The harshness of the terrain, the remoteness of the project and the violence of the weather are testing the skills and patience of tunnellers on the Alimineti Madhava Reddy (AMR) Project, India. The mammoth 43.5km tunnel is being driven at a 0.03 per cent gradient from both ends using identical TBMs.

But while fiercely hard and abrasive rock is wearing and slowing the outlet drive, a monsoon flood has wrecked the TBM on the inlet drive. As the team press on relentlessly Robbins project managers Martino Scialpi and Elisa Comis warn: “This is a marathon, not a sprint.”

The tunnel will transfer floodwater from the Srisailam dam and reservoir on the Krishna River to arid regions of India’s Andhra Pradesh state, providing irrigation to 1,200 km2 of farmland and clean drinking water to 516 villages.

Contractor Jaiprakash Associates (JP) won the USD 413M engineer-procure-construct contract in 2005 from the Andhra Pradesh government to construct a head regulator and two tunnels, including the main 43.5km tunnel. On 26 May 2006, JP awarded a complete contract to Robbins for two 10m-diameter double shield TBMs, as well as conveyor systems, back-up systems, spare parts, personnel, and technical support.

Outlet drive
When the TBM launched on the outlet drive in March 2008, Scialpi explains the crew was expecting to bore through hard granite with strengths of between 100 and 230MPa UCS. In reality he thinks the machine is facing strengths of up to 300MPa UCS. Scialpi says, “We measure this by comparing the thrust and the advance rate, we can see clearly that we are boring a rock harder than 230MPa.”

To calculate the exact strength of the rock being bored, the team has begun taking samples to test the quartz content and the main features of the rock including the compressive strength and tensile stress. “These tests can give us a better understanding of the mechanical behavior of the rock under our cutters,” adds Comis.

Details of the geological conditions along the route are sketchy as there was only a limited amount of investigation work carried out. The tunnel’s entire length passes under a tiger reserve, greatly restricting access.

The outlet drive launches into severely blocky ground, which tears the conveyor belt and slows the tunneling process. Large rock blocks make 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 is reduced and additional bars are added so the boulders cannot pass onto the conveyor system. Grill bars are 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.

Scialpi says, “Where you find fractured rock you like to push as hard as you can but you have to restrain yourself as you risk delivering big blocks to the conveyor belt that could damage it. So you have to find a compromise between the real efficiency of the machine, which is able to obtain a penetration of 3m per hour, and the geological features you are boring.”

By 2010, the outlet machine entered into better conditions with ideal stability, but is still encountering abrasive wear and high cutter usage. At some points the ground has been so tough to bore the advance rate has been less than 10mm per minute. “It is like boring a mirror, some days you look at the face and it is incredible, it is completely flat and you can see perfectly all the tracks of your cutters. And during the boring the conveyor belt is quite empty, just chips, even under your greatest thrust.” The TBM advances 300m per month.

Inlet drive
The Inlet machine is one week away from launch in October 2009 as a 100-year monsoon hits the region. Flood control doors are not opened in time to release the water downstream, causing the significant rise in water levels. The cofferdam wall at the inlet site is not designed to withstand a major flood, and is breached by the floodwaters. The launch pit is inundated with more than 20m of water, leaving the crown of the TBM beneath more than 10m of water for approximately 15 days until it could be pumped out.

The flood strikes in the early hours of the morning while the pit is empty, says Comis, “Nothing was saved, not even the office laptop.” With little warning there is nothing the crew can do but watch.

The TBM and backup are jacked back 12m from the tunnel face to allow removal of the cutterhead and inspection of the main bearing. Cleanup lasts approximately two weeks, and includes jet washing the machine and removing silt 300 to 400 mm thick that is left on the machine.

“The amount of damage was so high that they simply pumped out all of the water and started to do random checks on the equipment to get an idea of the extent of the damage. Most of the electrical equipment was completely destroyed and brand new replacements have been bought. A good 50 per cent of mechanical equipment has been repaired.”

All the components are removed, assessed and repaired or replaced. The main bearing is so clogged with silt it is shipped back to the factory for checking. The insurance claim takes about a year to be agreed and in October 2010 rebuild work on the TBM picks up pace. The team are skilled in constructing the TBM on site as both the inlet and outlet machines were built on site using the Onsite First Time Assembly method, whereby the TBMs are not assembled first in a factory but instead the parts shipped directly to site.

The safety wall around the pit is raised 5m to prevent the same incident occurring again, it is now taller than the dam.

On 22 June the inlet TBM launches, boring the first 2m on the same day. The machine launches into quartzite, expected to be from 85 to 230MPa UCS for much of the drive but with possible spikes as high as 450MPa UCS. The quartzite is very abrasive; even more so than the granite on the outlet drive.

Having twins
Finding the silver lining in the one and a half year delay to the inlet project, Scialpi says, “The flooded TBM will benefit from our experience gained on the outlet tunnel. We have learnt what to expect [under] these conditions, how the machine should sound and feel. All this experience will make for a smoother operation of the inlet project. The main points are the relationship between the TBM, the cutterhead and all the wear parts, the contact with the rock and the rock itself.”

The inlet and the outlet drives are being driven by identical TBMs. The machines are 10m diameter Robbins double shields. A double shield is made of three shields, a large shield behind the cutterhead, a telescopic shield that moves in and out of the front shield as the TBM advances and a tail shield, which protects workers installing the lining. There is a two-storey backup on the machines with all the usual equipment.

Both machines have to undertake very long drives through hard abrasive rock. There is no intermediate access so all repairs and maintenance need to be carried out during the drive. To ensure the machine lasts the length all of it parts exposed to the rock have been given extra protection.

Comis explains, “The machines are especially made for hard rock. There is an additional skin all around the shield to cope with the wear over the very long drive. Extra wear plates also protect the cutterhead.”

“Everything on the TBM was designed for very hard rock,” says Sciapli “so the cutters are extremely heavy duty, the cutterhead is completely shielded with a hard facing, the cutter housing is especially made for this kind of rock but this is not yet enough. It is a long drive and with the rock we have to consider the real length.”

With the long life of the project the machines need to be well maintained to ensure they can complete the drives. Cutters are checked every day and the cutterhead goes through a major overhaul every year to 15 months. The first overhaul on the outlet drive comes after 5.5km and sees an extensive refurbishment of the cutterhead, replacing all the wear plates. On 9 June the machine reaches the 10km mark and two weeks later is shut down for 10 weeks for its second overhaul.

Daily maintenance is carried out by the day shift. The machine is usually shut down for about four hours while disc cutters are inspected and replaced. The machines feature back-loading 20-inch diameter cutters for longer cutter life in the abrasive conditions, with twin discs in the centre. There are 67 cutters on the face. A skilled maintenance crew will need to change up to 25 cutters in a shift.

Brand new cutters are installed in the gauge positions and the used cutters from the gauge positions are removed and installed in the face positions. The same cutter will work several positions through its life. The gauge cutters are changed almost every day because the wear of the gauge position is so severe. The cutter can often last two to three weeks in the face before being sent back to the cutter workshop to get new rings and bearings. The maximum wear for the cutters is 35mm on the 20-inch disc.

Muck removal and lining
The restricted access to the tiger reserve above means that ground monitoring was ruled out. With no concerns about settlement, overbreak is measure by monitoring the advance rate against the amount of muck removed.

The muck is carried out on a continuous conveyor also being supplied and operated by Robbins. There is a drawback to using a continuous belt, especially in blocky conditions, if there is a crack or tear in the belt the whole system needs to be shut down and replaced. To counteract this issue the main contractor JP is installing intermediate chambers along the route, the first one at 12km. A new conveyor system will start from the chamber and feed muck from the machine to the first conveyor. Spacing chambers at 12km intervals limits the belt length to 24km. The chambers are being lined with wire mesh and shotcrete.

The tunnel is being finished with RCC precast bolted segments 300mm thick, 1.5m wide in a 6+1 arrangement. The segments are cast locally in a precast factory by the main contractor and trucked to site. There are no gaskets between the segment and the annular space is filled with pea gravel and grouted.

The tunnel will be under a small water pressure as the water level will be kept at a level slightly higher than the tunnel crown. The maximum pressure will be 3 bar.

Working in a remote part of India throws up some unusual challenges for the team. Towards the end of May the crew are carrying out routine cleaning of the telescopic shield when a viper snake is spotted on the TBM. The viper, which carries a lethal venom, had been brought to the TBM in empty sacks. Just a few weeks later the machine is home to another alien, a stray dog on the site has had puppies and one puppy decides to take the train to the TBM. It took several grown men several hours to trap and remove him.


The outlet portal TBM launched into hard blocky ground Figure 1, the AMR tunnel feeds the water canals for 516 villages A cutter disc unevenly worn after becoming stuck on the hard rock Figure 2, the inlet drive launches into quartzite and the outlet into granite The continuous conveyor system carries muck from the TBM