3. Grain Size Distribution (Das, Chapter 2) Sections: 2.5, 2.6, 2.7

Soils - What are they? Soils are natural material that are made up of particles that have different sizes. Soils differ from other engineering materials in that one has little control over their properties Broad Categories of soil particle sizes are: Coarse grained soils sands, gravels - visible to naked eye Fine grained soils silts, clays, organic soils – not visible to naked eye Particle size is related to mineralogy: - Gravelly and Sandy soils are formed due to decomposition of rocks containing quartz with high in silica content. - Silt and Clay formed from rocks which contain iron, magnesium, calcium, or sodium minerals with little silica

Soil Grain Shapes Soil grains have different shapes that somewhat difficult to quantify. An infinite number of shapes are possible, a few of which below: Bulky (sands and gravel) Flaky or Needle shape (clay)

Soil Grain Shapes The shape of a particle could be classified through microscopic analysis. A soil sample is spread on a microscope plate and the particles are observed. Their shape is classified using standard tables.

Soil Grain Shapes

sand 2

4

Soil- Grain Size Grain size of soil refers to the diameters of the soil particles making up the soil mass. This is however a loose description of soil since most soil particles have irregular shapes and are not round. The sizes of the soil particles are important factors which influence soil properties including : Strength Deformation Permeability Suitability as a construction materials like in dams and pavements

Soil- Grain Size gravel sand silt clay Depending on the predominant size of particles within the soil, the sizes of particles that make up soils vary over a wide range. Soils generally are called : gravel sand silt clay To describe soils by their particle size, several organizations have developed particle-size classifications.

Grain Size Distribution of Soils Grain size distribution is the determination of size of particles in a soil, expressed as a percentage of the total dry weight. Significance of GSD: To know the relative proportions of different grain sizes. An important factor influencing the geotechnical characteristics of a coarse grain soil.

Grain Size Distribution of Soils Two methods generally are used to find the particle size-distribution of soil: Sieve (mechanical) Analysis: for particle size greater than 0.075 mm in diameter. Hydrometer (wet) Analysis: for particle size smaller than 0.075 mm in diameter.

Sieve Analysis (for coarse-grained soils) Remark The sieve is given a number that correspond to the number of opening per LINEAR INCH of screen, for example NO. 4 sieve has four 0.187-in openings per inch.

Sieving- (for coarse-grained soils with D > 0.075mm)

Sieve Analyses

Sieve Analysis

Sieve Designation - Large Sieves larger than the #4 sieve are designated by the size of the openings in the sieve

Sieve Designation - Smaller Smaller sieves are numbered according to the number of openings per inch 10 openings per inch 1-inch # 10 sieve

Sieving procedure (1) Write down the weight of each sieve as well as the bottom pan to be used in the analysis. (2) Record the weight of the given dry soil sample. (3) Make sure that all the sieves are clean, and assemble them in the ascending order of sieve numbers (#4 sieve at top and #200 sieve at bottom). Place the pan below #200 sieve. Carefully pour the soil sample into the top sieve and place the cap over it. (4) Place the sieve stack in the mechanical shaker and shake for 10 minutes. (5) Remove the stack from the shaker and carefully weigh and record the weight of each sieve with its retained soil. In addition, remember to weigh and record the weight of the bottom pan with its retained fine soil.

Data Analysis: Obtain the mass of soil retained on each sieve by subtracting the weight of the empty sieve from the mass of the sieve + retained soil, and record this mass as the weight retained on the data sheet. The sum of these retained masses should be approximately equals the initial mass of the soil sample. A loss of more than two percent is unsatisfactory. (2) Calculate the percent retained on each sieve by dividing the weight retained on each sieve by the original sample mass. (3) Calculate the percent passing (or percent finer) by starting with 100 percent and subtracting the percent retained on each sieve as a cumulative procedure.

Hydrometer (Wet analysis) Analysis Based on the principle of sedimentation of soil grains in water. It is assumed that all the soil particles are spheres and that the velocity of soil particles can be expressed by Stokes’ law

Hydrometer (Wet analysis) Analysis

Hydrometer test - used for smaller particles Schematic diagram of hydrometer test

Hydrometer Analysis - Procedure:

Hydrometer Analysis - Procedure:

Hydrometer Analysis: (1) Take the fine soil from the bottom pan of the sieve set, place it into a beaker, and add 125 mL of the dispersing agent (sodium hexametaphosphate (40 g/L)) solution. Stir the mixture until the soil is thoroughly wet. Let the soil soak for at least ten minutes. (2) While the soil is soaking, add 125mL of dispersing agent into the control cylinder and fill it with distilled water to the mark. Take the reading at the top of the meniscus formed by the hydrometer stem and the control solution. A reading less than zero is recorded as a negative (-) correction and a reading between zero and sixty is recorded as a positive (+) correction. This reading is called the zero correction. The meniscus correction is the difference between the top of the meniscus and the level of the solution in the control jar (Usually about +1). Shake the control cylinder in such a way that the contents are mixed thoroughly. Insert the hydrometer and thermometer into the control cylinder and note the zero correction and temperature respectively.

Hydrometer Analysis: (3) Transfer the soil slurry into a mixer by adding more distilled water, if necessary, until mixing cup is at least half full. Then mix the solution for a period of two minutes. Immediately transfer the soil slurry into the empty sedimentation cylinder. Add distilled water up to the mark. Cover the open end of the cylinder with a stopper and secure it with the palm of your hand. Then turn the cylinder upside down and back upright for a period of one minute. (The cylinder should be inverted approximately 30 times during the minute.) (6) Set the cylinder down and record the time. Remove the stopper from the cylinder. Very slowly and carefully insert the hydrometer for the first reading.

Hydrometer Analysis: The reading is taken by observing the top of the meniscus formed by the suspension and the hydrometer stem. The hydrometer is removed slowly and placed back into the control cylinder. Very gently spin it in control cylinder to remove any particles that may have adhered. (8) Take hydrometer readings after elapsed time of 2 and 5, 8, 15, 30, 60 minutes and 24 hours

Hydrometer - Data Analysis: (1) Apply meniscus correction to the actual hydrometer reading. From Table 1, obtain the effective hydrometer depth L in cm (for meniscus corrected reading).

Hydrometer - Data Analysis: For known Gs of the soil (if not known, assume 2.65 for this lab purpose), obtain the value of K from Table 2.

Hydrometer - Data Analysis: Calculate the equivalent particle diameter by using the following formula: (5) Determine the temperature correction CT from Table 3

Hydrometer - Data Analysis: Determine correction factor “a” from Table 4 using Gs. (7) Calculate corrected hydrometer reading as follows:

Hydrometer - Data Analysis: (8) Calculate percent finer as follows: (9) Adjusted percent fines as follows: (10) Plot the grain size curve D versus the adjusted percent finer on the semilogarithmic sheet.

Example 1

Example 2

Grain-Size Distribution Curve The results of mechanical analysis (sieve and hydrometer analysis) are generally presented by semi logarithmic plots known as GRAIN or PARTICLE-SIZE DISTRIBUTION CURVES. 1. Effective Size D10 2. Uniformity Coefficient 3. Coefficient of Gradation

Characteristics of the Grain size Distribution Curve The appearance (shape) of the particle-size distribution curve depends on the range and amounts of the various sizes of particles in the soil sample. These in turn have been affected by: The soils’s origin The method of deposition Types of soils with regard to shape of their distribution curves: 1. Well-graded soil: The curve is smooth and covers a wide range of sizes. This indicates that the soil is NON-UNIFORM.

2. Poorly-graded soil: The curve is nearly vertical 2. Poorly-graded soil: The curve is nearly vertical. This indicates that the soil is UNIFORM. 3. Gap-graded soil: This is the case when intermediate sizes are absent. This could be the case when two separate soils are mixed.

Example 1 Well-Graded?