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DISPERSIVE SOILS

Dispersive soils are those in which the clay content has a high percentage of sodium.

This clay fraction readily breaks down to form a suspension in water.

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Some types of clay minerals are washed out with water flow even at small velocities.

-High sodium content is one of their characteristics.

-The presence of water will overcome and eliminate the inter-particle forces.

-The particles will move apart, forming a colloidal solution. The separated particles would move even with an existing

slow water flow.

However non-dispersive soils are eroded only when the water flow is strong enough to overcome the inter-particle

attractions.

What Are the Problems

If they are used to form an embankment but are not compacted properly, air voids will result within the wall.

When water seeps in and leaches out the clay fraction, small pipes form which quickly develop into tunnels, resulting in a breached embankment.Pipes usually develop in a very short timefollowing filling of the dam and failure can occur in a matter of hours.

When excavated, dispersive soils form strong, tough, impermeable clods. Considerable compactive effort is needed to break down these clods and compact the material.

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Problems Concerning Dispersive SoilsSink Holes in farms with a top layer of Dispersive Soils will make the

land unusable as the clay is washed out

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Piping of interest in Hydraulic Structures:

• Earth Dams, • Embankments and • Soil lined Irrigation canals

This is the most important damage.

Origin of Dispersive Soils

• Weathering of sedimentary rocks, transfers Sodium Ions from the rock components to the soil.

• In Dry climates, vaporization of water induces a high sodium content top layer, especially in case of low permeability underlying layers.

• Existence and flow of Salty water in the area (deserts)

• Fluctuating groundwater tables causes higher sodium content close to the surface.

In Planes having several ups and downs, hills dry soonerand may contend dispersive soils

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Dispersive Soils Are found in

• Planes Alluvium and Lacustrines

• Claystones and Shales formed under the seas

• Also found in many humid regions: Brazil, Vietnam, Australia, Mexico,Thailand, India, Venezuela and southern US.

Mechanism of Piping Failure

1. Erosion of dispersive clay particles starts from:

• Down-stream or where there is a jump in permeability

• Joints or cracks locations

• Close to Drainage structures

• Under the concrete cover of the irrigation canalsObserved in Russia, Iran, Syria and Iraq in several

cases

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2. Erosion continues towards higher water head.

3. The eroded area sums up to form a pipe

The water starts to flow as a main stream at a higher velocity which may cause deterioration.

This phenomenon can also be observed in very fine-grained cohesionless soils, which would be physical versus chemical reasons behind the same phenomenon in clays.

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Effective Factors in Dispersive Soils

• Type and amount of dissolved chemicals (mostly Na) in the pore water

Higher amount of dissolved salts will reduce the inter-particle forces.

• Ion Concentration in the water in touch with the soil.

The lower Ion Concentration, the higher the potential risk.

Presence of Ca Ions would be more effective in reducing the risk.

• Clay mineralsUsually some fraction is Montmorillonite. The plate

shaped particles influence the ease of particle dispersion

• Relative Compaction and Water content:The effect is UNCLEAR and more depends on other

conditions such as pore water ions, ion Concentration in the water …

• Shrinkage limit (SL), Atterberg LimitsSL should be less than wsat, otherwise cracks are

formed in the soil surface

Atterberg limits have been shown to plot above the A line

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Distinction and Recognition of Dispersive Soils1. Field ObservationsGeological Surveys should be carried out in order to

find proper clay quarries for earth dams.

Some of dominant characteristics of Dispersive Soils:• ColorHistorically reported to be Dark (Called Names: Black

Basic Soil or Solonetz)

• Steep slopesDeep cuts due to erosion in steep slopes

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• PlanesThe clay is washed out and a layer of SANDY SILT

is left behind.

- Erosion Gullies: Deep and Narrow Natural Culverts

- Sink Holes: vertical walled holes- Natural streams- Caves and Tunnels similar to limestone dissolved

Karsts- Natural water canals with sharp (90 degree) turns

• Arial Surveys

2. Laboratory Tests

Most common lab tests are:

1. Pin Hole test2. Chemical Tests, (Pore water Ion content)3. Double Hydrometer test4. Crumb test (or Emerson Test 1967

Australia)

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- Pin Hole is the best test for this purpose

- Agriculture specialists are more familiar with chemical tests

- Performing all four is recommended and also for as many samples as possible

- Disturbance of the samples should be avoided.

The Pin Hole test

• Models the flow of water in earth dams

• Yields very confident results

• Distilled water, with a constant head, passes through a hole made through the

sample horizontally.

Needle d = 1 mm Water pressures p = 50 180 380 1020 mm

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The water flowing out would be colloidal in case of dispersive soils

-If the soils is washed out under the 50 mm head and the exiting water is colloidal the soil is DISPERSIVE

-The test should be done with an undisturbed sample, however for remolding one would add water and wait for at least 24 hours.

-Observe: Water flow, Color of the water, Final Diameter of the hole

- The EXIT FLOW is calculated based on fluid mechanics theories (head loss inwater pipes) and is plotted versus HYDRAULIC HEAD

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Chemical TestsPore Water- Undisturbed sample is arrived to Liquid Limit by adding DISTILLED water-10 – 25 cm3 pore water of the resultant paste is extracted using vacuum.

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By performing chemical tests on the Saturated Electrolyte the amount of Na, Ka, Ca, and Mg ions are measured which will lead to following parameters

The Cation Exchange Capacity CEC of the soil is measured in “meq / 100 gr” of dry soil and from that:

ESP = Na / CEC Exchangeable Sodium Percentage

For typical values of:

TDS between 0.5 to 3 meq/lit,

ESP>10 and

SAR>2 values

were found to be representative of a number of Dispersive Clays

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From the aboveFrom the above--acquired data the ratio of Na/TDS is acquired data the ratio of Na/TDS is plotted versus Total dissolved minerals (plotted versus Total dissolved minerals (meqmeq/lit) to give /lit) to give us a measure of samples us a measure of samples dispersivitydispersivity..

3. Double Hydrometer Test (Volk 1937)

-The test is carried out, Dispersive agent (Sodium Hexa-Meta-Phosphate) is mixed with the soil using the shaker as the ordinary hydrometer test procedure

-The test is carried out for second time using distilled water instead without using the shaker.

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- Dispersion percent would be calculated as the percentage of particles smaller than 0.005 mm (F 0.005mm) from the second step (without dispersing agent) to the F0.005 mm from the first step (with dispersing agent)

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Typical values for:

non-dispersive clay would be between 10 and 40 percent

and

more than 50 percent for Dispersive clays.

The importance of Undisturbed samples is emphasized once more.

It is recommended that due to the variation of the soil, the test be carried out on at least 10 samples of the same soil.

4. Crumb Test (or Emerson Test 1967 Australia)

A sample of 6 to 9 mm diameter is put into a container of 150 cm3 distilled water the visual observations would be categorized as follows (Figure next page ):

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Practical Considerations

• The first choice is to avoid dispersive soils.Construction of Earth Dams would otherwise need a careful design of filter material

• Chemical TreatmentMixed with the soil:

- Lime treatment: 2-4% lime results in increased shear strength, reduced swell potential, no cracks induced.

-Aluminum Sulfate: Easier and cheaper to mix with soil and has much adverse effect on skin;

however it is much more expensive and has environmental effects on vegetation.

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• Mixed with the stored water

Gypsum: would also preserve the water quality by preventing colloidal solutions;

6566 Tons in 28 m3 of water in Australia reduced dispersion risk and improved the water quality.

•Using Filters:

containing a small percentage of fine sand (silt) would reduce the risk of dispersion by preventing leakage