Gammon, a leading Indian contractor, has recently celebrated the final breakthrough on its six-face 13.81 km headrace tunnel contract, forming part of the Teesta V hydroelectric project in Sikkim in North East India. Playing a key central role in this significant landmark were six purpose-ordered Tamrock Axera T68 – 296 two boom drill Jumbo, fitted with sandvik-rock-tools.

The Teesta V hydroelectric project is the first of a six-stage cascade plan in Sikkim to provide in total 3635MW of hydro power.

Teesta V is a run-of–river scheme, including a concrete gravity dam which is 95m high and 180m long at Dikchu, some 140km north of Bagdogra. The water level is raised upstream before being diverted through a 17.69km long headrace tunnel to the power house at Balutar. The project was started in 1999 and is due for completion in 2007. It was implemented by the National Hydroelectric Power Corporation (NHPC) and will generate 510MW.

Headrace tunnel contract

Gammon was awarded the contract for 13.811km of the 17.69km long headrace tunnel and three adits totalling 1038m in 2001 with excavations beginning in July of that year. Recognising that it required high production underground drilling rigs to successfully complete its section of the headrace tunnel working on six-faces, Gammon placed an order for six Tamrock Axera two boom drill jumbo; its first Tamrock drill jumbo and featuring Sandvik rock tools. Gammon started work on excavating the adits designated 2, 3 and 4 in July 2001 on delivery of the Tamrock Jumbos to provide access to the proposed six-faces; completing all three within seven months.

The twin boom Tamrock Jumbos fitted with Sandvik R38 drifter rods and 38mm spherical button bits were used to drill up to 70 holes on each of the three faces. Holes were generally 75cm-1m apart depending on the condition of the rock and 2-3m deep.

Rock conditions also determined rock bolting and shotcreting. Rock anchors were generally drilled 3m deep and 1.5m spacing although this constantly changed throughout excavation of the adits and into the headrace tunnels.

For Classes 1, 2 and 3 rock anchors and shotcreting has been specified affecting almost 75% of the project. With Class 4 and 5, full steel supports are implemented. For drilling duties on the six faces in the headrace tunnel Gammon is using 48mm diameter Sandvik button bits drilling up to 80 holes generally 75cm apart. The excavated heading features a 6.8m height and a 10.5m width. The 3.3m high bench provides a complete tunnel height of 10m with an excavated base of 7.6m.

Where conditions permit, the holes are drilled up to 4m deep but with the regular rock deterioration it can be just 1m. As with the portals, rock bolting and shotcreting or full steel supports are undertaken according to the classification and condition of the rock. The final breakthrough took place on 13th March 06 between the 2.454km long adit 2 face 3 and adit 3 face 4.

Whilst difficult conditions were experienced throughout drilling on all six faces of the headrace tunnel encountering numerous caverns, this section was particularly difficult – with two cavities being up to 20m long; the first taking six months to resolve and the second more than 12 months.

Rock tools performance

At the start of the project, Sandvik entered into a service agreement for 48 months which was recently extended for a further four months.

Initially one Sandvik site engineer had been allocated to each rig. With two rigs transferring to other Gammon projects and with the reduction of rig utilisation just a single engineer maintains the remaining four rigs. Throughout drilling the Axera rigs have achieved a 90% utilisation.

The Sandvik 45mm and 38mm spherical button bits are achieving an average 400m, despite the difficult rock conditions. This has been achieved by regrinding the bits on-site every 40m-50m, extending the bits’ life cycle. The Sandvik type R38 drifter rod is achieving a life cycle of 3500m, whilst the H35 shank adaptor and coupling sleeves are each achieving an average 4000m life.

By completion the successful combination of Sandvik rock tools and the six Axera Jumbo rigs will have drilled almost 3Mm with Gammon excavating more than 1.3Mm3 from the tunnel and adits.

Despite the problematic geology, Gammon has maintained its drilling schedule to such an extent that it is anticipating completion by January 2007, a full four months ahead of its May 2007 completion date.

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