15
9. Mark the rings, and draw a diagram of the box, so that the
rings can be replaced in exactly the sample place after
removal (Fig. 35).
10. Take the ring carefully out of the water basin and wipe off
any water drops hanging underneath the sample before
weighing it (accuracy of balance 0.01 g) (Fig. 36).
This weight (including ring, cloth and elastic) is used to
calculate water content at saturation, pF 0 (weight A, See
Chapter 6).
Record any irregularities that occurred during
saturation (e.g. swelling of clayey soils, changes in
soil structure, accidental loss of soil material).
1
2
3
4
Fig. 35 Mark samples before placing them
Fig. 36 Weigh the samples
Fig. 37 Press to ensure soil-sand contact
Water content measurements at pF 0 are relatively inaccurate:
• It is difficult to transfer the saturated sample to a balance without changing water content, especially with
sandy samples.
• The middle of the soil sample is used as the reference level for zero pressure, but the free water level
(h = 0) is in fact 1 cm below the top of the sample ring. The moisture tension thus ranges from +1 cm at the
bottom of the sample, to -4 cm at the top of the sample. Note that at lower pressures this difference due
to sample size becomes less important.
11. Place the ring on the sandbox. Press the ring slightly, to
improve soil - sand contact. (Fig. 37).
12. Slide the suction regulator so that a pressure of -2.5 cm
head is applied to the centre of the samples (this level is
equal to the level of the sand when standard 5 cm sample
rings are used.)
13. Leave the sample to equilibrate, (with the lid on the box to
stop evaporation). This will take a few days for sandy soil
and up to a week for clayey soils.
Fig. 38 Pressure of -2.5 cm head (pF 0.4)