Eastern areas of India’s Andhra Pradesh state receive only 200mm of rainfall per year—an amount comparable to Africa’s Kalahari Desert. The Andhra Pradesh Irrigation Department hopes to assuage the chronic drought conditions and contaminated drinking water in the region with a massive water transfer scheme consisting of over 120km of tunnels, all sourced at the Srisailam Dam on the River Krishna.
Tunneling with three Robbins TBMs has been ongoing at the Alimineti Madhava Reddy (AMR) and neighboring Pula Subbaiah Veligonda projects since 2008 and 2009, respectively. Isolated jobsites are located hours away from local villages, with at least three days road travel required for spare parts shipments and other project supplies.
“One of the biggest challenges of this project is logistics—getting spares and supplies from one place to the next,” says Andy Birch, Robbins field service project manager at the Veligonda site. Due to the remote locations and similar tunnel specifications, the three 10m diameter Robbins double shield machines, their back-up systems, and continuous conveyors are identical. Parts and supplies are the same for all of the jobsites, which are about three hours apart by road.
Once complete, the AMR tunnel will be the longest TBM-driven tunnel in the world with no intermediate access. The unusual alignment is a result of the project location: Both AMR and Veligonda tunnels pass directly underneath the Nagarjuna Sagar Tiger Reserve, the largest sanctuary for Bengal Tigers in India. The proximity to an environmentally sensitive area meant that traditional methods, such as drill and blast, had to be ruled out due to possible noise and vibration at the surface. The setup also meant that no intermediate access was possible for either tunnel project.
A combination of atypically hard rock, tunnel length, and jobsite location make the AMR and Veligonda tunnels among the most challenging projects in the country’s history. Crews have overcome multiple obstacles including blocky ground, continual power outages, and a once-a-century monsoon.
AMR Water Tunnels
Stretching from the left bank of the Srisailam Damto arid areas of Andhra Pradesh, the AMR tunnels will provide water to approximately 1,200 sq km of farmland. The new water scheme will also bring potable drinking water to 516 villages currently using contaminated sources. Local water use from groundwater wells has become highly polluted with waste bacteria and excess fluoride, resulting in various illnesses and fluorosis, for which there is no treatment.
Planned since 1983, the USD 400million AMR project includes the 43.5kmlong TBM driven tunnel, as well as several miles of open canals and a second 7.25kmtunnel excavated by drill and blast. The 43.5km main tunnel was awarded in 2006 to Indian contractor Jaiprakash Associates, with Robbins supplying two TBMs, continuous conveyor systems, spares, and operating personnel to complete the tunnel excavation.
Once in operation, the main tunnel will receive water from a head regulator currently being constructed on the foreshore of Srisailam Reservoir. Water will flow by gravity through a number of balancing reservoirs, using centra masonry in overflow sections and rock fill in non-overflow sections.
“Balancing reservoirs are required to cross all of the valleys and rivers along the system route. The reservoirs will also be used as storage facilities for overflow,” explained Anil A. Kamat, senior vice president of Jaiprakash Associates. At the Dindi River, a second, 7.3kmlong tunnel will then distribute the water to a network of canals. The 8.7m diameter, horseshoe shaped tunnel is slated for completion within four years.
Each TBM drive, at 22kmin length, is being excavated from portal areas at the opposing outlet and inlet ends of the tunnel. The outlet portal area was designed to accommodate the full length of the TBM and back-up at 45m wide by 160mlong. However, a shortened launch setup is needed at the inlet site, where space is restricted to 120mby 45m. The inlet site is protected by a bund wall at the side of the reservoir, and sits below the water level in limited space.
Veligonda Water Tunnels
Also on the Krishna River, on the right bank of the Srisailam Canal, lies the future inlet site for the Pula Subbaiah Veligonda Project. Once complete in 2014, the system will draw 1.2bn cubic meters of flood water annually from the foreshore of the Srisailam reservoir. Two parallel, 19.2km long tunnels will transfer water via a network of five canals to over 1,600 sq km of farmland in the three districts of Prakasam, Nellore, and Kadapa. Up to 243 cubic meters per second of water will travel through the bored tunnels to a feeder canal.
The USD 180 Million contract was awarded to the Coastal Projects (CPPL) / Hindustan Construction Company (HCC) JV. A Robbins double shield TBM and continuous conveyor system, as well as spares and key operating personnel, were sent to the jobsite to excavate tunnel number two starting from the outlet end. The machine was launched in June 2009. Tunnel number one is currently under excavation by the NSC Consortium – a JV of local firms Nuziveedu Seeds, Swathi Constructions, and Coastal Projects – using a 7.9m diameter Herrenknecht double shield.
Ground conditions
The three 10m diameter Robbins TBMs are identical, and are all designed to bore in the hard rock of India’s Deccan Plateau. Conditions at AMR include quartzite zones up to 450 MPa, layered and separated by shale for approximately 60 per cent of the length, with granite (160 to 190 MPa) for the remaining 40 per cent.
The Veligonda tunnel is located in sedimentary rock on the western margin of the Cuddapah Basin, where a number of faults and folds make for complex geology. Rock includes quartzite with interbedded shale (60 per cent) and shale with limestone and phyllite (40 per cent). Two major faults are expected along with some ground water.
In general the high quartz content of the rock in both tunnels causes high abrasive wear during tunneling, making a larger diameter, 20-inch (508mm) disc cutter the best option for longer cutter life. The quartzite sections with interbedded shales can also be very blocky in nature – the layering of hard quartzite with thin, weak shale makes overbreak in the crown and shoulder of the tunnels a possibility. Tests have shown the granite sections to be fairly competent and less of a concern.
Double shield TBMs
The three machines are mounted with back-loading 20 inch diameter cutters for more efficient excavation, particularly in the high rock strengths present. Specially designed drive motors also allow each machine to run at a higher than normal RPM, compensating for low penetration rates in the hard rock. In squeezing ground, each cutterhead is also capable of vertical movement to allow for overboring.
Each machine is installing 300mm thick concrete segments in a 6+1 arrangement to serve as a final liner, making the finished tunnel diameter 9.2m. Stability of the segment rings is achieved through a combination of crushed aggregate injection and grouting to fill the annulus outside the lining.
Probe drills on the three machines allow for verification of geology 30m ahead of the TBM. The drill is capable of 360 degrees rotation and can alternatively serve as a grout consolidation drill. Large 40kW dewatering pumps located on the back-up systems have been specially designed to pump any water away from the tunnel face.
To monitor TBM performance throughout the project, a newly designed data logging system was installed on each machine. Real-time meters allow the measurement of parameters including cutterhead motor amperage, RPM, cutterhead power, and gripper cylinder pressure. Information is relayed from the machines to computers viewable by the tunnel superintendent and engineers to allow monitoring and adjustment of all TBM equipment.
The bore paths behind all three TBMs are nearly straight, making powerful steel cable belt systems a good option. System components are identical, so that parts exchange and sharing of belt rolls between the jobsites is relatively easy. The Veligonda system tops out at 19.2km, making it the longest single conveyor flight Robbins has provided. The setup also allows maintenance and belt splicing to be performed outside of the tunnel in an optimal environment. The long steel cable belt system is powered by a total of four drive systems – one main drive with two 300kW motors at the tunnel portal plus three booster drives inside the tunnel.
Once outside the tunnel in the spoil handling area, muck is transferred to spoil dumps using trucks. The excavated granite from the TBM drives is then recycled for use as concrete aggregate and pea gravel. The same is planned at the inlet where the quartzites also make good aggregate.
AMR Outlet Tunnel
Initial conditions at the AMR outlet included intermittent power outages, which were supplemented by onsite generators, and unexpected geology. Severely blocky ground at the outset tore the conveyor belt and slowed the tunneling process. Large rock blocks made their way through the muck buckets, stopping in transfer hoppers and point loading the conveyor system. Crews found a solution by reducing the spacing of grizzly bars on the muck buckets and adding additional bars so the boulders could not pass onto the conveyor system. Grill bars were also added to the AMR inlet machine and the Veligonda machine in anticipation of similar ground conditions. In good ground, the grill bars can be removed to allow a higher flow of material into the muck hopper.
By March 2010, the Robbins outlet machine had entered into better conditions. “We are in very good ground, boring through sections of hard, massive rock. The ground is ideal as far as stability but does cause abrasive wear and higher cutter usage,” says Malcolm Moxon, project manager at the outlet site. The machine has excavated about 5.9km, with advances of up to 149 m per week. Crews had also installed the first conveyor booster drive.
AMR Inlet Tunnel
Work to restore the TBM at the AMR inlet tunnel is ongoing – the machine was a week away from launch in October 2009 when a 100-year monsoon hit the region. The natural coffer dam wall at the inlet site was not designed to withstand a major flood, and was breached by the flood waters. Flood control doors were not opened in time to release the water downstream, causing the significant rise in water levels. The launch pit was inundated with over 20m of water, leaving the crown of the TBM beneath over 10m of water for approximately ten days until it could be pumped out.
The TBM and backup were jacked back 12m from the tunnel face to allow removal of the cutter-head and inspection of the main bearing. “Cleanup operations took us a further 14 days, which included jet washing the machine and removing 300 to 400mm of silt deposited on the equipment. In order to return the machine to an ‘as new’ condition, we are currently replacing a major portion of the TBM components,” says Jim Clark, Robbins projects manager, India.
Veligonda Tunnel
As of March 2010, the Veligonda machine had excavated 1.5km of tunnel in soft and fractured rock. “We have had some water ingress, which was not excessive. There were points of heavy water inflows when we passed under three small rivers, which caused water in the head and ingress after each ring was built. We have successfully controlled water seepage in these sections,” says Birch.
At the Veligonda site, an international crew of workers keeps the TBM in operation for two 10-hour shifts, plus a 4- hour maintenance shift. Typical operations during the shifts include ring building, pea graveling, and grouting to control any water inflows. During rotations the crew stays at a campground and living facilities 2km from the portal site, with electricity, air conditioning, and internet service.
Hard rock tunneling in India
For all its challenges and successes, the extensive irrigation scheme is seen as a renewal of the hard rock tunnelling industry in India. “The AMR and Veligonda tunnels are the largest projects ever undertaken in India. In the past, hard rock TBMs have been used in the Deccan Plateau for water supply and sewage systems. Recent attempts have involved the use of TBMs in the Himalayan mountains for power generation—attempts that have been plagued by geological problems including tectonically active mountains, large underground water bodies, faults and shear zones,” says D.G. Kadkade, consultant for Jai prakash Industries.
“These latest tunnelling projects are good for the Indian economy. Previous methods for tunnelling primarily consisted of drill and blast, but TBM usage has made it possible to excavate in sensitive environments for hydropower, irrigation, drinking water, and metros. These improvements are contributing to the growth rate of the country, which is increasing at 7 to 9 per cent per year despite the global recession,” said Kapil Bhati, general manager for Robbins India.
According to Bhati, the possibilities for improved infrastructure using tunnels in India are immense: “There is still a huge potential for hydropower projects in India, particularly in the mountainous states of Himachal Pradesh, Jammu & Kashmir, Uttaranchal, and Arunachal Pradesh. In addition to this, there are many areas that need better systems for irrigation and drinking water. With so many of these projects in the planning stages, we expect the demand for hard rock TBM tunneling in India to rise substantially, by 20 to 40 per cent per year.”
Water will be sourced from the dam Three identical 10m diameter Robbins double shields are excavating India’s AMR and Veligonda water tunnels Blocky rock conditions at the AMR outlet tunnel prompted crews to find a solution by adding extra grill bars across the muck buckets Extra grill bars were added to all three Robbins machines to prevent rock blocks from damaging the conveyor belt
