Geotechnical Engineering

DHARM

270 GEOTECHNICAL ENGINEERING

s 3

s 3 s 3

s 3

s 3

s 3 s 3

s 3

Ds

s 1

Ds

s 1

ssDs

Ds

\Ds s s

13

13

=+

where = externally

applied axial stress

=( – ),orthe

principal stress difference,

often called the “Deviatoric stress”.

(a) Initially, upon application of

all-round fluid pressure,

or confining pressure

(b) After application of external axial stress in

addition to the confining pressure,

held constant until failure

Fig. 8.10 Principle and stress conditions of triaxial compression test

Air vent

Dial gauge to

measure axial

deformation

Proving ring

Ball contact

Piston

Water

Rubber ‘O’-rings

Soil sample

Triaxial cell of

Lucite or perspex

Porous stone

Pedestal

Studs with wing

nuts at 120°

ValveTo pore-

pressure-

measuring

apparatus

To burette

for volume

change Drainage line

Inlet for cell fluid

and cell pressure

Radial

grooves

Rubber

membrane

Steel tie bars

at 120°

Top cap

Base

Fig. 8.11 Triaxial cell with accessories
The assembly may be placed on the base of a motorised loading frame with a proving
ring made to bear on the loading piston for the purpose of measuring the axial load at any
stage of the test.

Test Procedure

The essential steps in the conduct of the test are as follows:

(i) A saturated porous stone is placed on the pedestal and the cylindrical soil specimen

is placed on it.