HE ANCIENT ‘Silk Road’ is perhaps the most famous trading route of all time running from China, through India, Pakistan and Iran into Europe. Over time this route grew to cover a broad range of trading paths, particularly at the European end, but one of the oldest, and most significant, sections ended at the Black Sea port of Trabzon in Turkey, with the final 130km or so being from Bayburt, over the Salmankas Pass at an elevation of 2,280m, following a fast fllowing stream and river to Arakli and then along the coast to Trabzon itself.

Now the Turkish Road Directorate has awarded a contract for a new road tunnel on this, the shortest road route between Trabzon and Bayburt, below the highest elevations to Turkish contractor Asyol Yapi. Project designer is TTS Uluslar Arasi Muhendislik ve Mimarlik.

The Salmankas tunnel is thus the most critical construction project on the new road section joining Bayburt and Trabzon which will shorten this historical Silk Road route and, most importantly, will make it accessible to modern day traffic year round. Currently the section of this existing road over the Salmankas Pass is unmetalled, in a poor state and can be completely closed off to traffic for up to six months of the year due to excessive snowfall and plunging temperatures which can drop to as low as minus 30°C. The road is tarmaced up to the point the tunnel commences at an elevation of around 1,940m above sea level.

The 4.03km long tunnel will, on its own, cut some 16km off the existing route traversing one of the most difficult sections of the old road. The full road enhancement project will also include the construction of three viaducts between the new tunnel and Bayburt.

The initial Asyol Yapi tunnel project only involves driving a single tube although a second is planned and the south portal for this has already been developed as part of the current contract, but the contract for the parallel tunnel itself has not been let yet and may be in the 2013 or 2014 Roads Administration budget timed to commence when the current tunnel is complete.

The current Salmankas tunnel project was awarded in mid-2011, with test drilling then undertaken by the contractor from surface along the tunnel length to check the rock types and competence of the ground to be worked. Principal rock types that will be encountered are basalt and andesite. Actual tunnelling work commenced in October 2011 and is due for completion in January 2014.

EquipmEnt The drilling units actually driving the tunnel are two Sandvik DD320-40 twin boom drill rigs mounting HLX5 drifters. One drills from the south end of the tunnel and the other from the north.

These drive the 10m high by 12.3m wide tunnel by first drilling and blasting a top heading, leaving a 3.5m bench which is then mined out after the top heading is mucked out. For the single tube tunnel safety refuges are being blasted out of the sidewall at 900m intervals. If and when the second tube is built the plan is to drive pedestrian accesses between the two tubes at 350m intervals as emergency exits.

The Sandvik DD320-40 is a compact two-boom electro hydraulic jumbo for drilling in drifting and tunnelling of 8 to 49m² cross sections.

The TB 40 universal booms have a large optimum shaped coverage and full automatic parallelism. The booms can also be used for crosscutting and bolthole drilling.

The powerful four-wheel-drive centre articulated carrier ensures rapid and safe tramming with good balance.

The Asyol Yapi drillers are satisfied with the performance rate, with the rigs currently drilling through the extremely hard basalt and achieving a 120 hole round using 4.3m drill steels and 45mm RT300 button drill bits (used because of the very hard ground) inside two hours. Some 360kg of explosive are used with the 120 hole, v-cut excavation round.

A design advance rate of 4m per round was given, though realised rate has been 3.75m on average. Mucking out is with a Cat 966 wheel loader tipping into contractor-ownd 30t on highway trucks. The muck is delivered to stockpiles near the site which may be used for roadstone by the Turkish Roads Authority. The tunnel is being driven from both ends with the south end of the tunnel at 1,200m in at the time of the site visit, much further advanced than the north end (only 150m advanced) – in part because the north portal was in much more difficult ground requiring jet grouting which slowed down the start-up – and also because the severe winter weather conditions, which last for the best part of six months, make access to this end of the tunnel by the contractor’s workforce, which is being accommodated close to the south portal, virtually impossible during periods of such extreme winter conditions. The current schedule calls for one 4m round to be drilled, blasted and mucked out per shift, with two working shifts per day.

Support At the southern portal ground conditions were better, but still required a 50m long pipe umbrella put in – at 55m width, sufficient to consolidate the ground above the portal for the second tunnel as well so it will be ready for constriuction start as soon as a contract is let for it. Some 9m long umbrella holes were drilled using a Sandvik DX700 rig drilling 115mm holes to accommodate 114mm diameter pipe to provide cover over the first 55m of the tunnel. Additional support in the portal section included steel arches, 6m long, 32mm diameter self drilling rock bolts, wire mesh and a 300mm thickness of shotcrete. In the main tunnel section, primary support is with 4m long 28mm diameter grouted rebar rock bolts, placed by the Sandvik drill rigs, with 5 to15 bolts per metre depending on rock conditions, supporting wire mesh and followed by 150-250mm of shotctrete.

The final tunnel lining of a concrete skin 500 to 600mm thick including PVC waterproofing, is being placed using locally manufactured formwork behind the advancing face. Finished tunnel size is 10.6m wide by 6.85m high from inside of concrete lining to roadway.

The concrete batching plant on site is from Turkish specialist supplier Meka. Formwork is provided by Ceka Beton Kalibi Imalat Insaat Sanayi ve Ticaret, and the shotcrete manufacturer is Titan.

Interestingly, because of the extreme weather conditions which can be experienced the plant is fitted with diesel-powered heater systems to prevent the concrete slurry freezing when the temperature drops significantly (-15°C). As Tunnels goes to press, some 64 per cent of the tunnel is excavated, and one quarter of the lining.