GEOTECHNICAL EXPERIMENT - DETERMINATION OF Particle Composition

1. Concepts:

Particle composition of the soil: The percentage (%) by weight of the particle size groups present in the soil.

Particle composition analysis is an essential test of soil, especially coarse-grained soil, the results of which show the relative proportions of different particle sizes in the soil. From there, it is possible to determine whether the main composition of the soil is gravel, sand, dust or clay. And to a certain extent it is possible to control some of the technical properties of the soil. For example,  determining the permeability coefficient of the soil , ...

The grain size curve is more significant when the soil is described for grain color and shape, as well as the packaging condition of the intact sample. However, the engineering behavior of the soil depends on a number of other factors such as mineral type, structure and geological history.

2. Experimental methods:

- Dry sieving method;

- Wet sieving method;

- Hydrometer method.

3. Experimental equipment:

- Electronic technical balance with 0.01g accuracy;

- Oven;

- Set of sieves;

- 1000ml measuring tube;

- Thermometer;

- Desiccator;

- Ground crushing equipment: Porcelain mortar and pestle;

- Jet sprayer,...

4. Applicable standards:

TCVN 4198 : 2014 Construction soil - Analytical method for particle composition in the laboratory.

ASTM D422 - 63 Standard Test Method for Particle-Size Analysis of Soils.

SOME THEORIES ON Particle Composition

Some definitions

Particle size analysis  ( particle size analysis ) expressed quantitatively by the weight ratio of the different sizes of particles in the soil.

Particle size  ( Particle size ) usually given in equivalent diameter.

Effective diameter  ( Effective size ) (D ₁₀ ) is the particle size of which 10% to 90% fine-grained and coarse-grained. That is, the particle diameter at the cumulative fine particle content is 10%.

Uniformity coefficient  ( Uniformity coefficient ) (C _u ) is the ratio of D ₆₀  and D ₁₀

Coefficient distribution curve grain composition ( Coefficient of curvature ) (C _c )

Hydrometer method:

In the experiment to analyze the particle composition of the soil by hydrometer method:

According to ASTM D422-63, there are two types of hydrometer (152H) and (151H) corresponding to hydrometer type (A) and hydrometer type (B) in TCVN 4198:2014.

Here is how to read hydrometer 151H.

On the hydrometer 151H, there are markings from  0.995  to  1.038 ; respectively we can read from  -5  to  38  for the data to be clear.

STOKE'S LAW

The particle size or diameter is determined based on Stokes' law: The speed  υ (m/s)  of a free-falling spherical particle in a liquid is given by:

Inside:

                  d: particle diameter (m);

                  ρ _s : grain density (kg/m ³ );

                  ρ _L : density of liquid (kg/m ³ )

                  η: viscosity of the liquid (Ns/m ² );

                  g: acceleration due to gravity m/s ² .

                  t is the settling time of the soil particles from the time the suspension is stopped stirring until the hydrometer reading is taken, in seconds (s);

                  L is the settling distance of soil particles from the surface of the suspension to the centroid of the hydrometer bulb corresponding to the settling time (t) when taking the hydrometer reading, in centimeters (cm).

REGULATIONS ON ADJUSTMENT OF GROWTH GROWTH

1. Adjusting the settling distance of soil particles:

        The hydrometer reading represents the density of the suspended suspension; The distance from the surface of the suspension to the center of gravity of the hydrometer bulb represents the effective settling depth of the soil particles. During the experiment, after dropping the hydrometer into the suspension to read the measurement, the surface of the suspension rose, making the settling distance of the soil particles larger than it actually was, so it was necessary to calibrate to eliminate the problem. subtract that error.

2. Correction of suspension curvature

         When reading the readings on the hydrometer float handle, the top of the surface of the suspension is taken as the standard, but because of the engraving on the hydrometer float, the bottom of the curved surface of the water is used as the standard, so it must be corrected.

3. Dispersant Correction

         When etching degrees on a hydrometer float, use distilled water as the standard, but in experiments with the particle composition of the soil it may be necessary to add a certain amount of chemical to the solution to disperse the soil particles, thus causing density changes, so correction is required.

4. Temperature correction

         Since the hydrometer etching is carried out in distilled water at a temperature of 20 ^°  C, when conducting the test at a temperature of the suspension other than 20 ^°  C, there will be a change in the density of the water and the expansion of the body. float volume of the hydrometer, affecting the accuracy of the hydrometer, so correction is required.

GROWTH CALCULATOR PROCESS

1. Settling distance of soil particles  corresponding to each graduation on the hydrometer scale

1.2.1. Determine the volume (V ₀ )

         Pour 900 cm ³  to 920 cm ³  distilled water at a temperature of 20 ⁰ C into a 1000 cm ³ volumetric cylinder . Immerse the hydrometer to  the last division  (1.038 mark) and record the rise of the water level. The difference between the water level while the hydrometer is submerged and when the hydrometer is not present is equal to the volume (V ₀ ) of the hydrometer float.

1.3. Determine the value of a

         Value  a  is the distance from the center of the hydrometer float to  the lowest engraved line  on the float handle (mark 1,030), in centimeters (cm): Pour 900 cm ³  of distilled water at a temperature of 20 ⁰ C into the measuring cylinder. area 1000 cm ³ . Glue a piece of lined paper to the hydrometer float. Drop the hydrometer into the cylinder until the water in the tube rises to exactly half the volume of the float (V ₀ /2). Mark the point of contact between the rising water and the buoy, which is the center of the buoy. Measure the distance from the lowest engraved line on the hydrometer float (mark 1.030) to the center of the float with the value a, in centimeters (cm).

For simplicity, measure the distance from the lowest mark to the bottom of the hydrometer float and divide by 2.

2. Correction of suspension curvature

         Drop the hydrometer into the cylinder with distilled water at 20 ^° C, read the measurements on the float handle along the lower and upper edges of the curved surface. The difference of the two readings is the correction value of the surface.

3. Dispersant Correction

          Pour 950 cm ³  of distilled water at 20 ⁰ C into a 1000 cm ³ cylinder . Drop the hydrometer in and take the reading along the upper edge of the curvature, then take the hydrometer out and put it in the cylinder containing distilled water;

          Add an amount of dispersant equal to the amount used in the test (of the same type and concentration as used for the test) to the cylinder. Then pour water into the measuring cylinder until 1000 cm ³ , use a dedicated stirrer to stir from top to bottom, then from bottom to top, let the water surface stand still, drop the hydrometer in and read the measurement along the top edge of the surface. curved. The dispersant correction value was calculated according to the formula:

C = R' ₂₀  – R ₂₀

Inside:

C is the correction value of the dispersant;

R' ₂₀  is the hydrometer reading in solution of the dispersant;

R ₂₀  is the hydrometer reading in 20 ⁰ C distilled water .

4. Temperature correction

During the test, if the temperature of the suspension is different from 20 ^° C, the correction must be made, the temperature correction values ​​are listed in  Table 1.

Table 1: Temperature correction value

Table 2: Table to look up the viscosity coefficient of water

VIDEO OF DETERMINATION OF SOIL Particle Composition