Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 55

• 5 0 5 10 15 20 25 30 35 40
  Rh¢

32

24

cm 16

He

8

Fig. 3.13 Calibration graph between hydrometer reading and effective depth
For the first few readings for about 2 minutes, the hydrometer may not go down much
and He may be taken approximately to be equal (H + h/2).

Procedure: The method of preparation of the soil suspension has already been indicated.
The volume of the suspension is 1000 ml in this case. The sedimenation jar is shaken vigorously
and is then kept vertical over a firm base and stopwatch is started simultaneously. The hy-
drometer is slowly inserted in the jar and readings taken at elapsed times 30 s, 1 min and 2
min. The hydrometer is then taken out. Further readings are taken at elapsed times of 4 min,
8 min, 15 min, 30 min, 1 h, 2 h, 4 h, etc., by inserting the hydrometer about 20 seconds prior to
the desired instant. The hydrometer should be stable without oscillations at the time each
reading is taken. Since the soil suspension is opaque, the reading is taken corresponding to the
upper level of the meniscus. The temperature is recorded. For good results, the variation should
not be more than 2° celsius.
Certain corrections are required to be applied to the recorded hydrometer readings be-
fore these could be used for the computation of the unit weight of the suspension.
Corrections to Hydrometer Readings

The following three corrections are necessary:

1. Meniscus correction


2. Temperature correction


3. Deflocculating agent correction.

Meniscus Correction
The reading should be taken at the lower level of the meniscus. However, since the soil suspen-
sion is opaque, the reading is taken at the upper meniscus. Therefore, a correction is required
to be applied to the observed reading. Since the hydrometer readings increase downward on
the stem, the meniscus correction (Cm) is obviously positive.

The magnitude of the correction can be got by placing the hydrometer in distilled water
in the same jar and noting the difference in reading at the top and bottom levels of the meniscus.
Temperature Correction

Hydrometers are usually calibrated at a temperature of 27°C. If the temperature at the time
of conducting the test is different, a correction will be required to be applied to the hydrometer