KARSTIC COLLAPSES IN THE TOURNAISIS
Genesis, evolution, localisation, prevention (Tournaisis, Belgium)
Olivier KAUFMANNFaculté Polytechnique de Mons (Belgique)
JURY : I. GODFRIAUX, professeur
B. F. BECK, docteur
A. BINI, professeur
J. M. CHARLET, professeur
C. CONTI, professeur
C. DUPUIS, professeur
C. EK, professeur
Y. QUINIF, professeur
A. RORIVE
Soutenance publique de thèse de doctorat pour l'obtention du grade de Docteur en Sciences Appliquées, le 15 décembre 2000.
ABSTRACT
In the Tournaisis area (southern Belgium), localized and sudden ground collapses (cover-collapse sinkholes) have been reported since the beginning of the 20th century. The sinkholes are most of the time circular or elliptic and cylindrical or cone-shaped. Their dimensions can exceed 10 meters in diameter as well as in depth.
In this area, an unconformable cover that mainly consists of Cretaceous marls, sandy and clayey Tertiary deposits and Pleistocene alluvial deposits overlies the Carboniferous limestone bedrock. The thickness of that cover may vary from a few meters up to more than 50 meters.
Although the relief of the 'Tournaisis' area is not typical of karst terrains, quarry faces show that paleokarstic features affect the siliceous limestone of the area. These features result of a weathering which consists in the progressive dissolution of the carbonates along the rock joints. This weathering process leaves a porous weathering residue sometimes preserved without significant volume reduction. The weathering products are to be found in large cutters ranging between 1 to approximately 10 meters wide and centered on diaclases affecting the limestone bedrock. Weathered zones where a sound rock roof remains may be found in the continuation of these cutters. Several hundred meters wide and 30 to 50 meters thick weathered zones, formed by the widening of many cutters, are also observed.
A digging as well as geotechnical and geophysical surveys, undertaken within the scope of this project on sinkhole prone sites, show that the sinkholes occur right over paleokarstic cutters or on the edges of wider weathered zones. A microscopic study confirms that the materials found in these cutters result of the dissolution of the carbonates within the original fine siliceous limestone. Geotechnical tests performed within the weathering products reveal that very soft materials remain deep in the cutters.
Since the thirties, a drastic decline of the piezometric heads of the limstone aquifer have been recorded in the Tournaisis area. The lowering of the piezometric heads is a well-known triggering factor of sinkhole occurrences in karst terrains; in particular when the piezometric surface is lowered under the top of bedrock. Such a lowering brings about more rapid underground waterflows and thus underground erosion responsible for sinkholes. In the context identified in the Tournaisis area, the lowering can have an additional consequence, i.e. the compaction of the soft weathering products which creates voids within the paleokarstic features. Thus, even if voids existed before the lowering of the piezometric heads, some others can result of this lowering. All these voids are likely to enlarge as a result of the collapsing of their roof and the erosion and transport of weathering products due to an increase in underground water velocities. The successive failures of the void's roof lead to its upward migration. This process goes on until the soil roof is so thin that it cannot support its own weight leading to the sudden collapse of the ground surface.
An historic record of these collapses reveals itself essential for managing the sinkhole geohazard and associated risks. In order to establish and update a database of the cover-collapse sinkholes that occur in the 'Tournaisis' area, a computer application, based on a geographic information system, has been developed. Then, a geohazard map of sinkholes has been drawn up on a regional scale on the basis of this database and geological and hydrogeological data.
In order to take the identified geohazard level into account, recommendations are formulated as far as measures prior to new developments are concerned. In this perspective, the appropriateness as well as the advantages, disadvantages and limits of the various geotechnical and geophysical methods available to delineate sinkhole prone zones have been examined in the light of the identified sinkhole formation context. This evaluation was based, on the one hand, on the physical principle of these methods, and, on the other hand, on applications previously carried out in the 'Tournaisis' area. The study shows that, among the various geophysical methods, microgravimetry and electric resistivity methods seem to offer the most possibilities. Within the scope of this study, electric tomography proved a valuable investigation method in identifying and delineating sinkhole geohazard zones. The inversion of apparent electric resistivities combining measures with wenner-schlumberger and dipole-dipole arrays along lines spread out on the investigated site enabled to reconstruct a series of undergroud sections. These sections reveal the paleokarstic features affecting the underlying bedrock. They have been checked with systematic CPT tests and borings. The sections have been interpolated so as to draw up a series of resistivity maps corresponding to different depths. These maps have also been compared with a similar map drawn up on the basis of geotechnical tests. The comparison of the tomographies with geotechnical data enables to validate the earth resitivity tomography results. This shows that the method is espacially suitable for identifying and delineating paleokarstic features from which sinkholes form.
This study finally presents an overview of adequate measures that can be undertaken in the 'Tournaisis' area in order to protect exposed facilities in collapse prone zones and to remedy the damage which has been brought about.
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