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.

A combo pix showing the TBM emerging from the wall of the Pasar Rakyat MRT station. - Starpix by ART CHEN

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, where several complications arose.

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.

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.