Case Studies: Current Projects
Double Shield TBMs Assembled Onsite for Giant Water Tunnel
- Machine Type: Double Shield TBM
- Number of TBMs: 2
- Diameter: 2 x 10.0 m (32.8 ft)
- Tunnel Type: Water Transfer
- Tunnel Length: 43.5 km (27.0 mi)
- Owner: Government of Andhra Pradesh
- Contractor: Jaiprakash Associates Ltd.
- Location: Andhra Pradesh, India
Project Overview
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 in 2012. 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, JALawarded a complete contract with 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 will be initially assembled onsite using the Onsite First Time Assembly (OFTA) process. OFTA, rather than assembling the machine in a manufacturing facility, saves time on the construction schedule and money to the contractor.
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.
Geological conditions during the tunnel bore 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. 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.
The second Robbins machine will be assembled onsite later in 2008 and launched from the opposite end, at the inlet portal. After excavation of the entire tunnel is completed, a drill and blasted disassembly chamber will be used for removal of each TBM and back-up. The chamber will include poured concrete inverts and installed 170-metric-ton (187 U.S. ton) gantry cranes.
Updates of this project will be posted as boring continues.
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