Metro tunnelling: Controlled blasting to remove hard rocks

MUMBAI: The Mumbai Metro Rail Corporation Limited (MMRCL) is carrying out controlled blasting to remove hard rock layer for constructing stations on the 33.5-km underground Metro-3 line (Colaba-bandra-seepz).

MMRCL has undertaken controlled blasting at Cuffe Parade and Marol. The corridor will have 27 stations between Cuffe Parade and Aarey Colony in Goregaon (East).

According to MMRC officials, mechanised excavation of hard rock or rocks that are abrasive in nature take longer time and hence blasting is carried out to save time. SK Gupta, director (projects), MMRCL, said they had taken permissions from various authorities to carry out blasts “wherever required”.

Gupta said, “Blasting is done for construction of stations and not to bore tunnels. It is not our preferred option to excavate soil and rock. It is only done when the mechanised tools are unable to break rocks. It is a technique which is legal and can be carried out in populated urban areas because the impact of the blast charge is feeble.” The agency, which is excavating up to 25 metre deep to construct a station, encounters three types of rocks — basalt, breccia and tuff.

Dispelling doubts, Gupta said that the impact of the “controlled blast is not felt by humans”.

Meanwhile, tunneling on the Metro-3 line has been impacted owing to hard rock layer at certain locations. Gupta said MMRCL had bored 6.8km of the total 52.2km of tunnels. MMRCL bores 350 metres per week. Gupta said that the tunnelling was progressing “broadly” at a desired speed and the tunnel boring machines were designed specifically to carry out work in a particular geological condition. “We want to take out weekly target to 750m by October-end,” said Gupta.