SOIL MECHANICS - CHAPTER 23 pdf

Chapter 23

CELL TEST

An early version of the triaxial test was developed around 1938 by Keverling Buisman, see Figure 23.1. Actually, the triaxial test can be

considered to be an improved version of this Dutch cell test. The apparatus consists of a container with a cylindrical glass wall (the cell), with

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Figure 23.1: Cell test.

a fixed rubber membrane, in which the sample can be installed. The water

pressure in the cell can be controlled. In contrast with the triaxial apparatus,

in which the cell pressure also acts on the top of the sample, in the cell test the

pressure in the cell only acts on the cylindrical surface of the sample, because of

the way of installing the membrane, which is glued to the top and bottom plates

of the apparatus. The usual testing procedure consists of applying a vertical

load, by means of a gradually increasing dead weight, and to measure the

vertical deformation. Note that in the standard triaxial test the rate of vertical

deformation is imposed, and the vertical load is measured. The cell test is stress

controlled, whereas the triaxial test is strain controlled. In horizontal direction

there is no difference between the two tests: in both the horizontal stress is

controlled by the cell pressure. The size of the sample in a cell test usually is

6.5 cm diameter, which is somewhat larger than the size of the samples in a

triaxial test (3.8 cm). This is of minor importance, however, and there exist

larger cells, especially for tests on course material, such as gravel.

The original purpose of the cell test was to investigate the stresses that would

occur in a loaded sample while the strains remained small, without reaching

failure. For this purpose the lateral deformations should be kept to a minimum,

close to zero. This can be accomplished, approximately, by filling the cell with

water, and to prevent volume change of this water, by closing the cell. The cell

pressure will then increase if the vertical stress is increased. In this classical

form of the cell test the cell pressure and the vertical deformation are measured as a function of the vertical stress. The measured states of

stress can be considered as safe, and correspond to small deformations only. A disadvantage of this procedure is that the cell pressure, and thus

the state of stress in the sample, depends upon the stiffness of the system of cell, water and rubber membrane. An air bubble in the cell, or a

fold in the membrane, may lead to greater lateral deformations and lower horizontal stresses. An advantage is that the sample is never brought

to failure, so that the behavior at various stress levels can be tested on a single sample.

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