Desiree Willis, Technical Writer, The Robbins Company

In May 2009, Onsite First Time Assembly (OFTA) had the second of three Robbins Double Shields ready for launch in Andhra Pradesh

Beneath India's largest tiger sanctuary, the Nagarjuna Sagar National Park, tunnel boring machines are orchestrating one of the largest water transfer schemes in India. The second of three 10m (32.8ft) High Performance (HP) Robbins Double Shields that will bore under the park was assembled in late May 2009. The machine is scheduled to be launched presently for a 19.2km (11.9 mile) long tunnel in the Prakasam District of Andhra Pradesh State.

The TBM will bore tunnel No. 2 of the Pula Subbaiah Veligonda project for Coastal Projects Pvt. Ltd (CPPL), of the CPPL/Hindustan Construction Company (HCC) JV. The reservoir intake will be located approximately 60m (200ft) away from the 7m (23ft) diameter tunnel No. 1, which is currently being excavated.

The 10m (32.8ft) Robbins machine was assembled onsite successfully, despite high daily temperatures

Using Onsite First Time Assembly (OFTA), the double-shield machine was assembled in approximately four months. The process allows machine components to be assembled initially at the jobsite, rather than in a manufacturing facility, typically providing savings in terms of man-hours and shipping costs. "The real savings in using OFTA were time-related," said Jim Clark, Robbins' Field Service Project Manager. "The lead time between the TBM order and components arriving onsite was much shorter. The machine did not have to be assembled and disassembled at a shop prior to shipment."

Assembly went well despite harsh local temperatures, which can climb to 45ºC (113ºF) daily. In addition, some components could only be installed at night due to thermal expansion in the midday heat. Robbins is now providing a formal training program for contractor personnel, covering all aspects of TBM maintenance and operation prior to the launch.

A third, identical 10m (32.8 ft) HP double shield will be built using OFTA at the nearby Alimineti Madhava Reddy (AMR) Tunnel in June. The machine will join another double shield of the same diameter that has been boring at the opposite end of the tunnel since May last year (2008). Both AMR and Veligonda projects draw water from Srisailam Reservoir and are part of the Andhra Pradesh government's massive scheme to provide ample irrigation and drinking water in the drought-prone region.

Utilizing back-loading 20in cutters, the Veligonda machine will excavate a mixture of quartzite, shale, and phyllite from 90 to 225 MPa (13,000 to 33,000 psi) UCS. As the TBM bores, it will erect 300mm (12in) thick concrete segments in a 6+1 arrangement, making the final tunnel diameter 9.2m (30ft). Muck haulage will require one of the most extensive conveyor systems ever used in India. The continuous steel cable belt, provided by Robbins, will extend 19.2km (11.9mi) and will require four main drives and three booster drives.

All tunnels are scheduled to go into service in 2014.

References

Robbins leads India's reintroduction to mega-TBM advantages at AMR - TunnelTalk, Jan 2009