To carry out in situ research concerning the confining properties of geological formations. Research takes place in underground laboratories call "LEMI": Laboratoires d'Etudes Méthodologiques et Instrumentales.
The objectives of the LEMI are to provide the opportunity to carry out in situ experiments and to get samples in natural deep conditions, in order to be able to determine the properties of geological medium and to set a transfer model. The LEMI of Tournemire is devoted to the study of clay.
In situ research work concerning argillaceous formations has been started at the end of 1990. The selected site is a hundred year old disused railway tunnel, 1885 metres long, in the close vicinity of the village of Tournemire, in the South of France. This tunnel crosses a 200 metres thick Toarcian clay formation; the overlying limestone layers are 270 metres thick Toarcian clay formation; the overlying limestone layers are 270 meters thick, so the geotechnical and hydrogeological conditions can be considered as representative of those of a deep repository.
Many permeability tests at different levels have been performed (mainly pulse tests) as well as geophysical loggings. The results of the permeability tests show that the permeability of the two Toarcian and Domerian clay formations is very low. The permeability of the calcareous Carixian and Aalenian formations is substantially higher.
A detailed structural study has been performed, based on observation in quarries and on core samples. Three states of stress have been identified: a north south Pyrenean compression (Eocene), an east west extension (Oligocene), and an Alpine compression. Limestone formations are extensively fractured; in Toarcian clay, some faults have been observed but they are filled in by calcite.
The two main results were: the transient period to reach the steady state after a significant perturbation was very long (several years); the value of the ground water head at different levels in the Toarcian is close to the value of the topographic level at this point. Combined with the very low permeability of the rock, it means that the flow discharge rate by drainage which can be anticipated even in a drift 100 m long, will be very low. Mineralogical and petrographical investigation by X-ray diffractometry has provided the mineralogical composition of the clay, which is mainly composed of quartz, calcite, kaolinite and mica; it contains also about 10% of illite-montmorillonite and, in some samples, a few amount of chloride, pyrite, dolomite and siderite. The water content, very low, is between 1% and 3%. The main porosity is referred to a radius of access to pores equal or lower than 0.02 um. Permeability is between 1.0e-13 m/s and 1.0e-14 m/s, to be compared with values measured in boreholes. Geomechanical testing showed that the seismic wave velocity in Toarcian clay, measured in laboratory, was high (approximately from 3000 m/s to 4000 m/s for P waves and 1500 to 2200 m/s for S waves). The Young's modulus (E) is between 10000 and 15000 MPa and the Poisson's ratio is between 0.15 and 0.20.
The general IPSN research programme at Tournemire concerns geotechnical, hydrogeological and later on, thermal properties of Toarcian clay. To meet these objectives geologic and hydrogeologic survey, rock sampling for laboratory analyses, hydraulic test in boreholes, isotopes study, scale effect experiments and modelling are contemplated. For supporting this research regarding a very low permeability medium, it is intended to develop new methods, apparatuses and devices, if it is necessary.
In situ research work concerning argillaceous formations has been started at the end of 1990. The selected site is a hundred years old given up railway tunnel, 1885 meters long, in the close vicinity of the village of TOURNEMIRE, in the South of France. This tunnel, crosses a 200 m thick toarcian clay formation; the overlying limestone layers are 270 meters thick, so the geotechnical and hydrogeological conditions can be considered as representative of those of a deep repository.