Assessing Karst-Hydraulic Hazards in Tunneling—The Brunnmühle
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Author's personal copy Environ Earth Sci DOI 10.1007/s12665-015-4655-5 THEMATIC ISSUE Assessing karst-hydraulic hazards in tunneling—the Brunnmu¨hle spring system—Bernese Jura, Switzerland 1 1 1 1 Pierre-Yves Jeannin • Arnauld Malard • Demian Rickerl • Eric Weber Received: 23 December 2014 / Accepted: 10 June 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract Drilling a tunnel in a karst region requires the Keywords Tunnel Á Karst Á Hydraulic pipe flow model Á assessment of disturbances in the hydrogeological func- Inception horizons Á 3D Á Jura Á Switzerland tioning of aquifers and flow-systems as well as disturbances for the tunnel itself. A complete characterization of the karst hydrogeological functioning and the establishment of pre- Introduction vention measures are essential in order to insure a suitable construction. The paper relates investigations which were Communities of Twann and Ligerz (Bernese Jura, CH) are conducted in the frame of the construction of a safety gal- crossed by heavy traffic load. A 5 km long highway tunnel lery (Sisto) in the bernese Jura (Switzerland). This gallery is was built in the late 1980s. However, due to geological and located close to the outlet of a regional karst system and hydrological difficulties—partly due to the karst hydraulics— parallel to an existing highway tunnel, which met numerous , as well as to land-use conflicts the tunnel had to be shortened. disorders in the past due to the intersection of active karst Actually, road authorities want to add a safety gallery parallel conduits with a discharge reate exceeding 1 m3/s. The to the existing highway tunnel, and in the future, plan the KarstALEA method was applied along the Sisto trace in prolongation of the existing tunnel, as it was initially planned. order to identify sections with a high probability of karst However, the nearby communities are exclusively supplied by occurrences and to characterize the type of karst related the Brunnmu¨hle spring, which emerges from the karst aquifer, problems. The main issue turned out to be related to water directly downstream of the existing tunnel and the planned head and discharge, which could potentially be cut by the galleries… An assessment of the karst aquifer was mandatory tunnel construction. The system discharge rate may fre- prior to the construction of the security gallery, especially quently exceed 10 m3/s, and most of this water could about potential issues on the water quality. This assessment potentially flow into the tunnel. Therefore, a hydraulic had therefore two main issues: (1) potential impact of the model was set up in order to assess expected flow rates and tunnel construction on the tapped spring; (2) potential impact heads if conduits are intersected during the construction. of the karst hydraulics on the tunnel construction. Mitigation Based on this model and on meteorological and hydrolog- measures had to be planned for both aspects. ical real-time measurements, a predictive model was The KarstALEA method (Filipponi et al. 2012) has been established in order to anticipate high-flows events. applied on the site as it reveals the most appropriate method for assessing karst hazards (voids location, expected size and filling) along the tunnel trace and espe- cially for locating zones where the conduits upstream of the & Pierre-Yves Jeannin permanent and overflow springs are supposed to develop. [email protected] Besides KarstALEA, a hydraulic model was established, Eric Weber enclosing all natural and artificial aquifer outlets (perma- [email protected] nent springs, drainage devices, overflow springs, wells, 1 Swiss Institute for Speleology and Karst Studies, rue de la etc.). Geometry of the modelled conduit network was Serre 68, 2301 La Chaux-de-Fonds, Switzerland inferred from field and cave surveys, hydrological 123 Author's personal copy Environ Earth Sci monitoring and local dye tracing tests, as well as from This multitude of outlets and drainage devices induces a literature information. Based on this model, various sce- high degree of complexity of the hydraulic scheme (Bol- narios of conduits intersection (diameter, hydrological linger and Kellerhals 2007). Furthermore, no relevant stage, etc.) have been tested. These provide the range of the measurements of discharge rates did exist for all of these expected discharge rates in the gallery during the con- points, excepting for those of the Brunnmu¨hle spring and struction, which is crucial for dimensioning outflows the D-Stollen since 2012. devices and for initiating precaution measures. As indicated in Fig. 1, the trace of the Sisto develops between the Brunnmu¨hle permanent spring and the group of springs emerging in the Twannbach gorges. It seems Context and problem then obvious that active conduits are present in this domain and could be intersected by the gallery. The key-question is The drilling site is located along an anticline foothill in the then related to the expected position of the conduits Eastern part of the Jura Mountain (canton of Bern) along upstream of the Brunnmu¨hle spring and the provenance of Lake Biel (see Fig. 1). Geologically, this area is composed the groundwater from the upper part of the system. Does by South-East dipping pile of Jurassic and Cretaceous this groundwater flow along the Twannbach gorges axis or limestone, underlain by Oxfordian Marls (aquiclude). may it come from another part of the aquifer? The safety gallery (Sisto) develops approximately along the strike of bedding planes. The Brunnmu¨hle karst spring emerges at the base of the massif, at the contact between Methods the Portlandian limestone (top of the aquifer) and the Purbeckian marls, close to the elevation of lake Biel. It is KarstALEA mainly fed by the Malm aquifer (upper Jurassic limestone, from Sequanian to late Portlandian, *600 m of thickness) The assessment of karst-related hazards during under- which is dammed by the Purbeckian marls (early Creta- ground construction could be performed by applying the ceous). The projected cross section (Fig. 1) gives a repre- KarstALEA method (Filipponi et al. 2012). This method sentative overview of the geological context. results in the combination of four basic 3D models (Fig. 2,) 300 m to the north of the Brunnmu¨hle, several other and provides an interpretation of karst potential zones springs emerge in the Twannbach gorges: along the construction trace. – The Sauser spring is considered as a permanent spring; Models have been established on a rectangular zone it emerges at the foot of the gorges and its high-flow (6 km 9 4 km) encompassing the gallery project. discharge rate is expected to reach 300–500 L/s – The geological model is based on the 1/25,000 – Further upstream, the Wasserhooliloch, Gischeren and available geological map and cross-sections (Ha¨feli Schu¨ttstein caves act as overflow springs. While the 1964; Scha¨r 1971). It considers also the additional data Wasserhooliloch is active several times per year, which were collected in the frame of the highway Gischeren and Schu¨ttstein do overflow less frequently. tunnel (Kellerhals 1982). The model really focuses on As their entrances are located 44 m above the elevation the geometry of the identified aquicludes bounding the of the Brunnmu¨hle spring, it suggests that the hydraulic Malm aquifer (Oxfordian marls and Purbeckian marls). head considerably rises during high-flow conditions. Thank to dense and accurate information and data, it These caves are known along a few dozens of meters. was possible to define a relatively high level of Wasserhooliloch is 33 m deep. resolution. All these outlets are expected to belong to a regional – The hydrogeological model consists in adding specific karst system, which extends up to the top of Chasseral hydrological features into the geological model once it mounts. The catchment area of the system extends over has been fixed. Specific features are: location and more than 30 km2 as previously assessed in the frame of elevation of the main springs, information and location the Swisskarst project (http://www.swisskarst.ch). of underground sumps or flooded conduits and indica- A part of the karst groundwater was drained by the tion on the water tables that have been recorded in exploration tunnel (‘‘Stollen’’) drilled in the late 1980s, and different boreholes or piezometers of the site (only was diverted from natural springs. In the eastern section of those which intersect a karst conduit or enlarged the exploration tunnel (S-Stollen), a part of the karst fractures in the phreatic zone). The hydrogeological overflowing groundwater is diverted into the Twannbach model provides the distribution of the phreatic and gorges via F-Stollen, while in the western part groundwater unsaturated zones within the massif and—depending on is drained outside by through D-Stollen. the available data—fluctuations of the epiphreatic zone. 123 Author's personal copy Environ Earth Sci Fig. 1 Overview of the site and geological structuration; Br Brunnmu¨hle spring, Ws Wasserhooliloch cave, Gs Gischeren cave, Scht Schu¨ttstein cave, Sr Sauser spring – The speleogenetic model makes it possible to distin- to the previous stages of karstification. It is mainly guish the type and the potential infillings of the based on speleological data (caves surveys, in-cave expected voids or conduits within the massif according geological description) and geomorphology. 123 Author's personal copy Environ Earth Sci Fig. 2 Basic models of the KarstALEA method and their combination for hazards assessment – The inception model derives from the geological one. It Field campaigns provides the 3D geometries of the stratigraphic and tectonic horizons where the karstification preferentially In addition to the establishment of the models, continuous develops. As for the speleogenetic one, this model monitoring and measurement campaigns have been per- based on speleological, geological and geomorpholog- formed in order to understand the hydraulic relationships ical observations.