Final breakthrough on the tunnelling for the headrace of Beles Multipurpose (“Beles II”) project, in Ethiopia, was achieved early this month.
Seli completed the tailrace tunnel last year but the headrace was longer and more difficult drive. Both tunnels have been excavated under subcontract to Salini Costruttori.
Geology along the alignment of the headrace was mostly basalt of up to UCS 350MPa but there is a zone of almost 2km of local faults and loose, lake deposits.
The 12km long tunnel was excavated with a 8.1m diameter double shield universal TBM designed also by Seli for EPB mode excavation.
The TBM is one of the company’s most advanced shields, capable of switching from DSU to EPB mode in the complex ground. In addition to usual DSU functions, the design provides ability to handle squeezing ground and face treatment. The switchover in excavation mode takes between two and three weeks.
Progress achieved by the EPB-DSU shield through the loose soils was up to 15m-20m per day. Tunnel lining was precast concrete rings (6+1) with segments of 300mm thickness.
The EPB-DSU shield started excavation on the headrace in late 2006. Finishing works on the headrace are due to be completed by January next year.
Separately, Seli used a conventional, 8.03m diameter DSU to bore the 7km long tailrace tunnel through basalt. The TBM was launched in the second quarter of 2007 and holed through in May 2008. The tailrace will discharge into the Blue Nile river (T&TI, August 2008, p4). In addition to the two TBMs, Seli also supplied backups, rolling stock and ancillary equipment for the drives.
The client is the Ethiopian Electric Power Corp. It awarded the EPC contract to Salini in 2005.
