New design TBM tames the Kuala Lumpur karst 14 Jan 2014
Shani Wallis and Peter Kenyon, TunnelTalk
The world's first Variable Density TBM (VDM) breakthrough is achieved in Malaysia for the Klang Valley MRT Project.
Contractor team celebrates breakthrough at Pasar Rakyat

Contractor team celebrates breakthrough at Pasar Rakyat

The TBM was the second of two Variable Density machines (VDMs) that started the tunnelling works on the project in May 2013. Its partner machines mined through the Pasar Rakyat Station zone on the lower drive of a stacked configuration for the twin running tunnels and is heading towards a breakthrough at the Inai interim shaft before Chinese New Year in February. A second pair of VDM machines are progressing in the opposite direction from the Cochrane launch box towards the Maluri Station and on to the transition portal to the surface works on the southern end of the city's first MRT line. A third pair of VDMs are to launch from the shafts at the intermediate Inai working site and drive stacked running tunnels towards the Bukit Bintang Station box.
The first two VDM machine have now completed of more than 2km of 5.8m i.d. running tunnel and without any incidents of either sinkholes or blowouts. This is a considerable achievement through the highly karstic nature of the Kuala Lumpur Limestone formation and a confirmation of the Variable Density concept and technology designed specifically to deal with the challenging geology. The machines can work in EPB mode and in a modified slurry mode with the screw conveyor in place and extracting the material from the excavation chamber when using both systems.

World's first VDM breakthrough

The Variable Density TBM concept is the joint development by Herrenknecht and MMC Gamuda, the JV contractor for the underground section of the MRT project. The same partnership learned considerably from the shared experience of the earlier dual purpose SMART tunnel in Kuala Lumpur also through the same kartic limestone formations. Conventional slurry Mixshields performed well for the SMART project but suffered many losses of the thinner slurry medium into the kasts as well as blowouts and pressure-loss sinkholes. Approximately 100 incidents were recorded during that project. Fortunately none caused any serious damage or collapse of private property, but they did cost time and caused operational challenges, and provided information for the development of the new Variable Density concept.
The breakthrough machine is one of six designed and jointly developed by the MRT Project's underground works contractor, MMC Gamuda, and German TBM manufacturer Herrenknecht, specifically for tunnelling through the challenging karstic limestone formations that are to be found along a significant part of the 9.5km underground section of the Sungai Buloh-Kajang Line.
Two Chinese-manufactured (by CRTE) and two more Herrenknecht EPBM machines are being used for tunnel excavation in the western part of the city where the more consistent Kenny Hill Formation is to be found. The first of these broke through just before Christmas.
Fig 1. Two distinct geological formations along underground section

Fig 1. Two distinct geological formations along underground section

VDM Cochrane 2 was launched from the Cochrane Launch Shaft in Cheras on 22 July 2013, completing a distance of approximately 1km – including excavating under the city's SMART tunnel- by breaking through the headwall at Pasar Rakyat Station last Thursday (January 9). The trouble-free drive marks the completion of the first section of the twin running tunnels between Cochrane and Pasar Rakyat Station.
Mass Rapid Transit Corporation Acting Chief Executive Officer Encik Haris Fadzilah Hasan said the breakthrough was a significant achievement for the construction of the MRT Sungai Buloh-Kajang Line. "Within seven months of Prime Minister Dato’ Sri Mohd Najib Tun Abdul Razak launching the first TBM, we have completed the first section of tunnels. I am confident that we will be able to complete constructing the entire length of tunnels by mid-2015 according to the programme," he said.
Encik Haris said it was critical to ensure that the line becomes operational by 31 December 2016 for Phase I (between Sungai Buloh and Semantan) and by 31 July 2017 for the rest of the line.
The latest breakthrough follows an earlier one which took place at the KL Sentral MRT Station worksite on 24 December 2013. That machine, Semantan 1, was a conventional EPBM, and it will be relaunched to excavate through to Pasar Seni MRT Station.
A total of ten TBMs will be used for the construction of the tunnels for the underground portion of the MRT Sungai Buloh-Kajang Line. With the completion of Cochrane 2, six TBMs are in the ground at various stages of tunnelling for the Project.
Variable Density TBM design
The multi-mode TBM with EPB and slurry-supported mode is the most complex form of a convertible machine currently in existence, and so its use is only cost effective in very special cases – like tackiling the specialised geological formations to be found in Kuala Lumpur.
Design of the VDM from Herrenknecht

Design of the VDM from Herrenknecht

The Variable Density TBM applies innovative technology by combining the advantages of both methods in one machine. Without mechanical modifications the machine can switch between four different tunnelling modes directly inside the tunnel. This means that geological and hydro-geological changes along the alignment can be managed flexibly.
The excavated material is removed from the pressurised excavation chamber through a screw conveyor both in the earth pressure and in the slurry mode. Depending on the mode used, the support pressure is controlled via the screw conveyor speed and advance rate or using slurry that is automatically controlled by an air cushion. The submerged wall opening is replaced by communicating pipes. In EPB mode the screw conveyor drops the excavated material onto a belt conveyor. An additional slurryfier box at the end of the screw conveyor makes it possible to drive the TBM with a hydraulic slurry circuit in slurry mode. In the latter case, either a normal bentonite suspension or a high-density suspension can be used. By introducing a small amount of slurry into the excavation chamber, the consistency of the material maintains high viscosity and high density levels, reducing the risk of slurry loss into karsts or blow outs to the surface. In slurry mode, the higher density material is extracted from the chamber by the screw conveyor and discharged into a slurry box where it is mixed into the circulation slurry for pumping out to the treatment plant on the surface. This makes the range of application of the VD-TBM even larger and makes the machine the all-rounder for loose soils of all kinds.
First breakthrough on Klang Valley MRT - TunnelTalk, December 2013
First TBMs launch on Klang Valley MRT - TunnelTalk, June 2013
Chinese TBM maker goes international - TunnelTalk, February 2013
CRTE of China takes over Wirth TBM business - TunnelTalk, November 2013
New-design TBM ready for Malaysia challenge - TunnelTalk, November 2012
MMC-Gamuda orders six TBMs from Herrenknecht - TunnelTalk, April 2012

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