INTRODUCTION The term ‘Soil’ has different meanings in different scientific fields.

It has originated from the Latin word Solum. To an agricultural scientist, it means ‘‘the loose material on the earth’s crust consisting of disintegrated rock with an admixture of organic matter, which supports plant life’’. To a geologist, it means the disintegrated rock material which has not been transported from the place of origin. But, to a civil engineer, the term ‘soil’ means, the loose unconsolidated inorganic material rock with or won the earth’s crust produced by the disintegration of rocks, overlying hard without organic matter. Foundations of all structures have to be placed on or in such soil, which is the primary reason for our interest as Civil Engineers in its engineering behavior.

4-1 Test for particle sizeDifferent soil has different characteristics affecting the

performance of each individual soil by moisture content (MC) and density(ρ).

WATER CONTENT OR MOISTURE CONTENT, MC OR w

is defined as the ratio of the weight of water to the weight of solids (dry weight) of the soil mass.

It is denoted by the letter symbol w and is commonly expressed as a percentage :

MC or w = (Ww / Ws) x 100%

There are several kinds of physical tests that have been developed to measure soil performance issued and recommended by AASHTO

(American Association of State Highway and Transportation Officials) and ASTM (American Society for Testing and Materials.)

THE SIEVE ANALYSISIs the process being used to determine the particles sizes

of gravel and fine aggregates.A sample of the material is thoroughly dried and shaken

through series of sieves ranging from course to fine.

The amount on each sieve is weighed and recorded.

The AASHTO standard sieve sizes for soil aggregates are presented as follows:

Sieve no. in inches (“) By no. opening in millimeters (mm)

2 50

1 -1/2 37.5

1 25.0

3/4 19.0

3/8 9.50

4 4.75

10 2.0

40 .425200 .075

Material finer than No. 200 sieve (.075 mm) is not feasible in determining the particles sizes Instead, these are determined by observing the rate at which the grains settle through liquid or gas.

Under the stoke’s law“ the rate of settlement of solid through a given liquid or gas is proportional to

the square of the diameters of the solid.”v = (1/18) . [(γs – γτ)/μτ] . D^2

Where;γs = unit weight of the material of falling sphere in g/cm^3

γτ = unit weight of the liquid medium in g/cm^3

μτ = viscosity of the liquid medium in g sec/cm^2

D = diameter of the spherical particle in cmV = the terminal velocity, is obtained in cm/s

The AASHTO method as specified is called hydrometer test AASHTO designation T-88

Hydrometer is a device which is used to measure the specific gravity of liquids.

hydrometer test1. Sample of the material passing No. 10 sieve is mixed thoroughly with water

and dispersing agent that dissipate any electrolytic bonds in the sample that might cause flocculation and accelerate settlement.

2. After 12 hours, the mixture is agitated making all particles suspended in water.

3. The mixture is then placed in a graduated flask, allowing the solid to settle under the pull of gravity.

4. The bigger particles settle first, followed by the smaller then the smallest one. The specific gravity of the liquid decreases. The changes are recorded by special hydrometer and read at prescribed intervals.

5. Change in specific gravity is related to the grain size of the material by stoke’s law.

6. Careful control of temperature and other possible variable is strictly observed to obtain satisfactory test results.

4-2 test to evaluate the effect of moistureconsistency of soil varies from semi solid to plastic then, to liquid if water content is increased.

Consistency is the term used to describe the degree of firmness (e.g. soft, medium, firm, or hard) of a soil.

1. LIQUID LIMIT TEST (LLT)

2. PLASTIC LIMIT TEST (PLT)

3. PLATIC INDEX TEST (PIT)

4. SHRINKAGE TEST (ST)

5. HAND FEEL TEST (HFT)

6. SAND EQUIVALENT TEST (SET)

test to evaluate the effect of moisture

LIQUID LIMIT TEST

The AASHTO designation T-89 on liquid limit signifies the percentage of moisture at which the sample

changes by decreasing the water from liquid to plastic state.

When the sample is wet than the liquid limit, a grooved sample of the soil in a cup will flow when lightly jarred 25 times. On the other hand, if the sample is jarred at the liquid limit, water separates the soil particles just wide enough to remove the soil mass shearing strength. Several tests on each sample are required to reach the moisture content wherein the groove in the sample is required to close at exactly 25 blows.

example problem of liquid limit: The following results were obtained from a liquid limit test on a clay using the casagrande cup device. Use the graph in figure.

What is the liquid limit of this clay?

No. of blows 12 20 28 32

Water content (%) 47.1 42.3 38.6 37.5

PLASTIC LIMIT TESTAASHTO Designation T-90 signifies the percentage of moisture wherein the sample

changes with lowering wetness from plastic to semi-solid condition.

At this stage, the soil mortar starts to crumble when rolled into threads 1/8” diameter. Additional water will make the soil plastic, but sands are non-plastic material that its non-plastic limit cannot be determined.

PLASTIC INDEX TESTAASHTO Designation T-91 is defined as the numerical difference between its liquid limit and its

plastic limit.

PI = LL-PLIt is also referred to as the percentage of dry weight. In some

combination, it measures:

a.) the fitness and shapes of the soil particles.b.) the interplay of the attractive forces tending to hold the clay

mineral flakes together.c.) the thickness and velocity of the water film and,d.) the quantity and electrical charges of the cations.

Example of plasticity index: “CE board exam may 2003”The following data were obtained from the atterberg limits test

for a soil.

Liquid limit = 41.0%Plasticity limit = 21.1%

What is the plasticity index of the soil?Solution:

PI = LL-PLPI = 41.0% - 21.1%PI = 19.9%

SHRINKAGE TESTShrinkage test measures the changes in volume and weight

that occur as partly mixture of soil and water (except No. 40). Sample is dried from the near liquid limit to constant weight of 110°C. The results are stated in terms of shrinkage limit, volumetric change, and lineal shrinkage.

higher value of shrinkage factor showed that the soil will give more trouble and problem. The volume of a certain soil increases when it absorbs water. Soils that are suitable for sub-grade and base course are those that expand very little when moistened. And those that swell more are considered as poor materials. A common soil specimen requires volume change limit of 1% for base coarse materials.

HAND FEEL TESTExperienced soil Engineer employ the “ hand feel test” to

approximately predict the plasticity index of the soil. The test may include:

a.) thread toughness at moisture content approximating the plastic limit.

b.) the air-dried strength.

c.) Dilatancy - the tendency of a substance to become more viscous or solid when affected by an outside force or agitation.

SAND EQUIVALENT TESTAASHTO Designation T-176 is for field determination of the presence of undesirable quantities

of clay like materials in soil. Aggregate mixture is based on the volume, rather than the on

weight. The sand equivalent is the ratio between the height of the sand

column (laboratory experiment test) and the combined height of sand and expanded saturated clay expressed in percentage.

Higher values of sand equivalent indicate superior materials. The allowable moving average value from the California

specification is 21 for sub base, 31 for aggregate base and 45-50 for aggregates of various type asphalt concrete and 76 for concrete sand.

4-3 density test of soil The density of soil or weight per cubic foot

varies with the peculiarities of the soil itself, the moisture contents, and the compacted device plus the method of their use. The standard weight of soil per cubic foot cannot be fixed, but should be determined in particular instance.

The main variables in the soil proper are:1. Specific gravity of the soil particles that may vary from 2.0 to 3.3

that is normally between 2.5 and 2.8.

2. Particle size distribution of the soil. A mass composed of entirely spheres of one size in the densest possible condition will contain 75% solid and 25%. The smaller the sphere in the mass the higher the percentage of the solid, hence, particle size distribution may greatly affect density.

3. Grain shape of soil particles. Sharp angular particles will resist shifting from loose to a compacted state. Flaky particles in soil will decrease its density because they are difficult to compact.

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