Triaxial Compression Test - Soil Properties: Testing, Measurement, and Evaluation - page 354

If the specimen's height and diameter are assumed to decrease in
the same proportions during consolidation, area of the specimen after con-
solidation ( A c ) can be derived from its initial area A 0 using the equation

a V c
V 0

b 2

>

3

A c A 0 (22-3)

and the height of the specimen after consolidation H c can be computed by

a V c
V 0

b 1

>

3

H c H 0 (22-4)

For each applied load, the axial unit strain can be computed by

ε

dividing the change in height of the specimen, H , by its height after

Δ

consolidation, H c . In equation form,

¢ H
H c

(22-5)

ε

Each corresponding cross-sectional area of the specimen, A , can be com-
puted by the equation

A c

A (22-6)

1

ε

where A c is the initial area of the specimen after consolidation. Each cor-
responding applied axial load can be determined by multiplying the
proving ring dial reading by the proving ring calibration. Finally, each
unit axial load can be computed by dividing each applied axial load
by the corresponding cross-sectional area. These computations must be
repeated for each specimen tested.

[C] Stress-Strain and Pore Pressure-Strain Graphs

A stress-strain graph should be prepared by plotting unit axial load
on the ordinate versus axial strain on the abscissa. From this graph,
the unit axial load at failure can be determined by taking the maxi-
mum unit axial load or the unit axial load at 15% axial strain,
whichever occurs first. The unit axial load at failure is denoted by p .

Δ

versus axial strain should also be pre-
pared by plotting pore pressure on the ordinate versus axial strain on
the abscissa. The same scale for axial strain in the stress-strain graph
should be used in this graph. The pore pressure corresponding to the
unit axial load at failure can be determined from the graph.

A graph of pore pressure

[D] Shear Diagram (Mohr Circles for Triaxial Compression)

Results of CU tests are comm o nly presented with Mohr circles p lotted
in terms of effective stress . T he effective lateral pressure

and

σ σ 3

major effective principal stress can be computed as follows:

σ 1

(22-7)
(22-8)

σ 3 σ 3 μ

σ 1 σ 1 μ