reading material :UNIT 3 Texture & Mechanical Analysis

Particle Size Analysis Lab

Terminology:
Dispersion
Flocculation
Aggregates
Polyvalent and monovalent cations
Suspension
Hydrometer
Colloids
Stoke's Law

Introduction
The textural class of a soil is determined by its particle size distribution; namely sand, silt, and clay content. Texture represents a rather stable soil characteristic and exerts an influence on many soil physical and chemical activities (see Exercise 2). This influence is directly related to the amount of surface activity presented by the mineral particles. Surface activity is a function of both particle size, which determines total specific surface area; and clay type, which determines relative surface reactivity. Particle size distribution analysis quantifies particles size categories, but does not determine clay type. Particle size distribution provides the information necessary for determining soil class on the textural triangle, an important standard for categorizing soil physical and chemical behavior on the basis of surface activity.
 

Part One:Sieve Analysis

Mechanical sieving can be used to separate coarse grains from soil separates, to divide sand separates into classes, and to separate sand fractions from silt and clay fractions in the soil. The silt and clay fractions cannot be distinguished from one another by sieve analysis. The method used in the second part of the lab will separate silt from clay paticles.

Procedure:
1. Remove all pieces of visible organic matter from the soil sample and crush soil clumps, using a mortar and pestle.

2. Weigh approximately 50 gms soil. Record the exact weight as Ws.

3. Stack the sieves, with the largest mesh on the top and successively finer meshes beneath. Place the pan under the stack of sieves and the cover on top of the stack. Pour sample into top sieve. Place the lid on top of the stack and secure the stack to the sieve shaker. Turn on the shaker and let it run for 10 minutes.

4. Carefully collect residue from each sieve for weighing. Weigh soil from each sieve, including that in the pan at the bottom of the stack. Record weights as follows:

W1 = weight of fragments  retained by Sieve #10 (larger than sand; >2mm)

W2 = weight of Soil retained by Sieve #40 (coarse sand)

W3 = weight of Soil retained by Sieve #80 (medium sand)

W4 = weight of Soil retained by Sieve #230 (fine sand)

W5 = weight of Soil remaining in Pan (silt and clay)

Calculate percentages of each of the above, as a percentage of the original soil weight (Ws), and record in your notebook.
 

Part Two: Bouyoucos Hydrometer Method

Quantitative determination of the proportions of differently-sized solid particles is called mechanical analysis. This exercise uses the Bouyoucos hydrometer method of mechanical analysis which relies on the principles of dispersion and sedimentation.

Dispersion: Individual soil particles must be dispersed (separated from each other) in an aqueous solution and remain dispersed to enable determination of particle size distribution. However, soils naturally exist as aggregates and not as a dispersed mixture of particles. Aggregates are secondary particles formed by cementing a mixture of primary particles; sand, silt, and clay. Cementing agents include organic matter, mineral oxides, or polyvalent cations (ions possessing more than one + charge). Dispersive methods remove or inactivate these binding agents. Only after binding forces have been negated can individual particles separate and their settling rate be properly analyzed.

Complete dispersion requires both mechanical and chemical assistance. Mechanical stirring overcomes weaker binding forces in large aggregates, but chemical agents are also necessary, especially to deflocculate clays. Polyvalent cations (normally Ca+2 and Al+3) flocculate clays by forming interparticle, electrostatic links. Chemical dispersing agents (such as sodium hexametaphosphate) are effective in dispersing these clay bundles because:

a.The sodium monovalent cation (Na+) replaces polyvalent cations adsorbed on clays, breaking the interparticle linkage. The displaced polyvalent cations form insoluble complexes with phosphorus which prevents re-establishment of floccules.

b.The adsorption of sodium, a highly hydrated cation, brings increased hydration of clays. This condition diminishes the binding strength between clay and cation which raises a clay particle's electronegativity and, hence, their repulsion from other clays.

The mixture of dispersed soil particles in water is called a suspension. Once a true suspension state has been achieved, differential settling rates can be used to distinguish particle size distribution.

Sedimentation: Sedimentation rate, the settling rate of a mineral particle in water,  depends on the size of the particle. Large particles settle out of suspension more rapidly than small particles. Analytical techniques based on this direct sedimentation relationship allow quantification of particle size distribution.

The connection between particle size and settling rate is expressed by Stoke's Law. This relationship shows that small particles, those exposing high specific surface area (m2 g-1), produce more resistance to settling through the surrounding solution than large particles and, hence, settle at slower velocities
 
 
 

Stoke's Law :                    D2g (d1-d2)
                               V =     _________
                                            18 n
 
 

The formula shows that the settling velocity, V, is directly proportional to the square of the particle's effective diameter, D; the acceleration of gravity, g; and the difference between the density of the particle, d1, and density of the liquid, d2; but inversely proportional to the viscosity (resistance to flow) of the liquid, n. The density of water and its viscosity both change in a manner so that particles settle faster with increased temperature. Hence, it may be necessary to apply temperature correction factors as explained with the procedure.

Stoke's Law can be condensed to V=kD2 by assuming constant values for all components except the effective diameter of soil particles. Then, for conditions at 30 degrees C, k=11241. For particles size values in centimeters, the formula yields settling velocity, V, in centimeters per second. Because soil particles do not meet the requirements of being smooth spheres, exact conformance to Stoke's Law is not realized.

Soil erosion into surface waters provides an environmental application of sedimentation principles and illustrates another criteria of Stoke's Law; namely, that the settling solution should be still. Moving water maintains particles in suspension that would otherwise settle in still water. Thus, sediment loads in streams and rivers are determined by water velocity and turbulence. Sediments segregate by particle size during settling. Fast moving water con transport even very large particles, but as water flow slows, first sand particles and then silt are deposited. These deposits can bury an existing surface, alter subsequent water flow patterns, and reduce reservoir capacity. Clays, on the other hand, settle only when water movement has virtually ceased. Since clays constitute the most surface active fraction and bind chemicals that can constitute pollutants, clays deposits are frequently sites of environmental pollution.

This procedure uses a hydrometer to quantify the solid material remaining in suspension at each stage of the sedimentation process. The hydrometer is calibrated to measure the density of a suspension at the hydrometer's center of buoyancy in units of grams per liter. Research has determined that within 40 seconds, sand particles (0.05 mm and larger) have settled below the buoyancy center of the hydrometer. Within two hours, silt particles (0.05-0.002 mm) have settled and no longer influence the hydrometer. Thus, measuring the density of the soil suspension 40 seconds after shaking and again at 2 hours provides the information necessary to calculate the percentages of sand, silt, and clay in the soil.

Procedure:

1. Weigh 40 g oven-dried soil into a stirring cup (sieve the sample first to remove any fragments larger than sand). Fill the cup half full with deionized water and add 10mL of dispersing solution. Mix and let stand for 10 minutes to initiate chemical dispersion.

2. Place cup on mixer and stir 3 minutes for coarse-textured soils and 4-5 minutes for high clay soils. This step completes mechanical and chemical dispersion.

3. Quantitatively transfer stirred mixture to a sedimentation cylinder with a stream of deionized water. Fill cylinder to the 1000 mL mark with deionized water

.

4. Agitate mixture to uniformly suspend all material throughout the liquid by inserting a stopper and inverting  the cylinder several times (but do not shake in a manner that would produce circular currents in the liquid as this can alter the settling rate)

5. When suspension is complete, set cylinder on table and mark the time. After about 30 seconds, slowly lower the hydrometer into the suspension and release it. Then read the scale at the meniscus exactly 40 seconds after agitation was stopped. Remove hydrometer. Repeat steps 4 and 5 until hydrometer readings within 0.5 g of each other are obtained and record that number on the data sheet.

6. Hold a Celsius thermometer in the suspension for about 3 minutes, record the temperature, and calculate the corrected hydrometer reading.

---For each degree above 18oC, add 0.25 g/L to the original hydrometer reading.
---For each degree below 18oC , subtract 0.25 g/L from the original hydrometer reading.

7. Mix the suspension again as described in Step 4 and place the cylinder where it will not be disturbed. After 2 hours, insert the hydrometer into the suspension and take another reading. Record the temperature and correct this hydrometer reading as described in Step 6.

8. Discard the liquid portion from the sedimentation cylinder into sink drain. Transfer settles materials to a designated waste receptacle.

DATA AND CALCULATIONS to be recorded in notebook:

PART ONE:
1. Soil identification number and name_________________________
2. Ws__________
3. W1__________
4. W2__________
5. W3__________
6. W4__________
7. W5__________
8. % rock fragments (larger than sand)__________
9. % coarse sand ______________
10. % medium sand _______________
11. % fine sand _________________
12. % silt and clay ________________

PART TWO:
1. Soil identification number or name ________________
2. Soil weight, g __________________
3. Average 40-second hydrometer value, g/L _______________
4. Temperature of suspension @ 40-sec hydrometer value, g/L _______________
5. Temperature corrected 40-sec hydrometer value, g/L ______________
6. Two-hour hydrometer value, g/L _____________
7. Temperature of suspension @ 2 hour reading, 0C______________
8. Temperature corrected 2-hour hydrometer reading, g/L ______________
9. Grams of sand (Line 2 - Line 5) ______________
10. Grams of clay (Line 8) ______________
11. Grams of silt (Line 2 - Line 9 - Line 10) ___________
12. Percent sand (Line 9 Line 2) x 100 ___________
13. Percent clay (Line 10 Line 2) x 100 _____________
14. Percent silt (Line 11 Line 2) x 100 _______________
15. Soil textural class, from textural triangle  __________________
 

Questions:
 

1.Use the condensed version of Stoke's Law (V=11241 x D2) to calculate the following. (Note: Values for D must be in centimeters and the resulting value of V will be in centimeters per second). How long would it take the largest medium sand particles to settle to the bottom of a quiet pond that is 2 meters deep? How long would it take the smallest silt-sized particles?

2.Why are sandy deposits found near the banks of a flooding river, while silt-laden sediments are found farther away from the river channel?

3.Explain how a hydrometer works.

4.Suppose that all the silt and clay was transferred from the mixing cup to the settling cylinder, but that some of the sand remained in the cup. How would this affect your calculated values for sand, silt, and clay percentages? Explain.

5.Describe a natural illustration of the principles of sedimentation studied in this exercise.

6.Erosion can cause soil dispersion and form separate deposits of sands, silts, and clays. Why does each of these deposits present unique environmental hazards?