Abstract The Riedberg Tunnel, part of the National Road A9 in the canton of Valais in Switzerland, presents special requirements due to its alignment through the active Riedberg landslide area. Managing the expected differential displacements and deformations along the tunnel was a focus of the planning process and required innovative technical solutions. An important component of the overall project is the monitoring of the slope and the structure, as well as understanding the slope behaviour, which also forms the basis for measures aimed at reducing sensitivity and stabilizing the slope's movement.
Abstract The Semmering Base Tunnel was constructed from several intermediate access points, which required the establishment of the necessary construction sites. At these sites, slope cuts with related support measures had to be carried out. This article describes the approach taken in planning the support measures for two of these slope cuts, as well as the adjustments that became necessary during the different planning phases as the project progressed.
Abstract The Auebach Valley is the morphological expression of one of the main fault systems in the Semmering area. The varying lithological composition of erosion‐prone fault rock and groundwater‐bearing carbonates poses challenges for design and construction. In difficult conditions, overlapping exploratory drillings in front of tunnel drive are essential to improve the geological‐hydrogeological model and to assess geotechnical risks. However, under the prevailing geological‐hydrogeological conditions, the drilling methods reached their limits. Despite the ongoing exploration and concluding risk assessment, a very large‐scale water and ground inflow with subsequent tunnel face collapse and development of a sinkhole on the surface could not be prevented. The article outlines the reasons for the occurrence and the formulation of methods to deal with the issue. It addresses geological‐geotechnical analyses of the incident and explores the limitations of geotechnical surveillance in complex heterogeneous rock masses. Finally, an evaluation of the costs associated with managing the incident is conducted.
Abstract The Semmering Base Tunnel (SBT) crosses several geological units that contain significant ground water reserves. These aquifers are under special protection, as stipulated in the Environmental Impact Assessment (EIA) decision. The affected zones are predominantly located within carbonate rock mass blocks, whose properties ‐ particularly regarding permeability and strength ‐ vary considerably between formations. These variations impose stringent requirements on the planning and execution of tunnel excavation. Consequently, customized drilling and grouting concepts were developed for each formation. Particularly challenging for excavation are the intensely fractured and highly permeable carbonate rock mass of the Grassberg, as well as the marginal zones strongly affected by tectonic deformation. During the initial investigative drilling, water inflows exceeding 100 L/s were encountered, accompanied by significant material washout. The paper focuses on the excavation of the Grassberg carbonate unit and describes the technical measures implemented to protect the groundwater. To ensure safe tunnelling, stepwise measures were applied, including top‑down injections and a thicker primary lining with an almost circular profile. In addition, groundwater pressure‑relief drilling, which achieved discharges of up to 215 L/s, reduced the groundwater pressure from approximately 8.7 bar to 0.4 bar, further enhanced the effectiveness of these methods.
Abstract The excavation of the Semmering Base Tunnel (SBT) through fault zones under high overburden presents significant challenges, especially when these zones transition directly into a highly water‐bearing formation with groundwater pressures of up to 10 bar. Excavation under these conditions demands advanced tunnelling and support measures. To permanently reduce groundwater inflow, extensive pre‐excavation grouting was implemented to enhance the rock mass properties within the fault zone and reduce the permeability of the adjacent carbonate rock. Systematic exploratory drilling revealed a complex sequence of geological units at the transition zone from fault zone into the water‐bearing carbonate formation. As a result, grouting umbrellas with borehole lengths of up to 100 m were installed. Large‐scale face instabilities within the fault zone required further adaptations of the tunnelling and grouting measures. Among other measures, geotechnical monitoring was significantly expanded, as the tunnel passes beneath a motorway and an anchored retaining wall located approximately 160 m above the tunnel crown.
Abstract An unexpected huge water inflow in one of the caverns of construction lot SBT1.1 of the Semmering Base Tunnel (SBT) triggered immediate emergency procedures to avoid a complete loss of access. The provision of safe construction conditions required the construction of large settlement basins underground to enable the temporary discharge of the water to the surface via approx. 250 m deep shafts. The final configuration of the underground structures needed to be adapted to meet the encountered conditions. Extensive preparation and injection works were caried out to create sound and safe conditions for the operational phase of the tunnel system. This was only possible by an open minded, solution oriented and common approach of all stakeholders involved in the project.
Abstract The Swiss municipality of Albula/Alvra has adopted an approach to handling the risks emanating from natural hazards which is based on a comprehensive risk dialogue. Since 2019, persons affected by a large landslide have been receiving up‐to‐date information, background knowledge and countless opportunities for dialogue and exchange. The dialogue establishes trust, which is the foundation the community needs to overcome the challenging and difficult situation.
Abstract Switzerland is planning a deep geological repository to safely dispose of all radioactive waste. After a multi‐stage site selection process involving nationwide screening, geological investigations and extensive deep drilling, Nördlich Lägern was identified as the safest location due to its tectonic stability and the favourable properties of Opalinus Clay, the host rock offering long‐term containment. The general licence application was submitted in 2024. A provisional repository concept was developed, comprising a surface facility and an underground facility accessed by three shafts. At 800–900 m depth, radioactive waste will be emplaced in drifts or caverns. Long‐term safety is ensured through a multi‐barrier system combining engineered and natural barriers. Construction is planned in phases, with emplacement starting in 2050. Extensive geotechnical investigations confirm the anisotropic yet predictable mechanical behaviour of the host rock. Tunnel boring machine (TBM) tunnelling and shaft sinking require careful design to address squeezing ground, lining overstressing and potential water inflow in the shaft in the layers overlying Opalinus Clay. The project will evolve further based on site‐specific data and future operational requirements to realise a safe, feasible and adaptable repository.
Abstract Tunnelling through a fault zone under high overburden presents significant challenges for all parties involved. The use of ductile shotcrete lining in combination with yielding elements has proven to be an effective and meanwhile is state of the art in deep rock tunnelling. This support concept was selected from the outset of the project for the excavation of the Grassberg‐Schlagl fault system—a major fault zone within Construction Lot SBT1.1 of the Semmering Base Tunnel (SBT). Based on extensive investigations conducted during the design phase, various support types were defined, with the most robust design including the installation of an additional rigid 2nd shotcrete lining. Using this approach, approximately 75% of the fault zone was successfully excavated. The transition to a roughly 600‐meter‐long section without re‐ consolidation posed significant challenges and required continuous adaptation of the tunnelling and support concept. Through the construction of a pilot tunnel and using innovative HS‐EPS yielding elements, this particularly demanding zone was ultimately traversed successfully. This article presents the development from the initial design approaches to the ongoing modifications of the support concept during execution, also accounting for the difficulties experienced during construction.
Abstract The design of flood protection levees requires careful consideration of the prevailing subsoil conditions. Where a highly permeable aquifer is overlain by a fine‐grained cover layer, such as silty sands, its comparatively low hydraulic conductivity plays a decisive role in limiting seepage flow underneath the dams. Due to these geological conditions, the Machland Nord flood protection system described here is built without the use of vertical sub‐surface sealing elements. Following the legal review, the operator, Machland‐Damm GmbH, commissioned a systematic delineation of areas in which interventions in the cover layer may adversely affect the hydraulic performance and structural integrity of the system. The developed assessment framework is based on the geometry of flood protection structures, hydraulic head differentials, and site‐specific subsurface conditions. The resulting recommendations define a “core zone”, assigning the components of the protection system overall priority, and an adjacent “verification zone”, where any intervention affecting the cover layer is subject to a defined set of technical assessments, including hydraulic analyses, global and local stability verifications, and checks against hydraulic failure. Using zoning maps, the framework provides the operator, regulatory authorities, and potential project owners with a basis for evaluating risks to the flood protection system and, where required, for designing appropriate mitigation measures.
Abstract Grouting has become indispensable in modern tunnelling. Not only does it serve to improve rock mass integrity, but it also makes a significant contribution to groundwater protection amid growing sensitivity toward water resources. The decision regarding the choice of grouting system and the justification for its application requires careful consideration of technical, economic, and ecological factors. During the construction of the Semmering Base Tunnel (SBT), geological and geotechnical boundary conditions in several sections necessitated post‐excavation grouting measures. The following paper presents insights and experiences gained from the post‐excavation grouting on the SBT, illustrating the applied measures, the achieved results and the lessons learned in relation to the surrounding geology.
Abstract In the Semmering Base Tunnel's construction lot SBT3.1, the western tunnel drives went through the large‐scale tectonic unit of the Semmering cover. The immediate area surrounding the structure is composed of karstified carbonate rocks and quartzite from the Permo‐Mesozoic era, as well as overlying mica schists and quartz phyllites from the Semmering crystalline base. In order to ensure the serviceability of the structure over its service life, it was necessary to detect the karst structures of the carbonate rocks in the surroundings of the structure and to implement rock improvement measures using appropriate methods. This article provides an overview of the karst exploration methods used for construction lot SBT3.1, the practical implementation, and the measures carried out.
Abstract During construction of the Semmering base tunnel, two shafts with a depth of approx. 250 m were excavated from the intermediate access Göstritz in lot SBT 1.1. During shaft sinking in poorly consolidated brecchiae, regressive erosion occurred along existing fractures behind the shotcrete lining, causing water and material ingress into the shaft. An extensive grouting campaign was executed, finally allowing for a significant reduction of water ingress and erosion phenomena. Subsequently, the modification of the groundwater management turned out as a key factor: The original external water collection system with ring drains outside the shotcrete lining was abandoned in favor of systematically draining the groundwater into the shaft. This modification significantly minimized new water pathways behind the shotcrete lining, thereby reducing potential erosion channels. For similar projects, it is recommended to catch groundwater in non‐erodible layers, avoid external drainage systems, grout spots with murky water ingress immediately, and minimize drilling through the lining.
Abstract The use of yielding elements is a highly effective method in deep rock tunnel construction for protecting the shotcrete, particularly in the early stages, from excessive stress caused by high rock loads. Installing multiple rows of yielding elements results in a ductile and flexible support. By deliberately allowing additional deformations, stresses are transferred into deeper rock regions in accordance with the NATM philosophy. Consequently, the stress‐strain relationship of the yielding elements must be adapted to the time‐dependent properties of the shotcrete in relation to the excavation rate. Experience at the SBT2.1 construction site has shown that, when using yielding elements, not only the precise location of the elements within the circumference but also their positioning within the cross‐section is key to success. It is also essential to determine the correct timing for closing the yielding elements in order to achieve a load‐bearing ring closure in a timely manner and avoid excessive loosening of the rock mass. Detailed geotechnical monitoring and continuous assessment of the observed system behaviour are crucial. This article discusses positive and negative experiences and describes measurement methods and procedures with the aim of successfully utilising yielding elements.
Abstract The article describes excavation and investigation measures for the Semmering Base Tunnel that became necessary due to the occurrence of swelling rock mass, particularly anhydrite. The swelling behaviour was primarily governed by the specific surface area of the anhydrite and its structural characteristics. Massive, scarcely jointed anhydrite exhibits low swelling potential, whereas fractured zones pose significantly higher risks. Extensive investigations during excavation, geological documentation, and laboratory analyses enabled a detailed assessment of the geological conditions. Based on these findings, the typical cross‐sections — especially the geometry of the base — as well as support measures to resist swelling pressures were defined. These measures include avoiding water ingress, applying sealing grouting to reduce the water flow into the tunnel, and designing the inner and outer linings to allow for time‐dependent swelling pressures. The complex geological structures resulted in uncertainties and required situational adjustments of the support systems during tunnelling. Overall, the article demonstrates that a flexible, geologically informed approach and close coordination between geology, tunnel design, and construction are essential to effectively manage the risks associated with anhydrite‐bearing ground.
This article presents a case study of a permanent avalanche protection embankment constructed at the Kaltenbachgraben avalanche path in the Gesäuse National Park, Austria. Given the strict environmental constraints and site‐specific boundary conditions, a vegetated geosynthetic‐reinforced soil structure was selected instead of a conventional massive stone‐faced embankment. The applied system consists of a prefabricated facing combined with polymeric coated steel woven wire mesh (PWM) reinforcement, allowing a steep dam geometry, rapid construction, and long‐term ecological integration. The geotechnical design was carried out in accordance with ONR 24806 and the EBGEO recommendations. Internal and global stability were verified using limit equilibrium analyses with the software GGU‐Stability, considering of load cases in accordance with ONR 24806, including horizontal avalanche loads. Construction was performed in a staged manner using locally sourced fill material. Construction‐accompanying static plate load tests were conducted to assess compaction quality and stiffness, revealing material‐dependent variability and underlining the importance of continuous geotechnical quality control. The results demonstrate that vegetated geosynthetic‐reinforced soil structures represent a technically reliable and environmentally compatible alternative for avalanche protection embankments in sensitive alpine regions.
Abstract The continuous excavation in contract section SBT2.1 of the Semmering Base Tunnel posed high demands on advance exploration, prediction, and the selection of countermeasures due to complex geological and geotechnical conditions. Rotary‐percussive drilling was applied to investigate unfavourable ground sections. However, drilling‐related influences and varying spatial orientations of geological units led to significant prediction uncertainties in certain sections, which affected the selection of the planned supplementary and special measures. The special measure SM3 (injection pipe umbrella) proved to be of limited effectiveness due to groundwater inflow into the ground, high requirements regarding prediction accuracy, low flexibility, and considerable technical effort. In contrast, the supplementary measure ZM1 (foam injection) demonstrated high effectiveness and flexibility. It enabled rapid stabilization of collapsed material, filling of voids, and a controlled resumption of excavation even in heavily disturbed ground sections. By providing additional openings in the shield, the injection range and thus the effectiveness of the measure could be further increased. The results show that flexible, non‐stationary support measures offer significant advantages in deep tunnelling under complex geological conditions. Based on these findings, practical “lessons learned” are derived for future TBM projects.
