La Craie

The stability of very high chalk embankments
A. DE RAGUENEL
Observations and studies made on the Val-Guyon embankment on the A 13 autoroute have provided
an explanation of the cause of the settlements and cracks observed.
These disturbances cannot be explained by classical theories. It is shown that the settlements of
the bed of the embankment are attributable to a densification of the latter resulting from a disintegration
of the low-density chalk blocks in the course of time. This densification may lead to pressure
being exerted on the pore water and an increase in the Poisson's ratio of the chalky mixture. This
variation may in turn entail, as shown by a study of stresses using the method of finite elements,
the development of tensile stresses at the head of the embankment which would explain the longitudinal
cracks observed.
These calculations made it possible to define the influence of the mechanical characteristics of the
chalk, the height of the embankment, and its gradient, on the development of such cracks. This
led to the possibility of laying down recommendations for the use of soft chalk in constructing
embankments of various geometries.
La craie, Bull. Liaison Labo. P. et Ch., special V, oct. 1973, p. 57.
Problems of earthworks in chalk
J. PUIG
The authors give the results of research conducted on the problems of earthworks in chalk, and the
practical conclusions that may be drawn from them.
Conventional geotechnical tests (particle size, Atterberg limits, water content, Proctor test) are
unsuitable for identifying chalk, which is essentially an evolutive material, and consequently they
cannot be used for predicting its behaviour.
The percentage of paste or fineshas a predominant influence on working conditions. This percentage
is essentially dependent on the nature of the « rock chalk », which may be more or less easy to break
up, and also on the nature of the handling to which it is subjected.
Research has led to the development of empirical tests enabling both these factors to be determined
simultaneously : the vibrocrushing test and the repeated compaction test.
On-site observations of the construction of chalk embankments and their stability in the longer
term have made it possible to lay down recommendations concerning the geotechnical study of
chalky materials and their conditions of use in the light of their nature and of the geometry of the
structure.
La craie, Bull. Liaison Labo. P. et Ch., special V, oct. 1973, p. 81.
The study, by vibration grinding, of the aptitude of chalks for compacting
R. STRUILLOU
The use of vibration grinding is envisaged for the purpose of predicting in the laboratory the difficulties
of compacting chalks on working sites. Crushed dry samples in the form of 5/20 aggregates
are ground for 30 s at various water contents. It is observed that for most chalks the quantity of
elements below 1,25 mm which are formed varies linearly with the water content. Every chalk can
therefore be characterized by a line of wear which gives its sensitivity to fragmentation.
Furthermore, the fines of a given chalk are characterized by their liquid limit WL with effect from
products below 400 microns formed by vibration grinding of the chalk in an aqueous medium under
constant conditions (so as to allow for the sensitivity of the Atterberg limits of many chalks to operating
conditions).
By combining the characteristics of wear, natural water content w„, liquid limit WL of finesproduced
by vibration grinding, and empirical coefficients whose purpose is to allow for various conditions
on site, it is possible to determine the critical water content w c
beyond which serious difficulties may
be encountered during compacting.
La craie, Bull. Liaison Labo. P. et Ch., special V, oct. 1973, p. 99.