The tunnels of the KVMRT SBK Line will be built through high-precision mechanised tunnelling using TBMs. These machines will excavate rock and soil with special tools fixed onto a rotating cutterhead while building a permanent precast concrete lining of the tunnels at the same time. The 2 types of TBMs used to construct the underground tunnels of the SBK Line are:
- Earth Pressure Balance (EPB) TBMs to tunnel the Kenny Hill Formation
The EPB TBM will be used in geology associated with the Kenny Hill Formation – 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.
- Variable Density (VD) TBM to tunnel the Extreme Karstic Limestone Formation
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 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.
Kuala Lumpur limestone formation covers large areas of the city and over the years has weathered to Grade V (extreme karst) that contains interconnected fissures and cavities mostly filled with much softer soils and water. The slurry TBMs used in SMART project, when encountering such fissures and cavities, were unable to maintain face pressures with slurry escaping into the voids, resulting in softer ground entering the TBMs or slurry pressures causing blow outs to the surface. These caused large craters on the surface with the overlying soils entering the cavities below.
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 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.
The VD TBM works on a two-pronged approach towards minimising tunnelling risk, primarily caused by ground settlement:-
- Higher viscosity slurry prevents loss of slurry into the cavities and this in turn prevents sinkholes
- Higher viscosity slurry prevents the slurry from flooding the ground surface, hence helping to maintain the pressure between the cutterhead and the excavated surface, therefore minimising incidence of sinkholes
Technical specifications of the VD TBM:-
- Diameter : 6,620 mm
- Cutterhead power : 1,280 kW
- Max. thrust force : 42,751 kN (350 bar)
- Cutting wheel torque : 4,239 kNm (kilonewton metre)
- Cutting tools : 33 single discs, 4 double discs, 54 cutting knives, 8 buckets
- Total weight : Approximately 1,100 tonnes
- Total length : 135 m