Civil Engineering Reference
In-Depth Information
pressure ensures that air does not come out of solution and, by applying the same increase in pressure
to the value of the cell pressure, the effective stress situation is unaltered.
The technique can also be used to create full saturation during the consolidation and shearing of partially
saturated natural or remoulded soils for both the drained and consolidated undrained triaxial tests. In
these cases, back pressure values often as high as 650 kPa are necessary in order to achieve full
saturation.
Example 4.6:
Consolidated undrained triaxial test (i)
The following results were obtained from a series of consolidated undrained triaxial tests
carried out on undisturbed samples of a compacted soil:
Cell pressure (kPa)
Additional axial load at failure (N)
200
342
400
388
600
465
Each sample, originally 76 mm long and 38 mm in diameter, experienced a vertical
deformation of 5.1 mm.
Draw the strength envelope and determine the Coulomb equation for the shear
strength of the soil.
Solution:
π
4
Volume of sample
= ×
38
2
× =
76
86 193
mm
3
86 193
76 5 1
Therefore cross-sectional area at failure
=
=
1216
mm
2
.
−
.
Major principal
stress
σ
1
(kPa)
Cell pressure
σ
3
(kPa)
Deviator stress (
σ
1
−
σ
3
) (kPa)
0 342 10
1216
.
×
6
200
=
281
481
0 388 10
1216
.
×
6
=
319
400
719
0 465 10
1216
.
×
6
600
=
382
982
The Mohr circles and the strength envelope are shown in Fig.
4.23.
From the diagram
φ
′
=
7°; c
′
=
100 kPa.
Coulomb's equation is:
c
′ +
σ
tan
φ
′ =
100
+
σ
tan
7
° =
100
+
0 123
.
σ
kPa
