The measurement of the deformation properties of Cowden Till at small strains.

The work described in this thesis was firstly concerned with developing
and evaluating automated soil testing equipment and associated instrumentation.
The equipment consists principally of a triaxial stress path
cell of the Bishop-Wesley type, a microcomputer and two pressure controllers.
Inductive displacement transducers have been mounted inside the cell to
measure axial and radial strains locally on the specimen boundary and axial
strains between the end caps. The local axial strain measurements have
proved superior to the end cap measurements which can be adversely affected
by bedding errors and misalignment of the transducers relative to the
loading axis.
Following the development, the system was used to investigate the
stress-strain behaviour of Cowden Till, particularly at small strains
(0.01 - 0.10%). Cylindrical blocks of 250mm diameter were retrieved from
the site and stored under isotropic stress. Eight specimens of 100mm
diameter were trimmed from these blocks and subjected to either a drained
or undrained compression test under load-controlled conditions.
Cowden Till has been shown to exhibit strongly non-linear stress strain
behaviour, even at small strains, and most of the shear strain is
irreversible. The stress-strain characteristics were in acceptable agreement
with those derived from a 865mm diameter plate loading test with
under-plate instrumentation. Although the interpretation of the plate test
is still being investigated, it is concluded that plate tests provide no
better information about the stiffness of the material than triaxial tests
of the type described in this thesis.
The experimental stress-strain behaviour during compressive loading
has been compared with the predictions of some mathematical models. The nonlinear
elastic model of Atkinson (1973) appears to be applicable to Cowden
Till, for which the behaviour is approximately isotropic. Simple stiffness
predictions on the basis of critical state soil mechanics are inadequate
at small strains. However, the model of Pender (1978) predicts the
behaviour reasonably well.
(ii)
An attempt has been made to analyse the compression (bedding error)
which occurs at the end of a triaxial specimen as the axial strain is
increased. A quantification of the compression is hindered by the random
nature of surface variations and by the limitations of present theories.