TRIAXIAL CELL “MATRIX” (UNIVERSITY OF NAPOLI, FEDERICO II) II)

Some drained and undrained monotonic tests have been performed in a MaTrix triaxial cell at the University of Napoli, Federico II. It was designated at the University of Tokyo (Tatsuoka et al., 1994, Santucci de Magistris et al., 1999), where it is very common.

The basic components of MaTRIX are: a triaxial cell, a unique mechanical axial loading system and several transducers, connected through A/D and D/A converters to a microcomputer that controls the tests and records the data (Tatsuoka 1988). The operating scheme of this device is shown in Figure 4.6a, while a picture is reported in Figure 4.6b.

The specimen is placed in a triaxial chamber, put inside an iron frame, which supports the motor and contrasts the advancement of the ram in its application of the deviator stress. The pressure cell is made of Plexiglas and reinforced with aluminium bands at its ends. In order to reduce the disturbance of the soil specimen, its design allows to work with the cell open. The specimen can be placed on the pedestal by hand, guaranteeing a correct alignment between the loading piston, the specimen cap, and the specimen.

Moreover, the pedestal is smooth, avoiding shear stress at the base of the specimen and then a perfect triaxial stress state. Owing to that, the drainage is on the lateral surface of the pedestal.

Furthermore, the base pedestal can move avoiding the non-uniformity of strains. A detail of the base pedestal is shown in Figure 4.7.

(a)

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Figure 4.6. MaTRIX triaxial cell (University of Napoli, Federico II): functioning scheme (a), cell of geotechnical laboratory of University of Napoli Federico II (b).

Figure 4.7. Detail of the base pedestal of MaTRIX triaxial cell (University of Napoli, Federico II).

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The stress path can be controlled by an advanced axial loading device and an electro-pneumatic transducer. The axial load application system allows to maintain a constant strain rate and to apply very small unload-reload cycles with an axial strain amplitude of the order of 0.001% or less, without a noticeable time lag when reversing the loading direction.

The loading device was designed to automatically switch the motor on and off and select the upward and downward loading shaft direction through a personal computer and a D/A converter.

The cell pressure is regulated by an electro-pneumatic transducer (Fujikura Transducer Mod. RT: E/P) that receives the commanding signal from a computer. Alternatively, cell pressure can be controlled by a manual regulator (Fairchild). By now, the pore pressure is controlled only through a manual regulator.

The load cell used in the present study, designed at the University of Tokyo is pressure-insensitive; i.e., the reading (output voltage) from the load cell does not change with changes in the cell pressure. Moreover, to eliminate the effects of piston friction, it is placed inside the triaxial cell.

This load cell (Fig. 4.8) is made of a very stiff material (i.e., phosphor bronze) and is essentially non-compressible when subjected to changes in the cell pressure σc within the range used in the present study (i.e., σc=0~600kPa), which is negligible compared to the compressive strength of the load cell material. Forces applied on the load cell induce deformation in the top weakened part of the transducer, to which four electrical resistance (ER) strain gauges in a full Wheatstone bridge are attached. Thus the load cell deformation is directly connected both to the output voltage and to the axial force.

Figure 4.8. Load cell of MaTRIX triaxial cell (University of Napoli, Federico II).

The effective confining pressure is measured accurately and directly through a liquid-liquid High Capacity Differential Pressure Transducer (HC-DPT) produced by Fuji Electric (FCX-A type FCH/I). The two channels of HC-DPT are connected respectively to the pore pressure water and to the cell pressure water (Fig. 4.9).

The generalized equation for the evaluation of the effective confining pressure acting at every level in a saturated sample is given by:

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𝜎′_𝑟 = 𝑝_ℎ− 𝑢 + ∆𝜎_𝑟𝑚 (4.2)

Where ph is the liquid pressure applied on the high pressure face of the HC-DPT and evaluated as:

𝑝_ℎ = 𝜎_𝑐 + (ℎ_𝑐𝑙+ ℎ_𝐷𝑃) ∙ 𝛾_𝑤 (4.3)

With σc cell air pressure; hcl height of cell water from the sample bottom, hDP distance of HC_DPT down from the specimen bottom, γw unit weight of water.

The term u in eq. (4.2) is the pore water pressure applied on the low pressure face of the HC-DPT. u is given by:

𝑢 = 𝜎_𝑏𝑝+ (ℎ_𝑐𝑙+ ℎ_𝐷𝑃) ∙ 𝛾_𝑤 (4.4)

where σbp is back air pressure, while hcl is the height of burette water surface from the sample bottom.

Finally, the last term in eq. (4.2): Δσrm is the stress correction for membrane force.

Also the volume change measurement method adopted is illustrated in Figure 4.9. It utilizes a low capacity differential pressure transducer (LC-DPT) from Fuji Electic (FCX-A type FHC/I). This instrument is directly connected to a double burette system: a very useful trick to compensate for water evaporation assuming that the rate of water evaporation is probably the same in the two burettes.

One burette is used to give a reference level for the reading of LC-DPT, the other one is directly connected with the specimen pore water. A back pressure is applied to water inside burettes. The pressure helps dissolving air (if present) in the pore water fluid, thus improving the quality of the measure, and helps reducing the evaporation phenomenon.

Figure 4.9. HCDPT and LCDPT of MaTRIX triaxial cell (University of Napoli, Federico II).

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Regarding to axial strains, two different devices can be used: an LVDT and a gap sensor.

The former is produced by Kanetec, it has a maximum measuring range of 40 mm, while its output voltage is ± 5 Volt.

On the other hand, the gap sensor is a proximity transducer AEC-5509 (Applied Elettronics Corporation) placed in opposition with a steel target integrated with the top cap. The electromagnetic field produced by the instrument changes with the changing in its distance from the target. Its maximum measuring range is 2 mm.

4.2.2.1 SPECIMEN’S PREPARATION AND TEST METHODOLOGY

The monotonic tests carried out in the MaTrix triaxial cell have been performed on specimens prepared by 1D- compression technique. It consists of mixing dry sand with a fixed amount of water to have the desired degree of saturation (Sr), in this case 50%. The moisture is then poured in a mould and compacted to have the following dimensions:

d=50mm and h=100mm (Fig. 4.10a). Before starting a test, the specimen is covered by filter paper (Fig. 4.10b) to improve the drainage of water toward the pedestal drainage (Fig. 4.7).

As traditional triaxial test, the testing phases can be divided in:

- Saturation;

- Consolidation;

- Application of the deviatoric load.

(a) (b)

Figure 4.10. Preparation of a specimen for a MaTrix triaxial test in a mould (a) and specimen covered with filter paper (b).

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The saturation occurs by means of a percolation into the specimen due to a hydraulic gradient imposed between the bottom and the top of the specimen. Measurement of B Skempton coefficient is necessary to establish if the specimen is saturated that is if, the saturation phase can be concluded. The next step is the isotropic consolidation, where cell and back pressure are imposed to reach a given effective stress. When the volume strains tend to a constant value, deviatoric stress is applied by means of a drive able to push forward at a speed chosen by the operator. In this research, a velocity of 0.5 mm/h and 1.0 mm/h has been applied respectively for drained and undrained tests.

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4.2.3 CYCLIC TRIAXIAL JAPANESE CELL (UNIVERSITY OF TOKYO)