Hydro-Mechanical Properties of an Unsaturated Frictional Material

6.2. SOIL-WATER CHARACTERISTIC CURVE 117
to 1.5 kPa) most amount of water is retained in the specimen. After passing the air-entry
value, that is between a suction of 1.5 and 1.9 kPa, air starts to enter the water filled pores.
Thus water is leaving the soil and the time period to reach equilibrium significantly increases
for drainage path. The uniform grain-size distribution of the investigated material leads to
uniform pores and therefore most water is flowing out when passing the air-entry value. With
further increase in suction the volume of water that is leaving the specimen is decreasing. Viz
versa the time period for reaching equilibrium condition increases when passing the residual
suction (between 2.8 and 1.5 kPa) along the imbibition path, where the specimen starts
to absorb water. Fig. 6.2 presents the experimental results of the soil-water characteristic
curves derived from back calculation using the final water contents and the amount of water
outflow (drainage curve) and inflow (imbibition curve). Both the suction versus volumetric
water content θ(ψ) and suction versus saturation S(ψ) drainage and imbibition relations are
given for loose and dense specimens. The results for specimens prepared in both a loose and
dense state show significant hysteresis behavior. Thus during drainage cycle a certain suction
corresponds to a lower volumetric water content (or saturation) then during the imbibition
cycle.
Volumetric water content (%)
Saturation (-)
50
40
30
20
10
0
process Imbibition process Drainage
Volumetric water content (%)
1.0
0.1 1 10 100 0.1 1 10 100
1.0
Suction (kPa)
Suction (kPa)
0.8
0.6
0.4
0.2
0.0
Saturation (-)
Loose specimen
0.1 1 10 100
Suction (kPa)
50
40
30
20
10
0
0.8
0.6
0.4
0.2
0.0
0.1 1 10 100 specimen Initial void ratio = 0.66 Dense
Suction (kPa)
Initial void ratio = 0.89
Figure 6.2: Experimental results of soil-water characteristic curves from steady state tests
performed in the modified pressure plate apparatus (loose specimen-left, dense specimenright)