Hydro-Mechanical Properties of an Unsaturated Frictional Material

6.2. SOIL-WATER CHARACTERISTIC CURVE 125
and than at the bottom of the specimen. The sensors on the bottom of the cell (TDR/T 450
mm) are located in the saturated zone through the complete testing procedures. Therefore
the volumetric water content is constant through the complete testing procedure (TDR 450
mm: θs,loose = 46%, θs,dense = 39%). The other sensors give results for saturated as well as
unsaturated conditions.
As shown in Fig. 6.8 the tensiometer as well as the TDR probe measurements enable a
direct measurement between negative pore water pressure (matric suction, capillary pressure)
and volumetric water content that finally lead to the observation of the soil-water character-
istic curve. For loose as well as dense specimen the measurements are presented as volumetric
water content versus matric suction and saturation versus matric suction. The main drainage
path, 1st imbibition and drainage paths (flow rate ≈ 30 ml/min) as well as 2nd imbibition
and drainage paths (flow rate ≈ 100 ml/min) obtained in the different depths are shown. It
was observed that after the first wetting, complete saturation (S = 1) was not again recov-
ered. The occluded air could not be displaced during subsequent imbibition paths. Different
placement depths of the sensors resulted in different initial suctions and volumetric water
contents, respectively, in the unsaturated zone during further imbibition cycles. Subsequent
curves are located within the main drainage and imbibition loop and are known as scanning
curves.
Similar to the flow experiments in the sand column test I, the tests performed in the column
testing device II were carried out under transient state condition. Therefore measurements of
pore-water pressure by using tensiometer sensors and volumetric water content by using TDR
sensor were carried out in several depth. Pore-water pressure versus time measurements are
given in Fig. 6.9 for the loose specimen and for the dense specimen. Corresponding TDR sensor
measurements are given in Fig. 6.10 as volumetric water content versus time measurement
results. After application of air-pressure ua = 3.5 kPa that is conform to a suction ψ = 3.5 kPa
to the specimens from the upper part of the apparatus the specimen starts to drain. At the
beginning of the test tensiometers measure positive pore-water pressure, which is higher at
the bottom than at top of the cell (see Fig. 6.9). When applying air-pressure the pore-
water pressure is decreasing first in the top layer than in the bottom layer until reaching
negative-pore water pressure. Initially the specimens are water saturated and a saturated
volumetric water content of θs,loose = 47% and θs,dense = 41% was measured (see Fig. 6.10).
After application of air-pressure to the specimens from the upper part of the apparatus the
specimen starts to drain. First the volumetric water content in the top layer is increasing and
than the volumetric water content in the bottom layer is increasing. Cumulative water outflow
measurement results derived during column testing device II for loose and dense specimen are
given in Fig. 6.11. Larger amount of water flow out from the loose specimen than from the
dense specimen. Measurements stopped when water was flowing out no longer.
Similar to the tests performed in the column testing device I the TDR and tensiome-
ter measurements are directly linked to the soil-water characteristic curve, that is given in