Seli is set to relaunch its DSU-TBM on the inlet drive of the Gilgel Gibe II headrace tunnel in Ethiopia after many months of recovery work to overcome extremely difficult soft ground dominated by high-pressure mud.

The 6.98m diameter shield unexpectedly encountered a fault zone with mud at pressures up to 40 bar, which pushed the TBM back several metres and partly filled the tunnel for some distance (T&TI, April 2007, p6).

Mud problems were met approximately 4.2km into the drive after already having dealt with unexpected ground conditions that required chemical grouting to stabilise the face, bypass digs and overcutting to counter squeezing ground. Competent basalt had been anticipated, except for a few local faults, which consequently led to a standard DSU being specified for the job.

The company has, however, enjoyed far better geological conditions on the drive from the outlet portal.

The 6.98m DSU-TBM has bored more than 14km and has been regularly achieving advance rates of 500m-600m per month, and up to 900m in an early best month. The power tunnel is 26km long and Seli expects about half the length to be in poor ground.

Elsewhere in the country, Seli has also enjoyed success on the Beles Multipurpose (‘Beles II’) project with the completion of excavation of the tailrace tunnel, work having started in mid-2007.

The firm completed the 7km long tailrace drive in 11 months using a standard, 8.03m diameter DSU-TBM.

For the headrace tunnel though, one of the company’s most advanced machines – its first EPB-DSU with capabilities including handling squeezing ground and face treatment.

The 8.1m diameter EPB machine was selected and specified to handle a long zone of soft lake deposits that is expected to stretch over approximately 2km while most of the bore is through granite. Almost two-thirds of the headrace tunnel – 7,400m – has been excavated since the machine was launched in October 2006.

Seli said that the TBM is now approaching the poorest ground on the drive and is preparing to switch over the excavation mode from open to EPB. The company has also supplied backups, rolling stock and ancillary equipment on the drives.

The main contractor on Gilgel Gibe II and Beles II is Italian firm Salini Costruttori, and the client is the state power utility, the Ethiopian Electric Power Corp.

Seli’s TBM on inlet drive of the Gilgel Gibe II project, Ethiopia