At 43.5 km (27 mi), the Alimineti Madhava Reddy (AMR) Project will be the longest tunnel without intermediate access in the world when complete. The tunnel will transfer floodwater from the Krishna River to arid regions of India’s Andhra Pradesh state, providing irrigation to 1,200 km2 (400,000 acres) of farmland and clean drinking water to 516 villages.
Contractor Jaiprakash Associates Ltd. (JAL) won the USD $413 million engineer-procure-construct contract in 2005 from the Andhra Pradesh government to construct a head regulator and two tunnels, including the main 43.5 km (27 mi) tunnel. On May 26, 2006, JAL awarded a complete contract to The Robbins Company for two 10.0 m (32.8 ft) diameter Double Shield TBMs, as well as conveyor systems, back-up systems, spare parts, personnel, and technical support.
The first of the two machines was launched in March 2008 after an unprecedented onsite assembly. Both of the machines were initially assembled onsite using the Onsite First Time Assembly (OFTA) process. OFTA, rather than assembling the machine in a manufacturing facility, saves both time and money to the contractor in terms of personnel and shipping costs.
Assembly of the first machine took place in a large launch pit at the outlet portal site, using gantry cranes to hoist components into place. Machine parts including the cutterhead, gripper system, forward shield, and telescopic shield were then assembled in a concrete “cradle”. The assembled TBM and back-up then crawled forward by reacting against invert segment pieces installed progressively up to the tunnel entrance. The second Robbins machine was assembled onsite at the opposite end, or the inlet portal.
Geologic conditions consist of quartzite zones up to 450 MPa (65,000 psi) UCS, layered and separated by shale for approximately 50% of the length, with granite (160 to 190 MPa/ 23,000 to 28,000 psi UCS) for the remaining 50%. Both machines feature back-loading 20-inch diameter cutters for longer cutter life in the abrasive conditions. Other design modifications include specially designed drive motors to run each machine at a higher than normal rpm for optimal penetration rates in the hard rock.
Initial conditions at the AMR outlet included intermittent power outages, which were supplemented by onsite generators, as well as difficult 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. To counteract this problem, the spacing of grizzly bars on the muck buckets was reduced and additional bars were added so the boulders could not pass onto the conveyor system. Grill bars were also added to the AMR inlet 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.
The Inlet machine was one 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 20 m (66 ft) of water, leaving the crown of the TBM beneath over 10 m (33 ft) of water for approximately ten days until it could be pumped out.
The TBM and backup were jacked back 12 m from the tunnel face to allow removal of the cutterhead and inspection of the main bearing. Cleanup lasted approximately 14 days which included jet washing the machine and removing silt 300 to 400 mm (12 to 16 in) thick which was left on the machine. A major portion of the TBM components were replaced to get the machine back to running condition.
The beginning of 2010 (6th km) was marked by the first cutterhead refurbishment, deemed necessary to restore bucket lip housings and face wear plates that were severely worn out after excavation of unusually abrasive and hard rock. After refurbishment, the monthly advance slowly improved to within the range of 300-330m. However, consistently challenging conditions during the same time period in 2010 meant average machine utilization was 21.7%.
To help avoid sudden breakdowns or delays caused by the constant extremely high vibrations propagating from the cutterhead up to the back-up bridge gantry, a careful inspection of all the exposed components was implemented during and after each stroke, or at any time that a sudden change in boring values or other anomalies occurred. All the cutter fasteners, bucket lip bolts, grill bars, main bearing studs, seal clamp rings, seals and wear bands suffered from the excessive stresses induced and needed to be checked constantly. With the implementation of this regime it was possible to reduce the number of blocked cutters and any possible damage to the head. A second cutterhead refurbishment was planned and carried out in the 12 weeks between June and September 2011 by replacing and removing all damaged carbide wear plates, in particular in the outer/gage section of the head.
When tunneling with limited geological data in a remote location, it is quite often necessary to find technical and practical countermeasures, particularly in the case of AMR where adverse excavation conditions where a large diameter machine is boring one of the world’s longest tunnels. Constant monitoring of the boring data, and a routine maintenance regime play a significant role in the system performance. While hard rock conditions are continuing to be a challenge for both the inlet and the outlet portal tunnels, training of the crew members as to how to deal with the unusually abrasive and hard rock has been invaluable.
As of November 2020, both machines have excavated at least 70% of their respective tunnels. Since the jobsite has continually experienced very hard, abrasive rock conditions, it has allowed for research and testing of harder, more durable cutter rings because of the extreme conditions.