The Concept of Effective Stress in Unsaturated Soils - Multiscale Geomechanics: From Soil to Engineering Projects - page 165

0.7

24

Drying-wetting path
Compaction points

Drying-wetting path
Compaction points

0.6

Initial
conditions

18

Initial
conditions

0.5

12

0.4

6

0.3

0

1 10 100 1000 10000

1 10 100 1000 10000

Capillary pressure p c (kPa)

Capillary pressure p c (kPa)

Figure 6.11. Drying and wetting paths of specimens of Perafita silty sand
compacted to modified Proctor optimum water content and maximum
density and compaction points of triaxial tests specimens

6.3.2.1. Precision triaxial cell for large samples (LPTT)

The precision triaxial cell for tests on large samples, 320 mm high and 160 mm
in diameter, is equipped with three LVDT (Linear Variable Differential
Transformer) for measuring axial strains and three LVDT for measuring radial
strains. The frame and the armature rod of the vertical LVDT are fixed to target
studs placed in the specimen during compaction. The same type of studs is also used
to measure radial strains, the frame of the LVDT being supported by a ring mounted
in the triaxial cell. The strain resolution is around 10 -6 with a 16-bit A/D converter.
A standard pressure transducer and a sensitive load cell located inside the triaxial
cell are used to measure the confining stress and axial force. The specimen is in
contact with air at atmospheric pressure through semi-permeable membranes at the
top and bottom.

The test procedure uses the multistage technique. For each confining pressure
(26 and 52 kPa), the test starts with five unloading-reloading cycles of very small
vertical stress amplitude. The amplitude of the cycles is controlled to ensure that the
cycles are closed and linear, in order to evaluate the elastic Young's modulus. Then,
deviatoric loading is applied up to an axial strain of about 5 x 10 -4 to obtain the
decay curve of the secant Young's modulus with vertical strain. The strain rate of
the tests is approximately 1.4 x 10 -7 s -1 .

During the unloading process, very small unloading-reloading vertical stress
cycles are performed at different steps. Figure 6.12 shows a typical result for a
confining pressure of approximately 52 kPa. At the end of all these tests, an
isotropic stress path is followed in order to evaluate the anisotropy of the soil.