Particle Size Analysis of Soil

The particle size analysis of a soil sample involves determining the percentage by mass of particles within the different size ranges. The purpose of the analysis is to derive the particle size distribution of soils. Particle Size Distribution is an important property of soil. Two methods are used to determine the particle size distribution of soils:

1. Sieve analysis: for particle size larger than 75 μ (0.075 mm)

(2) Hydrometer analysis: for particle size smaller than 75 μ (0.075 mm) using a sedimentation process.

The British Standards give soil classification as follows

Cobbles        >60mm

Gravel          2mm to 60mm

Sand             0.06mm to 2mm

Silt               0.002mm to 0.06mm

Clay             <0.002mm

The particle size distribution is usually shown as a graph of ‘percentage passing by weight’ as a function of particle size.

Sieve Grain Size Analysis

Sieve analysis is the sieving operation by means of shaking a soil sample over a set of sieves that have progressively smaller openings. Two methods of sieving can be used: wet sieving and dry sieving. Wet sieving is the definitive method applicable to essentially cohesionless soils. Dry sieving is only suitable for sands and gravels which do not contain any clay.

Required equipment

• Test sieves: 75 mm, 63 mm, 50 mm, 37.5 mm, 28 mm, 20 mm, 14 mm, 10 mm, 6.3 mm, 5 mm, 3.35 mm, 2 mm, 1.18 mm, 600 µm, 425 µm, 300 µm, 212 µm, 150 µm, 75 µm.
• Lid and receiver.
• A balance readable and accurate to 0.5 g.
• Riffle boxes.
• A drying oven capable of maintaining a temperature of 105oC to 110oC.
• Evaporating dishes.
• Metal trays.
• Scoop.
• Sieve brushes.
• Sodium hexametaphosphate.
• Rubber tubing about 6mm bore.
• Mechanical sieve shaker (optional).

Test Procedure

1. Select representative test specimen by quartering and riffling.
2. Oven dry specimen at 105—110°C, and weigh.
3. Place on 20mm sieve.
4. Wirebrush each particle retained on the 20mm sieve to remove fines.
5. Sieve particles coarser than 20 mm. Record weights retained on each sieve.
6. Riffle particles finer than 20mm to reduce specimen mass to 2kg (approx.). Weigh.
7. Spread soil in a tray and cover with water and sodium hexametaphosphate, at a concentration of 2 g/liter. Stir frequently for 1h, to break down and separate clay particles.
8. Wash the material through a 75 µm sieve, allowing the material passing sieve 75 µm to run to waste. Ensure that neither test sieve is overloaded in the process, either with material or with water
9. When clean, transfer all the material retained on the sieve into a tray or evaporating dish and dry in an oven at 105—110°C. Allow it to cool and weigh.
10. Sieve the dried fractions through the appropriate sieves down to the 75 µm test sieve. Weigh the amount retained on each sieve and any fines passing the 75 µm test sieve and record.

Calculation

The weight of the soil retained on each sieve is calculated by subtracting the weight of the empty sieve from the recorded weight of the sieve after the test. The total weights of particles retained are added and compared to the initial weight of the soil sample.

The percentage retained on each sieve is determined by dividing each weight retained by the initial weight of the soil sample. Subsequently, the total percentage passing from each sieve is calculated by subtracting the cumulative percentage retained in that particular sieve and the ones above it from totality.

The Uniformity Coefficient (C_u)

The uniformity coefficient (C_u) expresses the variety in particle sizes of soil and is defined as the ratio of D₆₀ to D₁₀. The value D₆₀ is the grain diameter at which 60% of soil particles are finer and 40% of soil particles are coarser, while D₁₀ is the grain diameter at which 10% of particles are finer and 90% of the particles are coarser. Therefore, C_u is estimated as:

When C_u is greater than 4, the soil is classified as well graded; whereas when C_u is less than 4 the soil is classified as poorly graded/uniformly graded.

Hydrometer Grain Size Analysis

The Hydrometer method covers the quantitative determination of the particle size distribution in a soil from the coarse sand size to the clay size by means of sedimentation. The test is normally not required if less than 10% of the material passes the 75 µm test sieve in a wet or dry sieving analysis.

A hydrometer is a device designed to measure the relative density of a liquid which refers to the ratio of the actual density of the substance to the density of the water. The apparatus consists of a cylindrical stem and a bulb that contains a specific portion of mercury or lead at the bottom, calibrated to float upright in the liquid. The liquid is poured in a tall cylinder usually made out of glass and the hydrometer is placed inside until it is stabilized. The test is based on the principle that in a low-density liquid, the hydrometer will sink deeper until it balances.

The hydrometer contains a scale which is used to record the relative density of the liquid based on its submersion.

The hydrometer grain size analysis takes advantage of the change in the relative density of a soil-water mixture as the soil particles sink. The test relies on the fact that when the soil is poured in the liquid, the relative density of the soil-water mixture will rise. As the soil particles sink the density decreases until it reaches the initial density of the liquid. The heaviest particles (larger in diameter) will sink first.

Required equipment

• Hydrometer
• 2 nos. 1L graduated measuring glass cylinders of about 60 mm diameter
• Thermometer readable to 0.5^oC
• Mixer
• Drying oven capable of maintaining the temperature of 105 °C – 110 °C
• Distilled water
• Test sieves comprising at least 2 mm, 600 µm, 212 µm, 75 µm, and receiver.
• A balance readable to 0.1 g.
• Stopwatch
• Plastic wash bottle
• Evaporating dish
• Dispersing agent, Sodium Hexametaphosphate solution
• Nomographic Chart (ref. Stoke’s law).

Hydrometer Test Procedure

The dry mass of soil required depends on the type of soil. Appropriate quantities are about 100 g for a sandy soil and 50 g for a clay or silt.

The typical testing procedure consists of the following steps:

1. Weigh the sample to 0.1 g to obtain its initial dry mass
2. Place the sample in a wide-mouthed conical flask.
3. Add 100 mL of the dispersant solution to the soil. Shake the mixture thoroughly until all the soil is in suspension.
4. Mix the suspension in the mixing machine for about 5 minutes until the soil is broken down to individual particles.
5. Transfer the suspension from the flask to the 75 µm sieve placed on the receiver, and wash the soil using a jet of distilled water from the wash bottle. The amount of water used shall not exceed 500 mL.
6. Transfer the suspension that has passed through the sieve to the 1 L measuring cylinder, and make up to the 1 L graduation mark. This suspension shall be used for the sedimentation analysis.
7. The material retained on the 75 µm sieve shall be transferred to an evaporating dish and oven-dried.
8. When cool, resieve this material on relevant sieves down to 75 µm.
9. Weigh the material retained on each sieve to 0.1 g.
10.  Add any material passing the 75 µm sieve to the measuring cylinder.
11. Make a separate solution in a 1 L measuring cylinder consisting of 100 mL of the dispersant solution and dilute with distilled water to the 1 L mark. This cylinder shall be placed alongside the cylinder with the soil suspension to achieve the same temperature.
12.  Mix the soil suspension in the measuring cylinder by placing the palm of one hand over the open end and turn it vigorously end-over-end about 60 times in 2 minutes.
13.  Place the cylinder quickly on a table and start the timer.
14.  Immerse the Hydrometer in the suspension and allow it to float freely.
15.  Take hydrometer readings at the upper ring of the meniscus after periods of approximately ½ min, 1 min, 2 min and 4 min. without removing the Hydrometer.
16.  Remove the Hydrometer slowly, and rinse it in distilled water and place it in the other cylinder with the dispersant solution. Record the top of the meniscus reading, R₀.
17.  Reinsert the hydrometer in the soil suspension and record readings after periods of approximately 8 min., 15 min, 30 min, 1 h, 2 h, 4 h, and 24 h from the start of sedimentation. Insert and withdraw the hydrometer after each reading.
18.  Observe and record the temperature of the suspension once during the first 15 min and then after each subsequent reading.