La Craie

ABSTRACTS
The geology of chalk in the Paris Basin
G. BIGNOT et M.P. AUBRY
The authors describe the petrographic nature of chalk, which consists of detrital fragments and microfossils,
ostracodes and foraminiferas, embedded in a gangue comprising a mixture of nannofossils
and very small particles of calcite. The loose structure of this gangue, observed under the electron
microscope, explains the mechanical behaviour of clay.
The stratigraphic divisions obtained by a study of the microfauna and paleography of the Paris
Basin enable geologists to define the variations of facies : chalk which tends to be detrital on the
edge of the Basin, and chalk containing a high percentage of CaC03 in the centre.
Knowledge of the nannofacies helps us to understand the particularities of the mechanical characteristics
of the rock. Stratigraphic and petrophysical data, situating the rock in time and space,
make it possible to consider geotechnical problems in a more general context.
La craie, Bull. Liaison Labo. P. et Ch., special V, oct. 1973, p. 11.
The petrophysics of chalk
M. MASSON
The petrophysical study of chalk has revealed the influence of the fine structure of the material
on its physical and mechanical characteristics.
We note in particular the importance of the factors brought to light in relation to the value and
dispersion of mechanical strength (compressive strength, micro-hardness, speed of wave propagation)
and the importance of the microscopic gangue in relation to physical characteristics (density,
porosity, suction, capillary rise).
The analysis of these different parameters and of their relation to geophysical problems has made
it possible to define a method of identifying chalks by means of simple tests and to class them in
three families : hard, semi-hard, and soft. With this method of sample study it is necessary to combine
a more general method of studying chalk masses, taking account of heterogeneities due to
cracking and weathering.
La craie, Bull. Liaison Labo. P. et Ch., special V, oct. 1973, p. 23.
The texture and behaviour of chalk
A. LE ROUX
Examination under the electron microscope shows the importance of the parameter of texture
in the behaviour of these materials. But the intergranular bonds revealed in the whole of the materials
studied vary in number from one sample to another ; there may be few of them (as in the Incarville,
Sauqueville and Pacy-sur-Eure chalks) or they may be abundant, with the result that the material
may pose numerous on-site problems, or it may behave satisfactorily.
The study of a material which can be considered as intermediate between chalk and limestone confirms
the existence of « bridges » which in this case are very clearly in evidence. Relations existing
between intergranular bonds and the mechanical characteristics of the material are also confirmed.
In the presence of water, the bonds are weakened (swelling of clay, for example), but they are still
strong enough to prevent the material loosening. This explains, in the absence of any o ther phenomenon,
the satisfactory behaviour of chalk under water ; and also explains why some chalks loosen
when they are subjected to vibrations (e.g. the passage of on-site truck-mounted machinery).
The preliminary study of textures should make it possible to predict behaviour and thereby provide
the contractor with information about the materials he will have to work with.
La craie, Bull. Liaison Labo. P. et Ch., special V, oct. 1973, p. 49.
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