When CEI visited Bombay work on the Worli tunnel was almost complete, and Dywidag had just begun driving at Bandra.
India's first undersea bored tunnels are respectively 3.4km and 3.7km long with a gradient of 1:250 so that they can be drained back to the pressure shaft for inspection. The drilled, blasted pressure shaft at the head of each tunnel is 7.5m diameter. It is 63m deep at Worli; 70m at Bandra. The shaft is shotcrete lined with a PTFE lining on the top 5m to resist rise and fall.
At the far end of each tunnel is a 240m diffuser section where the inside diameter reduces from 3.5m to 1.2m to maintain flow. Adits of 1m internal diameter connect the diffuser section to ten risers, bored and installed as part of the marine works. The adits are excavated by hand and backfilled with concrete around the GRP lining.
D&W has carried out work in basalt and tuff rocks from 15MPa to 300MPa using a refurbished TBM from Wirth, Germany. Unfinished tunnel diameter is 4.08m with minimum rock cover of 16m. Mucking out, segment delivery, etc. is provided by two 16t Schoma locomotives with Muelhauser rolling stock.
Work at Worli progressed at a minimum speed of 3m/hour but a number of small collapses requiring temporary support slowed progress. Elsewhere 1,500 litres/minute water ingress caused three weeks' machine downtime. Meanwhile, heave in rock between the face and lining, erected 25m behind, required additional hand excavation.
The lining is made of 200mm thick reinforced concrete elements precast to M45 strength using Cifa forms. There are eight elements per section, bolted together. The tunnel is backfilled with concrete and the joints grouted.
Just before the diffuser section there is a Y branch, one arm of which will be sealed off. If, at a future date, the tunnel needs to be extended it will enable work to be carried out with minimum disruption to the existing structure.
Because bored tunnels are extremely rare in India, local workers were learning on the job in the Worli tunnel. Faster progress is expected at Bandra. Completion of Bandra is expected by February 2001.