A growing population in north east Turkey is challenging engineers to manage and dispose of wastewater.
Andrew Mylius reports.
Tunnellers as a rule seem pretty unphased by difficult ground. So when Gordon Shaw, project manager for contractor Balfour Beatty on construction of the Pasakoy wastewater tunnel, east of Istanbul, Turkey, says 'we've been through terrible geology', you know things must have been bad.
'About 65% of the length has been shocking, ' Shaw states matter of factly.
The wastewater tunnel is being built to take effluent from Pasakoy, which has a growing population and a corresponding sewage management problem, under one of the hills encircling it to the River Riva. It will cope with an eventual flow of 7m 3/sfrom a new sewage treatment works nearing completion.
In all there are three sections to the project - 5,915m of 3m diameter tunnel in the upper reaches, reducing to 1,500m of 2.2m diameter tunnel and a 1,044m long box culvert discharge channel.
Client the Istanbul Sewerage & Water Institution (ISKI) carried out schematic design before handing the $10.6M project over to Balfour Beatty and its local design adviser EN-SU Engineering & Construction in November 1999.
'We considered using a tunnel boring machine, but quickly discarded that as it's not at all the norm in Turkey, ' Shaw says.
Instead, excavation of the tunnel would be carried out mainly using drill and blast, giving a horseshoe section. Backhoe excavators and a twin headed Jumbo roadheader were also used over short sections.
In weak areas Balfour Beatty and EN-SU designed for a primary shotcrete lining, reinforced where necessary with wire mesh, and with provision for steel arches and rock bolts.
Once excavation was complete a 200mm to 300mm secondary, circular section concrete lining would be cast insitu.
Though the ground was known to be difficult, in practice it proved even more challenging.
There are rapid transitions between highly faulted and folded rock, consisting of sedimentary and metamorphic rocks - mainly quartzite, arkose, greywacke, siltstones and claystones. Water poured in through the faulted rock at 20 litres/s.
Soft ground made up of loose, gravely-silty-clay sand to very stiff clay, was highly saturated.
'Very little of the tunnel would stand up on its own, ' says Shaw.
'In places we were putting up arches every metre.'
Consolidation grouting has been widespread and longer rockbolts at closer centres were needed over much of the tunnel length. Forepoling was also used extensively to support ground ahead of the face.
In several locations Balfour Beatty was forced to backfill with lean mix concrete and reexcavate. While the contractor managed 325m of progress in its best month, for much of the time ground conditions slowed it to 100m a month.
'By the time we broke through at the beginning of this year we'd lost quite a bit of time, ' Shaw admits. 'But our guys have come up with a really good shuttering system for the secondary lining, ' he adds with pride.
'We've just about caught up with the programme by now and should be complete for 15 December.'
The EN-SU designed and locally built steel shuttering enables the secondary lining to be cast in two 36m parts: first the invert, followed by the walls and crown. Shuttering is positioned using telescoping walers, braced from a sub-frame mounted on temporary rails. For speed and accuracy, the home-grown system is unmatched Shaw boasts - he is pressing EN-SU to apply for a patent.