Hydro-Mechanical Properties of an Unsaturated Frictional Material

94 CHAPTER 4. EXPERIMENTAL SETUPS
The measurements in Fig. 4.19 prove that both sensors, the TDR sensors and tensiometer
sensors, are suitable for investigation of unsaturated sand in a column device. The sensors
react immediately and the measurements are reasonable. As soon as water is removed from
the specimen the tensiometers are responding to changes in water pressure. The difference
in change of water-pressure is decreasing when reaching negative pore-water pressure (matric
suction), due to the fact that the pores are retaining water. As soon as the air-entry value is
reached, the water starts to enter the largest pores in the soil and thus the volumetric water
content is decreasing.
4.7 Summary
Testing unsaturated sand is a challenge for testing cells and equipment. When testing sand
it has to be taken into account that the suction-water content takes place in a narrow range
of suction, where suction as well as water content have to be precisely measured and con-
trolled. Hanging water column technique and axis-translation technique fulfill these require-
ment. Moreover in the present study burettes with high resolution as well as TDR and
tensiometer sensors with high accuracy are used to perform measurements. For observation
of volume changes in the specimen during loading and unloading path the controlled-suction
oedomater cell is equipped with a dial gauge with a high accuracy. The testing devices and
equipments introduced in this chapter are appropriate to perform experiments on sand that
investigate hydraulic (modified pressure plate apparatus, sand column testing device I and
II) as well as mechanical behavior (controlled-suction oedometer cell) of unsaturated sand.