Geotechnical Manual for Slopes

47
excessive.
Filter paper side drains should not be used in triaxial tests, because
they can lead to errors in strength measurement and are generally unnecessary
for the soils of Hong Kong. Membrane corrections must be made in the usual
way.
Saturation by back pressure can only be obtained by applying a small
effective stress to the specimen. Specimens that start with a very low degree
of saturation can be difficult to saturate. In these cases, saturation can
be carried out by first percolating deaired water under a small hydraulic
gradient through the specimen until air stops bubbling from it. A back
pressure can then be applied to complete the saturation.
The strain rate for drained tests with pore pressure monitoring should
be such that the pore water pressure fluctuation is negligible, and in any case
the fluctuation should be no greater than 5% of the effective confining pressure.
For undrained tests, the rate should be selected so as to allow complete
equalisation of pore water pressure throughout the specimen. It is desirable
that the strain rate does not exceed 2% per hour.
For undrained tests, failure can be defined either as the maximum deviator
stress or as the maximum obliquity (σ 1 '/σ 3 '). For fully-drained tests, these
two criteria coincide.
3.8.2 Interpretation of Results
For ease of interpretation, it is recommended that the results of CU
triaxial tests are plotted as p'-q stress paths (Figure 3.1), where
p' = (σ 1 ' + σ 3 ')/2 and q = (σ 1 - σ 3 )/2 (Lambe & Whitman, 1969). The shape of
a stress path indicates the tendency for a specimen to compress or dilate during
shear. The p'-q plots also enable the most sensible strength envelope to be
drawn as the boundary to a family of stress paths.
For CD tests, the p'-q stress paths are of no significance. Actual volume
changes during drained tests should be measured throughout the shear process.
Strength envelopes determined from triaxial tests will often not be
linear, and they will sometimes exhibit an apparent break-point in the region
of a definite ‘critical’ pressure. This is because the stress-strain
behaviour of the material is dependent upon the confining pressure under which
it is sheared. Specimens that are tested at low confining pressures in the
triaxial test tend to dilate during shearing. At high confining pressures,
specimens tend to compress. These different stress-strain behaviours are
indicated clearly by the different shapes of the respective stress paths
(Figure 3.1). In Hong Kong soils, the critical pressure can be considered to
be analogous to the maximum past pressure for a sediment.
It is important to remember that, where a strength envelope is not linear,
the portion of the envelope used for design purposes must be that for the correct
design stress range.
In the interpretation of triaxial test data, especially in the low stress
range, the following sources of error should be borne in mind :