INVESTIGATIONS ET CARTOGRAPHIE DES ALEAS KARSTIQUES DANS UNE ZONE TECTONISEE COMPLEXE DES AVANT-MONTS DU JURA NORD-OCCIDENTAL (FRANCE)

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Abstract

ABSTRACT. Investigation and mapping of karstic risks of a complicated tectonic zone of the mountain front in north-west Jura (France). In the sector North West of the Jura Massif construction of the high speed train TGV required the definition of a method for resolving karstic hazards. The geographical zone, with a length of 140 km and 1 km width , spreads between the valleys of the Saône to the West and of the Savoureuse river to the East.. This sector includes some thrust faults and transverse faults affecting karstified limestones dated Triassic to upper Jurassic. Nu­merous dissolution forms were associated to water flows. 110 springs, 206 dolines, 6 underground caves and 21 swal­low holes were noted. Different types of maps have been plotted: geomorphological features, diameter size of dolines and depth of dolines. The synthetic map of karstic risks is a combination of the three previous maps and shows various zones indexed by 3 digits. The value of the total surface S_tf is assessed for each mapsheet. The zonal risk coefficient R is provided for each mapsheet by R = S.. a.+... + S_n. a_n where S. is the surface of the zone number i, a. is the weighting factor of the zone i and n is the maximum number of zones. The definition of the risk index I_k is estimated on the surface repartition of geomorphological features with the evaluation of the maximum risk coefficient Rmax in relation with six numbers Rmax = S_tf x 6 . The risk coefficient R ranges from 4.7 to 72.9 and the risk index ranges from 26.1% to 53.7%. This kind of evaluation is very sensitive to problematic sites where a special investigation is necessary with complementary geophysical and geotechnical data.

The field readings, statistically analyzed, led to a synthetic mapping of the karstic hazards. A multivariate statistical analysis shows a different time activation on the field of observed dolines. Two different cases of dolines have been observed: active ( 112) or passive (55). By means of a statistical analysis a time-dependent behaviour of dolines could be observed. The hypothesis is double: a modification of the transport of clays away from fissures or a chang of hydrological and morphological conditions. The depth of active dolines is a good factor and is well linked with diam­eter. This fact is not noted for the passive dolines. Another statistical treatment by the multivariate discrimination (Mahalanobis) method shows the same behaviour. Karstic phenomena are generally visible on the Ognon river fault system across the Triassic and Jurassic limestones with sometimes some dissolution disorder inside anhydritic marls.

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