Final Breakthrough on Teesta v hydroelectric Projects’s Six-Face Headreace Tunnel

Final Breakthrough on Teesta v hydroelectric Projects's Six-Face Headreace Tunnel

Indian contractor Gammon has passed a significant landmark on the Teesta V Hydroelectric Project in India with the final breakthrough on the six-face headrace tunnel. Six Tamrock Axera T68 - 296 two boom drill Jumbo and Sandvik rock tools have, reports John Hooper, provided the key to the successful breakthrough.

Gammon, a leading Indian contractor has recently celebrated the final breakthrough on its six-face 13.811 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.

Teesta V

The Teesta V hydroelectric project is the first of a six stage cascade plan in Sikkim to provide in total 3635 MW of hydropower. Sikkim is dominated by the Teesta River which rises above the northern boundary with China from glaciers fed in the high Himalayas.

Teesta V is a run-of–the-river scheme, including a concrete gravity dam which is 95 m high and 180 m long at Dikchu, some 140 km north of Bagdogra. Water level is raised upstream before being diverted through a 17.69 km 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 510 MW.

Headrace Tunnel Contract

Gammon was awarded the contract for 13.811 km of the 17.69 km long headrace tunnel and three adits totalling 1038 m in 2001 with excavations beginning in July.

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 was quickly impressed with the units performance placing orders for a further four drill jumbo for use on other hydroelectric projects in India; bringing its fleet to 10 jumbos – making it the largest fleet of Axeras in the region.

The headrace tunnel features four curves each located at the adits with the fourth between faces 3 and 4. The tunnel also incorporates a 1 in 609 gradient towards the powerhouse.

It will have a maximum 600 m and minimum 130 m cover over its 17.69 km, with a downstream power station at an elevation of 526 m and the dam elevation at the bottom of the tunnel at 555 m.

The headrace tunnel has been designed for a flow of 292,137 m3/sec.

Geology

Located in the foothills of the Himalayas, considered to be the youngest group of mountains in the world, the project’s geology has posed major problems for the contractor.

“For the geologist, the range of rock conditions is like a paradise but for the contractor, it is a nightmare,” says Sandvik’s Underground Equipment and Services Business Line Manager, V. L. “Raja” Rajasekaran.

The rock is a Phylatic Quartzite with constantly changing percentages to become a Quartzite Phylatic and back again.

For the headrace project the rock has been classified as Classes 1, 2, and 3 being medium hard and Classes 4 and 5 as a soft to medium.

“But this does not tell the story,” says Gammon Senior Project Manager, Ashish Gupta, elaborating, “The constant pressure of moisture can quickly turn a Class 3 into a 4 having very serious consequences on the constantly changing geology.”

In a region of numerous tributaries and small streams flowing from the top of the mountains and crossing above the tunnel alignment, the rock is saturated and has been eroded over time.

“Major shear joints are also commonplace, often coupled together proving very difficult to handle,” says Gupta.

The worst section is directly below the largest tributary, the Rangchong Khola, with water flowing all year round. With heavy rainfall virtually six months a year, water is a significant problem for the contractor to such an extent that more than 2500l/min are constantly being pumped from each adit round-theclock.

According to Gammon’s Vice President (Projects) K C Panda, he has never seen such diverse and difficult geology.

“I have worked on more than 10 international projects around the world and numerous jobs in India, yet I have seen nothing like this,” he says, continuing, “The geology is constantly changing blast by blast.”

Adits

Final Breakthrough on Teesta v hydroelectric Projects's Six-Face Headreace Tunnel
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 – 2, 3 (adit 2); 4, 5 (adit 3) and 6, 7 (adit 4) for the 13.811 km contract.

Adits 1 and 5 formed part of a separate contract for the contractor M/s Jaiprakash. This also included a total of approximately 3.8 km of headrace tunnel at both ends to complete the 17.69 km tunnel.

Featuring a 7 m wide x 6.5 m high cross section, the contractor had completed all three adits by February 2002.

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

Rock conditions also determined rock bolting and shotcreting. Rock anchors were generally drilled 3 m deep and 1.5 m 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 is implemented.

“The constant presence of moisture can quickly turn a class 3 rock into a 4, having serious consequences on the constantly changing geology,” says Gupta. He continues, “At times, because of the moisture problem, it is necessary to use multidrift techniques, preparing the top 20% of the heading across a 2 – 3 m base and widen ‘piece by piece.’ This was a direct result of the water flow and poor conditions of the rock – often little more than slurry

Headrace Excavation

For drilling duties on the six faces in the headrace tunnel Gammon is using 48 mm diameter Sandvik button bits drilling up to 80 holes generally 75 cm apart.

The excavated heading features a 6.8 m height and a 10.5 m width. The 3.3 m high bench, provides a complete tunnel height of 10 m with an excavated base of 7.6 m.

Where conditions permit, the holes are drilled up to 4 m deep but with the regular rock deterioration it can be just 1 m.

On some faces Gammon has experienced ‘squeezing’ with the steel ribs twisting. This has been overcome with “lots of grouting, additional anchors and replacement of the twisted ribs,” says Gupta.

Throughout this remedial treatment the contractor is using consolidated drilling techniques with the Tamrock rigs drilling 5-6 m deep 45 mm holes and grouting at a 5 bar pressure.

To eliminate further squeezing additional 4 m long anchors are installed 1.5 m centres and 1 m where needed.

Where the roof condition has particularly deteriorated, shorter holes have been drilled ensuring standing time is kept to a minimum, introducing temporary supports and ribs as quickly as possible to prevent collapsing.

When Gammon was able to hit hard rock, the rig was able to drill 2 m/min. In the soft rock, drilling rates of 4 m/min can be achieved but the rock bit can choke as the rock fractures causing drilling to slow down as necessary to retrieve and flush.

According to Gammon’s Ashish Gupta, the great advantage of the Tamrock rigs is that higher production can be achieved by faster drilling.

In the softer rock, Gammon is blasting once a day whilst in the harder rock 21/2 blasts are carried out every 24 hours using a power gel explosive in a slurry mix.

Generally, the contractor is carrying out bench drilling up to 0.5 km – 1 km behind the header; if trouble is experienced in the header face.

The notable exception was face no. 2 however, where the full 2.12 km long heading was cleared before starting the bench.

In a couple of instances and to maintain the drilling schedule, where serious problems were encountered, the contractor cleared only half the heading and began part benching excavations with vertically drilled holes.

Final Breakthrough

The final breakthrough took place 13th March 06 between the 2.454 km 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 20 m long; the first taking six months to resolve and the second more than 12 months.

“These two areas, due to the geology were a mess, a real mess,” says Gupta, continuing, “It was

really a case of trial and error as no one had experienced these conditions. It was not nice but we have succeeded.”

For the first cavity more than 100 m3 of cement was injected as thick slurry at a pressure of 7 bar.

The second cavity proved even more of a problem.

“Everything just kept collapsing,” he explains, adding, “Initially we tried to remove the rock but realised it just kept dropping across the complete hole of 60 m2 over 20 m.”

“We were unable to identify how high it went and could hear boulders falling high above but when they arrived, they were totally sheared and small fragments with no strength.”

He continues, “Samples were tested with cement to determine its strength just to understand if it would bond with grouted cement. We experimentally added grout in an inching process piece-by-piece.”

This major collapse covered the complete heading but the contractor recognised that most of the problems were on the right hand side of the tunnel. They therefore blocked the heading with sand bags, wire mesh and shotcrete.

Insert grouting at an incline was carried out on the right hand side to clear a small opening and insert a temporary support. A second 300 mm clearing was then made to extend the access and the operation repeated across almost half of the heading.

With multi-drifting a passage was cleared to allow equipment to pass and continue drilling over a compete face. This also allowed Gammon to tackle the difficult 20 m section on two faces. By mid-March this had been reduced to a 6 m long stretch to clear.

It has been evaluated that this will require 36 ribs, “going rib by rib, supporting and grouting before starting the next rib,” says Gupta.

When Gammon first excavated the cavity problem they inserted pipe roofing through holes drilled around the perimeter with 89 mm seamless pipe up to 20 m long and filled with grout.

“This is considered the best approach for these conditions, yet it still collapsed,” adds Gammon’s Vice President (Projects) K C Panda.

In addition to the final breakthrough and completion of face 2 and 3 header and bench, Gammon

had by mid-March completed all headers except the troublesome 6 m between faces 3 and 4.

All benches had also been completed with the exception of 2.5 km between faces 3 & 4 and 5 & 6. A total of 8 km of concrete lining had been completed and 1.5 km of concrete floor laid.

By completion the headrace tunnel will be lined with a 300 m thick M25 concrete featuring quick setting plasticides.

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. “Whilst we always want more,” jokes K C Panda, “I have been very impressed with the performance of the rigs,” he adds, continuing, “Perhaps more importantly perhaps so to our hq which has placed orders for a further four Tamrock rigs.

Throughout drilling the Sandvik 45 mm and 38 mm spherical button bits are achieving an average 400 m, despite the difficult rock conditions. This has been achieved by regrinding the bits on-site every 40 m – 50 m, extending the bits’ life cycle.

The Sandvik type R38 drifter rod is achieving a life cycle of 3500 m, whilst the H35 shank adaptor and coupling sleeves are each achieving an average 4000 m life.

The contractor admits to trying to stagger operations so as to improve the utilisation of plant, particularly the haulage fleet with a mix of 18 t, 21 t, and 25 t trucks.

But sometimes drilling was performed at the same time on the different faces so as not to fall behind schedule,“ says Ashish Gupta.

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

Explaining one reason for the successful drilling element, Sandvik’s V. L. Rajasekaran (Raja) endorses the close bond between his team and the contractor, “We have never been made to feel like the supplier but instead made to feel very much a key member of the Gammon team.”

Early Completion

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. This has not gone unnoticed by the project’s client, NHPC, which has instructed Gammon to continue drilling for a further 150 m from its face 7 into M/s Jaiprakash’s tunnel.

NBMCW August 2006

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