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PLASTICITY
INDEX “PLASTICITY”
It shows the size of the range of the moisture contents at
which the soil remains plastic. In general, the plasticity index depends only on the amount of clay present. It indicates the fineness of the soil and its capacity to change shape without altering its volume. PLASTICITY INDEX
The plasticity index (PI) of a soil is the
numerical difference between its liquid limit (LL) and its plastic limit (PL) Why is plasticity important in soil? The plasticity of a soil is its ability to undergo deformation without cracking. It is an important index property of fine grained soil, especially for clayey soils. The adsorbed water in clayey soils is leads to the plasticity of soil. Absorbed water in the clay particles allow the particles to slip over one another. Apparatus
1. Dish: porcelain evaporating dish or similar
mixing dish approximately 115 mm in diameter. Apparatus
2. Spatula: spatula or pill knife having a blade
about 75 mm to 100 mm in length and about 20 mm in width. Apparatus 3. Containers: suitable containers made of material resistant to corrosion and not subject to change in weight or disintegration on repeated heating and cooling. Containers must have close- fitting lids to prevent loss of moisture from samples before initial weight determination and to prevent absorption of moisture from the atmosphere following drying and before final weight determination. One container is needed for each moisture content determination. Apparatus
4. Balance: the balance must have sufficient
capacity, be accurate to 0.02 g or 0.1 %, and readable and sensitive to 0.01 g. Apparatus
5. Oven: a thermostatically controlled drying
oven capable of maintaining temperatures of 140ºF ± 9ºF and 230ºF ± 9ºF. CALCULATION
Calculate the plasticity index of a soil as the
difference between its liquid limit and its plastic limit, as follows: PI = LL − PL CALCULATION Report PI as non-plastic (NP) when:
1. The liquid limit or the plastic limit cannot be determined, or
2.The plastic limit is equal to or greater than the liquid limit.
PART IV TESTS OF CEMENT TREATED
OR LIME TREATED SOILS PROCEDURE Follow the procedures described in this test method with the following exceptions: 1. Determine percent of cement or lime to be added to the soil. For field samples that contain 10 % or more aggregate retained on the No. 4 sieve, use the following example to determine the percent of cement or lime to add to the soil passing the No. 4 sieve. PROCEDURE
Example: Two percent of cement is to be
added in the field to an aggregate graded 100 percent passing the ¾ in. sieve and 60 percent passing the No. 4 sieve. PROCEDURE
Let C = Percent cement required for the portion
passing the No. 4 sieve. Then 100/60 = C/2 60×C = 200 C = 3.33 % PROCEDURE
2. Add cement or lime to the material passing
the No. 4 sieve prior to grinding and sieving over the No. 40 sieve. PROCEDURE
3. Add water and mix. Leave the mixed
sample in an uncovered evaporating dish in workroom for 24 hr. PROCEDURE
4. Use spatula, plus hand operated mortar and
pestle when necessary, and break up crust and soil-cement (or soil-lime) aggregations. PRECAUTIONS 1. In order to get good test reproducibility, it is necessary to use seasoned soil, mold into ellipsoidal shape prior to rolling, exert the proper pressure during rolling, use the proper rate of rolling, use the proper length of soil thread, and prevent further moisture loss when the end point is reached. PRECAUTIONS 2. Reproducibility of test results is very difficult unless the test is performed exactly the same way each time. When training new operators, have them and an experienced operator conduct tests on duplicate samples. Do not use the new operator’s test results until they have mastered the test technique well enough to satisfactorily duplicate the test result of the experienced operator. PRECAUTIONS 3. Conform exactly to the specified times for mixing, curing, and testing because variations can cause erroneous test results. Some soils are slow to absorb water; therefore, it is possible to add the increments of water so fast that a false liquid limit value is obtained. Because this is particularly true when the liquid limit of a clay soil is obtained from one determination, it is of greatest importance that the soil and water be thoroughly and uniformly mixed and that at least two closures be observed to ensure that the accepted number of blows is truly characteristic of the soil under test. PRECAUTIONS
4. Replace grooving tool tips
that become worn. PRECAUTIONS 5. Take moisture sample immediately after the soil flows together for a distance of ½ in. Keep edges of watch glasses free of soil particles so that a moisture proof seal will be obtained. Moisture samples are small and evaporation losses can cause erroneous results. HEALTH AND SAFETY It is the responsibility of the user of this test method to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Prior to handling, testing or disposing of any materials, testers must be knowledgeable about safe laboratory practices, hazards and exposure, chemical procurement and storage, and personal protective apparel and equipment.