The Kerenzerberg Road Tunnel is part of the A3 national road and is located within the region of the Canton of Glarus, between the Weesen and Murg junctions. In 1986 the tunnel measuring 5691 m was released for traffic (towards Chur) and has not experienced any major maintenance work since then. In order to adapt the Kerenzerberg Road Tunnel to the guidelines and standards applicable today, the road tunnel is undergoing a safety upgrade and maintenance. The key element of the maintenance measures is the new construction of a parallel safety gallery.
2.1 Route Alignment and Project Elements
The safety gallery lies on the lakeside and has a length of 5504 m, running parallel to the Kerenzerberg Road Tunnel (Fig. 1). The tunnel centreline distance is between 20 and 25 m. In the Hofwald area, the safety gallery lies up to 65 m away from the existing road tunnel in order to bypass the Hofwald ventilation centre. Instead, the safety gallery crosses the adjoining Hofwald ventilation and escape tunnel. The Gäsi portal in the west with the Gäsi ventilation centre lies between the entrance portal to the Kerenzerberg Road Tunnel and the exit portal from the Ofenegg Tunnel travelling towards Zürich. The east portal in Tiefenwinkel is located around 180 m north-west of the tunnel on the Kerenzerberg road. Pivotal to this portal location is a deep landslip area near to the east portal of the Kerenzerberg Road Tunnel, which was responsible for slope movements of up to 25 cm back during the construction period. Besides relocating the portal outside of the landslip area, the Tiefenwinkel ventilation centre was moved approximately 200 m into the mountain due to the geological conditions in the portal area. This underground ventilation centre consists of a longitudinal and a transverse cavern, a lock cavern as well as a 150 m high ventilation shaft, which leads to the outside with a 5 m high smoke outlet in the Hochschleipfen area (Fig. 2). Two smaller caverns in the quarter points of the safety gallery complement the power supply as sub-centres.
2 | Tiefenwinkel centre longitudinal section
Credit/Quelle: INGE K2
2.2 Standard Tunnel Cross Section
Due to the combined use of the safety gallery as an escape tunnel and an exhaust duct, the cross section is divided into two separate areas (Fig. 3).
3 | Standard cross section of safety gallery
Credit/Quelle: INGE K2
4 | Cross section of exhaust gallery
Credit/Quelle: INGE K2
Credit/Quelle: CSD Ingenieure AG
Between the portals of Gäsi and Tiefenwinkel, the safety gallery passes through the often flat limestone and marl layers of the Swiss Mürtschen nappe and the Glarus thrust below it. Four geological sections are distinguished (Fig. 5). In the first section there are predominantly thin to thickly bedded, compact limestones with thin marls between them. Karst phenomena can occur particularly in this area. The rock also features geogenic arsenic contamination on the first 1400 m of this section, which needs to be taken into account when utilising the excavated material. The second section runs through the so-called Salleren breccia zone, an area of the Mürtschen nappe subject to varying tectonic loads, where water ingress is increasingly expected. Sections three and four pass through the Quinten Formation of the Glarus thrust. The limestone here is severely shattered, whereby particularly intensive fissuring prevails in the fourth section and instances of clayey crack filling can occur.
2.4 Construction Workflow
In a first stage, the safety gallery is created and equipped with the control centres and ancillary structures. The cross passages as well as the exhaust galleries are excavated in this construction phase up to the vault of the Kerenzerberg Road Tunnel. The cross passages and exhaust galleries are not connected to the road tunnel until later, together with the maintenance of the Kerenzerberg Road Tunnel.
3 Invitation to Tender
The breakout of the safety gallery is divided into two lots and is excavated from both the west and the east. Lot 1 from Gäsi portal includes excavation with the tunnel boring machine up to the lot of the Tiefenwinkel control centre, where the tunnel boring machine is dismantled for its removal. Besides excavating the two sub-centres, the 52 exhaust galleries plus 18 of the 20 cross passages are part of Lot 1. Lot 2 includes the excavation in the opposite direction from Tiefenwinkel portal with the breakout of the safety gallery, the underground caverns and the ventilation shaft as well as the construction of the smoke outlet in Hochschleipfen. The complete interior work on the safety gallery, the lining and interior work on the caverns and sub-centres as well as the construction of the Gäsi portal control centres are done by Lot 1. The logistics for the interior work can therefore be optimised by Lot 1 and carried out, if necessary, from two portal sides.
In both lots, the client only specified the date for the start of construction in the submission. Neither the end of construction nor the construction workflow were predefined. The end of construction was also explicitly not assessed in the evaluation of the tenders. Particularly in Lot 1, the intention was therefore to give the tendering parties the opportunity to optimally align their offers in terms of construction matters to the tunnel boring machine excavation, the numerous cross passages and exhaust galleries as well as the interior work on the 5.5 km long safety gallery. In the tender evaluation it was revealed that the tendering parties approached the construction process and the logistics in different ways, which came to light in the construction time and the costs. The main difference between the tenders was the timing of the breakout work for the cross passages and exhaust galleries. With the five tenders for Lot 1, five different construction workflows were presented. The construction time varied accordingly between 47 and 70 months. The price span of the five tenders for Lot 1 was 57 %. In the case of the tenders with a shorter construction time, the breakout of the exhaust galleries and partly also the cross passages was planned alongside excavation with the tunnel boring machine. The tenders with a longer construction time either took into account only excavation of the cross passages alongside excavation with the tunnel boring machine or a completely serial excavation of the safety gallery, the cross passages and the exhaust galleries When it came to selecting the logistics concept too, differences were revealed with road (Multi Service Vehicles) or rail-based concepts.
In Lot 2 the price span between the four tenders submitted was a mere 9 %. The construction time varied between 20 and 28 months. Due to the geology as well as the limited access in Hochschleipfen, the breakout method for the vertical ventilation shaft with a diameter of 4.5 m was not prescribed to the tendering parties. A pure raise boring method, sinking by means of blasting or a combination of both methods was able to be tendered. All tenders were ultimately based on a pure raise boring method.
4 Construction Work – Initial Experiences
4.1 Lot 1 – TBM Excavation on Gäsi Side
The installation areas lie in the Linthebene, an area shaped by agriculture, directly near the confluence of the Escher Canal into Lake Walen as well as the Gäsi campsite. The main installation site is divided into two and is located on the left bank of the Escher Canal. The technical installations, the workshops and the daytime storage area for the lining segments are placed between the two carriageways of the national road as an extension to the gallery shaft.
6 | Gäsi installation sites
7 | Auxiliary bridge for Gäsi pre-cut
Back when the Kerenzerberg Tunnel was constructed, an extensive karst cave system was encountered after 200 m. In depth surveys in the immediate area revealed an increased risk of further karst phenomena when constructing the safety gallery too. In order to avoid constructional difficulties during the initial stage of the TBM excavation, the developer decided to extend the launch tunnel to 250 m and hence excavate the critical area by drilling and blasting. After construction of the pre-cut with an up to 20 m high anchor wall, excavation of the starter tunnel was able to commence. During the drill and blast process, only minor karst phenomena appeared in the rock. Further investigations using hybrid seismic and georadar technology also resulted in no larger cavities under the floor, meaning that the TBM was successfully able to be pushed into the completed starter tunnel (Fig. 8).
The safety gallery is being excavated with a double shield machine. A segmental lining ring consists of five segments and one invert segment (Fig. 9). To connect the cross passages and exhaust galleries the segmental lining rings are cut open. Prior to partly opening up the segmental lining rings, these are provisionally secured with solid compound anchors. In its final state, shotcreted lattice girders are used for the exhaust galleries and in situ concrete bars for the cross passages to ensure the load-bearing safety in the connection area.
The executing consortium makes the cross passages and exhaust galleries as well as the two sub-control-centres alongside the TBM excavation work. Although, with a breakout diameter of 7.1 m for a safety gallery, there is plenty of space, the parallel execution of the various excavation works also revealed the limits of this cross section. In particular, the excavation of the exhaust galleries from a special work platform around 600 m behind the cutterhead is proving to be logistically challenging (Fig. 10).
10 | Working platform for breakout of the exhaust galleries
Credit/Quelle: INGE K2
11 | Breakout work on the working platform
4.1.3 Hofwald Intersection
The safety gallery crosses the existing Hofwald ventilation and escape tunnel at the same level. The single escape route in the operating road tunnel as well as the supply and exhaust ducts used in the present ventilation system are interrupted by this (Fig. 12).
12 | Hofwald crossroads area situation
Credit/Quelle: INGE K2
13 | Work to raise the tunnel roof at Hofwald intersection
4.1.4 Handling of Geogenically Contaminated Excavated Materials
On the first 1400 m of the safety gallery, the rock features geogenic arsenic contamination. This approximately 170 000 tonnes of excavated material is considered uncontaminated in accordance with the “Ordinance on the prevention and disposal of waste” (VVEA Art. 19. Para. 1) and should therefore be recycled as much as possible. Nevertheless, geogenic contamination may be relevant to the risk to ground water, surface bodies of water and soil depending on how the material is reused or put into landfill. In the course of developing the disposal concept, the relevant authorisations therefore had to be obtained from the local cantons. 40 000 tonnes of the geogenic excavated material contaminated with arsenic will be reused as backfilling for the prefabricated concrete elements in the project. The remaining 130 000 tonnes must be sent to the relevant landfill based on the level of the arsenic content. The excavated material will be transported to the intermediate storage area by conveyor belt and poured and sampled in aggregate heaps of approximately 1000 m³ using the pivoting belt discharge. Due to the limited space on the intermediate storage area, the time that the excavated material is held is only between two and three working days, which is the period available to determine the arsenic content. The arsenic occurs in the rocks primarily in the rock matrix, but also secondarily on the fissure surfaces.
Depending on where the content is measured, the measured values show very different concentrations. Different measurements from the same heap revealed very large differences. This prompted the developer to come up with a binding method approved by all parties for taking the samples and for determining the representative arsenic content. This is done with a weighted average of fissure surfaces and matrix values. Thanks to the agreed procedure, it is possible to determine that the correct disposal method for each heap is being complied with quickly and clearly for all parties.
4.2 Lot 2 – Excavation in the Opposite Direction Using Drilling and Blasting
In Tiefenwinkel, the east portal of the safety gallery lies on a steep slope on the cantonal Kerenzerberg road. Before it was possible to start sinking the pre-cut, rock clearing, rock covering and the preparation of rockfall protection nets were necessary. Due to the tight space available for the pre-cut (Fig. 14), a temporary lane reduction on the cantonal road was necessary.
14 | Tiefenwinkel pre-cut
The creation of the ventilation shaft by means of raise boring in Hochschleipfen first required an expansion of the steep access routes to the construction site area. Only after this it was possible to drill the 143 m deep pilot hole for the raise boring. The pilot hole cut through two fault zones, whereby as the result of too much drilling water being lost it was necessary to dismantle the drill rods and to fill the drill hole with cement to seal the rock. After successfully sinking the pilot hole and raise bore to the final diameter of 4.5 m (Fig. 16), it was possible to finally line the shaft with 25 cm of steel-fibre-reinforced dry shotcrete from a two-deck shaft sinking stage. The concreting work for the circular and 5 m high smoke outlet concluded the construction work in Hochschleipfen.
Whilst Lot 2 was scheduled to be completed by mid 2022, the construction work in Lot 1 will probably run to the end of 2024. The operating technology and equipment will then be installed in the new safety gallery and the maintenance work for the road tunnel will be started. The whole renovation process is expected to be fully completed at the end of 2026 or the start of 2027.