Basic soil tests - Hydrometer analysis

Procedure

If the size of the soil particles are smaller than 0.075 mm, it is very hard to use sieving to separate particles. Therefore hydrometer is used for obtaining particle size distribution of soil smaller than 0.075mm.

Soil passing 0.425 mm (NO 40) sieve is used for the test. but sample should not be oven dried. therefore, a portion from the same soil is oven dried to obtain the moisture content of the soul sample. ( normally 50g use)

after selecting the sample it should mix with ''Sodium Hexametaphospate" (125 ml or 50 g). Mixture is thoroughly wetted and be allowed to soak for at least 16 hours, and transferred to a dispersion cup and sample is further dispersed using mechanical stirring apparatus, after adding suitable amount of water.

Then soil-water slurry is transformed to a sedimentation cylinder and distilled or distilled water is added to until the total volume is 100ml.




Then leave the container on level surface and start the stopwatch. Then put the hydrometer to the solution and get readings. (0.5 min, 1 min, 2 mins, 4 mins, 8 mins, 15 mins, 30 mins, 1hr, 2 hrs, 4 hrs, 8 hrs, 16 hrs, 24 hrs)
Note: Insert the hydrometer 30-40 seconds before the reading. after 1 min reading get out the hydrometer to measure 2 mins. otherwise particles placing on the hydrometer surface.

Before this we need to prepare another identical container but without soil to take some readings / correction factor.

Please watch this video
https://www.youtube.com/watch?v=78sKJKsaUvY


Correction factors

1). Meniscus correction (Cm)
Need to reduce this value to get the actual value. for this we need to use 2nd identical container.

 RH = RH' + Cm

Numbers are increasing downward direction of the hydrometer. therefore Cm is add to the RH'. 

2). Dispersing agent correction (Cd)
this is the reduction of the effect from the Sodium Hexametaphospate.

The final equation should be,

                                           RH = RH' + Cm - Cd

Percentage of particles remaining in the suspension (P)

The percentage of soil (P) remaining in suspension at the level, where hydrometer is measuring density, may be obtained by,

                                           P = ((RH x a) / W ) x 100%
where,
RH = The corrected reading
a = Correction factor
W = Total weight of soil

Example: P = 40% means,

 Calculation of the diameter of particles (D)

                                           D = K ( L/t )^0.5
Where,
D = Diameter
K = Constant depend on temperature and specific weight 
L = Distance to the center of the hydrometer
t = Time (in mins)

using above data we can plot Particle Size Distribution curve (PSD Curve)