Geoscience Reference
In-Depth Information
The screw-plate load test permits a convenient method for measuring the in situ
vertical compressibility, bearing capacity and dynamic stability of loose to medium
dense cohesionless soils. It also provides a practical means for avoiding some of
the significant technical difficulties associated with the use of rigid plate bearing
load tests for these purposes (http://astm.org).
A review of the literature on in situ testing in peat indicates that the state of the art
and practice are not as developed as for inorganic soils. It is important to recognize the
differences between various organic deposits. Organic soils (with organic content of
25% or less) can be treated similarly to inorganic clays, and many of the same in situ
testing tools, such as vane shear, cone penetrometer, pressuremeter, dilatometer and
regular or screw plate load tests can be applied. While there may be some questions
regarding the interpretation of the test data in determining mechanical parameters
for design, it appears that the standard approaches can be followed with a greater
degree of cross-calibration of the various tests and care for greater material variability
and compressibility. At the other extreme, there are serious questions regarding the
applicability of the conventional in situ tests to fibrous, high-organic content surficial
peat (i.e. not buried and compressed). The presence of fibres, inherent anisotropy,
tendency for high compressibility and rapid drainage, and the low and highly variable
strength of these materials make using conventional field tests and interpretation of
mechanical parameters unviable. The use of large test tools (vanes, cones etc.), more
sensitive measuring devices, and more rapid loading rates to minimize compression
may improve the prospects. However, the irrelevance of the various modes of failure
induced in the field as well as laboratory tests relative to fibre interaction, anisotropy
and compressibility results in unusual values for mechanical parameters and incon-
sistencies between various tests. For instance, the shearing surface is vertical in vane
shear, ill-defined in cone penetration, inclined in triaxial compression and horizontal
in direct, simple or ring shear tests. This situation has led some investigators, such
as Landva, to recommend test fills as opposed to solely relying on laboratory or field
tests when designing embankments on such deposits. Amorphous peat is somewhat
intermediate between organic soils and fibrous peat. There are reports of successful use
of in situ tests. Such materials must be handled on a case-by-case basis. Interpretation
of in situ test results requires corrections usually calibrated based on local experience
with organic deposits and laboratory strength tests. The combined use of extensive
sampling for the definition of site variability, in situ tests and laboratory mechanical
property tests, and where possible the use of test fills, provides a reasonable approach
to dealing with these difficult organic deposits (Edil, 2001).
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