TBMs a main focus at Salzburg GeoColloquium 22 Nov 2018

Roland Herr for TunnelTalk

As well as presenting a focus on TBM excavation operations, the 2018 Geomechanics Colloquium in Salzburg celebrated the 50th anniversay of the Austrian Society for Geomechanics (OeGG). In their presentation past president Georg Vavrovsky and current president Wulf Schubert traced the development of the society from a private colloquium about rock engineering and technological questions organised by eminent Austrian engineer Leopold Müller in his own apartment in 1951. In 1962 the group, known as the Salzburg Circle, founded the International Society for Rock Mechanics (ISRM) with Müller elected as its first President. In 1968, a national group of the ISRM, founded the OeGG Austrian Society for Geomechanics.

Vavrovsky (right) and Schubet presented society history
Vavrovsky (right) and Schubet presented society history

The aim of the Society is to:

  • promote scientific research in all fields of geological and geotechnical engineering;
  • establish and maintain links to related societies;
  • exchange experience with all those involved in geotechnical research, construction, and administration;
  • improve design and construction methods; and
  • research and publish papers and guidelines.

Within the Society, there are four sections:

  • Engineering Geology
  • Soil Mechanics
  • Rock Engineering
  • Tunnelling

Presently the OeGG has about 598 regular and 40 supporting members, representing one of the largest national groups of the ISRM. Each October the Society organizes the Geomechanics Colloquium in Salzburg, which includes every two years, the Tunnel Day conference of the Austrian National Committee of the International Tunnelling and Underground Space Association (ITA) to discuss international and Austrian design, planning and construction concepts and present the latest state of the art of tunnelling technology.

Stipek President of Austria National Committee in the ITA
Stipek President of Austria National Committee in the ITA
Galler, Chairman of the Austrian National Committee of ITA
Galler, Chairman of the Austrian National Committee of ITA

During the 2018 Tunnel Day, Wolfgang Stipek, President and Robert Galler, Chairman, presented the Innovation Award of the Committee to the development of a water-permeable annular grouting material for a single-shell draining segmental lining as developed by STUVA, the Research Association for Tunnels and Transport Facilities in Germany and its research partners IMM Maidl & Maidl (Germany), Porr (Austria) and MC-Bauchemie Müller (Germany). The development succeeds in exploiting the cost advantages of a single pass tunnel construction under high groundwater pressures.

Until now, segmentally lined tunnels under groundwater pressures of 5 bar and above have required a complex and expensive double-shell construction, with a drainage layer between the inner and outer shells. With the newly developed pump-able annular gap material, this is no longer necessary, as the material has a sufficiently high permeability to be used as the drainage layer. The core idea of the development is the defined foaming of cement suspension to achieve an open-pore, water-permeable structure with sufficient strength to guarantee the integrity of the single-shell segmental lining.

Among presentations at the 2018 conference there were several that focused on TBM operations.

TBM proves effective for Bossler drives

Stuttgart-Ulm rail route
Stuttgart-Ulm rail route

In his presentation, Kurt Joham of Porr, explained the technical and contractual challenges encounded due to the changes from a cyclic to a continous tunnelling operation for the Bossler tunnel contract on the Stuttgart-Ulm mega railway project in Germany. Construction of the 8.8km long twin tube tunnel had originally planned the majority of the length to be excavated using conventional open-face methods with a double-layer shotcrete lining and about 2.8km intended for TBM excavation. In the course of further exploration, geological conditions turned out to be more favourable than expected and the TBM was used to excavate nearly 7.8km of the first tube and 8.8km in the second tube - almost the entire tunnel distance. The work progressed quickly and with completion of the Bossler tunnel, a total of 47km, or about 75%, of the tunnels on the new Stuttgart-Wendlingen-Ulm line have been excavated.

Challenges and special measures during tunnelling in lacustrine clay

Excavation of the 665m long twin-track Untersammelsdorf Tunnel on the Koralmbahn rail line between Graz and Klagenfurt in Austria, is through extremely challenging ground conditions consisting of silty to fine sandy lacustrine deposits and under a maximum cover of about 33m. Alexander Bahr of Baresel, Germany, reported on the comprehensive experience with the very challenging ground conditions describing wide-ranging underground engineering measures adopted to provide for successful excavation. Numerous challenges considered in the design phase were verified by carrying out extensive trials and investigations in the preparatory period or for which fallback levels had to be provided for the support system. Excavation work is almost complete and the tunnel is to be handed over to the client by the end of 2020.

Untersammelsdorf double-track 133m<sup>2</sup> cross section
Untersammelsdorf double-track 133m2 cross section

Follo Line TBM experiences

Johannes Gollegger of Bane Nor discussed relevant parameters for selecting double-shield TBMs for excavation of the 22km long twin tube Follo Line railway project in Norway (Table 1).

The open gripper TBM, he explained, is most efficient in competent rock with little fracturing and low water ingress. If support measures have to be applied right after the shield the productivity reduces significantly. Road and railway tunnels usually require the installation of an inner lining after the open gripper TBM excavation, which introduces an extra construction step and extends the construction time.

The single shield TBM can cope with varying rock mass behaviour including water ingress. The main drawbacks are the required excavation stop for ring building and the application of thrust force to the segmental lining. While this introduces a risk for damages, it avoids influencing the rock mass behaviour.

The double shield TBM is efficient if it can be driven in double shield mode, which requires stable rock mass with little outbreaks. To get a double shield TBM watertight requires between one and two days, if the sealings are still working when they are needed. Due to the working environment and possible rock mass instabilities, there is a high risk that the installed sealings may be subject to damages.

Table 1: Pros and cons of relevant TBM type characteristics
Parameter for TBM Open Gripper Single Shield Double Shield
Safety Exposure to
unsupported rock mass
Protection through
shield and lining
Protection through shield and lining
Rock support installation About 5m behind the face, preferred to install in
backup area
About 10m behind the face, drill distance to face about 5m About 13m behind the face, drill distance to face about 10m
Capability to deal with high rock load Short shield, rock support Risk of getting stuck or damaging the shield, lubrication of shield may be required Higher risk of getting stuck or damaging the shield, lubrication of shield may be required
Capability to deal with varying rock mass behaviour Flexible due to adjustable rock support According to shield and segment design, pre-support drill distance to the face to be considered According to shield and segment design, pre-support drill distance to the face to be considered
Influencing rock mass behaviour Gripper can have negative influence Little influence Gripper can have negative influence
Ring building NA Time pressure Parallel to excavation, no time pressure
Annular gap filling NA Sufficient time during ring building Short time during regripping
Water management Pre-excavation grouting, drain water during construction, final solution depending on the drained or undrained lining concept Pre-excavation grouting, watertight segmental lining, TBM can be closed easily for emergency protection against high water ingress Pre-excavation grouting, water tight segmental lining, challenging to close the TBM for emergency situation against high water ingress
UtilizationI availability Highly dependent on required rock support Between 30% and 40% Between 40% and 50%

An optimized segment lining design for single shield and double shield TBM mode and for the various load situations including rock and water load, reduces the required amount of reinforcement and accordingly saves costs. On the other hand, such segments have a reduced capacity to cope with irregular load situations, such as local pressure during annual gap filling or post-grouting activities.

From the early stages of design, it was known that water ingress would be one of the main issues to deal with on the project. Accordingly, procedures have been developed and certain requirements introduced in the contract. For future projects with expected water ingress an even higher focus on the water management is required. This includes the design of the tunnel structure consisting of watertight segmental lining with water stop barriers in the annular gap, annular gap filling materials, as well as the cross-passage design. Last but not least the required water treatment should be well studied and proper equipment should be planned for the TBM and its backup that allow for efficient pre-excavation grouting or the installation of fore polling measures if needed.

The four 9.96m o.d. double-shield TBMs selected for the project are boring through various kinds of hard rock gneiss with banding and lenses of amphibolite and pegmatite. They are capable of coping with hard rock conditions and the potential risk of very low penetration rates and high cutter wear has not materialised to date. More than 75% of a total of 36km have already been excavated with pre-excavation grouting applied over several kilometres to control water ingress.

Planning main construction of the Brenner Base Tunnel

Kurt Mayer am Tinkhof of Amberg Engineering, Austria, explained how design of the main Brenner baseline railway tunnel come to TBM application from the geological investigation. “It is state of the art to drive long tunnels in Alpine areas using hard rock TBMs,” said Mayer, “but mechanised tunnelling has much less flexibility than conventional excavation, so a deeper analysis of the available geological and geotechnical fundamentals is needed. In the course of planning the mechanised drives of the main tunnels on the Austrian side of the Brenner Base Tunnel, a detailed tunnelling concept was created, which deals with the project-specific features and boundary conditions.”

The exploratory tunnel for the project, which is used for geological and geotechnical exploration for the construction of the main tunnels, is currently being excavated. Through additional analyses, the geotechnical model could be verified and a reliable update of the rock parameters and the associated critical decisions has been achieved. All further specifications regarding TBM requirements, additional and auxiliary measures, support concept, bill of quantities and construction time model were based on the prognosis of system behaviour.

Brenner Base Tunnel progress at March 2018
Brenner Base Tunnel progress at March 2018

The authors of the presentation are of the opinion that the client should be provided with any relevant information about the geological and geotechnical conditions to be expected in a clear and understandable form. At the same time, the entire tendered tunnelling concept must agree with this information. This fundamental strategy was applied thoroughly in the course of the design of the Brenner contracts on the Austrian side in order to enable fair and transparent collaboration and decision-making during the construction phase. The methods of analysis, which give reliable statements about almost all relevant aspects of system behaviour, are meanwhile state of the art - they just have to be correctly implemented. The systematic evaluation and transmission of the findings from the driving of the exploration tunnel enables a validation of the results, which backs up these statements. “It is not correct to concentrate on numerical analysis alone,” said Mayer. “Rather the appropriate analysis process has to be found for each expected failure mechanism.”

Colloquium for 2019

Save the date for the next Geomechanics Colloquium in Salzburg on 10 and 11 October 2019 when session topics will discuss urban tunnelling: a technical and logistical challenge and challenges during construction of the Brenner Base Tunnel. Prior to the Colloquium a one-day session on 9 October will include the young researcher's day and workshops on the requirements of maintenance and operation of tunnels and the design and handling of risks in geotechnical engineering.

In closing the 2018 event, the near 900 delegates from 22 countries enjoyed the networking and social events that are legendary at Colloquium gatherings in Salzburg, and took advantage of the 65 companies and 12 universities in the exhibition, which was always crowded and well presented a market place for discussions, talks and meetings.

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