The south bypass Küssnacht – The black hole in the Swiss cheese

The more you take away,
the bigger I get.
The more you add,
the smaller I get.

What am I?

Of course, a hole! Like the Americans, who are proud of the holes in their doughnuts, and the Scots, who are proud of their lake monsters, the Swiss are proud of the very holes that make the Emmental-cheese so famous. Where they do not like holes, though, is in (besides socks) the tunnelling industry – because in this case the one big hole which runs through the mountain is more than enough. Those caused by ingress of water, collapses and other undesirable occurrences, however, are not wanted.

Unfortunately, this was the case with the project of the municipality Küssnacht in the canton Schwyz. Here, the resulting southern bypass will relieve the historic village centre of traffic in the future – but before that it was necessary to stuff some unplanned holes. Who wonders how such an unplanned hole looks like, should have a look at the pictures and videos in blog of Radio Pilatus. Patrick Gabriel, civil engineer at the Züblin Spezialtiefbau GesmbH, will tell us more about the activities in Küssnacht.

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Extensive measures for propulsion securing at the ascend of a shallow lying loose-rock-propulsion within the city area

The heart of the diversion of Küssnacht is the tunnel Burg. It underpasses the existing buildings with low covering in the loose-rock-advancement. The majority of the tunnel lies below the groundwater level and runs almost exclusively through glacial lacustrine and gravels. Large parts of the glacial lacustrine are firm to very stiff clay that shows layered deposits of silty fine sands with dense storage. The glacial lacustrine show high strength and very low permeability. The gravels on the other hand consist of alternating strata of sandy-gravels and sandy strata with a thickness in the decimetre to metre range. Also here, the storage density is high.

picture of the working face

picture of the working face (Credit: Züblin Spezialtiefbau GesmbH)

Almost along the entire route of the tunnelling operations are residential buildings on the surface. Starting from the North, the tunnelling commenced in September 2016. Around 14 sections were created by a propulsion securing of pipe machines, working face anchors and shotcrete. The foundation was homogeneous as expected, the deformations measured on the surface were high (with values from up to 4-5 cm), however, they were still in the forecast range for unfavourably assumed subsoil characteristics.

The problem

In July 2017, the tunnelling operations had to stop at the 15th sections due to an unforeseen mud ingress of about 10-15 cubic metres. Subsequently, a new exploration program from the surface was started in this section. The soil model had to be corrected based on the new probing because instead of cohesive clay, sandy cohesion-less glacial lacustrine with a thickness of more than a metre were sounded. The water level at this position was about 10 metres above the ridge. A water and mud ingress with unfavourable implications to the stability could not be precluded for the further tunnelling without any further auxiliary construction measures. Under great time pressure, alternative concepts were searched.

Even while the measures for the north side were clarified, it was decided that to start the cladding work below ground and to begin with the tunnelling work on the south side. In course of the tunnelling from the south, a strongly rolling, softened, gravelly material encountered, which is why the outbreak of the calotte was done in multiple subareas due to reasons of work safety. On the 20th of September, subsequently a collapse occurred caused by the pipe-roof pipes which led to subsidence damages at 17 metres coverage. The collapse funnel in the south had an approximate volume of 350 cubic metres and reached a diameter of 5 metres on the surface. After this incident, the tunnelling work from the south was also stopped.

The new concept/the solution

In July 2017, the tunnelling operations had to stop at the 15th sections due to an unforeseen mud ingress of about 10-15 cubic metres. Subsequently, a new exploration program from the surface was started in this section. The soil model had to be corrected based on the new probing because instead of cohesive clay, sandy cohesion-less glacial lacustrine with a thickness of more than a metre were sounded. The water level at this position was about 10 metres above the ridge. A water and mud ingress with unfavourable implications to the stability could not be precluded for the further tunnelling without any further auxiliary construction measures. Under great time pressure, alternative concepts were searched.

Even while the measures for the north side were clarified, it was decided that to start the cladding work below ground and to begin with the tunnelling work on the south side. In course of the tunnelling from the south, a strongly rolling, softened, gravelly material encountered, which is why the outbreak of the calotte was done in multiple subareas due to reasons of work safety. On the 20th of September, subsequently a collapse occurred caused by the pipe-roof pipes which led to subsidence damages at 17 metres coverage. The collapse funnel in the south had an approximate volume of 350 cubic metres and reached a diameter of 5 metres on the surface. After this incident, the tunnelling work from the south was also stopped.

The new concept/the solution

After an intense research of the cause and a reappraisal of the consisting security concept by the project manager and the executive tunnelling companies, the advancing concept was changed. At the north-side, a concept of groundwater lowering was created. This is done by multiple filtering wells from the surface, foregoing drainage drillings, injections and horizontal jet grouting pillars between the pipe-roof pipes which cause a consolidation of the ground between the pipe-roof pipes. On the south-side, a concept with horizontal jet grouting pillars between the pipe-roof pipes was also chosen. Before reassuming the excavation, the collapsed object was filled with injections and stabilised. For this work, the Züblin Spezialtiefbau GesmbH was subcontracted.

Auxiliary construction measures at the construction of the south bypass of Küssnacht

Auxiliary construction measures at the construction of the south bypass of Küssnacht [1, page 13]

From that point on, work was done alternately from both portals: either the excavation or the erection of auxiliary construction measures. The execution of the horizontal jet grouting pillars was done with a one-armed drill with a 28 metres long gun carriage. A depth of 20 metres was drilled, the horizontal jet grouting pillars featured a length of 17 metres according the plans.

one-armed drill with 28 metres long gun carriage

one-armed drill with 28 metres long gun carriage (Credit: Züblin Spezialtiefbau GesmbH)

Systematic monitoring of the deformations in the tunnel, in the rock and on the surface was part of the concept. In the ridge pipe of the pipe-roof pipe, the sag was monitored in real-time by an inclinometer measuring chain. Additionally, the deformations in the tunnel of the outbreak securing and at the working face were monitored. On the surface, the surveillance of the site and the buildings was done by automated tachymeters, water level gauges and manual levelling.

All´s well that ends well

Overall, more than 16,000 metres horizontal jet grouting pillars (of up to a length of 17 metres), more than 1,100 metres of blowout prevented drainage drills (of up to a length of 20 metres) and around 1,300 cubic metres working face drainages (of up to length of 24 metres) were produced.

The extraordinary collaboration of all involved parties established the most important conditions for a successful breakthrough on 29 May 2019. Up to this point, around 43,000 cubic metres of rock, clay and gravel were excavated out of the mountain, and 8,100 cubic metres of shotcrete were installed as well as 21 high-concrete-soles and 13 vault blocs.

Finally, it can be noted that the chosen concept for outbreak securing was suitable to reduce the settling to an acceptable degree without complex compensation measures in the soil. The erection of horizontal jet grouting pillars in addition to the pipe-ridge is recommended for similar geological conditions to solve stability problems and minimise settling.

eguana provides overview

The realisation of the project was also exciting and pleasing for eguana since we could use eguana SCALES.jetting for the first time for horizontal jet grouting. Like in Karlsruhe (link), it was the intention to give the constructor a live-view on the status of the works in the tunnel. This was done with the help of eguana maps and step by step visualisation of the screens. By the superimposing of construction sketch, road map and objects, on the basis of the site plan it can be quickly and easily identified that the tunnelling advancement from the south started later and that the securing concept was established after section 14. In the cross-section, the jet grouting pillars in the pipe-ridge and the working face pillars are referenced with the respective data set. The colour gives feedback on the status of the work (green, since all pillars are finished and shared).

visualisation of the jet grouting works on the map and cross-section visualisation.

visualisation of the jet grouting works on the map and cross-section visualisation. (Credit: eguana)

The construction management and the client could directly see the significant progressions of the drill and jet parameter with a chart overview. Thus, the logging and verification was simple and intuitive so one could focus on the extraordinary challenges of the south bypass of Küssnacht. Merci to Patrick Gabriel and the whole on-site team – we are looking forward to exciting future projects with the Züblin Spezialtiefbau GesmbH!

Author: Patrick Gabriel

 

Can you already be a complete civil engineer with 30? Patrick Gabriel can! The passionate climber and talented musician (he plays 7 instruments – and that by no means sinful but heavenly good) was already as a young boy fascinated by constructions and could decipher all plans at the soft age of 5. Already during his studies at the Technical University of Vienna, which he finished with honours and almost in minimum study time, he worked with numerous planning companies and completed large projects in Ireland and Switzerland together with the Züblin Spezialtiefbau GesmbH. After his degree, he was drawn to geotechnical planning to further deepen his specialist knowledge, until he returned to Züblin Spezialtiefbau GesmbH via detours in 2018. Since then, he has been working in the technical back office where he combines his planning skills with his experience from the construction site. Chapeau!

Patrick at his first construction site.

Patrick at his first construction site. (Credit: Alexandra Gabriel)

Source directory and further information:

[1] Zimmermann, A.; Schneider, A.: Südumfahrung Küssnacht, Tunnel Burg – Lockergesteinsvortrieb unter anspruchsvollen Bedingungen, Swiss Tunnel Congress 2019

[2] Bote der Urschweiz: Spektakulärer Durchlag im Tunnel für die Südumfahrung, Seite 1; Seite 17, Ausgabe vom 31.05.2019

[3] Freier Schweizer: Ein Durchstich fürs Geschichtsbuch, Seite 2, Ausgabe vom 31.05.2019

 

Cover photo:
High concentration during the execution of auxiliary construction measures for the construction project Küssnacht South Bypass. Credit: Züblin Spezialtiefbau GesmbH

Michael Ouschan

Michael holds a diploma in Civil Engineering and Infrastructure Management from the Vienna University of Technology, focusing on construction and geotechnical engineering. Before we won him over, he gained professional experience at -among others- PORR Bau GmbH or Züblin Spezialtiefbau GmbH. For us at eguana Michael is in charge of costumer contacts, from technical telephone support in the early morning, to ideas and improvements for SCALES develop and implement, or even directly on site in the tunnel. It seems that he spends most of his well earned free-time on vacation and we enjoy great photos from such exotic places as Brazil, Dubai, Australia, Cuba or Carinthia (yes, we have a real choice Viennese here).