Different scenarios are emerging to explain the order of a world record 17.6m diameter TBM by the Dragages/Bouygues joint venture for its design-build contract to excavate the 4.2km long undersea Tuen Mun - Chek Lap Kok Link (TM-CLK) highway project in Hong Kong.

Hong Kong TM-CLK diagram published by Dragages in March 2014

As reported last week by TunnelTalk (18 September), a mega-TBM is on order from Herrenknecht, but it remained unclear how the machine was to be applied or how one large machine changed the original scope for two TBMs of more than 14m for the twin tube crossing.

With Herrenknecht restricted from commenting further by its TBM supply contract, and a reluctance by Dragages/Bouygues to discuss the project details further, TunnelTalk has learned of different scenarios from different sources, including engineers met at the BTS tunnelling conference in London earlier this week.

One source said that concern about excavation under 5-bar pressure of up to 42 cross passages (one every 100m), and at up to 50m below sea level, had forced a project rethink to convert the twin tube tunnel to a single larger diameter bore to accommodate two decks of two lane traffic on each deck. This would eliminate the need for excavating the cross passages and include the refuge and emergency escape passages within the interior of the double-deck larger diameter tunnel.

In addition to the larger 17.6m diameter machine, it is understood that a second, smaller diameter TBM is on order with Herrenknecht for the project, and for this a different project scenario was explained. The information was that the larger diameter machine would be used at the start of one of the tunnel drives, if not both, to provide sufficient cross section space within the exits of traffic for the additional ventilation and smoke controls needed for the up-gradient flow of highway traffic that is predicted to be high density and to include a high percentage of heavy good vehicles.

Tuen Mun – Chek Lap Kok Link, Hong Kong

Excavating the exit ramps with a larger diameter TBM reduces the depth of the open-cut approaches to the bored tunnel sections and reduces the amount of land reclamation required at each portal area, particularly at the TBM operation and launch area on the northern end at Tuen Mun. The explanation then was that a smaller 15m diameter TBM would continue the drives under the sea using the same backup as the large diameter TBM and much of the same internal systems, and complete the undersea drives to provide for an increased three traffic lanes in each tube.

In the original design the deep-level cross passages were to be constructed using ground freezing support for each and with watertight emergency doors installed prior to break-out through the segmental lining of the bored tunnels.

In a technical presentation at the BTS conference in London, Chris Fesq of Buoygues explained the challenges of opening up the five cross passages for the company’s Miami Port highway project. It may be no coincidence that Buoygues is using this experience to reevaluate the challenges presented by the TM-CLK Link in Hong Kong.

A new mega-TBM, larger than the current world record Hitachi Zosen machine currently undergoing repair in Seattle, is on order by the Dragages-Bouygues joint venture for excavation of the 4.2km long twin tube Tuen Mun - Chek Lap Kok Link in Hong Kong.

Alignment of undersea highway tunnel to the airport

It was revealed exclusively to TunnelTalk that a 17.6m diameter Mixshield is currently being manufactured for the job. According to Herrenknecht the machine will be ready for operation at some point during 2015.

In September last year (2013), Engineering Manager Stephane Polycarpe of the Dragages Hong Kong-Bouygues Travaux Publics joint venture told TunnelTalk that the new undersea highway link is to be excavated with two TBMs of about 14m diameter. Polycarpe confirmed that the JV had consulted NFM, Herrenknecht and Hitachi. Herrenknecht and Bouygues did not want to provide any further details on the TBM order, or explain the upgrade to a 17.6m diameter shield. It is also not clear if a second TBM is currently on order or in manufacture.

TunnelTalk will follow the project developments closely and provide an update as soon as more information becomes available.

Aerial view of Hong Kong and its new undersea and fixed ocean link highway connections

The JV started construction on its awarded $1.5 billion contract in August 2013. The Tuen Mun - Chek Lap Kok Link (TM-CLKL) is a 4.2km twin-tube subsea tunnel with a minimum two traffic lanes in each tube. With the Herreknecht TBM operational during 2015, the civil works of the project, including cut-and-cover approach tunnels of 530m and 670m at either end, are scheduled for delivery by the end of 2018.

In 2013, Polycarpe told TunnelTalk that the TBM systems for the project will be equipped with brand new technology developed in-house by the Bouygues Construction Research & Development department. Mobydic, a system of sensors incorporated in the disc cutters, will make it possible to continuously monitor their state of wear while allowing real-time geological mapping of the rock face. Snake, which is a new remote-controlled exploration arm equipped with a high-pressure jet, will clean the cutterhead and eliminate clogging to facilitate inspection. In addition to Snake, the excavation chamber will be equipped with a video system to provide real-time viewing of the works and conditions during man entry interventions.

The complex geology is one reason why this project requires the implementation of these latest developments in TBM technology. The tunnel will pass through all the Hong Kong geological layers, from harder fresh granite to softer weakly consolidated marine deposits, and from more permeable alluvial gravel to more impermeable clay meta sediments.

News of the new mega-TBM order brings the list of super-sized TBMs used in the world to date, those of 14m diameter and greater, to more than 30.

The new machine will be marginally larger in diameter than the current world record 17.5m Hitachi Zosen EPBM that is working on the Alaskan Way viaduct bored tunnel replacement project in Seattle and which is currently undergoing recovery for repair of failed bearing seals.