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GDS HE |
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What is it? |
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The GDS Hall Effect Local Strain Transducers
provide on-sample small strain measurements of axial and radial
strains. Accurate determination of soil stiffness is difficult to
achieve in routine laboratory testing. Conventionally, stiffness of
a triaxial test specimen is based on external measurements of
displacement which include a number of extraneous movements. True
soil strains can be masked by deflections which originate in the
compliances of the loading system and load measuring system. Such
equipment compliance errors add to a variety of sample bedding
effects to give a poor definition of the stressstrain
behaviour of the material under test, particularly over the small
strain range. Most triaxial tests therefore tend to give apparent
soil stiffnesses far lower than those inferred from field
behaviour (Jardine, Symes & Burland, 1984). |
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Download full
specification datasheet as
pdf
[64kb] |
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| Why Measure
Small Strain |
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Recent work has demonstrated the rather surprising finding that
soils can be equally as brittle as rocks and that an understanding
of their behaviour at levels of shear strain below 0.05% is very
important. Indeed, K-zero for normally consolidated clays may
reach peak strength in the triaxial apparatus at axial strains as
low as 0.1%. Moreover, even when the behaviour is not brittle,
the strains prior to yield are usually very small (loc. cit). |
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| Why measure
locally on the specimen? |
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In the conventional triaxial test, surface friction arises between
the unlubricated ends of the test specimen and the end platens of
the test apparatus. The ends are therefore restrained laterally and
hence vertically also. Accordingly, the test specimen deforms
non-uniformly with a gradient of axial and radial deformation from
zero at the ends to a maximum at the middle.
It is widely believed that triaxial test specimens with a height to
diameter ratio of 2 have end zones which are more or less restrained
while the middle third is more or less unrestrained.
Therefore, it is highly desirable that radial and axial deformations
are measured locally in this region if realistic deformation moduli
are to be found.
The measurement of axial deformation based on the relative movement
between the top cap and the base pedestal is subject to bedding
errors. These errors arise because of the difficulty in providing
perfectly plane, parallel and smooth ends on the triaxial test
specimen. The top cap can rest on surface asperities of the test
specimen or make contact imperfectly, perhaps on one edge of the
specimen. Owing to this "point" loading effect, rapid deformation
will occur during the early stages of triaxial compression until the
top cap is properly bedded down.. |
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| GDS
Instruments is an ISO 9001:2000 credited company. |
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