La Craie

Foundations in chalk
F. BAGUELIN.
After referring to the difficulties encountered in taking chalk samples which can be used in the laboratory
and the resulting dispersion of mechanical characteristics, the author shows the advantages
of in situ measurements, in particular pressiometric measurements, for assessing the overall reaction
of chalk in the mass.
Observations and experiments have shown that calculations based on these various tests were pessimistic,
especially dynamic driving formulae employed in the case of driven foundation piles. Chalk
tends to liquefy under the effect of impacts, and recovers its mechanical strength after driving either
by dissipation under excess pore pressure or by a thixotropic restructuring of the chalk of which
little is so far known.
La craie. Bull. Liaison Labo. P. et Ch., special V, oct. 1973, p. 113.
Underground cavities in the chalk soil of Northern France
J. LEPLAT.
The chalk of Northern France was formerly worked intensively to meet the needs of agriculture
and especially the building industry. In this latter sector, the production of buiding stone, which
at that time had to be extracted from below ground, led to the opening of a large number of quarries
which are being rediscovered by present-day major projects.
The frequent absence of reliable records, the gradual loss of information formerly handed down
from father to son, makes the prospection of these underground cavities very difficult. In any event,
the essential starting point is a thorough geological knowledge of the region, which makes it passible
to select and valorize data gathered during the field survey (for example by taking account of the
fact that building chalk is almost always located at the same stratigraphic level). The next stage should
be a gravimetric campaign, the only method of geophysical prospection applicable to all cases in
this region. Professor Gabillard's electromagnetic method is designed for the detection of cavities
of the « bottle » type in the region of Lille. Though it is accurate and effective in this particular case,
its possibilities over the whole area are relatively limited. When the gravimetric or electromagnetic
prospection reveals negative anomalies attributable to underground cavities, then mechanical
means of reconnaissance must be employed. In favourable cases (numerous access shafts, shallow
cavities) recourse can be had to devices of the penetrometer type ; otherwise, drilling is necessary.
As soon as a cavity has been detected, the final stage is to enter it in order to survey it.
For the treatment of underground cavities in chalk, two types of solutions may be adopted : filling
in or tidying up. Filling in is reliable but costly ; in this region it is done by introducing quarry
aggregate pneumatically, or by injecting a grout of fly ash and cement. The latter solution seems
to be more reliable and less costly. Tidying up must be apdapted to the particular conditions, and
consequently takes various forms, the simplest being the construction of access shafts allowing
the cavity to be supervised periodically.
La craie, Bull. Liaison Labo. P. et Ch., special V, oct. 1973, p. 123.
Chalk in electric power station construction sites
G. COMES.
The Division Geologie-Geotechnique of the Direction de l'Equipement de l'Electricité de France
has in the course of the past five years to study chalk as a foundation material as a consequence of
the evolution in the building and siting of electric power stations.
The mechanical characteristics of chalk are such that this material behaves very differently from
the rocks usually encountered in connection with these types of structures, such as granite, shale,
limestone, etc.
The sites studied are located at various points of the Paris Basin, taking that area in its widest sense.
The geological and geotechnical work undertaken was performed on Senonian chalk in the Champenois
and Cauchois localities.
The geotechnical characteristics of the chalks in question were determined in the laboratory and in
situ. The behaviour of the material in place is examined and analysed in this article.
It is possible to build underground structures in chalk provided that a certain number of precautions
of a technological nature are taken while the work is in progress.
La craie, Bull. Liaison Labo. P. et Ch., spécial V, oct. 1973, p. 149.
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