Hydro-Mechanical Properties of an Unsaturated Frictional Material

6.2. SOIL-WATER CHARACTERISTIC CURVE 123
depth. Following for instance the 1 st imbibition process in Fig. 6.6 the tensiometer T (70
mm) measures a lower negative pore-water pressure than tensiometer T (450 mm), referring
to a higher suction at the top of the sand specimen. With increase in water table due to water
inflow the suction is increasing. Saturated conditions are reached in the specimen when the
tensiometers measure positive pore-water pressures. The imbibition process is stopped when
the water table reaches the top of the specimen.
Following the experimental results derived from the 1 st drainage process in Fig. 6.6 ten-
siometer T (450 mm) measures a lower positive pore-water pressure than tensiometer T (70
mm), indicating a higher hydrostatic pressure at the bottom of the specimen. During drainage
of the sand specimen the positive pore-water pressures are decreasing until reaching negative
pore-water pressures. The measured negative pore-water pressures at the end of the draining
process is lower at the top of the specimen than at the bottom of the specimen. Consequently,
the suction is higher at the top of the specimen than at the bottom of the specimen.
Volumetric water content measurements are given in Fig. 6.7. As mentioned before the
initial condition for the initial drainage process is a water saturated specimen with the water
table located at the top of the specimen. For the initially saturated specimen the TDR sensors
measure the saturated volumetric water content (corresponding to S = 1), which is equal in
each depth.
It was also mentioned before, that the original state for the imbibition cycles correspond
to an unsaturated sand specimen, where the water table is located at the bottom of the
specimen. Thus TDR sensors measure the volumetric water content in the specimen, that
differs with depth. On the bottom a larger volumetric water content is measured than on the
top.
Following for instance the 1 st imbibition process in Fig. 6.7 the measured volumetric
water content at the top of the specimen is lower than the volumetric water content at the
bottom of the specimen. During imbibition process the volumetric water content is increasing
corresponding to a decreasing suction. During the imbibition process the volumetric water
content increases first at the bottom of the specimen and then at the top of the specimen.
The imbibition process is stopped when the water table reaches the top of the specimen.
Even when the water table is at the top of the specimen, measurements in sensors TDR (70
mm) to TDR (450 mm) refer to a degree of saturation less than 1 (apparent saturation S ′ ).
Thus the measured volumetric water contents are influenced by occluded air bubbles (also
called apparent volumetric water content θ ′ ). The TDR (450 mm) sensor at the bottom of
the specimen measures the volumetric water content referring to water saturated conditions,
S = 1. This portion of the specimen remains saturated during the entire testing procedure
and occluded air bubble cannot occur. The experimental results derived from the 1 st drainage
process in Fig. 6.7 show that the sand specimen is saturated and the measurements from the
TDR sensors, TDR (450 mm) to TDR (70 mm), correspond to a degree of saturation close to
1. During drainage the volumetric water contents are decreasing, first at top of the specimen