Boosting Tunnel Advance at Teesta VI

Teesta Stage-VI HE Project is a Run of River (RoR) Scheme in Sirwani Village of Sikkim to utilise the power potential of Teesta River Basin. The Head Race Tunnel (HRT) and Tail Race Tunnel (TRT), 12.86 km long each are horseshoe shaped. There are twin tunnels running parallel to each other. The source of the water is from upstream Teesta River and a barrage is constructed for intake of the water. The Hydro Powerhouse is 500 MW capacity. The geology of the rock varies from Class-3 to Class-4 with several cleavages and faults at certain locations. The construction of the project has been undertaken by Jay Prakash Associates (JPA).

The main objective of the study was to track tunnel advance rate using the high energy formulation of BEL MX80 (40 mm dia.), a non-permitted (SDNP) emulsion explosive specifically developed for small diameter applications (eg. tunnelling, development rounds in mining) over existing explosives in use.

Additionally, design of cut holes and accuracy of drilling and charging and post detonation fumes, were part of the study and it had been identified as focus area for the trial.

Methodology

Two blasts were conducted at both tunnels (one blast in each tunnel) by using small diameter non-permitted (SDNP) explosives.

The advance rate was measured by using survey instruments. An existing stationary reference point was taken with known levels and co-ordinates at some distance from planned blasting face. After blasting the movement of face was measured from same reference point. The difference in this horizontal length, from reference point, is called advance rate per blast.

For post detonation fumes dissipation time was considered as parameter. Dissipation time is the time gap between blasting and dissipation of fumes from face. For unbiased analysis, clear visibility of after blast face from 10 mtr (approx.) considered as dissipation of fumes.

Technical solution

A trial of two blasts were conducted by replacing wedge cut with burn cut designs to study its impact on tunnel advance rate. Burn cuts, if properly designed and executed are more suited for faster advance rates in mechanized operations with jumbo drills. The factors critical to burn cut design are face dimensions, geology, and site drilling capability. A four-reamer diamond pattern was used for the trial. In addition, team ensured timing design to allow adequate relief for rock to break out into the void holes.

The technical services benefits demonstrated at site in terms of advance rate and better post blast fume characteristics.

Observations

The observations are as follows:

• Limited sockets observed in the post blast face indicating improved advance per blast.
• Faster clearance of post blast fumes improved the development cycle time.

The blast results of BEL MX80 (40 mm dia.) gave encouraging results:

• Advance per round improved by 33 per cent over existing levels.
• The fumes clearance time was 50 per cent lower compared to that of existing explosives.

Acknowledgement

Smartchem Technologies sincerely acknowledges and appreciates the support rendered from Jaypee Prakash Associates (JPA) for this study. We would like to thank Pintu Majumdar (Site Incharge Tunnel Adit-1), Manoj Garg (Site Incharge Tunnel Adit-2) and the entire blasting team of JPA for all the valuable support provided during the study.