214
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
The CT-triaxial collapse tests of Q
3
loess have been done
under various mass of inundation. The scan images of CT-
triaxial inundation tests for sample 4
#
are show in figures 7 and
8. The following understanding can be summarized from CT
images:
1) For certain stress state, the original structure of the loess
is damaged during collapse and a new stable homogeneous
structure is formed simultaneously. In the CT images, the
fissures and the cavity in the undisturbed loess samples shrink
gradually, even disappear at last.
2) Either spherical stress or shear stress can lead to collapse
including volume strain and shear strain. The 1#, 2# and 4#
samples (Table 2) are in hydrostatic state of stress, and their
collapsible volume strains are 4.3 %, 1.27 % and 4.79 %,
respectively. It can be seen that the structure of 4# sample
occurs a substantial change from an non-homogeneous (even
with cracks) to a quite homogeneous state. For example, from
Fig.7-2b to Fig.7-2c, the mean value of the CT data increases
from 829.06 to 983.26 with an increment of 154.2. However,
from Fig.7-2f to Fig.7-2g and from Fig.8-3f to Fig.8-3g, the
mean values of the CT data change only in the fractional part.
The above two opinions proposed by the first author of the
paper in 1986 firstly (Chen, 1986a, 1986b) are proved by the
CT images, which give the visual evidence to the structure
changes of the loess in the process of collapse. These two
characteristics of collapse also show that collapse is different
from shear failure, in other words, shear deformation during
collapse process of loess is generally limited and taking the M-
C criteria as initial collapse condition is unreasonable, which
was pointed out by the first author of the paper in 1986 (Chen,
1986b).
4.2 Determining yield stress of intact loess with the help of CT
scanning data
Figure 9 shows the relations of CT number vs. stress of two
scanning sections of 3
#
simple Q
3
loess during shear (Table 2).
It can be seen that there is a characteristic point at each curve.
The CT numbers of pre- and post- the point change
significantly, which means that the point is yield point. Thus, a
method to determine yield point is proposed with the help of CT
scanning data.
100 110 120 130 140 150 160 170
840
860
880
900
920
940
960
980
H
p /kPa
2
5
0
50
100
150
200
840
860
880
900
920
940
960
980
H
q /kPa
2
5
Fig.9 CT number vs. stress of two scanning sections of 3# simple Q
3
loess during shear
5 CONCLUSIONS
(1) The macro-scale behaviors of the soil samples are closely
related with their mesostructural evolutions.
(2) Fissures sprout and grow during loading or during wet-
dry circles for undisturbed expansive soil for remolded
expansive soil.
(3) The original structure of the loess is damaged and a new
homogeneous structure is formed during wetting and for certain
stress state. The holes and fissures of intact loess may become
gradually small even disappear during loading or inundation
depending on stress state of soil.
(4) Based on CT data, the definitions of the structure damage
variable of the loess and expansive soil are given. The structural
damage evolution equations of two kinds of soils are obtained
in various test conditions.
CT technology give the visual evidence to the structure
changes of soils, and makes a solid test foundation to establish
the damage evolution equation and structure constitutive model
of soils.
6 ACKNOWLEDGEMENT
This work has been supported by the National Natural Science
Foundation of China (Grant No. 11072265 & 11272353).
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