Tunnelling industry would be aware of the name "Big Bertha" as it is the largest Tunnelling Boring Machine (TBM) built so far [1] (although there were plans to build even bigger TBM [2], they are held up due to various reasons [3]). There is so much of anticipation and excitement about its progress that, I wanted to share my two-cents on its current situation. This is just an attempt to summarise the prognosis that would have been carried out by a TBM engineer.

Workers walking into the Big Bertha Tunnel [WSDOT Flickr Page]

A little introduction about Big Bertha (for those who are not aware):

At 17.5m (57.5 feet) in diameter and around 100m (~300 feet) in length, Big Bertha is a Tunnel Boring Machine specially built for The Alaskan Way Viaduct replacement tunnel in Seattle, Washington, USA to replace a 60 year old Viaduct which is vulnerable to seismic events in the future. Big Bertha is named after the first female mayor of a major U.S city (Ms. Bertha Knight Landes). This $80 million machine was manufactured by Japanese company and the TBM started digging in July 2013 and as of January 29, 2014, Bertha had tunnelled around 312m (~11% of total tunnelling length - 2.8 km)[4]

What happened then?

In December 2013, the TBM was stopped due to an unexpected blockage with an 8 inch diameter steel pipe [4, 5]. After more than 158 hours of "Hyperbaric" interventions, the Tunnelling work resumed in January 2014, just to stop after few meters of Tunnelling because of clogged cutter head, high temperature readings and damaged seals!

Steel Pipe which caused the blockage in December 2014 [4]

Is this Unusual?

No & Yes! It's not uncommon to see damaged seals or bearings on TBMs. Contractor even had a budget of $5 million to have bearing on hand in case of a problem with the original. But fixing them isn't so simple and such huge repairs/replacements are never anticipated in the beginning stage of a TBM drive. Moreover, the cause and extent of damage to the seals remains a mystery (at least till they have opened the main bearing).

Bertha's Broken Seals [8]

Was the Choice of TBM Correct?

TBM selection is often the most crucial decision for any tunnelling job and probably the first aspect to be suspected in instances like this (my previous post discusses about various international guidelines and how to Select TBM). For Big Bertha, this aspect became controversial right from the beginning of the project. 

Geological Profile and Pressure Levels (maximum design face pressure is 7 bar) [6]

The tunnel has to pass through a heterogeneous mix of clays, silts, sands, gravels, cobbles and boulders along the geotechnical profile. EPB or Slurry Type could be used in such circumstances. The presence of sands, gravels, cobbles and boulders would have made, Slurry Type TBM as the ideal choice but EPB machine could also be tricked to suit such situation. The client (WSDOT) left the design choice with the contractor and the winning bid chose to use EPB Machine (which Big Bertha is). Some argue that it is impossible to use EPB machine for such soils and for such face pressure values. Conventional wisdom would predict that Slurry Type TBM only is suited for face pressure more than 6 bars.

Approximate suitability of TBM based on Grain Size

Along with many innovations applied for the first time to the EPB system, the manufacturer included the ability to change cutters at atmospheric pressure from within the arms of the cutterhead. This enables to replace disc cutters at atmospheric pressure itself.

Okay, its EPB Machine! So what? - Problems

• When an EPB Machine is being used in a geotechnical strata with cobbles and boulders, the disc cutters are expected to cut the rock pieces in to smaller fragments. However, in a TBM with openings as big as in the Big Bertha (minimum cutter head opening of 37%), it is expected to have a Secondary Crusher Mechanism [6, 7] which is missing in Big Bertha and this could lead to clog. Although ribbon screw type is envisaged to handle the presence of boulders during the excavation, the absence of crusher mechanism is being questioned by experts.

37% minimum opening ratio in Big Bertha [WSDOT Flickr Page]

• Handling high face pressure using EPB is questioned by experts. When the face pressure is more than 6 bars, achieving pressure distribution in the screw conveyor could be tricky and would depend on the consistency of the material churned.
• The EPB churns the soil which is relatively in a paste state and for a TBM of this size this would mean requirement of high torque to advance the TBM. This could result in a lot of heat generation and especially when the TBM is resuming its drive after a maintenance period, the torque required and heat generated will be very high. If not properly managed, this could damage the seals.
• Provisions of accessible cutter head made the cutterhead thick and heavy (~630 Tons). This means difficulty in handling during repair. Special cranes and special crane footings need to be designed to lift such cutter head in single piece.

Way Ahead

• It has been planned to create a ~40m deep and ~25m dia shaft (TBM is at ~20m below ground level), lower the ground water and excavate 20m below ground level and drive the TBM into the shaft so that the repair work can be carried out at atmospheric pressure. Piles shown on either side of the Tunnel alignment were installed before the Tunnelling started - to limit ground movement.

Access Shaft to Repair the TBM [4]

Following illustrations are taken from WSDOT website [4] which conceptually shows the proposed stages involved in the repair works.

Current situation above the ground [4]

Current situation below ground [4]

Proposed injection between the piles. Piles were installed before tunnelling started to limit settlements [4]

Identify and Divert the Utilities [4]

Proposed grouting behind the Machine [4]

Proposed construction of access pit (40m deep and 25m dia) [4]

Proposed installation of crane and dewatering system before digging the soil in access pit [4]

Proposed system for dismantling the cutter head and repairing the seals in main bearing [4]

References:

[1] Foley, Amanda (3 April 2013). "Big Bertha arrives in Seattle". Tunneling Journal. Retrieved 21 December 2013.

[2] World's largest tunnel boring machine (September 2011), World Highways. Retrieved 13 May 2014.

[3] World's largest TBM on Hold as Orlovsky tunnel cancelled (June 2012), Tunnels and Tunnelling International, Page no. 5 (link). Retrieved on 13 May 2014.

[4] Tracking Bertha, the SR 99 tunneling machine, Washington State Department of Transportation. Retrieved on 15 May 2014.

[5] What's blocking Bertha, The Seattle Times. Retrieved on 16 May 2014.

[6] Technical parameters of Seattle's mega EPBM, Tunnel Talk (Dec 2012). Retrieved on 16 May 2014.

[7] Why is Bertha Stuck, Crosscut Seattle (January 2014). Retrieved on 16 May 2014.

[8] Bertha’s big troubles started in Japan, The Seattle Times, (February 2014). Retrieved on 16 May 2014.