Civil Engineering Reference
In-Depth Information
Fig. 4.15
The triaxial apparatus.
4.8.3 The triaxial test
As its name implies this test (Fig.
4.15)
subjects the soil specimen to three compressive stresses at right
angles to each other, one of the three stresses being increased until the sample fails in shear. Its great
advantage is that the plane of shear failure is not predetermined as in the shear box test.
The soil sample test is cylindrical with a height equal to twice its diameter. In the UK, the usual sizes
are 76 mm high by 38 mm diameter and 200 mm high by 100 mm diameter.
The test sample is first placed on the pedestal of the base of the triaxial cell and a loading cap is placed
on its top. A thin rubber membrane is then placed over the sample, including the pedestal and the loading
cap, and made watertight by the application of tight rubber ring seals, known as 'O' rings, around the
pedestal and the loading cap.
The upper part of the cell, which is cylindrical and generally made of Perspex, is next fixed to the base
and the assembled cell is filled with water. The water is then subjected to a predetermined value of pres-
sure, known as the cell pressure, which is kept constant throughout the length of the test. It is this water
pressure that subjects the sample to an all-round pressure.
The additional axial stress is created by an axial load applied through a load transducer, in a similar way
to that in which the horizontal shear force is applied in the shear box apparatus. By the action of an electric
motor, the axial load is gradually increased at a constant rate of strain and as the axial load is applied the
sample suffers continuous compressive deformation. The amount of this vertical deformation is obtained
from a deformation transducer. Throughout the test, until the sample fails, readings of the deformation
transducer and corresponding readings of axial load are taken. With this data, the computer plots the
variation of the axial load on the sample against its vertical strain.
Determination of the additional axial stress
From the load transducer it is possible at any time during the test to determine the additional axial load
that is being applied to the sample.
During the application of this load, the sample experiences shortening in the vertical direction with a
corresponding expansion in the horizontal direction. This means that the cross-sectional area of the sample
varies, and it has been found that very little error is introduced if the cross-sectional area is evaluated on
the assumption that the volume of the sample remains unchanged during the test. In other words the
cross-sectional area is found from:
Volume of sample
Original length Verti
Cross-sectional area
=
−
cal deformation
