Geotechnical Engineering

DHARM

834 GEOTECHNICAL ENGINEERING

It may be observed that the stress-deformation relationship is linear upto a point such
as B. The slope of this line is known as the coefficient (or modulus) of subgrade reaction, used
in geotechnical engineering problems.

When the stress is released, the plate does not spring back to its original position as can
be seen from Fig. 20.20.

O D F

Elastic

deformation

Residual

deformation

Stress

Deformation
Fig. 20.20 Cycle test
It may be noted that there is a significant residual deformation even though the applied
stress is released even within the limit of proportionality.

In a repeated plate bearing test a small stress is applied on the plate and released. This
forms one loading cycle. In the subsequent loading cycles, the stresses are gradually increased
in small increments and released. After a significant number of load cycles, a stress deforma-
tion curve of the type shown in Fig. 20.21 is obtained.

Deformation

Stress

Fig. 20.21 Repeated plate bearing test data
If the elastic deformation values are separated in each cycle and plotted, essentially a
linear relationship of the type shown in Fig. 20.22 is obtained.

The constant of proportionality from Fig. 20.22 is, by definition, the coefficient of elastic
uniform compression, Cu, which is a constant for a given size of the plate.