Contractor Edmund Nuttall has completed the second and final drive of the $1.4M Portsmouth Wastewater Treatment Scheme in the UK. Breakthrough of the Lovat EPBM comes some six months behind schedule following suspension of boring in June last year when the contractor encountered "technical problems in difficult ground conditions".

Nuttall started work on the scheme’s $42.2M, 8km long tunnelling section in the summer of 1999. Using two Lovat EPBMs, the contractor was pushing two 4km drives in opposite directions from two central shafts at Kendalls Wharf.

The first of the two 2.9m id segmentally lined tunnels, from Kendalls Wharf to the Budds Farm Treatment Works, broke through in early 2000, leaving the drive to Eastney with about 1.6km to go for a scheduled May completion. But in June, just 200m short of breakthrough, work was halted on as a safety precaution after cracks were found in the concrete lining. An extensive ground freezing scheme was needed to stabilise the ground before tunnelling continued. Ground investigation prior to excavation had confirmed interbedded clay and sand deposits on the route and the machine was equipped with picks and roller discs to cope with the changeable geology.

No solid information has yet been made available on the cause of the cracking of the lining.

Senior project manager Damon Elliot said: "It was demanding work but we are pleased with the success of this tunnelling programme".

The tunnel will transfer treated wastewater from Budds Farm to Eastney Pumping Station for recycling through the existing 5.7km long sea outfall. The scheme will ensure compliance with EU directives on waste treatment.