Gdansk is a regional transport hub and needs to improve its road network. The "Slowacki Route" will help improve traffic connections between Lech Walesa Airport, in the west of the city, and the freight and ferry port.
The scheme is being achieved by a number of steps, including: increasing the share of environmentally friendly public transport; improving traffic safety; sections of upgraded of road, and construction of new links – the most important, expensive and difficult of which is Martwa Wisla tunnel.
The entire "Slowacki Route" is being built by the city authority (Gmina Miasta Gdansk), through its subsidiary GIK. EU and local funds are co-financing the scheme, which is budgeted to cost a total of PLN 1.42bn (USD 430M).
The subsidy package from the European Union is in total PLN 1.15bn (USD 350M), which covers approximately 81 per cent of the entire project construction cost, according to Czech.
While Martwa Wisla tunnel is of ‘paramount significance’ to the city’s goal, it is the fourth, and last, and most difficult, construction task on the "Slowacki Route". The other three projects are spread to the west of the city, and include a variety of road improvements and new road and bridge construction, and interchanges. The tunnel is at the eastern end of the scheme.
Located just on the north, coastal, side of the heart of the city, the tunnels cuts below the Martwa Wisla shipping waterway.
Its bored tunnel section, however, accounts for only half of the 2.16km long underground link; there are also portal structures and long cuttings in the road link, which stretches from Przerobka on the east side of the river to Letnica in the west.
GIK says the "Slowacki Route" is ‘hugely important’ to improving traffic conditions in Gdansk, and to support ‘further city development.’
Planning & procurement
Many options were considered to help the "Slowacki Route" cross the river. Alternatives ranged from building either fixed or draw bridges to constructing tunnels – and both immersed tube and bored tunnel were studied before the latter was chosen, says GIK’s Piotr Czech. But while the bored tubes are not the longest of marine-environment tunnels, their construction challenge would be complicated by the soft wet ground.
Geology along the tunnel route comprises variable alluvial sediments of mainly medium and fine-grained sands with gravel and pebbles. Locally, at the river banks, there are silty sands, silts and clays. Deeper along the alignment, there are glaciofluvial sediments and moraine clays, both with boulders.
Groundwater level is high – approximately 600mm below the surface. Features of the groundwater are the high content of chloride and electrolytic conductivity, important to some ground freezing operations.
GIK’s Piotr Czech says the hydrogeological conditions are the characteristic feature of the tunnel boring project.
As a result of the geological challenge, tunnellers opted to perform a major bentonite slurry TBM excavation. The design layout spaces the tunnels 25m apart at their centrelines, and links them with seven cross passages, each 13m long. Each 11m i.d. tunnel will carry two lanes of traffic.
The chosen vertical alignment leads to overburden that, beyond the launch and exit portals, varies from 8m to about 21.5m over the crown. The applied design water level at the tunnel axis is up to 27.7m. Given the depth to be reached over a short distance, the gradients at each end of the tunnel are relatively steep – each four per cent.
Groundwater pressure at the deepest points of the tunnel alignment reach up to 3.5 bar, says GIK’s Piotr Czech.
In addition to the bored section, the underground link also includes two other types of construction: portal structures, totalling 305m in length (192.5m, 112.5m); and, approach cuttings which have a combined length of 777.5m (630m, 147.5m). The longer approach is on the east (TBM launch) side of the river.
The Martwa Wisla crossing project also includes some other road construction, and the Marynarki Polskiej interchange just to the west of the tunnel.
Design was performed by two companies in consortium – Europrojekt Gdansk, and SSF Ingenieure GmbH from Munich, says GIK’s Piotr Czech; GIK’s own engineers are handling project management and construction supervision.
The main contractor leading on the tunnel works is OHL, which is head of a consortium, at the beginning of the contract, of . The contract began with OHL leading a consortium that included local firms PBG, PRG Metro, Hydrobudowa Polska and Aprivia.
GIK says the budget for this section of the "Slowacki Route" – covering the Martwa Wisla tunnel, road link and Marynarki Polskiej interchange – is PLN 885.6M (USD 268M).
The deep foundations subcontractor for OHL’s contract package at Martwa Wisla is Keller’s local subsidiary Keller Polska.
Tunnelling equipment
OHL contracted Herrenknecht to produce and supply a wide-ranging, integrated package of tunnelling equipment and systems to allow it to successfully execute the tunnel drives. The equipment includes: a single Mixshield TBM; the navigation system from VMT; a bentonite slurry system (separation plant and pumps); compressor station; formwork moulds to cast the concrete segments; and, manrider and materials transport.
The specification of the Mixshield TBM (S-745) includes a maximum torque of 16,841kNm, and nominal power of 3,500kW. To deal with the water pressure, the TBM had a triple brush seal and a quadruple sealing system between the skin of the shield and the installed segmental rings of the tunnel. The system was designed to enable the TBM to remain watertight under the increasing pressure while handling the relatively steep gradients at the beginning and end of the drives.
The segmental concrete lining design was for 2m long rings (6+key). In total, each of the twin bored tunnels has 538 rings, says GIK’s Piotr Czech.
The slurry equipment included a HK60 mud mixer, a HSP2400 separator, centrifuge, mud tanks, pumps, pipelines and transfer system. Operating in manual or automatic mode, the HK60 could produce up to 40m3/hr of drilling mud.
The HSP2400 separator comprised a container (23m long by 9m wide by 10m high) with built-in double-deck vibrating screens and both pre- and post (fine) filters. The separator could take a spoil load of up to 500 tonne/hr (boulders, stones, gravel and sand) in the slurry.
GIK’s Piotr Czech notes that the full plant system had a capacity of 2,400m3/hour of loaded slurry.
Factory acceptance of the TBM and support equipment was in September 2012, says Czech. The shield was then dismantled and prepared for transport from Germany to Gdansk, the final leg of the journey by ship which docked in the river near the project site. Re-assembly of the TBM and equipment began in November 2012.
The TBM was named "Damroka", after a 13th Century princess from the Gdansk region. Damroka was the daughter of Swietopelk II, the Great, who founded the church in Chmielno, inland from the city.
Tunnelling
Foundation excavations began in Gdansk on the portal approaches with deep walls placed for a cut and cover section. TBM assembly began in late 2012 with tunnelling due to start in the second quarter of 2013.
The shield was officially launched in late May 2013. Excavation of the South Tunnel got underway in earnest in June, working three shifts per day, round-the-clock, five days per week. Precast concrete segments were cast at a factory in Kokoszki.
By late August, having bored more than 450m (through sands), the shield approached a jet-grouted zone near the riverside quay at Nabrzeze Dworzec Drzewny. The concrete block was constructed to allow inspection and replacement of worn tools, under hyperbaric conditions, before the TBM proceeded below the river. Another would be available on the opposite bank, if needed.
Following the planned maintenance stop, boring resumed in mid-September and the TBM quickly reached the deepest part of the tunnel. All too soon the shield was beyond the river, boring below the quay at Nabrzeze Wislane and arriving at the west bank without the need for another planned maintenance stop in another jet-grouted concrete block.
The shield rapidly pushed through clays to close on the exit portal, completing the South Tunnel drive at the end of November. It had achieved progress rates of up to 122m per week.
Ground conditions were reasonably as anticipated, OHL and co-authors reported in a joint paper [1] to a symposium, held in February, on the tunnelling experience. They added, though, that there were more stones, pebbles and boulders than expected, and they were large – up to 350mm wide. They concluded, too, that the right choice of tunnelling equipment had been made for the ground conditions.
OHL’s project manager, Jose M. F. Remesal, said in his paper [2] to the symposium that subsidence on the first bore had been less than three-quarters of theoretical design values.
Subsidence was less than 50 per cent of expected levels for more than half of the drive. Bentonite consumption was 25 per cent less than expected, though the volume of grout needed for the annulus was a bit more than anticipated.
Power consumption was also much less than expected, allowing a torque of only 30 per cent of the maximum, he said. It was anticipated that rather than consuming 24GWh for the drive, both tunnels might be excavated with only 12GWh.
The South Tunnel had an excavation programme of 183 days. The drive plan for the short bore was split into seven sections with advance rates between 4m/day-8m/day. Early on, average TBM progress during was approximately 6m/day on a 24-hour, five days per week basis, and edged higher, but was curtailed by the rate of segment production, he noted.
Adjustments were made to the concrete mix design to allow the mould to be stripped much earlier – achieving 25MPa strength after 8-10 hours instead of 24 hours. Curing time was also shortened by a quarter, from 28 days to 21 days, by when 95 per cent of total shrinkage would have happened.
With the changes made, the contractor was able to build sufficient stock of segments during the TBM’s two-week maintenance stop. When boring resumed, to drive below the river and then west bank, the changes enabled the advance rate to almost double to around 13m/day-almost 17m/ day. The only challenge was having to slow to 6m/day, briefly, when passing through a clay zone, which increased the density of the bentonite slurry being carried for processing.
The first drive was completed in 143 days – 40 days ahead on the initial programme.
Higher progress rates – around 16m-18m/day – would be sought in the second drive, at the North Tunnel, working on seven days per week. The only slow stretch would be at the clay zone. Also, it was planned that the drive would start with a big enough stockpile of segments for about two-thirds of the length of the tunnel. It was anticipated that the tunnel’s 183 day excavation programme might be beaten by around 90 days.
Preparing for the second drive, OHL dismantled and moved the shield back to the east bank during the darkest and coldest part of winter. Meanwhile, the backup equipment was pulled back along the South Tunnel, and switched over to line up with the North Tunnel. By the time the symposium took place, preparations were being finalised for the second drive.
OHL launched the shield again in early March, the concrete block was reached by early April, and soon the TBM was boring below the river. The drive was officially completed on 9 June, and tunnellers had achieved a best weekly advance of 132m.
While boring was underway on the North Tunnel, the contractor was busy in the South Tunnel installing the precast concrete structural slabs, walls and slabs to support the road platform, and house the utility services.
Remaining Tasks
Since the end of TBM boring, and dismantling of the slurry shield and tunnelling support equipment, the follow-on works have focused on cross passage construction and further installation of the precast concrete elements for the road platform.
The cross passages are being constructed at intervals of about 175m along the tunnels, and two are below the river. They are have excavated dimensions of 5.5m high by 5m wide, and finished cross sections of 3m high by 3.5m wide, says GIK’s Piotr Czech.
OHL is working on the cross passages from the South Tunnel, performing extensive ground freezing at each cross passage location.
The treatment enables safe breakout from the main tunnel and excavation towards the other twin tube.
In the recently bored North Tunnel, works are focused on installation of the precast concrete elements for the road platform.
