Plasticity Tests for Soil

By : Admin
October 23, 2022
Education

Plasticity is an important characteristic in the case of fine soils, the term plasticity describing the ability of a soil to undergo unrecoverable deformation without cracking or crumbling. In general, depending on its water content (defined as the ratio of the mass of water in the soil to the mass of solid particles), a soil may exist in one of the liquid, plastic, semi-solid and solid states. If the water content of a soil initially in the liquid state is gradually reduced, the state will change from liquid through plastic and semi-solid, accompanied by gradually reducing volume, until the solid state is reached.

The water contents at which the transitions between states occur differ from soil to soil. In the ground, most fine soils exist in the plastic state. Plasticity is due to the presence of a significant content of clay mineral particles (or organic material) in the soil. The void space between such particles is generally very small in size with the result that water is held at negative pressure by capillary tension. This produces a degree of cohesion between the particles, allowing the soil to be deformed or moulded. Adsorption of water due to the surface forces on clay mineral particles may contribute to plastic behaviour. Any decrease in water content results in a decrease in cation layer thickness and an increase in the net attractive forces between particles

Liquid Limit (Cone Penetrometer Method)

The liquid limit is the empirically established moisture content at which a soil passes from the liquid state to the plastic state. It is the minimum moisture content at which a soil flows upon application of very small shear force.

Required equipment

Test sieves of sizes 425 µm
An airtight container
A flat glass plate
Two palette knives or spatulas
A penetrometer
A cone of stainless steel, 35 mm long with a smooth, polished surface and an angle of 30 ° having a mass of 80 g.
A metal cup 55 mm in diameter and 40 mm deep with the rim parallel to the flat base
An evaporating dish or a damp cloth
Apparatus for moisture content determination
A wash bottle containing clean water
A metal straight edge
A stopwatch

Test procedure

Take a sample of the soil of sufficient size to give a test specimen weighing about 400 g which passes the 425 µm sieve. This should be enough material for both Plastic Limit and Linear Shrinkage tests in addition to the Liquid Limit test.
Transfer the soil to a glass plate. Add water and mix thoroughly with two palette knives until the mass becomes a thick homogeneous paste.
Place the paste in an airtight container and allow to stand for 16 – 24 hours to enable the water to permeate through the soil.
Take the 400 g soil sample and place it on a glass plate. Mix the paste for at least 10 minutes using the two palette knives. Add more distilled water if necessary so that the first cone penetrometer reading is about 15 mm.
Push a portion of the mixed soil into the cup with a palette knife, taking care not to trap air, gently tapping the cup against a firm surface if necessary. Strike off excess soil with the straightedge to give a smooth level surface.
With the penetration cone locked in the raised position lower the cone so that it just touches the surface of the soil. When the cone is in the correct position, a slight movement of cup will just mark the soil surface. Lower the dial gauge to contact the cone shaft and record the reading of the dial gauge to the nearest 0.1 mm.
Release the cone for a period of 5 ± 1 sec. After locking the cone in position, lower the dial gauge to contact the cone shaft and record the reading of the dial gauge to the nearest 0.1 mm. Record the difference between the readings as the “cone penetration”.
Lift out the cone and clean it carefully.
A little more wet soil shall be added to the cup and the process repeated. If the difference between the first and second penetration readings is less than 0.5 mm, the average of the two penetrations shall be recorded. If the second penetration is more than 0.5 mm and less than 1 mm different from the first, a third test shall be carried out. If the overall range is then not more than 1 mm, record the average of the 3 penetrations. If the overall range is more than 1 mm, the soil shall be removed from the cup, remixed and the test repeated until consistent results are obtained.
Take a moisture content sample of about 20 g from the area penetrated by the cone and determine the moisture content.
The penetration test shall be repeated at least three more times using the same sample of soil to which further increments of water have been added. The amount of water added shall be such that a range of penetration values of approximately 15 mm to 25 mm is covered by the four test runs.
Each time soil is removed from the cup for the addition of water, wash and dry the cup.

Calculations

Calculate the moisture content of each specimen.

Where:

m₁ is the mass of the container (in g)

m₂ is the mass of the container and wet soil (in g)

m₃ is the mass of the container and dry soil (in g)

Plot the relationship between the moisture content and cone penetration with the moisture content as the abscissae and the cone penetration as ordinates, both on linear scales.
Draw the best straight line fitting the points.
The Liquid Limit (LL) of the soil sample is the moisture content corresponding to a cone penetration of 20 mm and shall be expressed to the nearest whole number.

Plastic Limit and Plasticity Index

The Plastic Limit is the empirically established moisture content at which a soil becomes too dry to be plastic. It is used together with the Liquid Limit to determine the Plasticity Index which when plotted against the Liquid Limit on the plasticity chart provides a means of classifying cohesive soils. The Plasticity Index is the difference between the Liquid Limit and the Plastic Limit. The Plasticity Index is the range of moisture content in which a soil is plastic; the finer the soil, the greater the Plasticity Index.

Required equipment

• Two flat glass plates, one for mixing soil and one for rolling threads

• Two palette knives or spatulas

• Apparatus for moisture content determination

• Clean water

• A length of rod, 3 mm in diameter and 100 mm long

Sample preparation

This test commonly is performed as a continuance of the Liquid Limit test, and material for the test could conveniently be prepared as part of the Liquid Limit test.

Otherwise, a 40 g sample should be prepared in the same way as specified for the Liquid Limit test.

Test Procedure

Take the 40 g soil paste sample and place it on a glass plate.
Allow the soil to dry partially until it becomes plastic enough to be shaped into a ball.
Mould the ball of soil between the fingers and roll it between the palms of the hands until the heat of the hands has dried the soil sufficiently for slight cracks to appear on its surface.
Divide this sample into 2 sub-samples of about 20 g each and carry out separate determination on each portion. (Divide each of the 2 sub-samples into 4 more or less equal parts).
Mould the soil in the fingers to equalize the distribution of moisture. Then form the soil into a thread about 6 mm diameter between the first finger and thumb of each hand.
Roll the thread between the fingers, from fingertip to the second joint, of one hand and the surface of the glass plate. Use enough pressure to reduce the diameter of the thread to about 3 mm in 5 to 10 complete, forward and back, movements of the hand.
Pick up the soil, mould it between the fingers to dry it further, form it into a thread and roll it out again as specified above.
The procedure shall be repeated until the thread shears both longitudinally and transversely when rolled to about 3 mm diameter. The metal rod may be used to gauge the diameter. The first crumbling point is the Plastic Limit.
Gather together the pieces of crumbled soil thread and transfer them to a suitable container for determination of the moisture content and replace the lid immediately.
Repeat the rolling procedure on the other 3 portions of the sub-sample, placing them all in the same container for determination of the moisture content.
Repeat the rolling procedure on the 2nd sub-sample as described above so that 2 completely separate determinations are made.

Calculations

1) Calculate the moisture content of both samples. If the 2 results differ by more than 0.5 % moisture content, repeat the whole test.

2) Calculate the average of the 2 moisture content values and express the value to the nearest whole number. This is the Plastic Limit (PL).

Plasticity Index

The Plasticity Index (PI) is defined as the difference between the Liquid Limit (LL) and the Plastic Limit (PL), and is calculated from the equation:

PI = LL – PL

This value is also reported to the nearest whole number

Linear Shrinkage

Shrinkage due to drying is significant in clays but less so in silts and sands. If the drying process is prolonged after the plastic limit has been reached, the soil will continue to decrease in volume, which is also relevant to the converse condition of expansion due to wetting.

The Linear Shrinkage value is a way of quantifying the amount of shrinkage likely to be experienced by clayey material. Such a value is also relevant to the converse condition of expansion due to wetting.

Required equipment

• A flat glass plate

• Two palette knives or spatulas

• A drying oven capable of maintaining the temperature of 105 °C – 110 °C

• Clean water

• A brass mould for the Linear Shrinkage test

• Silicone grease or petroleum jelly

• Vernier callipers or steel rule with an accuracy 0.5 mm

Sample preparation

This test commonly is performed as a continuance of the Liquid Limit and Plastic Limit tests, and material for the test could therefore conveniently be prepared as part of the Liquid Limit test.

Otherwise, a 150 g sample should be prepared in the same way as specified for the Liquid Limit test. A sample of material passing through a 425 µm sieve, or alternatively a sample of natural soil without coarse particles, shall be thoroughly mixed with distilled water until the mass becomes a smooth homogeneous paste with moisture content at about the Liquid Limit of the soil.

Test Procedure

Clean the mould thoroughly and apply a thin film of silicone grease or petroleum jelly to its inner faces to prevent the soil from adhering to the mould.
Take the 150 g soil paste sample at approximately the Liquid Limit.
Place the soil/water mixture in the mould such that it is slightly proud of the sides of the mould. Gently jar the mould, or carefully tap the mould against a firm surface, to remove any air pockets in the mixture.
Level the soil along the top of the mould with the palette knife and remove all soil adhering to the rim of the mould by wiping it with a damp cloth.
Place the mould where the paste can air dry slowly for 1 − 2 days until the soil has shrunk away from the walls of the mould.
Then complete the drying at 105 °C to 110 °C.
Cool the mould and measure the mean length of the soil bar by pressing it against the end of the mould where there is a better fit, while measuring the distance between the opposite side of the mould and the soil bar.

Calculations

Linear Shrinkage of the soil is calculated from the equation :