Objective:

To determine the particle size distribution, finer than 75-micron sieve, by sedimentation analysis using a density hydrometer and then plot the grain size distribution curve.

Apparatus used:

Density hydrometer, Glass measuring cylinders of 1000ml capacity, Sieve, Weighing balance, Distilled water, Dispersing agent, Electric blender etc.

 Description

Based on Stoke’s law, the hydrometer method of sedimentation analysis is used to determine the particle size distribution of soils finer than 75-micron.

STEP  1 

 Take 750ml of water in a 1000ml glass measuring cylinder, and note down the water level reading.

STEP  2 

 Immerse the hydrometer into the cylinder and note down the reading of water level. (The difference in the 2 readings gives the volume of the hydrometer.

Initial reading (ml)= 750 ml

Final reading (ml) = 835 ml

Final - Initial

ml

Volume of hydrometer, V_h =

STEP  3 

 Measure the diameter of the measuring cylinder and find the area of the measuring cylinder.

Diameter of measuring jar, d = 6.4 cm

Variables Used in Calculations

Diameter of measuring jar, d = 6.4 cm

cm²

^{π× d2}⁄₄

Area of measuring jar, A =

STEP  4 

 Place the hydrometer on a clean sheet and trace the instrument along with its graduations.

STEP  5 

  Measure the height of the bulb, H.

Distance between neck and bottom of bulb, H = 16.6 cm

STEP  6 

  Measure the height between hydrometer reading and neck, h.

Distance between hydrometer reading and neck, h = 12 cm

STEP  7 

  Calculate effective depth He for corresponding corrected hydrometer reading, Rh values.

Variables Used in Calculations

Distance between neck and bottom of bulb, H = 16.6 cm

Distance between hydrometer reading and neck, h = 12 cm

Volume of hydrometer, V_h = 85 ml

Area of measuring jar, A = 32.2 cm²

^{H + 1⁄₂ (h - V_h⁄_A )}

Effective depth corresponding to each reading, He =

cm

Similarly other values are calculated

STEP  8 

  Draw the calibration curve between He and Rh on a normal graph sheet.

View Data View Graph

Soil Sample Preparation

STEP  9 

  Weigh accurately 50 grams of soil passing through a 75-micron sieve.

50.0

STEP  10 

  Add 2-3 drops of dispersing agent and 70 ml of water and mix thoroughly.

STEP  11 

  Transfer this mixture to the blender jar and blend the mixture for 5mins.

STEP  12 

  Carefully transfer the mixture to a measuring cylinder and add distilled water to make the volume exactly equal to 1000 ml.

Sedimentation test with hydrometer

STEP  13 

  Shake the contents in the measuring jar and allow it to stand. Immerse the hydrometer into the jar gently, a little below the free surface, and allow it to float freely. Immediately start the stopwatch.

After 24 hours the hydrometer reading is noted

STEP  14 

  Calculate the corrected hydrometer readings after applying meniscus correction.

Variables Used in Calculations

Meniscus correction, Cm = +0.001

Hydrometer reading, Rh^' (cm)= 1.016

Rh = Rh^'+ Cm

Corrected Hydrometer reading, Rh =

cm

Similarly other values are calculated

STEP  15 

 Find out the corresponding readings of He for corrected hydrometer readings from He vs Rh curve.

Similarly other values are calculated

STEP  16 

  Calculation and Observation.

Variables Used in Calculations

Time elapsed, t = 0.25 min

Effective length, He = 12.4 cm

Corrected hydrometer reading, Rh = 1.017 cm

Meniscus correction, Cm	0.001
% of silt and clay (%)	0.5
Specific gravity of soil, G	2.57
n (poise)	0.008
Particle size, D (cm)	sqrt(30×n ×He⁄((G-1)×981×t) )
%Finer, N'	N'=G(Rh-1)×105⁄((G-1)×50)
N	%Finer ×% of silt and clay

Similarly other values are calculated

STEP  17 

  Plot graph of N^' vs D on a semi-log graph, which gives us the particle size distribution curve for the given fine-grained soil.