THE FIRST tunnel boring machine to be used in Paraguay completed its 3km drive of a sewer tunnel at the end of June.
The 2.4m diameter Brazilian Unideutsch earth pressure balance machine started work on the Itay sewer in the capital Asuncion in August 1998. It initially experienced problems and had to be extensively modified at the midpoint shaft, causing a nine-month delay.
The modifications improved production rates dramatically - 743 rings were installed in a month and 35 rings in a 24-hour shift, completing the drive four months ahead of the revised completion date.
Tunnelling was carried out by Argentinian firm Benito Roggio e Hijos, with a consortium of UK consultant Halcrow and Inarec of Paraguay supervising construction for stateowned water company Corposana. Halcrow has been involved since the master planning of the scheme in 1995 and was responsible for the detailed design.
The 1.6m diameter finished tunnel has a segmental concrete primary lining and 180mm thick cast insitu mass concrete final lining. There are three 5.5m diameter principal access shafts and eleven 3.5m diameter intermediate shafts. Most of the alignment passes through densely compacted sands as deep as 30m below an area that is locally known as the Beverly Hills of Asuncion.
Detailed settlement analysis, monitoring and building surveys were included in the contract.
Work also included more than 300m of pre-excavated tunnel through several basaltic intrusions. These were excavated using drill and blast techniques with initial support of steel arches and timber lagging.
The tunnel is an integral part of the scheme, which includes a new treatment plant, a long river outfall and 450km of collector sewers. It designed to provide sewage collection and primary treatment for about 250,000 people in the rapidly developing outskirts of Asuncion.
The US$90M (£56M, 1997 prices) scheme is funded by the World Bank, the European Investment Bank and the Paraguayan government. The tunnel is costing £10.5M.
Final work should be finished by the end of this month.