Albania highway: Ain't no mountain too high
10 June, 2009 | By Alexandra Wynne
When complete, Albania’s new motorway will offer a drive to rival the most scenic around the world. But those same mountains that provide such a beautiful backdrop have not made life easy.
Much of the route of the central 61km section of Albania’s new motorway has been determined by the decision to follow the Fani i Vogel valley, which has already carved a neat path through the mountains (see ‘Making the first move’).
The work being carried out by the Bechtel-Enka joint venture breaks down into three parts. The first is the nearly 19km stretch from Rrëshen to Reps. Section two runs for 27km between Reps and Thirrë, and the 15km long section three stretches between Thirrë and Kalimash.
Weaving along and beside the river valley involves building 29 different bridges, most of which will be in sections one and two. Section one’s highlight is a 40m high reinforced soil structure (see box at bottom of page). Section two is in the steepest and narrowest section and requires 17 of the 29 bridges.
And it is no mean feat as the landscape is an example of a rural idyll, infrequently occupied by farmers and vast swathes of land is uninhabited except by the odd herd of goats with their keeper.
The trickiest part of the route
Section three is home to one of the trickiest parts of the route. This is where Mount Runes reaches into the sky at an altitude of 1,858m. The terrain is so rough here that it was decided that the best way to negotiate the mountain would be via a twin tube tunnel.
When the road is not in a tunnel or on a bridge, rock drilling − with a little help from explosives in the trickiest areas − is being done to carve a path through the mountains for the two 3.75m wide carriageways with 2m wide hard shoulders in each direction.
“We as a project, we’re bigger than the valley. We’ve 3,800 people to feed, clothe and house.”But added to that is a requirement for Bechtel-Enka to run an associated project to build a further 25km of new local roads and 250km of haul roads, while helping to maintain the national roads in winter.
Mike Swinford, Bechtel-Enka
Despite having a wealth of experience, many of those close to the project were keen to tell NCE quite how unusual it is.
“The real challenge of the project is the logistics of getting things done, from transporting materials and equipment to site to coordinating everything from the tunnel to the excavations to the bridges,” says Bechtel-Enka project manager Mike Swinford.
“There’ll never be another one like this. We as a project, we’re bigger than the valley. We’ve 3,800 people to feed, clothe and house, compared with the 2,000 that live in the valley.”
Despite its relative size compared to the 61km central section, the 5.5km tunnel at Thirrë has been one of the most interesting and challenging parts of the project as it happens to sit in some of the most difficult ground on the route.
In advance of construction starting, it was decided that the New Austrian Tunnel Method (NATM) would be best suited − here the contractor is using drill and blast to create the tunnel crown first, followed by the invert to create a 6.4m high tunnel.
With NATM, site workers install rock bolts and spray concrete onto recently blasted sections of rock to counter the effect of the ground naturally wanting to move. When the walls of the excavation are considered stable, work moves forwards.
Standard tunnel cross section
Five Atlas Copco drilling jumbos − two in the south bore and three in the north bore − drilled 140 holes into the rock face to a specified pattern for each advance. The clever machinery means that these hole locations are programmed into software on the rig so the jumbos can automatically identify the locations for each advance. A gel explosive is installed into each drilled hole before being charged and detonated.
This method comes with its own difficulties. “The principle is to allow the rock to move a small distance to mobilise the inherent strength of the rock mass,” says Bechtel-Enka prime contract manager Darren Mort. “With NATM we allow the rock to stabilise itself, but we add flexible reinforcement to provide extra support.”
But again the variable nature of the rock has meant that life has been even more interesting for the tunnelling team.
A design philosophy had been determined for the various types of support that the tunnel will need in advance of construction starting and according to the different classifications of rock.
The tunnel’s initial design anticipated that five classes of rock might be encountered and provided different solutions for each of these. Class I was very good rock, class II was good rock, class III was fair, class IV was poor rock and class V very poor rock.
“Ok, so the rock type was adverse compared with what we expected. But at least it is a dry tunnel and we have not had any water.”For the three classes of relatively stable rock, the team typically used 6m long steel rock bolts, wire mesh and spray concrete to keep the tunnel stable. Despite the fact that the rock was extremely variable, it was estimated that 43% would fall into the class II category and 40% into class III, with a small proportion split between each of the three remaining classes.
Salih Alkan, tunnel construction manager
For the least stable categories of rock, the design requires heavy steel arches with wire mesh. The spray concrete is installed by remote controlled robots, which tunnel construction manager Salih Alkan says helps with quality, safety and productivity.
The team was prepared for an even split of the work falling between the lighter engineering support versus the heavy-duty approach. But nature had other ideas. Following each blast, geologists had to map the rock type at the face to determine the rock mass rating. What they found was that the quantities of each rock class were completely different from those predicted during the geological investigations.
The cruel twist was that no rock fell into the most favourable class I and II categories. It turned out that only 32% of the rock could be rated as class III, while class IV predictions following geological investigations made a massive hike from 7% up to 60%. That left 8% of the poorest class V rock. As a result the team had to face up to a far more intensive method of supporting the tunnel.
Investment and optimism
Despite the setbacks, resources have been invested heavily in the tunnelling with all four faces in the two tubes worked simultaneously, 24 hours a day.
A dose of optimism from the team no doubt helped. “You can only do as much as the mountain will let you,” says Mort. Alkan still looks on the bright side despite the ground conditions. “Ok, so the rock type was adverse compared with what we expected,” he says. “But at least it is a dry tunnel and we have not had any water, which means no mud rushing and no pumping the water out.”
Tunnel portals: all four faces were worked simultaneously
Although the tunnel has been dry during construction, the permanent lining is supplemented by a combination of geomembrane and other lining that will provide long-term waterproofing. Drainage at the base and on each side of the tunnel tubes will help deal with groundwater.
Heavy steel support arches are installed in the areas of class IV and V rock at intervals of between 1m and 2m. These are in seven segments, which are assembled with the uppermost five put in place first and the bottom two (one either side) installed for the foundations − this mimics the process of advancing with the crown followed by the invert.
An unusual twist
In another unusual twist to the traditional tunnelling programme, contractors began installing mechanical and electrical systems before breakthrough on either tunnel. This was intended to save time and keep the project on track.
Tunnel electrical and mechanical construction manager Gary Dobbs says: “I’ve just come off High Speed 1 and I’ve never been on a project where the equipment is being installed permanently before the excavation project is complete. It’s probably unheard of to try and get the work done that we’re doing.
“You can only do as much as the mountain will let you.”“Added to that, we got a late start on the M&E so we’ve been getting the equipment manufactured from all over Europe to save time. We’ve only had five or six weeks to get it.”
Darren Mort, Bechtel-Enka
Ultimately the twin tubes will be linked every 450m by cross passages − alternating between passenger and vehicle access − for safety.
The final contractural deadline for completion of both tunnels is July 2010, “but I’ve got all the confidence in the world we’ll be done before that”, says Dobbs.
The new motorway will be far straighter than the existing mountain road, but the steep, mountainous terrain means that 29 bridges must be built.
A bridge hotspot is in the section from Reps to Thirrë in a steep, narrow section of valley. “Through this section we follow the river for much of the way,” says Mort. “There are steep valleys here, which on the one hand is why it makes it easier to follow but it is also why there are so many bridges. Plus, this area was really closed off before.”
Seventeen of the bridges feature in this section. Each follows the same design principle, comprising three prestressed, precast concrete U-beams for each span in each direction. The U-beams are cast with horizontal flanges extending outwards from the top. The beams are a uniform 38m long and 2.5m deep and weighing in at 160t, while expansion joints are typically installed at every two piers.
Standard bridge cross section
But that is where the similarities tend to end. There is a wide range of bridge heights from 10m to 85m, and bridge lengths vary from 40m to 360m. Total bridge length along the central section of the motorway is 4.4km.
The alignment geometry through the valley and mountains ensures that every bridge is unique. “Because of the rough terrain, all of the bridges are on a curve,” says structures manager Ibrahim Bilge. “To design the road for speeds of 80km/h to 100km/h we end up with curves like those on a race track.”
And they have super-elevated curves, meaning they vary in height from end to end and side to side. What is particularly unusual, says Bilge, is that the curves are created by the angle of the flanges on each U-beam, which means every single one is cast for a specific location on a specific bridge. It adds an extra element of excitement because there is a lot of surveying to check the alignment will be right before the beams are launched into their final resting place.
With finishing work involving sidewalks, handrail installation and waterproofing, Bilge estimates the bridges to be 95% complete.
Earth moving and building walls
Having a dig: The project has seen a massive excavation effort
Among the many technical challenges of the project is a mighty muck shifting operation. Excavations for the route amount to 33M.m3 − which on a tight timeframe has required help from over 1,000 pieces of earth moving equipment.
Much of the dug out material − somewhere in the region of 20M.m³ − has been reused for creating level earth platforms for use by the locals. It is a much-needed commodity in such an unforgiving and steep landscape.
But while muck shifting is a massive job, so is building structures to support the bridges and the road and to stabilise the steep inclines next to the route. All of which calls for a combination of slope engineering methods.
Another first for the project is a 40m high reinforced soil structure within section one of the central part of the route. Bechtel-Enka believes it is the tallest of its kind in Europe.
“There’s an acceptance that some of the fractured rock will come away and that’s what the rock trench is for.”About 70 retaining walls are being built adding along a total length of 6.4km of the route.Concrete walls are used at heights of up to 15m. Above that, the walls are reinforced soil structures with Maccaferri Terramesh units, which hold compacted fill, and geogrids that provide extra reinforcement at the base and top of each unit.
Darren Mort, Bechtel-Enka
Perhaps less surprising to the team once it became familiar with the region’s ground conditions, is the fact the area is prone to landslides − a few major ones have occurred during construction.
As a result, along with monitoring slope stability, a number of extra methods have been designed to help secure the area.
The 40m high wall in section one
Where the rock is most competent slopes can be cut back on the vertical and require little more than a rock ditch at the side to catch falling ground. “There’s an acceptance that some of the fractured rock will come away and that’s what the rock trench is for,” says Mort.
However, the excavated cut slopes vary from purely vertical to slopes down to 37° from the horizontal, according to the ground type, which is typically a mixed Gabbro rock, clays or colluvium. Each slope is designed according to the ground conditions in each location.
Excavations typically form 8m high benches with a 2m to 4m back step between each lift. Slope engineering ranges from the most simple use of hydroseeding to wire mesh, sometimes bolstered by 6m to 9m solid steel bars grouted into the slope, with either mesh or cables and mesh running across the face of the slope.
In places, spray concrete is distributed over the bolts and mesh. Hydroseeding is helped with the installation of a layer of Maccaferri’s geotextile Macmat − a fibrous mesh that helps the hydroseeding bond.