Prediction of karst occurrences in underground engineering Summary

Transcription

1 Prediction of karst occurrences in underground engineering Summary The project will be dedicated to the identification of criteria for the geological and hydrogeological characterisation of a karstic rock mass. Academic research on karst genesis made significant progress along the last 25 years. Using these results a method allowing engineering-geologists to better forecast karst occurrences will be established. It will concern mainly preliminary studies and project elaboration. The research project will include three aspects: 1) Formulation of a methodology based on the ongoing SNF research project and on literature; 2) Application tests to real cases allowing to assess the effectiveness of the method and to improve the method (3 existing tunnels and if possible 1 in construction); 3) Recommendations making the method accessible for applied situations. Results of the research will be assembled into a document dedicated to professionals of underground works, providing a method and a series of recommendations adjusted to karst environments and accessible for engineering geologists. This will significantly improve anticipation of karst-related hazards. 1

2 Prediction of karst occurrences in underground engineering 1. Introduction 1.1. Context and aims of the project Recent tunnel constructions in Switzerland (see paragraph 1.2.), as well as over the World (e.g. China; Wang and Wang, 2006) have shown that karst related uncertainty is a major issue, leading to economic, social, security-related and environmental problems. Many specific problems occur in karst areas, which do not occur in other geological formations. Table 1 gives an overview of such typical karst-related problems. The main difficulty beyond these problems is the poor predictability of the location and characteristics of karst features (e.g. size of the conduits, phreatic/vadose, sediment filling). Typical problems for tunneling in karst areas Mainly for limestone and dolostone (gypsum and salt are excluded) Water Massive and abrupt water inflow during boring («water pocket») Temporary massive water arrival Large discharge rates in the unsaturated zone Highly variable discharge rates in the unsaturated zone Plugging of drainage work by calcite deposition Concrete corrosion Effect on the discharge rate of neighboring springs Effect on the water quality of neighboring springs Instabilities/settlements at ground surface due to drainage Voids Instabilities of the tunnel walls, roof and face Uncertainty about the stable rock thickness on top of the tunnel (epikarst, soils, conduits) Problem for boring/blasting/digging and rock bolting Problem for anchoring tunneling-machine Other problems (falling down of engines) Infillings Face instabilities Sudden debris-flow of karst sediments Problem for blasting/digging/boring Problem of support of the tunneling-machine Problem due to swelling of infillings Figure 1: Typical civil engineering risks related to karst 2

3 In their present state, the SIA norms 198 and 199 are quite laconic concerning karst problems during tunnel construction. Karst is mainly perceived as a risk specifically linked with some geologic formations, but no method for the risk assessment is provided or recommended. The assessed risk depends thus on the knowledge and the experience of the geologist working for the project. The frequency and importance of the encountered problems, however, prove that the karst related knowledge of the responsible geologists is often too general to allow an optimal evaluation. The understanding of the spatial distribution of karstic voids significantly progressed along the last two decades. It is now possible to quantify the probability of karst occurrences inside a karst massif by identifying the few inception horizons that guide the karstification at regional scale, reconstructing the hydrogeolgical history and identifying different speleogenetical zones (see paragraph 2.2.). The aim of the present project is to provide a method for the evaluation of the risk of karst occurrences for engineer geologists. The method will include recent scientific research results which have to be transferred towards the needs of applied geology. The proposed project will enclose three steps: 1) Method formulation: The method will be formulated based on conclusions of a recent SNF project 1 and on the scientific literature. It will mainly consist in translating selected aspects of fundamental knowledge of karst into applied recommendations for engineering geology. The method will roughly contain the following steps/considerations: Identification of lithological horizons (bedding planes), which are especially susceptible to dissolution (i.e. inception horizons); Characterization of today s and past hydrogeological conditions; Characterization of some parameters of karst conduits: size, degree and nature of infilling (water, sediments); Determination of zones with different probability of karst conduit occurrences ( karst risk ). 2) Method validation: The applicability and feasibility of the method will be tested by applying it to real cases (back analysis of existing case-studies (tunnels) and of 1-2 tunnels in construction). 3) Method improvement and final formulation: Recommendations allowing the method to be applied by engineer geologists will be formulated and discussed with various persons from the practice. At the end of this project a document will be available, designed for engineer geologists working on underground constructions. Geologists will thus benefit from a practical and applicable method allowing them to better anticipate the occurrences of karst phenomena. The method will also include some general concepts and methods, which will guide geologists on the way to estimate potential groundwater heads and flow regimes in the construction vicinity. The method will also deliver a useful background for engineers who deal with karst occurrences in underground constructions. 1 Karst-ALEA: a scientific base towards the prediction of dissolution voids geometry in limestone and dolostone, SNF Nr and

4 1.2. Karst related problems encountered in Switzerland Some cases encountered in Switzerland are shortly presented hereafter (NB: the list is not complete). It must be noticed that descriptions found were quite general but that a closer analysis of unpublished reports and data related to these case-studies could provide an interesting database. Furthermore karst incidents are often related to more than one of the problem type presented in figure 1. Water-related problems Tunnel of Twann (BE): Input of massive water arrivals into the tunnel (up to 4000 l/s). This lead to stop the tunnel construction since 1990 due to the uncertainty on the security of the construction and the impact on the nature (Bollinger and Kellerhals, 2007). Tunnel of Sauges (NE): Presence of karst voids, dry in dry periods, but presenting a discharge of up to 1000 l/s during floods. Flooding of the construction site and the machines in December 1997 (Jeannin and Blant, 1999). Tunnel of Flims (GR): In October 2002 a karstic conduit providing 800 l/s was hit. It induced an important modification of the underground flow in that area (Jeannin et al., 2007; Jeannin, 2007). Voids-related problems Tunnel of Vue-des-Alpes (NE): In January 1991 a large cave was found inducing the construction of a bridge inside the cave to continue the tunnel construction (Jeannin and Wenger, 1993). Tunnel Lopper near Alpnach (NW): Several fossil karst tubes filled with clay were encountered in December Some of them were of large dimensions (up to 10 m in diameter). Instabilities due to those caves led to the collapse of the ceiling. Much supplementary constructions (anchors, consolidations) had to be made to sustain the tunnel. Additionally, an intensive precipitation produced the arrival of l/s of water, from a karstified fracture. (Keller, 1984) Infillings-related problems Tunnel of Engelberg (OW): In August 2003 the tunnel construction hit a karst conduit filled with water under pressure. In June 2004, after pressure build-up due to snowmelt, 2000 m 3 of blocks and debris invaded the tunnel and partially destroyed the installations. The discharge was in the order of 1000 l/s. Total price of the tunnel construction has been increased by at least 70 mio CHF in relation with this karst occurrences (NZZ 23 rd of April 2009). Conclusion of this short overview of karst incidents in Switzerland This short overview shows that karst incidents are frequent and represent a major issue, since they lead to economic, social, security-related and environmental problems. The main challenge beyond these problems is to improve the predictability of the location and characteristics (e.g. active/fossil, phreatic/vadose, size, sediment filling) of the karst features. 4

5 2. State of the art This chapter is divided into two part covering two aspects. The first part presents the principles of karst development, and the second part the experience and knowledge available in predicting karst occurrences and their characteristics for civil engineering purposes Principles of karst development Most of large dissolution voids are produced by the process of karst. This process develops in soluble rocks, i.e. mainly in carbonates (limestone and dolomite), but also in gypsum, halite and sandstones and rarely in quartzite. Karstic rocks outcrops cover nearly 12 % of the continental surfaces (Ford and Williams, 2007), but carbonates and evaporite rocks stretch below more than 25 % of the continents. They cover about 20 % of Switzerland but expand in the underground over much larger areas. The presentation of the details of cave genesis is beyond the scope of this proposal. Only some selected elements relevant for the understanding of the project description will be presented here, for further details the reader is referred to the literature (e.g. Kiraly, 1975; Klimchouk et al., 2000). The dissolution kinetics of calcite as well as dolomite explains why dissolution capacity of seepage water can be active all the way through a karst massif: The dissolution rate strongly decreases when the solution reaches about 90 % of the saturation of dissolved calcite (e.g. Plummer and Wigley 1976, Dreybrodt 1988). This means that a solution in contact with limestone (e.g. in a fissure or a conduit) can still remain aggressive over kilometres (e.g. Groves and Howard, 1994; Dreybrodt et al., 2005). Therefore, the dissolution kinetics is a key factor for the development of a conduit network at kilometre scale (or even more) as observed in the nature. Authors investigating cave networks generally agree to say that conduits do not develop randomly but are related to discontinuities within the rock massif. Karstic rock massifs are pervaded by a network of discontinuities (joints, faults, bedding planes and beds). These are the primary flow paths for the groundwater and guide the cave development (e.g. Kiraly, 1975). Furthermore, various authors investigated the speleogenetic role of stratigraphic discontinuities observed that caves develop along a restricted number of bedding planes within the limestone series (e.g. Rauch and White, 1970; Waltham, 1971; Palmer, 1989). Lowe (1992, 2000) synthesised these works and introduced the inception horizon hypothesis. A number of case studies seem to confirm this hypothesis. However, only the results of our recent SNF-research project confirmed quantatively the role of inception horizons, showing that only a few discrete lithostratigraphical horizons (i.e. between 3 and 5 bedding planes in a given massif) guide more than 70 % of the phreatic conduits in the investigated karst systems (e.g. Filipponi, 2009; Filipponi et al., 2009). From a hydrogeological viewpoint a karst massif can be subdivided into three speleogenetic zones (inception, gestation, phreatic development and vadose development) with distinct speleogenetic processes as well as a specific dissolution void distributions (Filipponi, 2009; Filipponi and Jeannin, 2009). The phreatic cave development is concentrated in a zone of some tens of metres thickness that extends almost horizontally from the spring towards the recharge area (i.e. close to the water table). Above the ground water table the rock massif is unsaturated, and therefore the karst conduits development is mainly vertical (shafts) and often associated with fractures in this zone. In an alpine context such as in Switzerland the valley entrenchment by glaciers has always produced new hydrological boundary conditions to which karst systems had to adjust what caused a complex multi-phases speleogenesis (e.g. Audra et al., 2007) 5

6 A conceptual model could be derived from field observations: Karstification develops mainly along the first inception horizons located some tens (to hundreds) of metres below the groundwater table (fig. 2). This concept explains and makes it possible to predict, at least in a probabilistic way, the 3D pattern of different cave systems by using the position and orientation of the inception horizons as well as the history of the landscape evolution (i.e. the re- and discharge area) (Filipponi, 2009). Figure 2: Schematic 3D model of a karst conduit system: Karst conduits develop mainly along the first inception horizon located below the water table, along the intersection with joints. This provides a very good academic background for establishing an applied method for the prediction of the position of karst conduits for applied purposes. Summary of the Principle of karst development Limestone is soluble, what induces the development of a conduit network that may reach hundreds of kilometres in length. Conduit diameter ranges between a few centimetres to a few tens of meters. The conduit network drains the water from the catchment area towards the karst springs. The geometry and the characteristics of the conduit system depend mainly on the following factors: The position of the spring: Over geologic times, valleys are deepened. New springs and thus new conduit systems are formed, adapting the drainage to the entrenchment of the valley. Each fossil spring is thus correlated to a speleogenetic phase and its related passages. A study of the evolution of the spring position along geologic time considerably helps to improve the prediction. The presence of inception horizons: Inception horizons are geologic surfaces where karstification occurs preferentially in regional as well as locale scale. With the recent improvements of the inception horizons hypothesis, it is now possible to develop a method for the assessment of the probability of occurrence of karst ( karst risk ) at a regional scale: Although the reconstruction of the true geometry (i.e. the exact position of the karst conduits and their characteristics) is not possible yet, the method will provide tools and indications about the way to identify inception horizons in a rock mass as well as the position of paleo and present water tables. Based on these data the method will explain how to derive a 3D distribution of the probability of karst occurrences in a karst massive. 6

7 2.2. State of the art in predicting karstic voids for engineering purposes If much work is published concerning the genesis of karst systems, it is not the case for descriptions and analyses of karst-related problems in civil engineering. A few papers or books do exist (e.g. Waltham & Fookes 2003, Milanovic 2004, Jefferson et al. 2008), but we could hardly find any syntheses on the prediction of karst voids for underground constructions. It is surprising because a lot of experience has been acquired along the increasing number of tunnels drilled over the last 120 years. However, this experience is dispersed in unpublished reports. The following paragraph gives a short overview of what could be found in the published literature, although most of it does not address the subject directly. Many papers present case-studies related to the remediation of pollution after accidents, to problems due to water losses and/or floods depending on karst features or to ground stability problems (e.g. Eds. Yuhr et al., 2008). A good overview of applied problems in karst is given in the book written by Milanovic (2004). This author concluded that predictions of the position of karst features and caves failed quite often, leading, for example, to spectacular caves found. This is why Milanovic states: "The main problem [...] is determining the karst channel position [...]", and a few paragraph later: "Successful tunnel excavation requires reliable definition of cavernous zones to adjust the tunnel route to the geological conditions.". He also writes that "no geophysical method has been developed yet for reliably determining the spatial position of underground cavities [...]". The same type of statement occurs for instance also in Fazeli (2007) who writes in his conclusion: "Although the main purpose of these investigations is to determine the exact location of karst features; so far, there is not a single reliable method to achieve it.. Günay and Milanovic (2007) presented a study in Turkey where they wrote that borehole did not provide indications of the presence of karst features, although such features were seen at the surface (and even descended and measured by cavers)! The assessment of karst risks is relevant for the planning, excavation as well as operating of underground structures. Each phase has typical requirements on the site characterization. During the planning phase (which corresponds in the guidelines of the Swiss Association for Engineers and Architects (SIA) to the preliminary studies and projection phase; SIA-Norm 112) the delimitation of zones with different risk levels will help to decide about the location, geometry and dimensioning of the underground construction (e.g. to avoid the most vulnerable zones) as well as the choice of tunnelling technique. Tunnel section crossing zones of higher risk can be managed in an appropriate way e.g. by a preliminary reconnaissance in front of the working face (Pöttler et al., 2002). During the excavation phase (execution phase in SIA-Norm 112) the karst risk assessment requires the prediction and characterization of karst conduits (> 10 cm) some meters to tens of metres in front of the working face to prepare safety instructions (e.g. to control water inrush). The conditions encountered require often constructive and structural measures that should be chosen in a way to be safe and economically interesting (e.g. Marinos, 2001; Milanovic, 2003). During the operating phase the conditions must be set/controlled in order to guarantee the safety of the construction and workers/users at any moment. Karst hazards related to the potential evolution of nearby karst features should be evaluated too (Waltham et al., 2005). In the last years different techniques have been developed for the local detection of voids for the excavation and operating phase (e.g. detection of voids a few meters in front of tunnel working faces - e.g. Pöttler et al. 2002; Pesendorfer and Löw, 2004) and controlling of encountered karst structures 7

8 (e.g. Marinos, 2001; Milanovic, 2003). However, today no method is available for the prediction of karst occurrences at a more regional-scale (planning phases). However, the challenge consisting in predicting the position of karst conduits is not new and many attempts were made in the past. A rough prediction of the position of karst conduits can be obtained by tracing experiments. The map produced by Quinlan & Ray (1981) is a famous example of this type of prediction. However, the position is drawn only in 2D (plan view) and is not precise (± 500 m). By the way, it is based on hundreds of tracing experiments and thousands of borehole data! Another way to address this question is to study the hydrogeological conditions that prevailed along the speleogenesis. One first step in this direction was made at Centre d hydrogéologie de Neuchâtel (e.g. Kiraly, 1968; Kiraly et al., 1971). These authors measured the fracture characteristics (mainly their frequency) and derived conduit directions from these measurements. However, the strong changes in hydrogeological boundary conditions, and possibly even in the fracture characteristics, happening along the evolution of a karst conduit network were not considered in this approach. A series of authors, especially from France and Spain applied similar ideas between the late seventies and the early nineties. For example Eraso (1985) and Eraso and Herrero (1986) developed more sophisticated field measurement methods for the characterization of the medium (fractures and bedding planes). However, most of these authors did not consider the fact that dissolution changes the flow conditions along the speleogenesis. They only considered the geological setting and as the influence of non-geological boundary conditions (mainly the respective position of the recharge areas and springs) is at least as significant as that of geology, it was not possible to really understand and quantify the control of fractures on the development of karst conduit networks. Prediction of conduit position was therefore very limited with this type of approach. Numerical modelling is used to understand karstification. Models include the peculiar dissolution kinetics of calcite (e.g. Plummer and Wigley, 1976; Dreybrodt, 1988) and the change of flow conditions from laminar to turbulent when the size of the conduits increases (e.g. Dreybrodt et al., 2005). Such models allow to verify hypotheses and to generate systems according to a series of various initial/boundary conditions. However, so far models have not been much used for the purpose of conduit prediction (e.g. Kaufmann and Romanov, 2008), but rather for the assessment of the time duration at which conduit can develop. In other words, uncertainty in predictions did not significantly diminished until the development of the inception horizon hypothesis. For the practice no methodology is available for the assessment of karst occurrences at regional scale. to the main reason is a missing transfer of knowledge from karst research to karst applied domains (e.g. Bachus, 2005). Furthermore the social trend to increase the pressure to security, economic and environmental questions makes the planning phase increasingly more significant. The literature dedicated to applied geology and engineers is therefore vague and pessimistic about the prediction of underground karst voids. The present project intends to fill this gap and to provide the first real investigation method for predicting karst occurrences. The prediction of karst-related problems is so far associated uniformely to a geological formation (e.g. a limestone series), assuming that the occurrence of karst is random. The paper by Pöttler et al. (2002) sketches an approach aiming at improving the prognosis, but does not describe the method, which seems mainly related to the experience of the authors. 8

9 At a local scale geophysical methods have been developed recently for the detection of voids a few meters away from the front of a tunnel working face (e.g. Pesendorfer 2006; Ziegler, 2006) These methods are quite money and time consuming, especially if the whole limestone mass is considered as potentially karstified. Therefore, the proposed prediction method would help to focus the application of such methods on zones with a higher probability of karst occurrences, what would considerably optimize their use. The proposed scientific based method will provide a series of applied methods and tools for helping the project geologist to predict the potential occurrence of conduits as well as to assess some essential parameters of the karst massif (e.g. groundwater heads) necessary for any construction in this environment. Conclusion of the state of art In conclusion for this short state of the art, it can be noticed that much knowledge is available from the academic side and that a significant part of this knowledge can provide useful information for the assessment of karst problems with respect to civil engineering. No publication could be found in the literature on applied geology, concerning a prediction method for karst voids. The aim of the proposed project is to identify and synthesise aspects of the academic research which can be used for applied purposes and to translate them into practical and technical recommendations for tunnel geologists. They would also be useful for engineers for improving their understanding of karst environment. References Audra P., Bini A., Gabrovšek F.,Häuselmann P., Hobléa F., Jeannin P.-Y., Kunaver J., Monbaron M., Šušteršič F., Tognini P., Trimmel H., Wildberger A Cave and karst evolution in the Alps and their relation to paleoclimate and paleotopography. Acta Carsologica 36/1, Bachus R.C Geotechnical Analysis in Karst: The Interaction between Engineers and Hydrogeologists. Proceedings of the Multidisciplinary Conf. on Sinkholes and the Engineering and Environmental Impacts of Karst, Bollinger D., Kellerhals P Umfahrungstunnel Twann (a5): Druckversuche in einem aktiven Karst. Bulltin angewandte Geologie 12/2, Dreybrodt W Processes in karst systems Physics, Chemistry and Geology. Springer Series in Physical Environments 4, Berlin & New York, 288 p. Dreybrodt W., Gabrovsek F., Romanov D Processes of Speleogenesis: a modelling approach. Carsologica, ZRC Publishing, Ljubljana, 376 p. Eraso, A Método de Prediccón de las direcciones principales de drenaje en el karst. KOBIE (Serie Ciencias Naturales), n XV, Dip. Prov. De Vizcaya, Bilbao, España. Eraso, A., Herrero, N Propuesta de un Nuevo metodo de deduccion de las direcciones principales de drenaje en el karst. Jumar, Madrid. Fazeli M. A Construction of grout curtain in karstic environment case study: Salman Farsi Dam. Environ. Geology, vol 51/5, Filipponi M Spatial analysis of karst conduit networks and determination of parameters controlling the speleogenesis along preferential lithostratigraphic horizons. Thesis. Ecole polytechnique fédérale Lausanne. Filipponi M. & Jeannin P.-Y Is it possible to predict karstified horizons in tunneling? Austrian Journal of earth sciences, 99: Filipponi M., Jeannin P.-Y Prediction of karst occurrences by interpreting borehole data within the Inception Horizon Hypothesis.. 11th Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst, Tallahassee. Filipponi M., Jeannin P.-Y., Tacher L Evidence of inception horizons in karst conduit networks. Geomorphology, 106, Ford D., Williams P Karst geomorphology and hydrology. - Unwin Hyman, London, 601 p. Groves C. G., Howard A. D Minimum hydrochemical conditions allowing limestone cave development. Water Res. Res., vol. 30 (3), Günay G. & Milanovic P Karst engineering studies at the Akköprü Reservoir area, southwest of Turkey. Environ.l Geology, n 51/5, Jeannin P.-Y., Blant D Boyaux karstiques recoupés par les tunnels N5 de Sauges (NE). Rapport non publié pour les Ponts et Chaussées du canton de Neuchâtel. Jeannin P.-Y., Wenger R Grotte du TM 800, tunnel routier de la Vue-des-Alpes. Stalactite, organe de la Société Suisse de Spéléologie, n 2/1993, Jeannin P.-Y., Häuselmann P, Wildberger A Modellierung des Einflusses des Flimserstein-Tunnel auf die karstquelle des Lag Tiert (Flims, GR). Bull. angew. Geol., Vol. 12/2, Dezember 2007:

10 Jefferson I., Rosenbaum M., Edmonds C., Walton N Subsidence-collapse: Occurrence, impact and mitigation. - Introduction to a special issue of Quaternary journal of Engineering Geology and Hydrogeology, vol. 41(3), Kaufmann G., Romanov D Leakage of dam sites in karst terrains. Geophysical Research Abstracts 10, EGU General Assembly, Vienna, Austria. Keller F Geologisch-geotchnischer Befund beim Loppertunnel. Strasse und Verkehr, Nr 11, November 1984, Kiraly L., Matthey B., Tripet J.-P Fissuration et orientation des cavités souterraines; région de la grotte de Milandre (Jura tabulaire). Bull. Soc. neuch. des Sc. nat., vol. 94, Kiraly L Eléments structuraux et alignement des phénomènes karstiques (région du gouffre du Petit-Pré de St-Livres, Jura vaudois). Bull. soc. neuch. sci. nat., vol 91, Kiraly L Rapport sur l état actuel des connaissances dans le domaine des caractères phsyiques des roches karstiques. In :Burger A. & Dubertret L. (eds), International Union Geol. Sci., Series B, 3, Klimchouk A., Ford D., Palmer A. N., Dreybrodt W. (eds) 2000, Speleogenesis, evolution of karst aquifers. - National Speleological Society Inc. (publ.), Huntsville, Alabama, USA, 527 p. Lowe D The origin of limestone caverns: in inception horizon hypothesis. PhD thesis, Manchester Polytechnic, UK, 511 p. Lowe D Role of stratigraphic elements in speleogenesis: the speleoinception concept. In : Klimchouk, Ford, Palmer and Dreybrodt (eds): Speleogenesis, evolution of karst aquifers, Marinos P.G Tunnelling and mining in karstic terrain: An engineering challenge. - In: Beck and Herring (eds): Geotechnical and Environmental Applications of Karst Geology and Hydrology, Milanovic P Prevention and Remediation in Karst Engineering. Proc. of the Multidisciplinary Conf. on Sinkholes and the Engineering and Environmental Impacts of Karst, Milanovic P Water resources engineering in karst. CRC Press LLC, 312 p. Palmer A.N Stratigraphic and structural control of cave development and groundwater flow in the Mammoth Cave region. In White W.B. and White E.L. (eds): Karst Hydrology, Concepts from the Mammoth Cave Area, Von Nostrand Reinhold, New York, Pesendorfer M Hydrogeologic Exploration and Tunneling in a Karstified and Fractured Limestone Aquifer. PhD Thesis, Swiss Federal Institute of Technology Zürich, 312 p. Pesendorfer M., Loew S Hydrogeologic Exploration during excavation of the Lötschberg Base Tunnel (AlpTransit Switzerland). - In: Hack, Azzam, (eds): Engineering Geology for Infrastructure Planning in Europe Plummer L.N., Wigley T.M.L The dissolution of calcite in CO2-saturated solutions at 25 C and 1 atmosphere total pressure. Geoch. Cosmochim. Acta, vol. 40, Pöttler R., Schneider V., Rehfeld E., Quick H Grundkonzept zur Lösung der Karst- und Erdfallproblematik für den Bau von Verkehrswegen. Felsbau, vol 20 (3), Quinlan J.F., Ray J.A Groundwater basins in the Mammoth Cave region, Kentucky. Friends of karst, occas. public. 1. Rauch H.W., White W.B Lithologic controls on the development of solution porosity in carbonate aquifers. Water Resources Research 6, Waltham A.C Controlling factors in the development of caves. Transactions of the Cave Res. Group of Great Britain 13, Waltham A.C. & Fookes P. G Engineering classification of karst ground conditions. Quaternary journal of Engineering Geology and Hydrogeology, vol. 36, Waltham T., Bell F., Culshaw M Sinkholes and subsidence Karst and Cavernous Rocks in Engineering Constructions. Springer.382 p. Wang X. & Wang M Anaylis of the Mechanism of Water Inrush in Karst Tunnels. Proceeding of the Underground Construction and Ground Movement Conference, Ziegler H.-J Vorauserkundung im Karst der Dodenhorn-Decke. In: Löw (eds): Geologie und Geotechnik der Basistunnels. Tagungsband zum Symposium Geologie Alptransit 2006, vdf Hochschulverlag AG,

11 3. Research plan The project work will be organised in three main steps: 1) The formulation of a method; 2) The validation of the feasibility and applicability of the method by its use on case studies; 3) The formulation of practical recommendations 1) Formulation of a prevision method Principles The academic studies (literature and SNF project) provide a sufficient theoretical base which can be used for the elaboration of a method to predict karstic voids. The prevision method will be based on the assumption that a significant amount of karst conduits occurs along the intersection between inception horizons and cave development zones related to levels of (paleo)water table. The analysis of the respective position of inception horizons relatively to the recharge and discharge areas, and to the duration of the phase (time between two valley incision events) makes it possible to assess the potential zone of conduit development, mainly around the level of the (paleo)water table. The hydrogeological boundary conditions of most cave systems changed along the geomorphic history of the massif, related to valley incision (mainly the position re- and discharge areas) therefore the karst conduit networks show a superposition of different phases (i.e. coexistence of voids at different karstification phases). It is why paleo water tables have to be considered too. The method thus consists in linking various existing evidences on inception horizons and paleo water tables, derived from geological data, to a probability of karst occurrence. This will make it possible to assign a risk level to some specific 3D volumes of a karst massif (table 1). To say it more simply it will be possible to subdivide a karstic rock mass into zones of different risk levels by means of the identification of the position of inception horizons in a rock mass as well as the reconstruction of the hydrogeological history. The main orientation of conduits can also be inferred from this approach, and, in some cases, further characteristics (e.g. conduit size and sediment-type) may be inferred too. The prediction of karst occurrences requires a given knowledge of the following concepts, which will have to be explained in a clear and concise way in order to make the method applicable: 1) Karst genesis: The tunnel geologist has to be able to link the existing data of known karst occurrences in the project region to a speleogenetical model of the area. 2) Karst hydrogeology: The tunnel geologist has to be able to correctly position the tunnel (or any other underground project) within its hydrogeological setting. Methods specific to karst are necessary otherwise the evaluation may be completely inadequate. 11 of 21

12 Description Tunneling Problems Water Voids Infillings Risk Level The rock is in the deep phreatic zone. Karst development is low. Only small and rare dissolution features (<10 cm) are expected. low The inception horizon is in the deep phreatic zone. Karst development is low. Only small dissolution features (<10 cm) are expected to occur. (x) moderate Vadose zone between the locations where karst is expected to occur. Small dissolution features (<10 cm) may occur. Vadose vertical conduits (>>10cm) with possible massive water flow during rain events or snow melting can be encountered. (x) x considerable Inception horizon in the vadose zone. Small phreatic dissolution features (<10cm) are likely to occur. Vadose vertical conduits (>>10cm) with possible massive water flow during rain events or snow melting can be encountered. x x high Inception horizon near the water table. Large phreatic conduits (>> 10cm) are expected. Main risk related to water. x x (x) very high Inception horizon is the vadose zone. Large (fossil or semi fossil) phreatic conduits (>> 10cm) are likely to occur. Possible massive water inflow during rain events or snow melting. (x) x x very high Table 1: The KarstALEA zones assign risk levels, based on the occurrence of inception horizons and the position of actual and paleo water tables. ( KarstALEA zones aléa is the French word for hazard.) 12 of 21

13 3) Preferred zones: The way to recognize preferred levels will be developed in an applicable manner for engineering geologists. For instance, the evaluation of existing caves and their position is a good criterion. The analysis of the morphology of a valley is another criterion. The way to obtain such data and to analyse them will be described as well as the way to assess their value and the confidence to the levels inferred. 4) Preferred horizons: The way to recognise preferred (inception) horizons will be developed and presented in a manner an engineering geologist can easily apply it. Among others the way to identify those horizons in boreholes, outcrops or from the regional geological literature will be described. It will probably lead to some specific observations in the preliminary studies. A ratio between information input and investigation costs will be sketched. 5) Prevision model: The way to combine steps 1 to 4 in order to make a prediction will be presented. An assessment method for the confidence of the prediction will be developed too. Indications about the way to infer information about the conduits (orientation, size, sediment, shape, etc.) will be given too. Up to now scientific (academic) studies remained very general concerning the way of studying a karst massif in order to predict karst occurrences. For the proposed prediction method, we will select the most determining criteria and use a pragmatic approach, in a way that an engineering geologist can make a prediction, in the best case without the help of a karst specialist. Sketch of the method The method will provide recommendations for the tunnel geologist to focus investigations in an efficient way onto the karst relevant data and aspects. It should thus not significantly increase the volume of investigations compared to today s situation. At this stage the method is expected to enclose the followings aspect/steps. Step 1: Sketch of karst in the project area 1) This step will at least include the analysis of the digital elevation model (DEM), of the geological map, of the position of the tunnel (or project), of the existing boreholes and geological cross sections. If available, horizontal cross-sections and/or isoelevation maps of some geological horizons will be welcome. Data on the water levels (springs and boreholes) will be also assembled. This will be based on the recommendation of the SIA-norm 199 with some special aspects concerning karst. Based on this data, a first model (including interpretations in 3D) will be sketched. 2) To make a first assessment of the spatial probability of finding karst conduits ( karst risk ) the model has to include some further data on the present and past hydrogeological conditions (water table levels) and some interpretation concerning potential inception horizons. The first prediction model will be preliminary and schematic, and it will be further refined along the preparation of the further steps of the method application. Ideally this model should be constructed in 3D. Clear criteria concerning the extension of the model around the project will be given. By the end of this step a first overview of karst in the project region will be available. This first model will strongly help the tunnel geologist to identify the necessary data and observations required to refine the prevision. 13 of 21

14 Step 2: Data collection and analysis A series of data, which are more specific to karst and especially to the prediction of karst occurrences, will have to be obtained at this stage. The method will clearly describe the various data, which could contribute to improve the basic model sketched in step 1. The potential significance for the prevision and the potential effort necessary for obtaining these data will be given, as well as practical information in order to make those methods as accessible as possible. The number of data to be collected in this step depends on the uncertainty we can keep in a project. It also depends on the budged dedicated to provide further parameters on the potential karst occurrences (e.g. heads and discharge, conduit size, conduit sediments, etc.). The following list gives a first overview of the data we expect to include. For all these data three major aspects will be looked for: identification of potential inception horizons, identification of paleo water table and understanding the hydraulics of the karst system in which the tunnel will be drilled. Geological data The lithological data and description will give a first overview of the spatial distribution of rocks susceptible to the karstification. Borehole data Identification of inception horizons: micro-voids visible with a camera or a scan of cores or borehole s walls, diagraphy data, any measurement of the porosity or permeability of the rock at a local scale (typically less than 1 m). Understanding karst hydraulics: any method allowing for the identification of hydraulic heads (low and high water) and hydraulic conductivities (e.g. fluid logging, packer tests, flowmeter, ). Rock outcrop observation Any observation providing information for the identification of inception horizons, of paleo water table or of hydraulics of karst will be useful (e.g. dissolution features along bedding planes, gypsum beds, water outlets). Information about the fracture pattern and density may be useful too (direct observation, outcrop 3D-scanning and analysis, etc). The geomorphic expression of fractures or of some bedding planes could be an indication of inceptions horizons. Speleological data The inventory and interpretation of karst landforms (e.g. cave position and geometry, cave type (vadose/phreatic), cave characteristics) allows the identification of inception horizons as well as paleo water tables, and the description of the conduit distribution and characteristics. Hydrogeological data Identification of the main flow systems (e.g. karstic/non-karstic, local/regional), delineation of catchment areas, delineation of the phreatic zone (low/high water stages). Identification of paleo water table: same type of observation methods but for small conduits (mm to dm) Geomorphological data Identification of present and past recharge and discharge zones by the identification of erosion/accumulation terraces, relation between quaternary glaciers and karst development, raw reconstruction of the valley incision history. 14 of 21

15 Geophysical data Application domains and limits of geophysical methods concerning the detection of karst features will be listed. This is important especially for tunnels located very close to the land surface. All these data have to be included into the model resulting from step 1. The method description will explain how to acquire and process these data in an efficient way. It is expected that most of the investigations recommended in steps 1 and 2 of the method will be considered as an appropriate level of investigations for the hazard assessment recommended by the SIA 199. Step 3: Prediction model The prediction model will include the synthesis and interpretation of data collected in steps 1 and 2. The method will describe the way to interpret the data and to make the assessment of the distribution of potential karst occurrences. The principle will remain the same: looking at inception horizons and paleo water table, and at their intersections. Some further interpretation hints will be added with respect to steps 1 and 2 concerning the assessment of the potential thickness of the paleo water tables and the probability distribution of conduits. This will include the interpretation of fracture patterns. This requires still some synthesis work of the data available in the literature. We will also provide hints to assess hydraulic heads and potential discharge rates. An option will be added to the method for tunnels in the vadose zone. Ideally the interpretation model as well as the risk assessment model(s) should be constructed in 3D. This would lead to 3D volumes of different probabilities of karst occurrences and voids characteristics. Although the most accurate way to make the synthesis and the interpretation will be to use simple 3D models, one version of the method will describe a way to use classical 2D documents (plan view and cross-section). It will take 5 months for a post-doc to make a formulation of the first complete karst prediction method. Concerning the application of the proposed method we assess the work related to steps 1-3 to a few man-weeks. 15 of 21

16 2) Validation of the method: case studies The formulation of the method described above will mainly be based on academic concepts, which will be translated into an applied method. This first version will thus have to be tested on real known cases. The correctness, applicability and required effort for a stand application of the method will be assessed along this validation procedure. As many problematic cases of karst occurrences in Swiss tunnels are reported and (in some cases) well documented, we expect to apply the method on those cases. Some cases in foreign countries may also be analyzed. This application will demonstrate the method applicability as well as its ability to make predictions. We will focus on 4 main case studies for the test. Tunnels will be selected among the existing cases in order to cover a wide range of situations. Their analysis will probably require some complementary field work in order to acquire the adequate data. This will allow for assessing the time and costs required for gaining the required information. One or two tunnels in construction will also be used for this validation. A close collaboration with the geologists working on them is expected. One tunnel could be the tunnel de Choindez (JU-BE), another tunnel the Umfahrungstunnel Brislach in canton BL. For each case study, we will first establish an initial prognosis on the base of reports that were realized before the construction (preliminary documents, observations and measures at outcrops). Then we will refine the model by integrating stepwise the data obtained during the construction works (boreholes, pilot tunnel, main excavation). We will thus see exactly what is feasible in practice, the effort needed to determine each parameter, its utility for the model, and in the end, the validity of the model itself. For each case study the data collection and analysis will take about one month. The collection of (complimentary) data will take another two weeks and finally the application of the method and its critique will take about two or three weeks. We expect the work on 4 case-studies to last 8 months. This will include the update of the method according to the experience acquired through the application to the case studies. A special effort will be dedicated to describe the most efficient way to apply the method, i.e. to show which data are the most necessary in which context, and what method is efficient to obtain the data. 16 of 21

17 Preliminary case study on the Weissenstein tunnel A first preliminary validation has already been attempted on the case-study of the Weissenstein Tunnel (3.7 kilometres long railway tunnel between Gänsbrunnen and Oberdorf, Solothurn/Switzerland). This tunnel; constructed in 1908 crosses the Weissenstein anticline at around 700 metres a.s.l., probably just below the water table. Although the tunnel walls are now concrete-lined the documentation of the tunnel construction (Buxtorf, 1908) indicates that the tunnel crossed karst conduits at three locations. Two of them were filled with sediments with only a minor water-flow. The third one had a stronger water outlet, with a discharge of around 80 l/s. Along the SNF PhD thesis on inception horizons we investigated the Nidlenloch cave system and identified four main inception horizons, which could be located within the stratigraphic series. As the stratigraphical position of the karst incidents in the tunnel was well documented, we could compare both data sets. We could thus note that the three observed karst conduits in the tunnel do correspond very well to three inception horizons identified in the cave. According to the Karst-ALEA classification they would belong to the zones of very high risk level of karst occurrences. This shows that our approach provides meaningful predictions. Figure: The karst incidents within the Weissenstein railway tunnel are located along inception horizons identified in the nearby Nidlenloch cave system (geological section Buxtorf, 1908). 17 of 21

18 3) Practical recommendations Based on the method described in point 1 and on its validation procedure described in point 2 we will first update the method according to results of the validation procedure. Then we will formulate practical recommendations for the assessment of the risk related to karst for tunnels. A practical guide (booklet) will describe the proposed method in order to make it accessible to geologists that are in charge of the evaluation of karst massifs. The method will be constructed to allow refinement of the prediction with every new more precise dataset obtained. The method will thus be applicable already during the preliminary study and the selection of the best trace. This guide will then be useful for the project study phase, as well as for the realization phase. It will also contain elements helping the geologists to estimate uncertainties and limits of the method. We will try to formulate the new method in a way that it can be inserted as an appendix to the SIA recommendations for constructions in solid rocks. Also a short course on the method could be taught. Five months are expected for this work. 4. New aspects of the project Neither the currently applicable norms nor the education of geologists do integrate the significant progress made over the last years about the genesis of karstic systems. The project will deliver a method that will synthesise the cutting edge knowledge of karst science into an applied method which will make it possible to predict the risk to encounter karst conduits in underground works. Such a method does exist neither in Switzerland nor in the rest of the World. The method will be mainly dedicated to tunnelling but will be also useful for other topics for which the prediction of the position of karst conduits is useful (e.g. ground stability issues, Groundwater management, archaeology, etc.). So far, karst was generally considered to be spatially random. The main novel aspect of the project is mainly to consider karst as a medium where prognosis is possible if adequate methods are applied. We would like to pinpoint that a better understanding of karstic problems in the conception and realization of underground works will not only improve its construction, but also reduce its effects on the environment, especially on groundwater and the associated springs. One usual problem in karst areas is the drainage by tunnels of huge groundwater quantities. These waters are difficult to master once encountered, and the consequence is that they drain water from the natural systems and thus deplete the discharge rate of the neighbouring karst springs. The method proposed in this project is novel because no equivalent method does exist anywhere in the World. 18 of 21

19 5. Benefit for the underground works The main benefits of the project are as follows: Reduction of the "geological risk" linked to karst; Proposition of a scientifically founded, normalized and applicable method; Reduction of impacts of tunnel construction on the environment. The development of the proposed method will improve the prognosis of karst occurrences in underground constructions. The reduction of risks that is induced will be of benefit for: the project owner (better scheduling with respect to deadlines and cost) the mandated enterprises (better handling of the situation) the workers (reduction of exposition to hazard) the users (better scheduling with respect to deadlines and cost) the environment (impact reduction) The developed method has a good potential to be recognized Worldwide. 6. Description of the expected deliverable The recommendation including the method description will be brought out in form of a brochure with about 50 pages. It could be published as an issue of the VSS bulletin. The targeted public will be tunnel geologists. Engineers may be interested in, at least in order to acquire some background concerning karst. We expect to organize 2 SIA 1-day courses (one in French and one in German) to present the method and provide basics for its application. Presentation in Conferences is also expected. 7. Organisation Project partners The project is mainly based on mutual competences of two institutions: The Laboratoire de géologie de l ingénieur et de l environnement de l EPFL (GEOLEP; Prof. A. Parriaux) and the Swiss Institute for Speleology and Karst Studies (SISKA, Dr. P.-Y. Jeannin). The association of these two institutions covers in an optimal way the aspects of engineering geology, especially for tunnels, and the knowledge of karst areas. This team already worked together for the SNF project, demonstrating its efficiency. The former PhD student who worked on the SNF project will further work as a post-doc at EPF-L to achieve this project within the expect time frame. SISKA will closely exchange and collaborate in order to inject its know-how into the project. 19 of 21

20 Collaborators to the project A committee of 2 geologists and 1 engineer will follow the project to guarantee its applicability. Two engineering geologists are already interested in participating: François Flury (bureau MFR-SA à Delémont) and Dr. A. Wildberger (Bureau Dr. Von Moos, Zürich). Both are very experienced in tunnel construction in karstic areas. A civil engineer (responsible for projects) from a private bureau (to determine) will also collaborate. Several engineers already announced their interest. It might be M. Vuilleumier (formerly at Bonnard et Gardel, Lausanne), who already has shown interest in the project. It could also be another tunnelling engineer, e.g. R. Daneluzzi from GGT (Jura). The final choice will also depend on the selected case studies. Prof. Marinos (Athens), very experienced in the karst tunnelling domain, should also take part in some of the group meetings. The authors of the project wish that this group can be staffed also with representatives from the OFROU/ASTRA, the GTS commission and collaborators from some cantonal offices. Calendar The project is foreseen for 1.5 years (tab. 2). Project Step Expenditure of Time Months since project start Formulation of the prevision method Formulation of the workflow Selection of investigation tools 1 mt 2 mt Formulation of the method 2 mt Applicability validation on case studies Reviewing of reports and available data Additional data collection Data compilation and analysis Evaluation of the case study Case study report 2 w for 1 case study 1 w for 1 case study 2 w for 1 case study 2 w for 1 case study 1 w for 1 case study Case study 1 Case study 1 Case study 2 Case study 2 Case study 3 Case study 3 Case study 4 Case study 4 Final report and practical recommendations Conclusion of the case studies Formulation of a practical recommendation (booklet) Meetings with the supervision committee 1 mt 4 mt 4 * 1 d Table 2: Calendar of the Project. 20 of 21