1 Location and Traffic Situation
The Pfänder Tunnel forms the bypass for the Bregenz/A area and is located on the A14 Rhine Valley motorway, which is the most important north-south connection in the Voralberg Rhine Valley and a component of the trans-European highway network (Fig. 1).
The existing east tube, planned by ILF and opened in December 1980 is no longer able to cope with the growing volume of traffic. With an average daily frequency (DTV) of some 26,000 vehicles/d the Pfänder Tunnel is the most used single-tube road tunnel in Austria. Traffic prognoses indicate a DTV of around 46,000 vehicles/d by 2020, thus exceeding the capacity of the tunnel that operates with bi-directional traffic. As a consequence the operator, the Autobahnen- und Schnellstraßen-Finanzierungs-AG (ASFINAG), decided to set up the 2nd tube.
In August 2003 the ILF Beratende Ingenieure were commissioned with the overall planning of the 2nd tube including the accessing gallery structures and the water protection facilities. The preliminary draft for the 2nd tube was produced taking the prescribed route and gradient alignment for the 2nd tube (west tube), which was determined when the existing tube (east tube) was planned along with the existing gallery, cross-passage and ventilation structures on the basis of current guidelines.
In May 2005 was tabled for approval in accordance with ordinance §4 of the federal highways legislation; in December 2005 the corresponding ordinance was given the green light by the responsible Federal Ministry for Transport, Innova-tion and Technology (BMVIT).
In September 2006 the ASFINAG decided to plan the continuous (mechanised) driving method in addition to the conventional one and to ask for tenders to be accordingly submitted as an alternative.
In January 2007 the tendering documents for the construction work for the 2nd tube were published. The award process was concluded in September 2007 with the contract going to the Beton- und Monierbau GmbH/Alpine Salz-burg JV. Construction work commenced in October 2007 with a tunnelling machine being used for building an Austrian road tunnel for the first time.
The construction work and the opening of the west tube are scheduled for mid-2012. Subsequently the east tube is to be redeveloped so that both tubes will be fully operational again as from mid-2013.
3 Planning the West Tube
3.1 Technical Data
The 2nd tube is 6,586 m long. The centre distance to the existing tube was established at roughly 70 m when the first tube was being planned. The longitudinal gradient in keeping with the existing tube amounts to 0.4 % in the northern section, 0.5 % in the southern one and the change in gradient occurs in the middle of the tunnel. Access galleries with facilities for reversing the traffic flow are to be found at the portals.
3.2 Tunnel Standard Cross-Sections
The cross-section is governed by the clearance profile – 7.50 m wide and 4.70 m high, the air ducts, the pathway and accommodating the supply lines (Fig. 2). For the cyclical operation applying the New Austrian Tunnelling Method (NATM) the findings obtained from building the first tube were applied. A rigid support with deep base invert (Fig. 2, left) was chosen for areas, where a high swelling potential was expected. In addition provisions were made to ensure that the bench and floor were excavated from storage niche to storage niche with the crown drive at a standstill in order to minimise the effects of construction operations relating to swelling. The concreting of the inner vault was also scheduled to be carried out in this way in these sections. A flat base invert was foreseen in the case of a slight swelling potential and an even floor in sections unaffected by swelling.
A 2-shell lining was selected for the continuous excavation using a shield TBM, which permits the application of low-cost, unsealed segments 27 cm thick, backfilled with pearl gravel (Fig. 2, right). The underground water accumulating in the joints behind the umbrella seal is collected below the raised side shoulders and diverted into the floor drainage. The monocoque base segments are placed in a thixotropic mortar in order to seal the joints. Strict specifications were laid down in the contract for installing gravel in the roof and side wall areas as well as for mortaring the base segments.
3.3 Ventilation System
A full cross ventilation system had to be selected for the west tube just as had previously been the case for the east tube, as it was necessary to ensure that the west tube should be capable of operating with 2-way traffic without any restriction if the east tube had to be closed. Taking into account that the toxic substance emissions had dropped considerably since the first tube was planned in 1976, the ventilation shafts originally planned only for the first tube could also be utilised for the second one. In this way it was ensured that no additional negative effects on the environment occurred as far as the approved project was concerned. The size of the air ducts for the tunnel result from the low amount of fresh air required and the high demands on the exhaust air removal system amounting to 120 m³/s. The 2 ventilation centres are housed in enlargements in the longitudinal direction of the tunnel, with 2 roughly 35 m long chambers forking off crosswise to the tunnel axis, which are set up at a centre distance of 50 m. This concept that was chosen when the first tube was built possesses the advantage that the chambers can be produced independent of the tunnel drive. A fresh air and an exhaust air fan are installed above the intermediate ceiling in each of these chambers, which cater for the 4 ventilation sections. The fresh air and exhaust air ducts are connected to the existing ventilation shafts of the first tube via inclined shafts at the ends of the chambers (Figs. 3 and 4).
The existing ventilation shafts are some 400 m high (north shaft) and around 230 m (south shaft). The connections to the existing shafts are to be executed when the tunnel is closed, something that will only take place at night during the week in order to reduce any inconvenience to traffic passing through the existing tunnel.
3.4 Operating and Safety Installations
The operating and safety installations take into consideration the specifications contained in the EU guideline relating to minimum demands for tunnels on the trans-Euro-pean roadway network, which came into force in 2004 following a series of serious tunnel fires (Mont Blanc Tunnel, Tauern Tunnel, Gotthard Tunnel).
In the Austrian guideline prevailing when the planning was being carried out, a maximum gap of 250 m was foreseen for the cross-passages. Consideration was accorded the existing constructional set-up for the first tube when the installations were planned so that as a result the prescribed maximum distances were not adhered to (Fig. 5).
During the construction process the maximum gap for accessible cross-passages (GQ) was increased to 500 m so that as a result two-thirds of them could be disregarded so that only 8 were actually produced.
3.5 Portal Galleries
The access galleries at the portals are based on the neighbouring existing galleries as far as their nature and appearance are concerned (Fig. 6). Sliding gates between the neighbouring structures and at the outer sides of the galleries on the one hand ensure the noise protection required for local residents and on the other can be opened if need be by the ASFINAG should an incident occur.
The Pfänder belongs to the “upright molasse” of the Mittel-land Molasse Zone. The molasse rocks at the Pfänder form a Neogene layer (young tertiary) and are now to be found in the form of conglomerates, sandstones, marl sandstones, marl and clay marl.
The layers generally tend to dip gently towards NNW. The tunnel axis largely runs in a perpendicular to obtuse direction. In the south it follows an acute angle to the direction of the strata. In the southern part coarse clastic rocks predominate while fine clastic ones (clay marl and marl) are mainly to be found in the north with a high swelling potential (Fig. 7). The swelling potential is derived from the Montmorillonites that account for as much as 7 % of the overall sample. Swelling tests in the lab during the course of construction of the 1st tube indicate maximum unhampered swelling pressures of
3.5 MPa and maximum unrestricted heaving of 17.8 %.
The geological prognosis for the 2nd tube was derived from a concordant extrapolation of the 1st tube. Investigatory drilling was undertaken merely to determine specific characteristic values (CAI, compressive and tensile strength) for the tunnelling machine. The maximum overburden amounts to some 350 m.
In the course of the driving operations for the existing tube (east tube) relatively insignificant amounts of ingressing water were registered. Only slight presence of water is expected for the 2nd tube as well on account of the known groundwater and underground water conditions.
5 Design Concept for the 1st Tube
During the construction of the 1st tube initially the swelling phenomena were underestimated, as these first made their presence felt after months through heaving of up to 30 cm. The following design concept was applied for driving the critical north section: a reinforced base invert was produced in one working phase without longitudinal division. In the process particular care was taken to ensure that no industrial water was permitted to accumulate on the rock floor, which was directly sealed by means of shotcrete. The reinforced base invert with a minimum thickness of 0.40 m was reinforced by means of permanent floor anchors depending on the heaving tendencies that occurred. This approach was chosen because the swelling parameters recorded in the lab could not be transferred to the conditions prevailing at the face and it was also revealed that the swelling potential greatly varied in the rock layers, which changed every few cm. An anchor grid was devised, which was capable of being strengthened in 3 stages (Fig. 8).
In order to be in a position to estimate the local swelling potential, extensometers were installed in the floor, rock samples taken continuously and water formation tests carried out. Disintegration over 24 h indicated whether or not a high swelling potential was to be reckoned with. The following approach was selected as the criterion for installing additional anchors: in the case of heaves of less than 5 mm no anchors are placed during the first month; given heaves of 5–10 mm anchor type SN I, 10–20 mm SN II and in excess of 20 mm SN III are installed.
Should heaving not dissipate the intention was to install the next higher class of anchor. The situation was said to be sufficiently resolved if heaving of no more than 20 mm is to be expected given an extrapolation in a semilogarithmic scale over 20 years.
It was shown that anchor class SN I with a support resistance of 0.13 MPa sufficed and only had to be applied over 52 % of the north section. The results of the slide micrometer measurements revealed that heaves up to 4 m beneath the floor for the most part successively diminished (Fig. 9).
6 Design Concept for the 2nd Tube
6.1 Determining the Swelling Pressure
No further investigations were undertaken for the 2nd tube to determine the swelling potential instead it was decided to carry out dimensioning on the basis of recalculating the deformation ratio for the 1st tube. Towards this end volumetric expansions were applied in the swelling area. The following marginal conditions were selected:
– swelling extends up to ½ a tunnel diameter beneath the floor
– the maximum expansions occur at the centre of the tunnel and decrease towards the abutment, i.e. that anisotropic volumetric expansions were forecast
– the tension variant governs the swelling behaviour
– 50 % of the swelling expansions occur prior to the anchors being installed, 50 % after installation of the anchors (this is the outcome of a parameter variation of 25 %, 50 %, 75 %).
The calculations were carried out separately for cyclic and continuous drives by means of a FE programme.
6.2 Supporting the Base for cyclic Excavation
In contrast to the 1st tube a rigid support for the base was selected for the 2nd tube although the design concept for the 1st tube had been able to prove itself for practically 30 years. The reason for this is that the floor anchors can only be controlled with great difficulty in the long term. The form of the base invert was optimised with regard to the flow of forces, whereby the separation of the shotcrete vault from the in situ concrete one by means of a dividing layer turned out to be advantageous. A deep reinforced base invert was chosen with a minimum thickness of 50 cm at the centre of the floor, which is supported against the shotcrete and a 35 cm thick inner shell in the vault. The recalculation came up with a total swelling pressure of 250 to 450 MPa, which is distributed in the shape of a crescent and is higher in the abutment area as the support reacts more rigidly in this zone (Fig. 10).
6.3 Supporting the Base for continuous Excavation
The base support for continuous driving is prescribed by the TBM’s circular form, the floor segments are monocoque and possess a thickness of at least 55 cm, which evolves from the double-shell lining of the vault. As a chronological development of the swelling phenomena was observed during the building of the 1st tube, it was assumed that only a part of the total swelling pressure acts on the base segment before the inner vault is installed. Owing to the circular form the swelling pressure is evenly distributed; during the initial phase the swelling pressure amounts to 300 kPa at the centre of the base and 500 kPa in its ultimate state (Fig. 11).
7 Tendering and Award Procedures
7.1 Special Features
For building the 1st tube a pilot heading was successfully driven using 2 open TBMs. Consequently for the 1st tube a cyclic drive in keeping with the New Austrian Tunnelling Method as well as a TBM drive was contained in the tender. The outcome was that at the time the drill + blast drive was more economical. For the 2nd tube owing to market developments a TBM excavation with shield was regarded as competitive so that it was taken into account at the tendering stage.
For the continuous drive with TBM-S a review draft (Leit-LV) including a list of services was generated, which formed the basis for an alternative offer and for assessing the offers on a fair basis. In the tendering documents the ASFINAG determined that when handing over an alternative offer for tunnelling with a continuous drive in keeping with the tender no commensurate offer relating to the client’s draft for tunnelling using a cyclic excavation had to be provided. In addition special conditions governing verification, responsibility and carrying of risks, bearing of costs as well as calculating and compiling the prices were included in the tendering documents for the alternative offer “tunnelling with continuous drive”.
7.2 Special Conditions for continuous Drive
In the tendering documents, it was defined that the TBM alternative offer would also have to include
– a technical report with data on the TBM,
– static proof for the segmental ring as well as
– a list of services with determination of the masses given alterations vis-à-vis the TBM Leit-LV. A reimbursement was provided for working out the alternative offer.
The execution planning for undertaking construction with a tunnelling machine is the responsibility of the responsible contractors and must be provided by them.
As far as bearing risks is concerned for an alternative construction method the contractor took over a quantity and completion guarantee for the construction parts required for the alternative method as well as the acceptance of responsibility for the entire extent of planning for each part, which exerts an influence on the alternative method.
As suitability criteria for tunnelling machine alternatives, reference tunnel projects with a tunnel boring machine with shield or double shield, with a minimum boring diameter of 6 m, a segmental lining and a total length of 3,000 m were defined to verify the technical capabilities of the construction companies. Proof of appropriate on-site experience was also requested for key personnel – the construction manager for the mechanised drive, the man in charge of segment production and a TBM operator.
7.3 Criteria for Awarding the Contract
In order to assess the offers, their quality was taken into consideration in addition to the price. During the evaluation of the offers, the facts supplied by the bidder were assessed as shown in Table 1.
It must be emphasised that the length of the transport routes and the means of conveyance were included in the criterion “environment” with the aim of minimising environmental effects during the construction period.
7.4 Outcome of the Offers
A total of 8 offers were tabled by 5 groups of bidders namely:
– 3 offers for tunnelling using a cyclic drive
– 4 alternative offers for tunnelling with continuous drive and
– 1 alternative offer for tunnelling with a cyclic drive.
The costs for the contract section 2nd tube of the Pfänder Tunnel including the advance connecting areas amount to 123 mill. euros net without equipment.