Hydro-Mechanical Properties of an Unsaturated Frictional Material

92 CHAPTER 4. EXPERIMENTAL SETUPS
since the electrical conductivity of sand is negligible small, the density of the specimen is not
influencing this relationship and Eq. 4.3 is used for analysis of the experimental results. Ac-
cording to Topp et al. (1980) a polynomial function of third order was suggested for relating
the dielectric constant to the volumetric water content:
θ(ka)loose = −18.888 + 7.481 · ka − 0.448 · k 2 a + 0.013 · k 3 a
θ(ka)dense = −7.989 + 3.232 · ka − 0.052 · k 2 a + 0.0001 · k 3 a
θ(ka) loose/dense = −12.085 + 4.638 · ka − 0.161 · k 2 a + 0.003 · k 3 a
(4.1)
(4.2)
(4.3)
To asses wether the sensor measurements are reasonable and not delayed in response of
time, a saturated sand specimen was prepared in the column testing device. During removing
stepwise 1000 ml from the saturated specimen measurements of the TDR sensors as well as
tensiometer sensors were performed. Experimental results are given in Fig. 4.19. On the left
hand side TDR sensor measurements and on the right hand side corresponding tensiometer
sensor measurements are shown.
The specimen is 550 mm in height and has an initial void ratio of e0 = 0.68 . Layer 1
(TDR 1, Tensiometer 1) is located in a depth of 70 mm, Layer 2 (TDR 2, tensiometer 2) in a
depth of 160 mm and Layer 3 (TDR 3, tensiometer 3) in a depth of 260 mm. Layer 1 is the
top layer and layer 3 is the bottom layer. Initially the specimen is a water saturated specimen
and thus each TDR sensor measurement refers to a volumetric water content θs = 41%.
The tensiometer sensor measurements refer to positive pore-water pressure. The pore-water
pressure in the bottom layer is greater than in the top layer. Tensiometer sensor measurements
were taken every minute and TDR sensor measurements were taken every third minute due to
limitations in equipment. After a time period of 6 minutes 1000 ml of water were withdrawn
from the specimen. Tensiometer measurements are decreasing immediately from positive
pore-water pressure to negative pore-water pressure (matric suction) when removing the first
1000 ml of water. Even though the water level is falling from 550 mm to 260 mm and is
passing all three layers no changes in TDR measurements occur. Water still remains in the
pores because the air-entry value is not reached till now. TDR measurements in layer 1
are decreasing to θ = 26% after removing the 2nd 1000 ml of water from the soil. Here
the pores start to drain, whereas the volumetric water content in layers 2 and 3 remain
constant. From the tensiometer measurements an air-entry value of approximately ψaev = 2.0
kPa can be derived. Volumetric water content is further decreasing when removing the next
1000 ml. Tensiometer measurements are decreasing continuously with decreasing water table.
Removing the fourth 1000 ml of water, also sensor T2 is reaching the air-entry value and thus
the volumetric water content in sensor TDR2 is decreasing. The measurements in T1 as well
as in TDR1 are only slightly changing, because small pores retain the water in the soil.