2017 | Quarter 3

The Geological Condition of Klang Valley Makes Tunneling Beneath Kuala Lumpur Extremely Challenging

The MRT Sungai Buloh-Kajang Line's underground alignment cuts through two different geological formations, namely the Kenny Hill Formation which consists sedimentary rocks such as mudstone, shale, phylite and sandstone, and the Kuala Lumpur Limestone Formation with erratic Karstic features comprising eroded limestone rock beneath a layer of top soil.

The transformation zone between these two geological conditions is at Bukit Bintang, with the Kenny Hill Formation lying to the west of this zone while the Kuala Lumpur Limestone Formation lying to the east.

Because of the erratic Karstic feature, the Kuala Lumpur Limestone formation is extremely challenging to tunnel through. Erosion of the limestone mass has created deep cavities and underground caverns, which, if not discovered beforehand, can pose considerable danger when tunneling activities are carried out.

To overcome this, meticulous soil investigation work had to be done to gather as much information as possible about the soil condition of the tunnel path. On top of that, the right type of TBM had to be deployed for the right formation. Specifically for the Kuala Lumpur Limestone Formation, a special type of tunnel boring machine was developed to overcome the challenges.

Tunnel Boring Machines

There were two types of TBMs used to excavate the tunnel for the MRT Sungai Buloh-Kajang Line. For the alignment going through the Kenny Hill Formation, Earth Pressure Balance (EPB) TBMs were used while Variable Density TBMs, which was specially developed jointly by world renowned TBM manufacturer Herrenknecht AG of Germany and MMC Gamuda joint venture, were used to excavate in the Kuala Lumpur Limestone Formation portion of the alignment.

Earth Pressure Balance TBM

The EPB TBM used excavated material to provide support for the drill face, thereby preventing soft ground from falling in during the tunneling process. Nozzles at the front of the drill head inject foam into the soil before the drill face cuts into it, and turns the excavated earth into a more liquid and malleable substance.

The substance is stored behind the drill face and applies an outward pressure equivalent to the combined inward pressure exerted by external forces, preserving what engineers call "face control". An operator monitors the pressure and releases excess excavated earth to the surface using a corkscrew conveyor to maintain the pressure between the TBM face and the external terrain.

Variable Density TBM

The high groundwater table and erratic Karstic feature renders EPB TBMs were not suitable to tunnel in Kuala Lumpur Limestone Formation conditions. Slurry Shied TBMs are usually used for such conditions but lessons learnt from the SMART Project also revealed that the standard Slurry Shied TBMs were not able to prevent incidences of sinkhole formation and blowouts.

The Karstic environment means that the standard Slurry Shield TBM frequently encountered chambers or fissures into which the slurry will escape resulting in high volume and pressure loss. If the fissures lead to the surface, the slurry will escape to the surface, resulting in a blowout. On the other hand, if the slurry escapes into an underground cavity, it will result in a loss of face pressure, thereby creating sinkholes on the surface.

To overcome these situations, the Variable Density TBM were developed which enables the density and viscosity of the slurry to be varied. This prevents the slurry from escaping into cavities or blowing out from fissures leading to the surface. With this, the face pressure of the TBM is preserved, and the terrain is kept stable during the excavation process.

The main method of construction for the underground alignment was by tunneling using Tunnel Boring Machines (TBM) while stations were built by excavating the station box from the surface. As the city centre of Kuala Lumpur is already well-developed, tunneling was chosen as the construction method to minimise traffic disruption, road damages, business activities and the city’s landscape.

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