CW introduces Equipment Apps, a knowledge-sharing platform that presents case studies of construction methods and showcases application-centric equipment deployed for the same. This inaugural column focuses on the drill blast method of tunnelling in consultation with Hindustan Construction Company (HCC) Ltd.
Tunnels are an essential part of infrastructure projects like hydroelectric power, road transportation, railways, metro rail and water supply. Their locations range from mountains and rivers to urban areas. Despite the advent of modern and sophisticated tunnel boring machines, the most common method of tunnelling still remains drill and blast.
The drill and blast method of tunnelling has been applied in ongoing projects like Kishanganga Hydroelectric Project (HEP) for a tail race tunnel (TRT); penstocks and draft tube in Kashmir; Sainj HEP - head race tunnel (HRT) in Himachal Pradesh; Tehri HEP for access adits and penstock tunnels in Uttarakhand; Vishnugarh-Pipalkoti HEP diversion tunnel; T49 and T49A railway tunnels at Pirpanjal in Kashmir; and many other such projects in India. Upcoming projects include Ganderbal HEP near Srinagar and Samalkot road tunnel. These two projects will be started soon by HCC. In Bhutan, HCC has recently completed an HEP in Dagachhu. Another one is going on at Punatsangchhu where the excavation is almost complete.
The process starts by construction of access tunnels or adits to reach the main tunnel and then the construction of main tunnels in most of the HEP applications. Usually, adits are not required in road and railway tunnels. However, in deep underground tunnels, vertical access shafts are created at strategic locations for lowering all the equipment and resources to reach the main tunnel.
As SD Jeur, Head-Construction, HCC and Satish Kumar Sharma, Head-Engineering, HCC, tell us, the standard work sequence for drill blast method is as follows:
Additions to standard work sequence
Depending up on the geological conditions encountered during tunnelling and size of the tunnel, necessary additions have to be made to the standard work sequence. Some of them are:
Equipment required and application
Nearly 65 different categories of equipment are required to execute a complete tunnelling project. They can be grouped as survey instruments, excavation, concrete, miscellaneous and workshop equipment. Showcased below are equipment directly involved in drill and blast operations with their applications. They may be part of the standard work sequence or additional work processes, as the case may be.
Occupational hazards and safety
Tunnelling by the drill and blast method is full of occupational hazards, leading to safety concerns and huge construction delays. ¨Unpredictable geology is the most significant factor. In India, the Himalayan geology is one of the worst in the world,¨ say Jeur and Sharma. To this, R Madhavan, Project Director-Hydro EPC Projects, HCC, adds, ¨Major challenges lie in the terrain and geology of the tunnelling projects. Most projects are being executed in the Himalayan region.¨
Rocks are classified from one to five. Rock class one is the best while rock classes two and three are easily manageable by simple modification of the work process. However, when rock class four, five or even worse conditions are encountered, specialised and job specific treatment is necessary, according to Sharma. Anuran Ghatak, General Manager-Equipment, HCC, says, ¨If the rock class is good, one cycle can be done in eight to nine hours; this means that in 24 hours, we can do three cycles.¨ Jeur describes his most memorable experience, saying, ¨At one of our projects in Punatsangchu, Bhutan, in the Himalayan range, after drilling for 2 km, we had only 450 m left when we met the worst geological strata in the world. There, we adopted 120-mm diameter forepoles. First, we did forepoling of 15 to 20 m length. We required dry drilling machine, as wet drilling was not recommended. This was a high challenge, which required special equipment that could do dry drilling. We used the Casagrande dry drilling machine with a boom length of 15 m and we could insert the pipes. After that, we grouted and concreted the entire portion. Thereafter, we had to proceed slowly by providing supports at short intervals. In a month, we could only do 14 to 15 m length per face. There were two faces.¨ Most occupational hazards related to poor geology result in chimney formation above the tunnel and sudden collapse of tunnel roof or sides. Excessive water discharge inside the tunnel, presence of sub-terrain water bodies, hot or cold operating conditions inside the tunnel and inclement weather are other occupational hazards requiring suitable solutions.
¨Chimney formation is an execution fault and can be avoided by strictly adhering to the work method,¨ says Jeur. He quotes the example of the 11-km railway tunnel constructed by HCC in the Pirpanjal T48 project in Kashmir, wherein despite adverse geological conditions and immense water discharge, chimney formation was completely avoided.
Tunnelling - an art
All considered, the drill and blast method is a complex process as there are unpredictable variables expected at any stage despite conducting detailed studies before commencing work. Each tunnel has unique features in its job. Jeur sums it up, ¨Tunnelling is not only engineering; it is an art!¨
- SHANKAR SRIVASTAVA