2509
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
aBstract: this paper presents an experimental study carried out on undisturbed cored samples of tunis soft soil extracted
at 17.25 m depth at the lagoon of sejoumi. three types of oedometer tests had been performed: first type was a standard test
on tunis soft soil, the second was an oedometer test on the same soil improved by a prefabricated vertical drain mebradrain
88 (mb88) type and the third test is similar to the second test in which vertical drainage was prevented. then, the assessment
of Carrillo’
s theory is studied by quantifying the effect of radial and vertical consolidation from the observed global degree of
consolidation of improved tunis soft soil specimens by geodrains.
résumé:
ce papier présente une étude expérimentale réalisée sur des échantillons intacts du sol mou de tunis prélevés à
17,25 m de profondeur de la marécage
de Séjoumi. Trois types d‘essais
oedométriques ont été effectués : le premier est un
test standard sur le sol mou de tunis, le second était un essai oedométrique sur le même sol amélioré par un drain vertical de
type mebradrain 88 (mB88); le troisième test est similaire au deuxième test dans lequel seulement le drainage radial a été
favorisé . ensuite, l'évaluation de la théorie de carrillo est étudiée en quantifiant l'effet de la consolidation radiale et verticale
sur le degré de consolidation global.
1. introdUction
considerable attention has been recently devoted
worldwide to the problem of building structures on highly
compressible saturated soils and to the development of soil
improvement techniques for increasing stability, reducing
settlements, and accelerating consolidation of soft soils.
prefabricated verticals drains (pVd) with preloading
method was considered the most used improvement
technique to accelerate the consolidation of soft soils and,
consequently, to increase their bearing capacity.
the commonly used consolidation theory for designing
PVD’s
is the unit cell model, e.g., Barron (1948), hansbo
(1981) and terzaghi (1943). Because the solutions
considering both vertical and radial drainage are
complicated, those most used in practice ignore the effect
of vertical drainage, such as Barron’s theory. Barron
(1948) developed solutions for two types of boundary
conditions at the surface of improved soil suc
h as: (i) “free
vertical strain,” resulting from a uniform distribution of
vertical load, and (ii) “equal vertical strain”, which results
from imposing the same vertical deformation. however, in
some cases, the vertical drainage by pdV has a
considerable effect on the degree of consolidation of
improved soil; terzaghi (1943) suggested the well known
simple method for one-dimensional (1d) vertical
consolidation condition.
furthermore, for most cases in practice, the soil is not
homogeneous, and the deformation of pVd improved soil
does not occur
in 1D condition. Carrillo’s theoretical
solution (1942) is used to combine the vertical and radial
drainage effects to predict the global degree of
consolidation
U
:
(1
–
U) = (1
–
U
r
) (1
–
U
v
) (1)
U
r
and U
v
are respectively the radial and the vertical
average degree of consolidation.
Theoretically speaking, Carillo’s formula (Eq 1) is only
valid for instantaneously applied loading.
the consolidation of soft soil is related to the dissipation
of excess pore pressure generated by the surcharge load.
for radial consolidation problem with centered vertical
drain in oedometer cell, the governing differential
equation of excess pore pressure is (parakash et al, 1996):
r
u
r
1
r
u
c
t
u
r
2
r
2
r
r
(2)
c
r
is the coefficient of radial consolidation of soft soil
and
) (
r
u
=∆u(r, t) is the excess of pore pressure at
radius r and time t.
solution of equation (2) that uses the condition of equal
vertical strain without smear effect is given by (Barron,
1948):
nf
t8 exp 1 U
r
r
(3)
Assessment of Carillo’s theory
for improved tunis soft soil by geodrains
halima Jebali
1
, Wissem frikha
2
& mounir Bouassida
3
Unive
rsité Tunis El Manar; Geotechnical Engineering Research Team, Ecole Nationale d’Ingénieurs de
tunis, enit, Bp 37 le Belvédère 1002, tunis. tunisia.
email: (1)
(2)
; (3)
Assessment of Carillo’s Theory for Improved Tunis Soft Soil by Geodrains
Jebali H., Prikha W., Bouassida M.
Université Tunis El Manar, Geotechnical Engineering Research Team, École nationale d’ingénieurs de Tunis (ENIT)
Évaluation de la théorie de Carillo pour les sols mous de Tunis améliorés par géodrains
