Landslide hits tunnel portal at Arun-3 in Nepal 05 Jul 2018

Four trapped workers were safely rescued from a hydropower adit after its portal was buried by a major landslide during early civil works for the Arun-3 project, in Nepal.

Landslide blocked adit portal and trapped workers

Days of heavy rainfall triggered the landslide on 15 June, taking out part of the access road as well as choking the portal, reported project developer SJVN. The tunnellers were working with jumbos in Adit 1, which is part of the Lot 1 civil works being undertaken by Indian contractor Jaiprakash.

A combination of machinery including excavators, breakers and a compressor with a jack hammer was used to dig through the debris of large boulders and muck blocking the portal on the hillside above the river. It took the rescue effort almost 40 hours in the wet conditions to safely pierce the debris and reach the trapped workers, who were found to be safe and well.

Arun-3 is being constructed on the River Arun in the east of the country by a subsidiary of the Indian SJVN. The run-of-river hydropower project involves significant underground works, including diversion and intake tunnels; four desilting caverns of 420m long x 16m wide x 24m high; a 11.8km long x 9.5m diameter headrace; a 149m high by 24m diameter surge shaft; pressures shafts; a powerhouse cavern complex; a short 10m diameter tailrace; and various adits.

Tunnelling is a significant portion of the two large civils lots for Arun-3 with Civil Lot 1 awarded to Jaiprakash in September 2017 and Civil Lot 2 to Patel Engineering in March this year. While early works have been underway for a few months, construction of the long-awaited project was officially launched in May with the laying of a foundation stone. Jaiprakash is building the underground works at the upstream end of the project, including the first 3.1km of the headrace, and building the reservoir dam. Patel will build the downstream balance of the underground works, including the lower 8.7km of the headrace. The project has a planned construction period of five years.

Worker rescue after tunnel portal blockage

Arun-3 is being developed under a build, own, operate and transfer (BOOT) concession lasting 30 years from financial closure, in September 2017. The memorandum of understanding for the 900MW hydro project was signed ten years ago with the Government of Nepal and SJVN, and since then the concessionaire has worked on survey and development plans, and funding. In 2013, it established a local subsidiary, the special purpose vehicle SJVN Arun-3 Power Development Co (SAPDC), to execute the project. Much of the Arun-3 energy output will be transmitted across the border to India.

SJVN is a joint venture between the national Government of India and the State of Himachal Pradesh. It has more hydro projects getting underway involving major tunnel works, including the 60MW Naitwar Mori scheme in India and Kholongchhu in Bhutan.

The Naitwar Mori project is located in Uttarakhand State on a tributary of the River Yamuna. The run-of-river project has a four-year construction period and major underground works include a 4.3km long x 5.6m diameter headrace, an almost 52m high surge shaft and the powerhouse machine hall caverns. Jaiprakash won the civils works package in December 2017 and has early excavation works at various adits began after laying a foundation stone in March 2018.

Kholongchhu is a 600MW hydro project being developed jointly by SJVN and Druk Green Power Corporation (DGPC) of Bhutan through their special delivery vehicle Kholongchhu Hydro Energy. The project is part of a portfolio of hydro initiatives agreed by the governments of Bhutan and India in 2014 to be developed in the country. Procurement is underway for the civil works.

Underground works for Kholongchhu project include two desilting caverns of 350m long x 13m wide x 17.5m high; an almost 15.8km long headrace tunnel with a horseshoe-shaped section of 5.7m finished width; a nearly 153m high by 9.5m diameter surge shaft; a powerhouse cavern complex; and a 1.5km long tailrace tunnel of D-shaped profile, 6m wide by 6m high.

Other hydro projects with underground works that SJVN plans include Devsari and Jakhol Sankri in Uttarakhand, and Sunni Dam in Himachal Pradesh.