Civil Engineering Reference
In-Depth Information
In this case the PC can be used to control the test and to record the results. Details of
this equipment are beyond the scope of this topic.
With a hydraulic triaxial cell like those shown in Fig. 7.9 the axial and radial stresses
or strains and the pore pressure or volumetric strains can be changed independently.
You can illustrate the test path by plotting total and effective stress paths using the
axes
σ
r
. However, because we are interested in shear and vol-
umetric effects in soil behaviour it is more illustrative to plot stress paths using
the axes
q
and
p
(or
q
and
p
). From Eqs. (3.3) and (3.4), changes of total stress are
given by
σ
a
vs
σ
a
vs
σ
r
, and
δ
q
=
δσ
a
−
δσ
r
(7.13)
1
δ
p
=
3
(
δσ
+
2
δσ
r
)
(7.14)
a
and, from Eqs. (6.14) and (6.15),
q
=
δ
δ
q
(7.15)
p
=
δ
δ
p
−
δ
u
(7.16)
Hence, if you know
δσ
a
,
δσ
r
and
δ
u
, you can easily plot stress paths using the axes
q
vs
p
and
q
vs
p
.
Figure 7.10 illustrates four simple total stress paths and also defines terms like com-
pression, extension, loading and unloading. Note that in a triaxial apparatus
σ
a
and
σ
r
(provided that the loading
ram is attached to the top platen) and so
q
and
q
can be positive or negative.
In Fig. 7.10 the four total stress paths correspond to increasing or decreasing either
r
must always be positive; however, we can have
σ
<σ
a
σ
a
or
σ
r
while the other is held constant. Using Eqs. (7.13) and (7.14) with either
δσ
=
0
r
3
2
. In Fig. 7.10
or
δσ
=
0, you should show that the gradients d
q
/d
p
are3or
−
a
Figure 7.10
Typical stress paths available in hydraulic triaxial tests.
