1205
Technical Committee 106 /
Comité technique 106
⋅
=
0
pt
v
Ψ
Ψ
log I
ε
3.
2
Experimental results.
The experimental program intends to simulate the behaviour of
a collapsing soil when it is progressively wetted until saturation.
The wetting process is carried out decreasing progressively the
suction until zero.
Oedometer tests have been performed in the prepared
specimens maintaining suction values in the range initial
suction-zero. Specifically, the suction steps applied were: 95.4
kPa, 50 kPa, 15 kPa, 10 kPa y 0 kPa (suction for saturation).
Figure 4 shows the test results.
Figure 4. Results from oedometer test
Figure 5 shows the volume increase in samples tested at
different external pressures when suction is decreased from the
initial value.
3 SIMPLIFIED COLLAPSE-SUCTION MODEL.
Following the results extracted from oedometer tests, a
relationship has been sought between volumetric strain under
oedometric conditions, suction and applied external vertical
stress.
Tests carried out on expansive soils indicate that for
constant external pressure there is a linear relationship between
volumetric strain and log of suction (Meintjes 1992, Gordon
1992).
The model proposed in this paper is associated to the
behaviour of a collapsing soil instead of an expansive soil, but
uses the same linear relationship between volume strain (
v
ε
)
and log relative suction (Eq. 1).
(1)
where
Ψ
0
is the initial suction and
I
pt
is
the “Instability Index”, proposed by Aitchison et al. (1973) for
swelling and shrinking test son soils.
The Instability Index is a function of the vertical stress
applied.
The proposed relationship is valid until suction arrives to a
low value, corresponding to the field capacity, when volumetric
strain becomes constant (v. Figure 6). Water is not absorbed
anymore by the soil.
In Figure 5, the Instability Index is the slope of the
regression line relating volumetric strain and log (
Ψ
/
Ψ
0
) for
every vertical pressure.
Figure 5. Volumetric strain versus relative suction.
Figure 6 indicates a linear relationship between Instability
Index and vertical pressure drawn in semilog scale.
Figure 6. Instability Index versus log vertical stress.
This relationship can be expressed by equation Eq. 2.
(2)
Substituting Eq. 2 into Eq. 1, a relationship between vertical
stress and suction with vertical strain is obtained.
(3)
Figure 7 is a 3D picture of
the experimental relationship between vertical strain, relative
suction and vertical pressure. The lines corresponding to
constant
σ
v
values may be approximated by straight lines as
indicated by equations (1) and (3). The picture includes the line
when the field capacity is reached and the subsequent constant
volumetric strain indicated in Figure 5. The projection of this
line to the
(
)
0
ΨΨ −
/
log
v
ε
plane is a potential, corresponding
to the equation:
ε
v
=24.5·(
Ψ
/
Ψ
0
)
0..2
(4)
( )
v
pt
log C I
σ
⋅ =
( )
⋅
⋅ =
0
v
v
Ψ
Ψ
log
log C
ε
σ
