KMRC eyes mining tech to bolster cave-in zone soil

KOLKATA: The East-West Metro authorities have started technical feasibility studies for ground freezing to stabilise the soil at Wellington where construction of ventilation shafts to the twin tunnels is held up due to threat of soil subsidence.
Refrigeration of the soil, also called ground frosting or freezing, is a technique used to stabilize soil for drilling operations.The technique has been in use in the mining industry globally since 1862 but has not been used in Metro tunnelling operations in the country yet.
“Ground freezing converts pores in the soil into ice by continuously refrigerating the soil. To do so, liquid nitrogen at -200°C needs to be pumped into the earth. This freezes the entire area and makes it rock solid,” explained an engineer. “If the entire soil mass is frozen, it becomes a seal against water. The technique works best in porous soil,” a senior KMRC official added.
Contractor ITD-ITD Cementation has initiated talks with Norway-based ground freezing technology expert GeoFrost. Since liquid nitrogen is needed to be pumped into the soil to freeze it, talks are also on with Linde for oxygen supply.
Last month, ITD-ITD Cementation had to suspend work to link the two tunnels below Nirmal Chunder Street with the ventilation shaft at Subodh Mullick Square, fearing fresh subsidence. They suggested that since soil character was very bad at this site, a new ventilation shaft could be built near Technical School on SN Banerjee Road and the tunnels linked there.
With the Board of Directors of implementing agency KMRC turning down the proposal, the agencies can now either go in for a cut and cover approach to access work area. But that will mean a prolonged surface-level disruption.
The other option, a more expensive but less disruptive one, is ground freezing. While KMRC board may need some convincing to give a nod to use of the expensive technology, officials say it is a more practical option as it won’t require relocation of underground utilities. There is an old brick sewer line and three 33KV CESC cables that run under Nirmal Chunder Street where the shaft work is yet to be done.