2511
Technical Committee 211 /
Comité technique 211
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
3
radial drainage (rd) is allowed, vertical drainage is
prevented by mean of an impervious membrane which
covers the porous stone at the top and the bottom levels of
the specimen.
series 3 (V&rd): it corresponds to an oedometer test
performed on tunis soft soil improved by a single of
geodrain (mebradrain 88) type sized as that used in series
2. in these tests the vertical drainage radial drainage are
allowed.
results of the three series of tests presented in figure 2
show the variation of void ratio e in function of the
effective stress plotted in the logarithmic scale for loading-
unloading
–
reloading sequences for three series of tests.
figure 2. oedometer curves obtained from three
experimental series
compression c
c
and the swelling c
s
indices, obtained
from the three series of tests (Vd, rd and Vr&d) were
determined from oedometer curves and summarized in
table 1. notice that the compression index obtained from
series 3 (Vr&d) is roughly the double of that recoded in
series 1 (Vd) and 2 (rd). this can be explained by the
allowed vertical and radial drainage paths from which
follows enhanced consolidation of the compressible soil.
from table 1, it is understood the swelling is only
attributed to vertical infiltration of water with sample
unloaded.
table 1: Values of compression and swelling indices
serie of
tests
1 : Vd
2 : rd 3 : V&rd
c
c
0.16
0.16
0.30
c
s
0.022
0.022
0.023
4. stUdY of three dimensional
consolidation:
4.1
coefficients of consolidation:
coefficients of vertical and radial consolidation cv
and cr are determined from the evolution in time of
settlement for each increment of loading (from 50 to
800 kpa). from the results obtained for series 1 and 2:
cv and cr were determined by the logarithmic
method; which use the plot of thickness of sample
versus the logarithmic of time: log (t) (casagrande,
1938).
4.2
coefficient of permeability
Vertical
and
radial
hydraulic
conductivities
(permeability coefficients kv and kr) are determined by
the variable head permeability test. in fact, oedometer
apparatus (in series 1 and 2) is equipped with a
conventional measuring device (tubes connected to the
base of the specimen). the measurements are performed
for different levels of applied load form 100 kpa to 800
kpa (100, 200, 400 and 800 kpa).
figure 9 shows opposite variations of the ratios cr/cv
and kr/kv when the consolidation stress varies from 100
to 800 kpa. in this range, ratio cr/cv varies from 36 to
12 and ratio kr/kv varies from 4 to 12. obtained results
show that the assumption made e.g. cr/cv = kr/kv is
only valid at high levels of consolidation stress (Jia and
chai, 2010).
figure 3. ratios k
r
/k
v
and c
r
/c
v
versus consolidation stress
4.3
degree of consolidation:
in this paper, the global degree of consolidation U(t) is
predicted by two methods. the first one uses the
measured settlement at different levels of applied load in
series 3 (case of vertical and radial drainage, Vr&d):
s
)t(s )t(U
(11)
s(t) and s
∞
denote respectively the settlements at given time
and at the end of consolidation.
the second method consists in calculating U by the
Carillo’s equation (1). The radial degree of consolidation
Ur is estimated from the experimental results of series 2
(case of radial consolidation rc) and equations (7) and
(9). the vertical consolidation Uv is obtained from
recorded results in series 1 (case of vertical
consolidation Vc) by using equation (2).
figures 4a and 4b illustrate the variation of global
degree of consolidation U in function of time for vertical
consolidation stress of 400kpa and 800 kpa.
1
10
100
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Serie 1 Serie 2 Serie 3
log
(kPa)
void ratio,e
C
c
C
c
C
s
(Serie 1)
C
s
(Serie 2)
C
s
(Serie 3)
C
c
0 100 200 300 400 500 600 700 800 900
5
10
15
20
25
30
35
40
C
r
/C
v
k
r
/K
v
verical consolidation stress
(kPa)
Variation of C
r
/C
v
and K
r
/K
v
