In a double first, Robbins has a hybrid TBM is under assembly, in India, that can switch between distinctly different excavation modes – EPB and hard rock while also being able to handle more mixed ground conditions – without having had a prior build in the factory. Major components arrived recently at the Sleemanabad Carrier Canal project site in Madhya Pradesh state and last month work at the site moved to assembly of the 10m diameter machine.
The hybrid TBM features two key components: a ‘true’ EPB cutterhead that can be converted to scoop muck in hard rock mode like a standard single shield and a screw conveyor of larger-than-normal diameter required for solely EPB functions to also accommodate crushed rock when encountered. However, should a significant stretch of hard rock be expected then the screw conveyor can be switched out with a TBM belt conveyor in the tunnel.
Robbins said the machine build is also the first time one of its hybrid TBMs has undergone the company’s Onsite First Time Assembly (OFTA) process. The system allows TBMs to be initially assembled at the construction site rather than in the manufacturing facility and Robbins says it affords a contractor both time and cost savings, particularly for larger diameter machines.
The hybrid EPB-hard rock TBM is to be launched in early 2011 to excavate a 12km long water transfer tunnel for the canal, which is part of the Bargi Diversion Project being constructed to support irrigation and agriculture in the region.
Spreading water resources
While India’s fifth largest river – Narmada – is relatively close by, the limited water distribution infrastructure in regions of Madhya Pradesh has meant extremities in available water resources for many areas despite annual Monsoon rains. For about four months of each year, around Mid-June to September, the rains fall and there is abundance and lush growth along much of the more than 1,300km long Narmada valley (the majority in upstream Madhya Pradesh), but some of those wet areas, and others more so, suffer for the remainder of the year with conditions where droughts can be more prone.
A number of water supply projects are being constructed by the state government through the Narmada Valley Development Authority (NVDA). One such project is the 194km long Bargi Diversion Project, a trans-valley scheme that will transfer some of the water stored at the Rani Avanti Bai Lodhi Sager (Bargi Dam) on the Narmada River to more arid areas in the districts of Katni, Satna, Panna and Jabalpur. The scheme is to convey water from Bargi reservoir to the Son and Tons river basins for irrigation, and also to supplement domestic and industrial water supplies to Jabalpur and Katni towns.
The Sleemanabad Carrier Canal, also known as the Bargi Right Bank Canal, is located in the Upper Narmada Zone of the Bargi Hills and is a key link in the new Bargi-Son distribution network. The 9.2m i.d. tunnel and approximately 13km of canal together comprise a 25km long section of the canal that is being constructed under a turnkey package by a joint venture of Patel Engineering and SEW Infrastructure (formerly SEW Constructions).
The tender notice was issued in late 2007 for bids before the end of that year. The bid opening was originally scheduled for early 2008. Patel announced the JV’s contract award in March 2009.
Turnkey responsibilities include survey, planning, geotechnical investigations, design, estimations, preparation of permanent land acquisition cases, construction of the bored tunnel as well as a total of 900m of reinforced concrete transitions and the sections of open channel waterway between chainages 104km and 129km on the canal route.
The tunnelling subcontractor on the project is Coastal Projects Pvt Ltd.
‘The living standards of the people in these districts will be improved as we bring dry land into wet cultivation,’ says Coastal Projects’ Vice President (Technical), K Ragavendra Rao. He added: ‘This is a prestigious project for us,’ and ‘The tunnel will improve the prosperity of the area by irrigating land and supplying drinking water to surrounding villages.’
Separately, construction of large sections of the canal network have been underway and procurement of further packages continue, such as the 24km long section with a deadline for bids last month.
Geological and hybrid TBM concept
On analysing core samples along the potential corridor of the proposed Sleemanabad water tunnel, Coastal Projects say that the results indicated highly variable strata and ground conditions. The geology along the alignment ranges from sections of soft ground, clay and gravel, to those of jointed rock and marble (15MPa-180MPa UCS) interspersed with clay and gravel. A section of soft ground passes below a railway line. The elevation of the tunnel is below the water table.
Conventional options would have been a double shield, which might face challenges with groundwater in softer ground, or a standard EPB but that would likely make poor progress in sections of hard rock. Robbins says the potential for use of hybrid TBMs in varied sequences of strata is gaining in popularity and, depending on the ground conditions at a site, there can be various combinations – EPB-hard rock, EPB-slurry, and open type-shielded hard rock.
Coastal Projects chose the Robbins hybrid EPB following the core sample analysis and it also agreed on the OFTA approach to help expedite the project schedule.
Brad Grothen, Robbins’ engineering manager, says hybrid TBMs can help reduce risk and increase the possibility of executing bores in difficult ground. The scale and accuracy of the available geological information though is, as ever, key to the design of a TBM.
To produce the fastest overall advance rate for a project, the design of a hybrid TBM can be optimised towards either end of the scale – soft ground or hard rock – depending on the majority of the strata. For example, if a tunnel would be excavated through 80% soft ground then the overall design of a machine would be optimised towards EPB.
The design, however, becomes more complex if there ground types are not split into solid sections or where there is a fair balance overall between hard rock and soft ground – such is the case at Sleemanabad. Constant changes in the strata between these extremes are to be expected over short stretches, according to the core samples.
Sleemanabad hybrid TBM
The design of the hybrid TBM for Sleemanabad has been optimised towards EPB with capability of boring some hard rock without too much change, primarily as shift from pressuresed to nonpressurised mode. Should ground probing ahead of the face indicated be continued (possibly prolonged and extensive) suitable rock conditions then there is the facility to switch over to single shield hard rock excavation.
With much variability anticipated in the geology, and switching between EPB and single shield hard rock mode estimated to take about two weeks to complete, the full conversion can be seen as a potentially time-consuming route to follow at Sleemanabad.
However, in other tunnels with both soft ground and hard rock – though, crucially, with more clear-cut, long sections of either type of ground – then a hybrid EPBHard Rock machine could be designed towards having an ease of conversion between the two distinct excavation modes, says Robbins.
So, of the choice for design optimisation at Sleemanabad, Grothen says: ‘We wanted to create an EPB design that would work in short sections of hard rock, since full conversion to a hard mode would be too frequent to be feasible.’
The cutterhead of the Sleemanabad hybrid TBM is fitted with tungsten carbide, special knife-edge drag bits for soft ground. In EPB mode, the screw conveyor runs at a high efficiency of more than 90 per cent with normal rotation speed and a typical pressurised face.
When rock is encountered, the screw conveyor is capable of being rotated faster by the hydraulic system – by about twothirds – to compensate for reduced efficiency at atmospheric pressure while rapidly evacuating chips from the cutterhead chamber. The machine has an oversized screw conveyor – 1200mm diameter, which is 300mm wider than in original proposals to help pass the chips. However, neither the increased size of the screw nor the capability of switching it out in a transfer to full, open mode excavation required any major changes to the typical layout of the machine.
Moving from EPB mode, incoming water in the non-pressurised sections – in either brief stretches of hard rock or in mixed ground – will be minimised using pumps to transfer water from the TBM belt conveyor to the tunnel portal.
Robbins’ project engineer, Matt Greger, says: ‘We have also provided extensive dewatering in the shields to aid in hard rock mode should some water be encountered.’
For full open mode excavation, the cutterhead is fitted with 53 of the 17” disc cutters. Compared to standard EPB machines, different materials and abrasion resistance techniques are employed to help to deal with open mode rock mining. Bucket lips and hard facing are welded to open spaces for abrasion resistance. The screw flights have bolted, replaceable face bits.
In transferring to full hard rock mode, and switching out the screw conveyor, the centre bulkhead and the screw are both removed and a multiple-piece muck chute is then installed, and the conveyor extended.
Specialised, small grippers are also provided, located around the circumference of the shield to help stabilise the cutterhead in the harder ground and also pull the machine back from the tunnel face for cutter changes and maintenance.
To help decide the best mode for excavation, the TBM is equipped to offer a range of options to probe the ground ahead, straight or at angles. In each case there are about 22 probe holes through which to drive boreholes as well as undertake pre-excavation grouting.
For ground conditioning both ahead of the TBM and for back-filling, the machine is capable of using two different kinds of grout: A+B two-component back-fill grout or mineral grout, depending on the ground conditions – the former for EPB modes (pressurised and non-pressurised, the latter for open mode). Ultrafine cement is used for ground treatment ahead of the face.
Robbins noted that, in non-pressurised EPB mode the flow of the A+B grout will need to be carefully monitored to maintain the proper volume. The grout system is fitted with piston pumps and for the mineral grout there is also a high shear mixer mounted on the backup.
Design of the grout system and getting a second bulkhead in the shield for switching out the screw conveyor were among the main challenges in developing the TBM.
For spoil removal, a side-mounted, steel cable belt conveyor system up to 12.2km long will be employed. The system also includes receiving hoppers to minimise spillage.
OFTA
Robbins’ OFTA system allows TBMs to bypass assembly in the manufacturing facility and go straight to the construction site, although critical subsystems are tested before shipping, such as the electrical and ventilation packages. Key aspects of the process are quality control measures to ensure precision components and proper fit, and also ensuring the sub-suppliers in the supply chain use a template when manufacturing for the job.
OFTA has been used in projects in Canada, China, Mexico and India – for a 10m diameter double shield on the Veligonda water tunnel project, in Andhra Pradesh state. A crew from Coastal Projects with experience at Veligonda is working with a Robbins assembly crew from the manufacturer’s office in New Delhi.
Assembly of the hybrid EPB-hard rock machine started in September in a 41m by 200m pit. The cutterhead and shield structures are to be assembled in a concrete cradle and a double-trolley gantry crane is being used to install the “inner core” components, such as the main bearing, screw conveyor, and inner cutterhead piece, followed by outer components including the upper and lower shields. The back-up is being assembled on rails in a 23m by 80m area.
The OFTA process is expected to be complete by the end of the year and launch is anticipated for February 2011. The machine will begin operations using a seal and thrust frame rather than a starting chamber, allowing it to push off of free-air segment rings.
As the first 3 km is expected to consist of mainly soft ground, the crew will complete assembly in full EPB mode. After driving the initial section of the bore, the TBM will be operated in either nonpressurised EPB mode or as a hard rock single shield, depending on the ground conditions established to be ahead.
The tunnel is expected to be completed and the canal operational by 2013.
Hybrid future
Robbins notes other machines it has built previously, such as an EPB-hard rock design for the Kavacik-Beykoz tunnels in Turkey and an EPB-slurry machine for a project in Tampa Bay, Florida.
Of the hybrid TBM in Turkey four years ago – a 3.1m diameter machine – Robbins says that the screw conveyor could be changed out for a TBM belt conveyor depending on the ground conditions. While the cutterhead also had interchangeable disc cutters and carbide bits, its architecture did not alter underground – it was not converted between standard EPB design and standard hard rock design and so was unlike the arrangement that has been created for Sleemanabad, which is consequently a first for the manufacturer.
Tough, in India, the estimated two weeks needed for a full changeover between EPB and hard rock modes is ‘in no small part’ due to the changes to the cutterhead, says Grothen.
For the latter project, almost a decade ago and which did not involve hard rock, the manufacturer produced an EPB-slurry hybrid machine to drive a pair of 2.8km long bores for a project being constructed by contractor Kenko for Tampa Bay Water. The geology was extremely fractured, highly permeable limestone topped by a layer of clay and then loose, fine sand. As water pressures increased during the tunnelling the mucking system was converted from EPB, plug and muck cars to slurry involving pumps.
With the use of hybrid TBMs expected to increase as more tunnels are built in mixed ground conditions, Grothen says their design is about minimizing complications. He adds: ‘Any job underground is not a simple one. Hybrid machines must be able to mine efficiently in the prescribed conditions while minimising cost to everyone involved.’
Location of the Sleemanabad Carrier Canal where a Robbins hybrid EPB-hard rock TBM will drive a 12km tunnel through highly varied ground conditions Schematic of the 10m diameter Robbins hybrid TBM that is designed to switch between full EPB mode to single shield, open mining of hard rock The 1200mm diameter oversized screw conveyor for the Robbins hybrid EPBhard rock TBM in the shop before being shipped to India The 10m diameter TBM cutterhead for the Robbins hybrid EPB-hard rock machine to drive the Sleemanabad water tunnel Key components of the Robbins EPB were tested in a factory prior to shipping to India for Onsite First Time Assembly (OFTA) Assembly of the Robbins hybrid EPB-hard rock TBM began last month at the Sleemanabad project site
