Robbins' tough Double Shields achieve Landmarks at AMRRobbins' TBMs are no strangers to challenging conditions, having persevered through severe floods and difficult ground at India's remote AMR water tunnel. On 18 June 2011, the 10 m (32.8 ft) Double Shield machine at the inlet end was launched following a once-in-a-century flood that inundated the machine and required an extensive rebuild. During the same week, an identical machine boring at the outlet end of the tunnel reached the 10 km (6.2 mi) mark.The outlet TBM is now stopped to undergo cutterhead maintenance and refurbishment due to the highly abrasive and blocky ground, consisting of granites up to 230 MPa (33,000 psi) UCS. "This TBM is the strongest machine I have seen in my life. It just keeps boring, even in these rock conditions," said Elisa Comis, Robbins Project Engineer, who has worked on multiple projects including tunnels in Ethiopia and Hong Kong. During its drive to date, the AMR outlet TBM has achieved rates of up to 512 m (1,680 ft) per month.The TBM's counterpart at the inlet portal is similarly robust. The machine was covered with 20 m (65 ft) of water following a devastating monsoon that breached the coffer dam wall of the inlet site and flooded the launching chamber. Crews from the contractor, Jaiprakash Associates, facilitated the painstaking cleanup, while Robbins engineers and field service began the re-design and rebuilding process.The TBM was retracted and the cutterhead welded onto beams and bolted to the tunnel portal for the refurbishment process. The main bearing was also disassembled from the machine shield and a new one was delivered to site. "The new main bearing was re-installed vertically-a very difficult process because they are normally installed while the machine shield is lying flat. We had to utilise a special crane hook fixture, and take time to align and center each bolt," said Comis. Nearly every part of the machine was worked on or cleaned, with many components being sent out to be reconditioned including all of the TBM's auxiliary thrust cylinders.The cutterhead was also optimised based on the ground conditions encountered at the outlet tunnel. The front muck buckets were plugged due to the extremely blocky ground and hardox plates were installed into the peripheral muck buckets to protect the cutterhead from severe wear.The inlet machine was launched in June using an abbreviated setup due to limited site space. The TBM has now bored ahead 20 m (65 ft) to allow installation of all back-up gantries and auxiliary equipment before continuing on.Once complete, the 43.5 km (27.0 mi) long AMR water tunnel will be the longest TBM-driven tunnel in the world with no intermediate access. The project is part of a massive 120 km (75 mi) scheme by the Andhra Pradesh Irrigation Department in order to assuage chronic drought conditions and contaminated drinking water in Andhra Pradesh state. The tunnels are all sourced at the Srisailam Dam on the River Krishna.