Faculté de l'environnement naturel, architectural et construit ENAC, Programme doctoral Environnement, Institut des infrastructures, des ressources et de l'environnement ICARE (Laboratoire de géologie de l'ingénieur et de l'environnement GEOLEP)
Spatial analysis of karst conduit networks and determination of parameters controlling the speleogenesis along preferential lithostratigraphic horizons
Thèse Ecole polytechnique fédérale de Lausanne EPFL : 2009 ; no 4376.Add to personal list
- The main objective of this thesis is to improve the understanding of the position and characteristics of karst conduits within a rock massif. Such a characterisation is an important issue in civil engineering and in hydrogeology. Today in practice dissolution voids are considered as random in most cases. However, it is obvious for karst researchers that dissolution void distribution is not random, but defined by parameters controlling the speleogenesis. We developed a method to analyse the 3D geometry of cave systems in order to demonstrate from a statistical point of view that karst conduits position is not random. The analysis of several among the largest cave systems in the World (more than 1500 km of analysed cave conduits) confirmed for the first time quantitatively that the development and position of karst conduits under phreatic conditions is strongly related to a restricted number of so called inception horizons. An inception horizon – a concept introduced by Lowe (1992) – is a part of a lithostratigraphic succession that is particularly susceptible to the effects of the earliest cave forming processes by virtue of physical, lithological or chemical deviation from the predominant carbonate facies within the formation. We demonstrate that probably less than 10 % of the existing bedding partings of a limestone sequence are inception horizons and guide more than 70 % of the phreatic conduits. Our analysis clearly confirms that the influence of these horizons onto the 3D geometry of cave systems is high. Based on the 3D analysis of cave systems as well as on field verifications, 18 inception horizons in six cave systems have been selected for field characterisation and sampling in order to identify the properties and processes that makes these particular lithostratigraphic horizons favourable to karstification. Around 200 rock samples from the horizons and the surrounding rock mass have been analysed. The results evidence that inception horizons have a thickness of some centimetres to decimetres and that it is possible to distinguish between 3 types of inception horizons: Inception horizons where the cave inception took place within the inception horizon (type 1); characterized by a slightly higher primary permeability, pyrite and quartz contents and lower matrix contents than the surrounding rock mass. Usually, fractures propagate through or occur within these horizons. Inception horizons where the cave inception took place at the contact with the inception horizon (type 2); characterized by a lower primary permeability and carbonate contents, but higher pyrite contents than the surrounding rock mass. Fractures usually are ending at these horizons. Inception horizons where the cave inception took place along bedding plane fractures (type 3); already a slippage of just a few millimetres, striation, brecciation and surface irregularities enhance openings along the sliding plane and cause a significant increase in permeability. Furthermore it can be assumed that for the inception of horizons of type 1 the primary permeability will be the relevant factor at beginning of karstification, whereas both the matrix and the pyrite contents are the key factors during the later phases of cave inception and gestation. Whereas for further cave development the total carbonate content will be crucial. For inception horizons of type 2, we can assume that the low primary permeability, the clogging of the pores by the clay minerals, the high content of pyrite (production of aggressive solutions within the horizon that concentrates the dissolution along the contact to the surrounding rock) and the ending of the fractures at the horizon are responsible for the enhanced karstification at the contact during cave inception and gestation phases. Using simple hydrogeological numerical modelling we show that an epigenic karstic rock massif can be subdivided into four speleogenetic zones: 1) vadose cave development zone above the water table, 2) the phreatic cave development and 3) gestation zone within the first tens of metres of the phreatic zone and 4) below them the inception zone. Each of these zones is characterized by typical speleogenetic processes as well as dissolution void distribution. Further, it was possible to explain and reproduce schematically the 3D pattern of different cave systems by using the position and orientation of the inception horizons and the history of the landscape evolution (i.e. the re- and discharge area). This forward analysis provides a first idea of the geometry of the conduits as well as a better understanding of the development of a karst system in time and space (vertical section). Finally, we evaluated the feasibility to combine the improved inception horizon hypothesis, to predict inception horizons, with other current applied methods to improve the prediction of dissolution voids. Furthermore, we proposed a scientific based risk assessment for underground engineering proposes. Essentially, it is now evidenced that it is possible to quantify the probability of karst occurrences inside a karst massif by reconstructing the hydrogeolgical history and identifying the few inception horizons that guide the karstification at a regional scale.