Stabilité des talus : 2. Déblais et remblais

From this angle, the principal elements gathered as a result of the study of the influence of the parameters
of tests on the measurement of shear strength of stiff clay are presented. It is noted that when
the dimensions and orientation of the samples and the rate of testing are varied, the shear strength
measured varies substantially in the case of the four French soils tested :
— Flanders clay (Ypresian)
— Provins clay (Sparnacian)
— Dozule clay (Callovo-Oxfordian)
— Sannoisien green clay (Oligocene)
An attempt was made to relate these results to the mineralogical texture of the soils tested, and for
this purpose recourse was had to the classification of different textures of argillaceous and marly rocks
developed by the LCPC and elaborated upon in this report.
To complement this data, three interesting cases of cuttings are presented, two of them very thoroughly
instrumented. At the Northern entrance to Lyon, on Autoroute A6, the Tronchon cutting (12 metres
deep), excavated in fairly stiff gravelly clay, gave rise to a tricky hydraulic problem which was treated
with an electric analog model adjusted to in situ measurements. Furthermore, the size of the coarse
elements present in the soil necessitated the use of a giant triaxial test apparatus (diameter 30 cm for
the samples) for the measurement of the mechanical parameters; this gave satisfactory results.
The Bosse-Galan experimental cutting was excavated in normally consolidated soft clay. An attempt
was made to determine the conditions of stability of this cutting in the short term to a height of 5 metres,
and the long term evolution of the hydraulic conditions. Calculations of stability under total stresses
were invalidated by experience (failure occured at a depth less than the predicted critical depth).
Calculations under effective stresses based on measured pore pressures were fairly accurate. The deformations
observed in the short term were not those of a soil evolving at constant volume; this may be
attributed either to non-saturation of the original material or to a more rapid long term passage than
was predicted.
Two slips of different kinds occured at Rognac on Autoroute A7 North of Marseille. One of them
occured while work was in progress in a 25-metre cutting excavated in an alternation of sandstone and
marl whose hydraulic conditions had been underestimated at the stage of preliminary investigation.
The slip was remedied by reducing the angle of slope initially planned and by adding large drainage
spurs. The second slip was the reactivation of a former slip in a zone analogous to the preceding one
but in which the alignment caused little change in the natural geometry. It was treated by adding a
drainage curtain to the slope and placing an abutment at the foot of the slope.
PART 2
EMBANKMENTS ON SOFT SOILS
The general report first briefly sets forth the conventional methods of studying stability. There follows
an examination of the real behaviour of embankments which have suffered disturbances, commencing
with a reference to structures observed by the Laboratoires des Ponts et Chaussées. Relating these
examples to those contained in the international literature on the subject, disparities between predicted
behaviour and real behaviour are revealed. The causes of these disparities are the subject of a detailed
analysis in which an examination is made of factors such as the shape of the failure curve, allowance for
strength in the embankment, and the effect of weather and anisotropy on the undrained cohesion C u
of the foundation soil. Also examined are the results of the calculation of effective stresses; and calculations
of movements are also presented.
The two following articles deal with the measurement of the parameters of resistance of the foundation
soil. First comes a description of a vane test apparatus with coaxial blades, making it possible to measure
separately shear strengt in the horizontal and vertical planes. An account is then given of a seris of
laboratory tests making it possible to determine the law of behaviour (CAU tests) allowing for the
initial state of the soil (K 0 state), rate of shearing, and anisotropy.
The final two articles describe two observed failures of embankments on soft soils. The first, induced
experimentally near Lorient, was the subject of numerous measurements which are briefly reported,
with emphasis on the analysis of pore pressures, the calculation of effective stresses, and the interpretation
of displacements by elastic analysis. The second failure was observed during the construction
of a trial embankment, and is an example of an unexpected failure; the calculated coefficient of safety
does not make it possible to foresee such disorders.
PART 3
REMEDIAL MEASURES
Two important points concerning the strengthening of embankments are dealt with :
— The strengthening of overall stability by means of drainage systems : drainage trenches, drainage
spurs, subhorizontal drains and shafts. The principles of these systems are explained, and diagrammatic
cross-sections are given, specifying the respective proportions and the way they are built. Examples
of applications of the various systems are given.
— Reinforcement in respect of surface erosion, which can be a serious problem, especially in regions
subject to rainstorms, as in the South of France. On the basis of studies and observations made in
Alsace and Provence, details are given of the different methods of stabilization which can be employed,
in the light of weather conditions, pedological conditions, and the geometry of the slopes concerned :
topsoil treatment, the use of sprayed chemicals, and stabilizing layers of various materials applied
mechanically.