The drive to improve public transport networks around the world has resulted in a rise in the number of tunnelled metro systems under construction.
While tunnelling may be the ideal solution to reduce congestion at ground level, the geological conditions through which tunnels are now being bored is creating some major challenges.
The Malaysian city of Kuala Lumpur is currently extending its mass rapid transit (MRT) system but the karstic limestone ground is riddled with fissures and crevices that present a real challenge for tunnel builders.
This challenge called for development of new tunnelling technology that has just proved its worth after reaching breakthrough.
Tunnel boring machine (TBM) manufacturer Herrenknecht called the achievement a major breakthrough for Kuala Lumpur’s metro and for mechanised tunnelling in difficult soft ground.
The 7M inhabitants of Kuala Lumpur have a huge traffic problem, which has created the need to expand the metro system. At peak hours, traffic in the Malaysian capital can be brought to a standstill for hours and the expansion of the metro is seen as essential to relieve this congestion.
Under the name of the Klang Valley MRT Project, three new metro lines are planned with sections above and below ground.
Estimated investment in the three new MRT routes
The system will use a “wheel and spoke” concept comprising two northeast to southwest radial lines and one circle line looping around the city in order to maximise the benefits and access to the system. Initial cost estimates suggested the network represented an RM36bn (€7.7bn) investment, making it the largest infrastructure project ever undertaken in Malaysia.
Construction of the metro began in 2011 with the 51km-long Sungai Buloh-Kajang Line, which is also known as the Blue Line. It extends from Sungai Buloh in the north to Kajang in the south of Kuala Lumpur and will have 31 stations along its length. While the majority of the route is above ground, the city centre is crossed via a 9.5km tunnel in the form of two single-track tunnels.
Work on the Klang Valley MRT Project’s Blue Line is being undertaken by the MMC-Gamuda joint venture and the contractor called on the help of Herrenknecht to develop a new type of TBM to tackle the ground conditions on the underground section of the route.
The heterogeneous geology of Kuala Lumpur, in particular a karst limestone formation with caverns connected by a network of crevices and fissures, was known to be a major challenge at the outset. Groundwater levels along the route of the tunnel were also very high and create a highly diverse subsoil.
Use of a liquid support medium ahead of a TBM in heterogeneous ground is risky as the support medium can easily – and suddenly – be lost through the cracks. This not only creates a dangerous situation for the construction team but it can also result in subsidence at ground level around the tunnel route.
Herrenknecht and MMC-Gamuda had already gained extensive experience of the ground conditions from their cooperation for Kuala Lumpur’s combined stormwaterand road tunnel Smart tunnel. The tunnel for the Smart scheme was excavated between 2004 to 2007 using two large Herrenknecht mixshield TBMs and, although the drive was very successful, the geology revealed its challenges during the course of the work.
Tunnelled section on the Blue Line
Around 90% of the Smart tunnel route ran through karstified Kuala Lumpur limestone with high levels of groundwater. Fissures and crevices sometimes resulted in losses of support medium into the ground. While the risks of subsidence could be accepted on the route of the Smart scheme, the Klang Valley project tunnel would take the scheme below a densely populated, built-up inner city area. The risk of subsidence needed to be avoided on this second collaboration between Herrenknecht and MMC-Gamuda.
The result was a variable density TBM that can be operated in several different modes in order to adapt to changing ground conditions and minimise the risk of ground movement at surface.
The variable density TBM developed by Herrenknecht with engineers from MMC-Gamuda is an advancement of the multimode TBM, which combines the advantages of the earth pressure balance (EPB) and slurry-supported (mixshield) modes.
The variable density TBM can prevent the loss or leaking of bentonite by increasing the density of the support medium. As a result, the machine offers four different tunnelling modes, which Herrenknecht believes is a world first.
In total Herrenknecht has supplied six variable density TBMs and two EPB TBMs to MMC-Gamuda for the Klang Valley project.
The innovation claimed international recognition when it was awarded theTechnical Innovation of the Year Award in 2014 at the International Tunnelling and Underground Space Awards in London.
Nonetheless, the development has now been tried and tested as the innovative TBMs have successfully completed the 9.5km tunnel just under two years after the first was launched.
Klang Valley MRT project client Mass Rapid Transit Corporation (MRT Corp) celebrated the completion of tunnelling for the first line on the project.
Star of the event was the Herrenknecht tunnel boring machine S-774 which was the world’s first variable density TBM. The TBM was launched in 2013 in the presence of Malaysian prime minister Datuk Seri Najib Tun Razak and the first drive was only 1km in length but took the machine through extremely karstified Kuala Lumpur limestone.
The machine started the advance in theusual slurry mode, and then switched to the new mode with a denser support fluid after the first 100m. In this mode it successfully completed the planned tunnel section from the launch shaft at Cochraneto the Pasar Rakyat station.
Challenges encountered during sinking a shaft at the station meant that the variable density TBM planned for the following section could not be used. Instead S-774 continued on past its target station and also completed the next 3km section. During this additional tunnel drive, the machine moved out of the limestone area and into the soft ground of the Kenny Hill Formation.
From here it bored its way through the Bukit Bintang station to the Pudu shaft in EPB mode. There it continued tunnelling in place of the planned EPB machine, crossed through the Merdeka station and finally reached the Pasar Seni station.
In total the machine drove 4.4km of tunnel through greatly varying geologies by switching between tunnelling modes.
“This is truly a milestone for the MRT project, our engineers have completed the tunnelling work successfully and smoothly,” says MRT Corp chief executive officer Dato´ Sri Shahril Mokhtar, speaking following the breakthrough. “At the same time the use of a new technology has proved itself.”
There are three types of ground conditions in Kuala Lumpur, hard granite rock, a softer mixed geology known as Kenny Hill Formation, and limestone areas. The new metro line runs through the last two and it is the combination – and the variations at the changes in ground conditions – that led to the need for the new type of TBM.
The variable density TBM solution was combined with development of a denser supporting fluid to reduce penetration depth and capillary rise in comparison to the previously used support fluids in a bid to prevent uncontrolled losses of face support.
The change from one mode to the other on the TBM takes place smoothly while fully maintaining face support and major modifications are not needed during the switch.
In all modes the excavated material is removed from the pressurised excavation chamber by a screw conveyor. In EPB mode the screw conveyor speed and the advance rate control the face support pressure.
The screw conveyor drops the excavated material onto a belt conveyor. In slurry mode the face support pressure is automatically controlled by an air cushion. Communicating pipes connect the front part of the excavation chamber, the pressure chamber, with the rear part, the working chamber. For the hydraulic slurry circuit a slurryfier box is used at the end of the screw conveyor.
In addition, in slurry mode it is possible to vary the density of the support medium in the excavation chamber. In cooperation with the Ruhr University Bochum, Herrenknecht developed a mix that is thickened with limestone dust.
The choice between the more fluid low density support medium (LDSM) and the denser high density support medium (HDSM) considerably increases the areas where a TBM can be used safely.
The high density mode closes the gap between mixshield and earth pressure balance operation. “With the experience from the Klang Valley project and with Herrenknecht’s expertise, the new variable density technology has opened a door that will allow us to tackle even more difficult geologies,” says Herrenknecht founder and chairman of the management board Martin Herrenknecht.
“The variable density TBM concept can be used not only in extreme karst, but also in other soils where only a few years ago mechanised tunnelling was still unthinkable,” he adds.
Malaysia’s Mass Rapid Transit extension – site plan
Even before the successful breakthrough on the tunnelled section of the Sungai Buloh-Kajang Line, the Malaysian government had already decided to push ahead on the second 59.5km-long Sungai Buloh-Serdang-Putrajaya, which will feature 36 stations. In July, MMC-Gamuda was appointed to deliver the construction for the new metro route and will be once again using the variable density technology developed for the first phase of the scheme.
The Blue Line is scheduled to open in 2017, while the second line is expected to be completed by 2021. The third line – the circle line – will connect the other two systems with existing metro and tram systems in Kuala Lumpur but no date has been set for the start of the work or the opening of the route.
For more information, visit www.mymrt.com.my
Klang Valley TBM fact file
The variable density tunnel boring machine:
● Diameter: 6,620mm
● Cutterhead power: 1,280kW
● Torque: 4,239kNm
● Four tunnelling modes:
Mode 1: EPB closed
Mode 2: EPB closed with additional bentonite support
Mode 3: Mixshield with low density support
Mode 4: Mixshield with high density support medium
Earth pressure balance tunnel boring machine:
● Diameter: 6,620mm
● Cutterhead power: 1,280kW
● Torque: 4,239kNm
● One tunnelling mode