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when the orientation of principal stress is varied more profoundly from the previous
loading. Being called stress-induced anisotropy, the effects of previous loading direc-
tion on deformation during a subsequent loading were demonstrated experimentally by
Oda(1972).Hismicroscopicobservationshowedthatthedirectionofgrain-to-graincon-
tact plane governs the overall behavior of sand and that the statistic nature of the contact
directions is changed by the stress history. One cycle of loading and unloading of princi-
palstress,
σ
1
,inaselecteddirectiontoalargedeformationgeneratesmanycontactplanes
perpendicular to this stress orientation, thus increasing soil rigidity against further load-
ing in the same direction. What is important in Oda's study is that the number of contact
planes in other directions decreases and accordingly the sand rigidity in other directions
decreasesaswell.YamadaandIshihara(1981)madethesamefindinginundrainedshear
tests on cubic soil samples.
The effects of varying
σ
1
orientation is further indicated in what follows. Figure15.6
shows the stress state in a torsion shear device in which stress difference of
(σ
v
−
σ
h
) /
2
and
τ
v
h
are loaded independently. Consequently, the orientation of the major principal
stress,
. A cyclic loading test was carried out on loose Toyoura sand
inanundrainedmanner,wherethetwocomponentsofshearstress,
σ
1
, is denoted by
β
(σ
v
−
σ
h
)/
2and
τ
v
h
,
angle changed by 90
◦
. Figure15.7 illus-
trates the stress-path diagram where the decrease of the effective mean principal stress,
P
were applied to a sample alternately and the
β
≡
σ
1
+
σ
2
+
σ
3
/
3, which is equal to the development of excess pore water pres-
sure, is plotted against shear stress components. It is therein shown that the first loading
of
τ
v
h
developed a substantial excess pore water pressure, the second and the third load-
ings of the same stress component did not reduce
P
so much because the orientation of
the principal stress did not change (
constant at 45
◦
), in contrast the fourth loading
β
=
0
◦
, resulting in more significant excess
pore water pressure, and the fifth loading came back to
cycle was associated with
(σ
v
−
σ
h
)/
2 and
β
=
45
◦
again; causing
τ
v
h
and
β
=
substantial excess pore water pressure.
β
σ
ν
σ
1
τ
vh
σ
h
σ
3
Fig. 15.6. Two independent shear stresscomponents and orientation
of principal stressaxis
