The civil teams of Jager-Gammon India and Larsen & Toubro-Patel are carrying out the work using Sandvik rigs and rock drills for drilling and bolting.
Jager-Gammon is using four Sandvik Axera T08-290 rigs and an Axera T08S- 290C, while L&T Patel has four Axera T08-290 rigs. The machines are equipped with Sandvik drill tools.
Tunnelling in the Himalayas is a notoriously difficult process, due to the varied nature of the rock encountered, the instability of some areas, and seasonal snowfall and rain.
The project is being undertaken by the National Hydroelectric Power Corporation (NHPC) in conjunction with the Himachal Pradesh government.
Stage III is based at Kullu, a picturesque district famous for its terraced agricultural fields and mountain scenery, and a popular destination for Indian and international tourists. It is being built at a cost of 23.04 billion rupees (approximately $562 million), with budgetary support from the central government and from financial institutions arranged by the NHPC.
Located on the Sainj River, another tributary of the Beas, Stage III is downstream of the Stage II powerhouse and will utilise regulated discharge from the earlier phase along with inflow from the Sainj. There is a 43 m high rockfill dam that will receive water from the tailrace of Stage II, and after power generation in its own tailrace and powerhouse, Stage III carries the water back to the Sainj. Consequently, no major diversion of the river is needed.
The rock being encountered on Stage III consists mainly of schistose quartzite, mica schist and granite gneisses, with joint infillings of clay and fluvio-glacial material.
Sandvik’s regional head of service engineering, Mahabirsingh Chauhan, said that the Jager-Gammon JV had taken delivery of four Axera T08-290 jumbo rigs equipped with two drilling booms and one basket boom. These are able to drill holes of up to 51 mm diameter at a rate of 3 m per minute.
There is also an Axera T08S-290C rig with two drilling booms and one basket boom, the machine differing from the other four units in that it is equipped with TCAD (Tamrock Computer Added Drilling) and TPC electrical controls. The high-power drifter makes drilling even faster, and the computer helps give even greater precision to the profile of the tunnelling, and this unit is being used where timing is particularly sensitive, for example on Gammon’s part of the headrace tunnel.
Gammon’s senior projectmanager Rajendra Singh said they are running to a 56-month schedule and their contract is due for completion in the second half of 2010. He said that at the end of April this year, Gammon had completed around 8% of the excavation work.
“Up to this period, we had drilled approximately 221,000 drill metres, using mainly 45 mm hole diameter on the face drilling, and 38 mm hole diameters for the rockbolt drilling,” said Mr Rajendra.
Sandvik has deputed three engineers to take care round the clock for all five machines since the date of commissioning, achieving availability of more than 90%.
Rajendra Singh said the tailrace tunnel is progressively ahead of schedule. As to the main access tunnel, he said the 1:10 slope had been a challenge for the drill-and-blast method.
“Because of the high gradient and the excessive seepage of water, the biggest challenge was to drill and charge the bottom hole,” he said. But we have been happy with the progress.
The Axera TCAD is in use on the 7.25 m diameter horseshoe shaped headrace tunnel, being undertaken with a full-face excavation.
“We are drilling 90 to 100 holes of 45 mm diameter and a depth of 4,000 mm per cycle, using a V-cut drilling pattern,” said Mr Rajendra. “Tunnelling work is being assessed in excavation cycles, with drilling, blasting, excavating and supporting constituting one cycle. Working on a 24-hour basis, the tunnelling is progressing at the rate of approximately three cycles every two days on average, with an average 3.2-3.5 m advance being achieved per cycle.”
The L&T-Patel consortium is using a fleet of four Sandvik Axera T08-290 jumbo drill rigs, also equipped with two drilling booms and one basket boom, and utilising high-power HLX 5 hydraulic rock drills. Again, a Sandvik engineer permanently on site has helped achieve 90% availability of the equipment.
“We are on schedule, even though the rock conditions are very poor,” said L&T-Patel project manager S K Upadhyay. “Outside the tunnelling works we also have instability to face when trucking the spoil down to the disposal areas, because rock falls and mud slides along the haul road can often disrupt the works.”
Both contractors are grinding their drill bits every 300 drill metres, with three to four grindings achieved for each bit and an average per bit of 600 drill metres. L&T has a mobile grinding unit that travels between the machines and collects the bits and grinds them on-site ready for refitting, while Gammon has a stationary grinder located inside its main workshop.
Drill rods for Gammon are R38 - R32 rods, 4.3 m in length, for the face drilling, and the average life of these is 4,500 to 5,000 drill metres. For rock bolting, R38 - R28 rods are being used, also 4.3 m in length, and returning an average life of 3,000 to 3,500 drill metres.
L&T is using Sandvik R38 - R32 drill rods of 4.6 m in length for the face drilling, and achieving an average life similar to Gammon. For rock bolting, L&T is using R38 - R28 rods at 4.6 m long with an average life similar to the Gammon drilling.
In each case the drill depth is 4 m, achieved with 90 to 95 holes being drilled per cycle at a total time of 2.5 hours.
Powergel water resistant explosive is used in the holes. The cartridges are packaged in plastic film, which readily splits during tamping to maximize coupling and bulk strength within a blasthole. The high resistance of this type of explosive to dynamic desensitisation makes it suitable for use in tunnelling and shaft sinking in Himalayan conditions.
Sandvik Asia Ltd supplied the machines and drilling tools through its Indian headquarters in Pune, and all spare parts are being supplied direct from Sandvik’s Asia-Pacific regional headquarters in Singapore. There is a permanent complement of Sandvik engineers on the site, reporting to the Pune office, as part of the company’s policy of working directly with a customer, rather than as an outside supplier.