HYDROMETER ANALYSIS OF SOIL – WHAT, WHY & HOW?

In geotechnical engineering, hydrometer analysis is mainly used to find the grain size distribution of a fine-grained soil. In this post I will share with you the following things.

  • What is hydrometer?
  • Why is hydrometer analysis done?
  • What is the principle behind hydrometer analysis of soil?
  • How to do hydrometer analysis of soil?
Figure-1 Hydrometer
Figure-1 Hydrometer

A hydrometer is an instrument used to measure the relative density of a liquid. The hydrometer is made of glass and mainly consists of two parts;


  • a cylindrical stem with graduation marks
  • a bulb loaded with mercury

The lower the density of the liquid, the more the hydrometer will sink. For example consider water and petrol. The density of petrol is less than that of water, so the depth of immersion of the hydrometer will be greater in the case of petrol than in water.

In the case of fine grained soils, the sieve analysis test does not give reliable test results. This is because a fine grained soil contains particles of various sizes starting from 0.075 mm to 0.0002 mm. And it is not practical to design a sieve with such a small screen size. Also, there is a possibility of the sample being lost during filtration. Hence hydrometer analysis is done for the grain size analysis of fine grained soils.

Hydrometer analysis is based on Stokes Law, According to this law, the velocity at which grains come out of suspension, all other factors being equal, depends on the size, weight, and shape of the grains.

In the case of clay, it is assumed that the soil particles are spherical and have the same specific gravity. Hence we can say that coarse particles in a soil water suspension will settle more rapidly than the finer particles.

If the final velocity of sinking of a spherical particle is V, then it is given by;

V = 1/18 [(Gs-Gw)/n)]*D2

Where,

V = terminal velocity of soil particle (cm/s)

D = diameter of soil particle (cm)

Yess = specific gravity of the soil particle

Yeswoo = specific gravity of water

n = viscosity of water (gs/cm.)2,

Device

  • hydrometer
  • Dispersion cup with mechanical stirrer with complete accessories
  • Two glass jars of 1 liter capacity
  • Deflocculating agent (sodium hexa metaphosphate solution prepared by dissolving 33 g of sodium hexa metaphosphate and 7 g of sodium carbonate in distilled water to make one liter of solution)
  • stop the clock
  • thermometer
  • scale

Process

  1. Take about 50 g in case of clay and 100 g in case of sandy soil and weigh it correctly to 0.1 g.
  2. If the soil contains a significant amount of organic matter or calcium compounds, pre-treatment of the soil with hydrogen peroxide or hydrochloric acid may be necessary. Pre-treatment should be avoided in case of soil containing less than 20 per cent of the above substances.
  3. In the soil thus treated, add 100 cc of sodium hexametaphosphate solution and heat it gently for 10 min and transfer the material to the cup of a mechanical mixer, using a jet of distilled water to wash away all traces of soil.
  4. Stir the clay suspension for about 15 minutes.
  5. Transfer the suspension to the hydrometer jar and make the volume up to exactly 1000 cc by adding distilled water.
  6. Take another hydrometer jar with 1000cc of distilled water to store the hydrometer in between consecutive readings of the soil suspension to be recorded. Occasionally pay attention to the specific gravity readings of the water and the temperature T0C.
  7. Coarsely mix the clay suspension, placing the palm of the right hand on the open end and holding the bottom of the denominator with the left hand, turning the jar upside down and back. When the jar is turned upside down make sure there is no soil stuck to the base of the graduated jar.
  8. Immediately after shaking, put the hydrometer jar on the table and turn on the stopwatch. Carefully insert the hydrometer into the suspension and take hydrometer readings at total times of , 1 and 2 minutes.
  9. After reading 2 minutes, remove the hydrometer and transfer it to the distilled water jar and repeat step number-8. Usually a pair of identical readings should be obtained before proceeding.
  10. Take subsequent hydrometer readings at 4, 9, 16, 25, 36, 49, 60 minutes and every hour thereafter. Each time a reading is taken, remove the hydrometer from the suspension and place it in a jar containing distilled water. Care should be taken while recording the hydrometer to see that the hydrometer is at rest without any motion. As time passes, the density of the liquid suspension decreases due to the fall of solid particles, which should be checked as a guard against possible error in the readings of the hydrometer.
  11. Continue the recording operation of the hydrometer readings until the hydrometer reads approximately 1000.

hydrometer calibration

The hydrometer will be calibrated to determine its actual depth with reference to the hydrometer readings (see Figure-2) in the following steps:

Fig-2 Hydrometer Calibration
Fig-2 Hydrometer Calibration
  1. Find the volume of the hydrometer bulb, VR. It can be determined in the following way:

By measuring the amount of water displaced, Fill a 1000-cc bachelor with water up to about 700 cc. View and record water level readings. Insert the hydrometer and look again and record the reading. The difference in these two readings is equal to the volume of the bulb and the part of the stem that is submerged. The error due to the inclusion of this latter quantity is so small that it can be neglected for practical purposes.

  1. set area, a, graduation in which the hydrometer is to be used by measuring the distance between two graduations. The area, A, is equal to the volume involved between the graduates divided by the measured distance.
  2. measure and record the distance from the lowest calibration mark on the stem of the hydrometer to each of the other major calibration marks, R,
  3. Measure and record the distance from the neck of the bulb to the minimum calibration mark. Distance, h1, corresponding to a reading, R is equal to the sum of the two distances measured in steps (3) and (4).
  4. Measure the distance from the neck to the tip of the bulb. Record this as the height h of the bulb. Distance, h/2, find the center of volume of a symmetrical bulb. If a non-symmetric bulb is used, the center of the volume can be determined with sufficient accuracy by projecting the shape of the bulb onto a sheet of paper and finding the center of gravity of this projected sphere.
  5. Calculate actual distances, hR, corresponding to each of the major calibration marks, R, from the formula:

hR = h1 + [h – (VR/A)]

  1. Plot the curve expressing the relationship between hR And R As shown in Figure 3. For a well-shaped hydrometer, the relationship is essentially a straight line.
Fig-3 Typical Hydrometer Calibration Chart
Fig-3 Typical Hydrometer Calibration Chart

Calculation

If the temperature is constant throughout the experiment, the following formula can be used to calculate the diameter of the soil particles:

D2 = kHR/Tea

Where

t = time in minutes

d = mm. diameter of soil particle in

k = 30n/(ggwoo,

Percentage fine N can be obtained from



n% = g * v / ((g-1) * w) * (r – rwoo)*100

Where

v = volume of soil suspension (1000 cc)

woo = weight of dry soil taken for testing

R = hydrometer reading in distilled water

Rwoo = hydrometer reading in soil suspension

Yes = specific gravity of soil particles

Since V = 1000 cc, the above equation can be easily represented as follows:

n% = k1 (Rh 1 – 1000) * 100

Where

K1 = g/(g-1) * (100/w)

Rh 1 = hydrometer reading = Rh +cM – CD ± CTea

Where,

Rh = actually observed hydrometer reading (upper meniscus)

CM = meniscus correction (ie 0.5)

CTea = correction for temperature (positive if the test temperature is higher than the temperature at which the hydrometer is calibrated and vice versa) (see Table-1)

CD = Correction for the dispersing agent. It is determined as below

Adding a dispersing agent to a clay suspension increases the density of the liquid and necessitates an improvement in the observed hydrometer readings. The correction factor, Cd, is determined by adding the amount of the dispersing agent used for the particular test in a 1000 mL graduation filled with partially distilled or demineralized water, adding additional distilled water to the 1000 mL mark, then pouring an is determined from. Hydrometer and looking at the readings. correction factor, CD is equal to the difference between this reading and the hydrometer reading in pure distilled or demineralized water.

Table-1 Temperature Correction (C)Tea) for hydrometer analysis
Temperature 0C CTea Temperature 0C CTea
20.0 Zero 27 0.00150
20.5 0.00009 27.5 0.00163
21 0.00017 28 0.00178
21.5 0.00027 28.5 0.00191
22 0.00037 29 0.00206
22.5 0.00049 29.5 0.00219
23 0.00058 30 0.00232
23.5 0.00068 30.5 0.00247
24 0.00081 31 0.00262
24.5 0.00092 31.5 0.00278
25 0.00102 32 0.00291
25.5 0.00116 32.5 0.00320
26 0.00127 33 0.00350
26.5 0.00139 33.5 0.00380

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.