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Abstract The Gotthard Base Tunnel (GBT) is a 57 km long railway tunnel, constructed in the Central Alps in Switzerland and extending mainly North–South across numerous geological units. We acquired 80 new gravity data points at the surface along the GBT profile and used 77 gravity measurements in the tunnel to test and constrain the shallow crustal, km-scale geological model established during the tunnel construction. To this end, we developed a novel processing scheme, which computes a fully 3D, density-dependent gravity terrain-adaptation correction (TAC), to consistently compare the gravity observations with the 2D geological model structure; the latter converted into a density model. This approach allowed to explore and quantify candidate rock density distributions along the GBT modelled profile in a computationally-efficient manner, and to test whether a reasonable fit can be found without structural modification of the geological model. The tested density data for the various lithologies were compiled from the SAPHYR rock physical property database. The tested models were evaluated both in terms of misfit between observed and synthetic gravity data, and also in terms of correlation between misfit trend and topography of the target profile. The results indicate that the locally sampled densities provide a better fit to the data for the considered lithologies, rather than density data averaged over a wider set of Alpine rock samples for the same lithology. Furthermore, using one homogeneous and constant density value for all the topographic corrections does not provide an optimal fit to the data, which instead confirms density variations along the profile. Structurally, a satisfactory fit could be found without modifying the 2D geological model, which thus can be considered gravimetry-proof. From a more general perspective, the gravity data processing routines and the density-dependent corrections developed in this case study represent a remarkable potential for further high-resolution gravity investigations of geological structures.

Abstract Drilling of the Gotthard Rail Base Tunnel through the Central Alps from 2005 to 2010 opened up fractured basement units and frequent water inflows provided access to the major fluid-rock interactions processes in orogenic crystalline upper crust. Construction of the 57 km long tunnel was divided into five different sections. Here we report data and observations from the 10 km long central Sedrun section 211 water samples were collected at inflow points 900 to 2350 m below the surface. The exceptional samples and data provide a comprehension of the hydrochemical evolution and solute acquisition of deep groundwater in basement units. The investigated tunnel section drilled through steeply dipping rock units and vertical fracture systems at high angle. It cuts across granite, gneiss and schist of the pre-Alpine basement and across two narrow zones of meta-sediments. Rock temperature along the Sedrun section varies from 30 to 45 °C depending on the thickness of the overburden. The fracture water is of meteoric origin and acquires its composition exclusively by chemical interaction with the surrounding rocks along the flow path. Water from inflow points in the basement of the Gotthard Massif has typically a high pH of about 10 and total dissolved solids in the range of 100 to 300 mg L−1. Sodium is the prime cation of most waters. Although plentiful in the rocks, calcium, potassium and magnesium are low to very low in water. The anions associated with Na are carbonate/bicarbonate, sulfate, fluoride and chloride in widely varying proportions. High fluoride concentrations of up to 15.4 mg L−1 are characteristic for most waters. As a result of the high pH dissolved silica (SiO2) reached concentrations of up to 58 mg L−1 and represents 25—30 wt% of the solutes. The meteoric recharge provides dissolved O2 and CO2 to the fluid-rock interaction processes. The solutes derive from the dissolution of feldspar (Na+, SiO2aq), oxidation of sulfides to sulfate (SO4 2−), alteration of biotite (F−), and fluid inclusions opened by brittle deformation (Cl−). The solids formed during fluid-rock interaction, mainly zeolites, chlorite (and other clay minerals) and secondary Fe-minerals, remove Mg, Fe, and K almost quantitatively from the water. The high pH results from hydrolysis of silicates. The data distinctly show that within the depth interval of 1.0–2.5 km below surface deep water in continental basement evolves to a low TDS, high pH, sodium carbonate and silica solution by interaction of gneiss and granite with infiltrating pristine meteoric water, snow and rain.

The Gotthard Base Tunnel (GBT) is a 57 km long railway tunnel, constructed in the Central Alps in Switzerland and extending mainly North–South across numerous geological units. We acquired 80 new gravity data points at the surface along the GBT profile and used 77 gravity measurements in the tunnel to test and constrain the shallow crustal, km-scale geological model established during the tunnel construction. To this end, we developed a novel processing scheme, which computes a fully 3D, density-dependent gravity terrain-adaptation correction (TAC), to consistently compare the gravity observations with the 2D geological model structure; the latter converted into a density model. This approach allowed to explore and quantify candidate rock density distributions along the GBT modelled profile in a computationally-efficient manner, and to test whether a reasonable fit can be found without structural modification of the geological model. The tested density data for the various lithologies were compiled from the SAPHYR rock physical property database. The tested models were evaluated both in terms of misfit between observed and synthetic gravity data, and also in terms of correlation between misfit trend and topography of the target profile. The results indicate that the locally sampled densities provide a better fit to the data for the considered lithologies, rather than density data averaged over a wider set of Alpine rock samples for the same lithology. Furthermore, using one homogeneous and constant density value for all the topographic corrections does not provide an optimal fit to the data, which instead confirms density variations along the profile. Structurally, a satisfactory fit could be found without modifying the 2D geological model, which thus can be considered gravimetry-proof. From a more general perspective, the gravity data processing routines and the density-dependent corrections developed in this case study represent a remarkable potential for further high-resolution gravity investigations of geological structures. [ABSTRACT FROM AUTHOR]

Drilling of the Gotthard Rail Base Tunnel through the Central Alps from 2005 to 2010 opened up fractured basement units and frequent water inflows provided access to the major fluid-rock interactions processes in orogenic crystalline upper crust. Construction of the 57 km long tunnel was divided into five different sections. Here we report data and observations from the 10 km long central Sedrun section 211 water samples were collected at inflow points 900 to 2350 m below the surface. The exceptional samples and data provide a comprehension of the hydrochemical evolution and solute acquisition of deep groundwater in basement units. The investigated tunnel section drilled through steeply dipping rock units and vertical fracture systems at high angle. It cuts across granite, gneiss and schist of the pre-Alpine basement and across two narrow zones of meta-sediments. Rock temperature along the Sedrun section varies from 30 to 45 °C depending on the thickness of the overburden. The fracture water is of meteoric origin and acquires its composition exclusively by chemical interaction with the surrounding rocks along the flow path. Water from inflow points in the basement of the Gotthard Massif has typically a high pH of about 10 and total dissolved solids in the range of 100 to 300 mg L−1. Sodium is the prime cation of most waters. Although plentiful in the rocks, calcium, potassium and magnesium are low to very low in water. The anions associated with Na are carbonate/bicarbonate, sulfate, fluoride and chloride in widely varying proportions. High fluoride concentrations of up to 15.4 mg L−1 are characteristic for most waters. As a result of the high pH dissolved silica (SiO2) reached concentrations of up to 58 mg L−1 and represents 25—30 wt% of the solutes. The meteoric recharge provides dissolved O2 and CO2 to the fluid-rock interaction processes. The solutes derive from the dissolution of feldspar (Na+, SiO2aq), oxidation of sulfides to sulfate (SO42−), alteration of biotite (F−), and fluid inclusions opened by brittle deformation (Cl−). The solids formed during fluid-rock interaction, mainly zeolites, chlorite (and other clay minerals) and secondary Fe-minerals, remove Mg, Fe, and K almost quantitatively from the water. The high pH results from hydrolysis of silicates. The data distinctly show that within the depth interval of 1.0–2.5 km below surface deep water in continental basement evolves to a low TDS, high pH, sodium carbonate and silica solution by interaction of gneiss and granite with infiltrating pristine meteoric water, snow and rain. [ABSTRACT FROM AUTHOR]

The multi-function station (MFS) Faido, with track crossovers located symmetrically around the transverse cavern, is accessed through a 2.7 km long access tunnel. The geology encountered in the transverse cavern and the first metres of boring the running tunnels was very different from the forecast. Technical, economic and scheduling considerations demanded the moving of the four crossovers by 600 m to the south. In the drill and blast excavation of the single-track tunnel to the north, deformations of up to 1.3 m occurred in the radius. In addition to the dangers of 'squeezing rock' and 'rock fall', unforecast and serious rock burst also occurred in the east and west running tunnels, which could not have been foreseen. Yielding temporary support measures had to be carried out along a length of 500 m and sometimes massively strengthened. Intensive probing in the form of hammer and core drilling and seismic methods (TSP - Tunnel Seismic Prediction) was carried out during the tunnel drives. The drives were also monitored with intensive rock mechanical measurements. The results of probing, the rock mechanical measurements and the interpretations of the encountered geology all confirmed that the decision was correct to move the MFS. Die Multifunktionsstelle (MFS) Faido mit symmetrisch um die Querkaverne angeordneten Spurwechseln ist mit einem 2,7 km langen Zugangsstollen erschlossen. Die angetroffene Geologie in der Querkaverne und den ersten Metern Vortrieb in den Einspurstrecken wichen massiv von der Prognose ab. Technische, wirtschaftliche und programmliche Gründe erforderten eine Verschiebung der vier Spurwechsel um 600 m nach Süden. Im Sprengvortrieb der Einspurtunnel Richtung Norden stellten sich Deformationen von bis zu 1,3 m im Radius ein. Zu den Gefährdungen 'Druckhaftigkeit' und 'Nachbrüchigkeit' kamen nicht prognostizierte starke Bergschlagphänomene in der Ost- und Weströhre hinzu, die nicht vorhergesagt werden können. Die Ausbruchsicherungsmaßnahmen mussten aufgrund der Bergschlagphänomene auf ca. 500 m Länge nachgiebig ausgeführt und zum Teil massiv verstärkt werden. Die Vortriebe wurden von einer intensiven Vorauserkundung in Form von Schlag- oder Kernbohrungen und mit Seismik (TSP - Tunnel Seismik Prediction) begleitet. Zudem wurden die Vortriebe mit intensiven felsmechanischen Messungen überwacht. Die Erkenntnisse aus den Vorauserkundungen, den felsmechanischen Messungen und den Interpretationen der vorgefundenen Geologie bestätigte die Richtigkeit der Entscheidung, die MFS zu verschieben. [ABSTRACT FROM AUTHOR]

The Sedrun section in the middle of the Gotthard Base Tunnel was critical in terms of scheduling and without doubt the section with the most complicated logistics. The article gives an overview of the complex overall construction measures and describes in detail the geotechnical and logistical challenges that had to be overcome. Der Teilabschnitt Sedrun im Zentrum des Gotthard-Basistunnels war zeitlich kritisch und zweifelsohne der logistisch komplizierteste Bauabschnitt, die bautechnischen Anforderungen lagen auf Weltrekordniveau. Der Beitrag gibt eine Übersicht über die komplexe Gesamtbaumaßnahme und beschreibt die zu bewältigenden geotechnischen und baubetrieblichen Herausforderungen im Detail. [ABSTRACT FROM AUTHOR]

The provisions of the planning approval for the Gotthard Base Tunnel required more than 1, 000 environmental measures for the Amsteg and Erstfeld sections, arranged in 80 detail projects. The environmental aspect was also of great significance for the client. Therefore AlpTransit Gotthard AG provided sufficient resources to be able to implement the project successfully from the environmental point of view. The article is concentrated on selected aspects and explains their special features. The integration of environmental monitoring into the local construction supervision team; soil protection and cultivation; the problem of nitrite in the site wastewater; and dust and noise emissions are dealt with in detail, along with the ecological opportunities of a large construction site. A description of the usual conditions and measures affecting a tunnel site is intentionally omitted. Die Plangenehmigungsverfügungen zum Gotthard-Basistunnel sahen für die Teilabschnitte Amsteg und Erstfeld mehr als 1.000 Umweltmaßnahmen vor, die in 80 Detailprojekten eingeordnet waren. Der Umweltaspekt war auch für die Bauherrschaft von hoher Bedeutung. Daher setzte die AlpTransit Gotthard AG genügend Ressourcen zur Verfügung, um das Projekt auch aus Umweltsicht erfolgreich umsetzen zu können. Der Beitrag konzentriert sich auf ausgewählte Aspekte und zeigt deren Besonderheiten auf. Im Einzelnen werden die Integration der Umweltbaubegleitung in die örtliche Bauleitung, der Bodenschutz und die Rekultivierung, die Nitritproblematik im Baustellenabwasser, Staub- und Lärmemissionen sowie die ökologische Chancen einer Großbaustelle behandelt. Auf eine Darstellung der üblichen Auflagen und Maßnahmen bei einer Tunnelbaustelle wird bewusst verzichtet. [ABSTRACT FROM AUTHOR]

The article describes how the great challenge of managing the material excavated during the construction of the Gotthard Base Tunnel was overcome. The recycling of material from mechanized tunnelling as high-grade aggregates and the development of high-quality concrete mixes demanded new methods and set new standards. At the same time, the environmental impact could be considerably reduced and considerable cost savings were achieved. Der Artikel beschreibt wie die großen Herausforderungen gemeistert wurden, die sich beim Bau des Gotthard-Basistunnels bezüglich der Bewirtschaftung des Tunnelausbruchmals stellten. Bezüglich der Herstellung von Ausbruchmaterial aus dem mechanischen Tunnelvortrieb zu hochwertigen Gesteinskörnungen und der Entwicklung von qualitativ hochstehenden Betonmischungen wurden neue Wege beschritten und neue Maßstäbe gesetzt. Gleichzeitig konnten die Umweltbelastungen deutlich reduziert und erhebliche Kosten eingespart werden. [ABSTRACT FROM AUTHOR]