World's first variable density tunnel borer travels its first kilometre

KUALA LUMPUR: The world’s first variable density tunnel boring machine (TBM) broke through the wall of the Pasar Rakyat MRT station here, heralding an important milestone in tunnel construction in the country.

The TBM, named Cochrane 2 (after the Cochrane station where it began its journey), was specially designed to tackle underground karst (limestone) that can be found at many places in Kuala Lumpur.

Known as extreme karst, this type of limestone profile is full of cavities formed after long periods of weathering.

The last time a TBM passed through karstic terrain was during the construction of the stormwater management and roadway tunnel (SMART) that began in November 2003.

Back then, many sinkholes and soil subsidence arose as the TBMs punctured various cavities that led to an instant loss of earth support pressure in front of the TBMs.

Arising from that experience, tunnelling contractor MMC-Gamuda KVMRT (T) Sdn Bhd worked together with the TBM supplier, Germany’s Herrenknecht, to modify the conventional slurry shield TBM to become a variable density TBM.

A variable density machine can vary the density and viscosity of the slurry to counter unfavourable conditions.

Cochrane 2 emerged after travelling 1km, starting from the Cochrane station on July 22 last year, progressing at an average rate of around 11m a day.

According to MMC-Gamuda construction general manager Ubull Din Om, the VD concept had worked as designed, keeping soil disturbance to a minimum even when passing through highly-built-up areas.

“Cochrane 2 is a star today because she has finished her mission and that is a great achievement for the machine, the company, as well as KVMRT as a whole,” said Ubull, who added that all tunnelling work was expected be completed by May next year.

Mass Rapid Transit Corporation Sdn Bhd acting CEO Haris Fadzilah Hasan said the breakthrough was a significant achievement for the underground portion of the Sungai Buloh-Kajang Line, which is slightly more than 50% complete.