Four Sandvik DT 820 underground drill rigs have virtually completed drill and blast duties on more than 6 km of tunnelling for Hindustan Construction (HCC), producing in excess of 1.8 million tonne of rock spoil on the country's first crude oil storage caverns at Vizag.Four Sandvik DT 820 underground drill rigs have been used throughout the construction of India's first underground crude oil storage caverns, located at Visakhapatnam, together with two Sandvik DX 700 surface drill rigs for benching works. The project is being constructed under a $75.5-million contract by Mumbai-based Hindustan Construction (HCC) for Indian Strategic Petroleum Reserves, a subsidiary of the Oil Industry Development Board of India under the Petroleum Ministry.Located at a hill known as Dolphin's Nose on land belonging to the Eastern Naval Command in Visakhapatnam - usually abbreviated as Vizag - the scheme will have a storage capacity of 1.33 million tonne of crude oil. Vizag, on the east coast and considered the eastern gateway to India, was selected as the site because it has a large seaport for the import of oil. Adjacent to the cavern site is South-east Asia's first underground LPG storage cavern and which also used Sandvik jumbos during construction. HCC began work on the cavern in 2008 in a 36 month schedule expected to be handed over in August 2011. The contract covers detailed engineering and design, underground excavation, access tunnels, water curtain galleries, main storage caverns, shafts and associated underground civil works.The contractor's Project Manager, Ramana Rao, confirmed that there are five caverns for the crude storage, all 30 m in height and 20 m wide, together with associated tunnelling. Three caverns are each 840 m long and each is connected at one end to form a 'w' shape. Two shorter 370 m tunnels are also joined to form a 'u' shape. Tunnel works have included excavation of two 6 m x 6.5 m x 1.3 km long water curtain galleries.Once completed, 75 m long x 102 mm dia boreholes were drilled horizontally from both sides of the galleries at 10 m intervals to cover the caverns. The boreholes are sealed and pumped in a continuous recirculating pump system with water to a pressure of 6 bar to seal any fissures in the rock around the caverns.An essential part of the project, the water will prevent seepage of crude fumes through the fissures, eliminating the possible risk of explosions. Similarly, the strictly enforced smooth finish specification of the walls, with just 400 mm tolerance on either side of the tunnels, is set to reduce the risk of voids in the caverns and the possible build-up of fumes, thereby once again preventing the risk of explosions. The complete length of the underground works exceeds 6 km and by completion of the excavations more than 1.8 million tonne of rock will have been excavated.The floors of the caverns are 80 m below mean sea level, which means that the working is accessed via an inclined access adit at ground level but with the hill above the site reaching an elevation of 130 m, two large shafts have also been sunk to house the operating equipment for the caverns. Rao said that the rock strata is Khondalite, a form of sandstone that follows an arc through India's southern and eastern states and which has been used throughout the subcontinent's history for architectural construction and ornamentation.HCC is using the four Sandvik jumbos simultaneously for much of the time, as the drilling and blasting is being undertaken on several fronts. "Our logistics are designed to supply power and water to four headings simultaneously, and to remove the rock and do the shotcreting," he says. "The four Sandvik machines are therefore working within this cycle." The jumbos are drilling at least 150 holes per blast cycle, at depths of between 2 m and 4 m, depending on the section of the cavern on which they are working. It takes an average two minute to drill one hole, with the holes closely following the tunnel profile. With a tunnel profile of 11 x 8 m holes are packed with a total of 480 kg of explosives. A 10 x 8 m profile requires 473 kg, and an 8 x 8 m profile needs 365 kg.Power Bulk Drive-T liquid explosives, supplied and charged by Orica Mining Services, have been used for only the second time on a construction project in India. The key advantages being faster cycle times and providing controlled blasting to ensure the specified smooth finish, plus improved safety.The drilling per blast cycle takes around three hours, and rock bolting is simultaneously being undertaken from a basket fixed to the jumbo. Once the drilling is complete, the jumbos are moved out of the tunnel where they are washed down after each drilling shift. With the blasting and rock removal complete, the jumbos are moved back into position for the next 16 hour cycle.Rao says that with work proceeding 24 hours a day, the production cycle allows three blastings to be carried out during a two-day period, and that each blast advances the work by 3.5 to 3.8 m. HCC's project mechanical engineer Kaushal Kumar Sharma says that the machines are hosed down at the washing area next to the access portal, and that routine maintenance is undertaken once a week. "We inspect the hoses and linkages, the drill bits, and oil and lubricate the machines. Though our own staff do this, Sandvik has an onsite presence and works very closely with our maintenance and operating teams."The jumbos were supplied to HCC by Sandvik India, whose Senior Engineer on site, Harish Pokharkar, says that spare parts are supplied via the company's Pune headquarters. The two Sandvik DX 700 surface drill rigs are being used around the two cavern benches, where they are undertaking vertical drilling: minimising explosives consumption and increased advance rates. Pokharkar says that both machines are fitted with 45 mm R32 drill bits, with an R 38 shank adapter and R 38 and R 32 drifter rods. "The DX 700s are drilling down to depths of 21 m on all three benches," he says. "We have a grinding facility for the drill bits installed at the site, and we have recorded that by grinding, we are achieving 30 per cent extra life with the bits. "We have also ascertained that machine availability has been slightly over 96 per cent per month."The Sandvik DT 820 is a two-boom electro hydraulic jumbo for fast and accurate drilling in tunnelling and cavern excavation of 12 to 110 sq m cross sections. It is equipped with an HLX5 rock drill and a TB 90 universal boom, which gives a large optimum shaped coverage and full automatic parallelism. The booms can also be used for cross-cutting and bolt hole drilling. An optional basket boom for bolting and utility works is available. The four-wheel-drive centre articulated carrier ensures rapid and safe tramming with good balance.