Tunnels have been a hallmark of human civilisation since prehistory and though their primary function is to create a direct route through an otherwise impassable medium, their actual use has been extremely varied. From the labyrinthine network of the London Metro and the New York subway system to the Cu Chi tunnels in Vietnam, tunnels are ubiquitous in modern society.
Tunnels are as much a part of the landscape here in Malaysia as well, with examples ranging from passages that cut through the mountainous interior of the peninsular to the underground LRT network beneath Kuala Lumpur city.
The SBK Line and SSP Line underground alignments will have strategic stops at the busiest commercial, residential and key employment areas in the heart of metropolitan Kuala Lumpur.
With part of the Klang Valley Mass Rapid Transit (KVMRT) alignments designed to traverse through heavily built and populated residential districts, business centres, commercial centres and key employment areas in the urban and suburban corridors of Klang Valley, MMC Gamuda KVMRT (T) Sdn Bhd, faced with land constraints, had little alternative but to take the alignments underground. Due to crucial reasons such as constructability, cost and operational efficiency, the only solution was to go underground to avoid impediments to the social and economic vibrancy of Kuala Lumpur city.
With part of the Klang Valley Mass Rapid Transit (KVMRT) Sungai Buloh-Kajang Line (SBK Line) and the Sungai Buloh-Serdang-Putrajaya Line (SSP Line) designed to traverse through heavily built and populated residential districts, business centres, commercial centres and key employment areas in the urban and suburban corridors of Klang Valley, some 9.5 km, for SBK Line, and 13.5 km for SSP Line, of the alignments were taken underground.
The tunnelling is primarily through three key geological formations, Kenny Hill, KL Limestone and Granite, with most of the tunnel alignment sitting in the Grade V karstic limestone, MMC Gamuda had proposed for this section a twin mode tunnel boring machine (TBM) that would operate in both ‘slurry’ and ‘EPB’ modes, plus additionally equipped with facility to use a higher density slurry.
The VD TBM has been attracting attention from industry specialists for its precise performance in tackling karstic limestone. This machine is a direct evolution from MMC Gamuda’s tunnelling experience on the dual purpose SMART tunnel project in Kuala Lumpur through similar karstic limestone, but obviously without the variable density provision and as a result with less control on ground loss.
The MRT system best serves its function by providing connectivity to the busiest population pockets, but these pockets are also extensively developed with narrow corridors and dense structures. An above-ground route especially in the heart of KL city for the MRT is simply unfeasible.
The most challenging task in the underground tunnelling works for the Klang Valley Mass Rapid Transit (KVMRT) for SBK Line is the tough geological formations found underneath Kuala Lumpur city. The underground alignment for the KVMRT SBK Line navigated past two distinctly different geological settings, with the extreme karstic limestone accounting for almost 50% of the underground alignment. Extreme karst is a geological formation consisting of weakly soluble bedrock such as limestone eroded by mildly acidic water over millions of years, leaving behind an elaborate labyrinth of unstable caverns, cliffs and pinnacles in the depth of Kuala Lumpur city. For the the Sungai Buloh-Serdang-Putrajaya Line, similar challenges are expected.
Thus, maintaining the equilibrium of the sub-surface geological system containing underground reservoirs and water-filled cavities is a challenge as any disturbances from underground activities could cause karst sinkholes and ground subsidence. Another possible consequence is bentonite blowouts, resulting in mudflows that rise to the surface during the boring process.
The MRT Sungai Buloh-Serdang-Putrajaya Line’s underground section passes through three distinct geological formations. This determines the type of tunnel boring machines to be used for boring works.
The main challenge tunnelling in limestone is the difficulty in controlling the sudden loss of face pressure due to the existence of unpredictable underground voids and channels, resulting in surface settlement or sinkholes. This is normally overcome by extensive mapping and investigation before tunnel boring starts. The Variable Density Tunnel Boring Machine (VD TBM) slows down the sudden loss of face pressure by applying more viscous slurry to prevent blowouts to surface through underground channels.
This geological formation consists of mainly sandy silt and sandstones and is generally stable for tunnelling. The challenge is from the abrasiveness of the material, resulting in excessive wear and tear to the TBM’s cutting tools. There are also sporadic intrusions of quartz lens or dyke that is very hard. The Earth Pressure Balance machine is well-suited for this geological formation.
Granite is generally hard and abrasive. The main challenge is the need for regular maintenance and change of tools, resulting in tunnelling at this section being slower. Weathering of the granite underground may result in water channels being trapped and hidden, making the conditions similar to karstic limestone. The use of the VD TBM manages the mixed condition and prevents the drawdown from the water lenses.
The EPB TBM will be used in geology associated with the Kenny Hill a soil condition with low permeability which is cohesive and plastic. The EPB TBM supports the excavation and tunnel face by using polymers to make an impermeable “paste” and constantly regulating the amount of material excavated with its screw conveyor and rate of advance of the TBM. Four EPB TBMs will be used to tunnel through the Kenny Hill formation of the MRT underground alignment.
The VD TBM is a slurry machine designed for several modes of tunnelling, both in slurry and EPB, with a facility to vary the density of the slurry. Increased levels of automation and controls were introduced to reduce human errors. The ‘variable density’ enhancement that led to this technological innovation is the first of its kind in the world. It is a direct result of MGKT’s previous experience from the involvement in tunnelling through same karstic limestone ground conditions in the vicinity of the Kuala Lumpur MRT project, albeit with a larger diameter single, dual purpose, tunnel designed to alleviate floods and carry traffic (SMART Project), where, regrettably, many sinkhole incidents occurred across tunnelling a similar length, with significant social impact and damage to properties.
Just a simple comparison of the number of major sinkholes on the two projects, each involving tunnels of similar lengths, albeit different diameters, the reduction from more than 41 numbers of major ground collapses on SMART project to just 2 minor ones on KVMRT SBK Line, provides a measure of ground control that has been possible with the Variable Density tunnel boring machines used for the KVMRT drives. This equates to 4.6/km sinkholes for SMART vs 0.3/km sinkholes for KVMRT, a major improvement by any standard.