Gauge Conversion works on Lumding-Silchar section: Tunnel No.7 was made through

Gauge Conversion works on Lumding-Silchar section: Tunnel No.7 was made through

Silchar (SCL): As a major step taken towards completion of the Lumding-Silchar gauge conversion project, tunnel No. 7 (1,687 metre length) near Mahur was made through on August 8 with underground excavation work from both ends meeting at a point.

This tunnel, located between Phiding and Mahur stations, is the second-longest tunnel being constructed under the gauge conversion project of the Northeast Frontier Railway (NFR), said DC Borah, NFR (Construction) senior PRO.

“As the area is geologically characterised by the Naga thrust on the west and Disang thrust on the east, the alignment of tunnel No. 7 obviously passes through numerous faults and thrusts. Moreover, the rock mass encountered comprises mainly of soft shale in highly weathered condition,” said Borah. He added, “In view of this, most adverse geological conditions were encountered during underground excavation for this tunnel and its completion is seen as a major challenge for the construction engineers of NFR.”

He said that while from Lumding end, moderate to heavy squeezing pressure was encountered during tunneling in most reaches of the tunnel, at the same time, tunneling at Silchar end had to be done in extremely soft ground conditions resulting into frequent chimney formations on account of low overburden substantiated with rock mass comprising mainly of soft shale in highly weathered condition.

“Notwithstanding the adverse geological conditions, all the challenges faced during tunneling were accordingly dealt with and the work was progressed with suitable changes in design as well as method of tunneling. It is noteworthy to mention here that tunneling in extremely soft ground condition from Silchar end was done successfully with pre excavation fore poling by 76 mm outer diameter self drilling bolts as pipe roof of length 15 metre,” he said.