The soil that lies below ground level and extends to such depth as may affect the support of the pavement.
Soil is an accumulation or deposit of earth material, derived naturally from the disintegration of rocks or decay of vegetation that can be excavated readily with power equipment in the field or disintegrated by gentle mechanical means in the laboratory. The supporting soil beneath pavement and its special under courses is called sub grade. Undisturbed soil beneath the pavement is called natural sub grade. Compacted sub grade is the soil compacted by controlled movement of heavy compactors.
1. Subgrade soil is considered as the integral part of the road structure.
2. It provides support to the pavement from beneath.
3. The properties of subgrade soil are important to the design of pavement structure.
4. Its main function is to give adequate support to the pavement.
5. It should possess sufficient stability under adverse climate and loading conditions.
The desirable properties of sub grade soil as a highway material are
3. Permanency of strength
4. Minimum changes in volume and stability under adverse conditions of weather and ground water
6.Ease of compaction
Index Properties of Soil
The soil properties on which their identification and classification are based are known as index properties.
Grain size distribution, Liquid limit, Plastic limit, Plasticity index, Shrinkage limit, Field moisture & compacted dry density.
1.Grain size analysis: It is found by mechanical analysis. That is Coarse grained soil by sieve analysis and fine soil by sedimentation analysis or Hydrometer method. The grain size analysis is carried out to determine the percentage of individual grain size present in a soil sample.
(1) Write down the weight of each sieve as well as the bottom pan to be used in the analysis.
(2) Record the weight of the given dry soil sample.
(3) Make sure that all the sieves are clean, and assemble them in the ascending order of sieve numbers (#4 sieve at top and #200 sieve at bottom). Place the pan below #200 sieve. Carefully pour the soil sample into the top sieve and place the cap over it.
(4) Place the sieve stack in the mechanical shaker and shake for 10 minutes.
(5) Remove the stack from the shaker and carefully weigh and record the weight of each sieve with its retained soil. In addition, remember to weigh and record the weight of the bottom pan with its retained fine soil.
According to size of grains soil is classified as gravel, sand, silt and clay. As per Indian standard classification the limits of grain size are as follows
Fraction of soils
Larger than 2.00mm size Gravel
Between 2.00mm – 0.06 mm size Sand
Between 0.06mm – 0.002 mm size Silt
Smaller than 0.002 size Clay
(1) Take the fine soil from the bottom pan of the sieve set, place it into a beaker, and add 125 mL of the dispersing agent (sodium hexametaphosphate (40 g/L)) solution. Stir the mixture until the soil is thoroughly wet. Let the soil soak for at least ten minutes.
(2) While the soil is soaking, add 125mL of dispersing agent into the control cylinder and fill it with distilled water to the mark. Take the reading at the top of the meniscus formed by the hydrometer stem and the control solution. A reading less than zero is recorded as a negative (-) correction and a reading between zero and sixty is recorded as a positive (+) correction. This reading is called the zero correction. The meniscus correction is the difference between the top of the meniscus and the level of the solution in the control jar (Usually about +1). Shake the control cylinder in such a way that the contents are mixed thoroughly. Insert the hydrometer and thermometer into the control cylinder and note the zero correction and temperature respectively.
(3) Transfer the soil slurry into a mixer by adding more distilled water, if necessary, until mixing cup is at least half full. Then mix the solution for a period of two minutes.
(4) Immediately transfer the soil slurry into the empty sedimentation cylinder. Add distilled water up to the mark.
(5) Cover the open end of the cylinder with a stopper and secure it with the palm of your hand. Then turn the cylinder upside down and back upright for a period of one minute. (The cylinder should be inverted approximately 30 times during the minute.)
(6) Set the cylinder down and record the time. Remove the stopper from the cylinder. After an elapsed time of one minute and forty seconds, very slowly and carefully insert the hydrometer for the first reading. (Note: It should take about ten seconds to insert or remove the hydrometer to minimize any disturbance, and the release of the hydrometer should be made as close to the reading depth as possible to avoid excessive bobbing).
(7) The reading is taken by observing the top of the meniscus formed by the suspension and the hydrometer stem. The hydrometer is removed slowly and placed back in to the control cylinder. Very gently spin it in control cylinder to remove any particles that may have adhered.
(8) Take hydrometer readings after elapsed time of 2 and 5, 8, 15, 30, 60 minutes and 24 hours
Highway Research Board (HRB) classification of soils
This is also called American Association of State Highway Officials (AASHO) classification of Revised Public Roads Administration (PRA) soil classification system. Soils are divided into seven groups A-I to A-7. A-I, A-2 and A-3 soils are granular soils, percentage fines passing 0.074 mm sieve being less than 35. A-4, A-5, A-6 and A-7, soils are fine grained or silt-clay soils, passing 0.074 mm sieve being greater than 35 percent. The classification of soils by three simple laboratory tests namely, sieve analysis, liquid limit and plastic limit.
A-1 soils are well graded mixture of stone fragments, gravel coarse sand, fine sand and non plastic or slightly plastic soil binder. The soils of this group are subdivided into two subgroups, A- 1-a, consisting predominantly of stone fragments or gravel and A-I-b consisting predominantly of coarse sand.
A-2 group of soils include a wide range of granular soils ranging from A- 1 to A-3 groups, consisting of granular soils and upto 35% fines of A-4, A-5, A-6 or A-7 groups. Based on the fines content, the soils of A-2 groups are subdivided into subgroups A-2-4, A-2-5, A-2-6 and A-2-7.
A-3 soils consist mainly, uniformly graded medium or fine sand similar to beach sand or desert blown sand. Stream-deposited mixtures of poorly graded fine sand with some coarse sand and gravel are also included in this group.
A-4 soils are generally silty soils, non-plastic or moderately plastic in nature with liquid limit and plasticity index values less than 40 and 10 respectively
A-5 soils are also silty soils with plasticity index less than 10%, but with liquid limit values exceeding 40%. These include highly elastic or compressible, soils, usually of diatomaceous of micaceous character.
A-6 group of soils are plastic clays, having high values of plasticity index exceeding 10% and low values of liquid limit below 40%; they have high volume change properties with variation in moisture content.
A-7 soils are also clayey soils as A-6 soils, but with high values of both liquid limit and plasticity index, (LL greater than 40% and PI greater than 10%). These soils have low permeability and high volume change properties with changes in moisture content. This is further classified in to A-7-5 & A-7-6.
Group index of soil
In order to classify the fine grained soils with one group and for judging their suitability as subgrade material an Index system has been introduced in HRB classification which is termed as Group Index. Soils are thus assigned arbitrary numerical number known as group index (GI).
Group index is a function of % material passing 75microns (0.075mm) sieve, Liquid limit & Plasticity index of soil and is given by
a=that portion of material passing 75micron sieve, greater than 35 & not exceeding 75% (expressed as a whole no. 0-40).
b=that portion of material passing 75micron sieve greater than15 & not exceeding 35% (expressed as a whole no. from 0-40).
c=that value of LL in excess of 40 and less than 60(expressed as a whole no. from 0-20)
d=that value of plasticity index exceeding 10 & not more than 30 (expressed as a whole no. from 0-20).
GI value is varying from 0-20. Higher the value poorer the soil as subgrade.
AASHTO Classification System
The AASHTO Classification System is based on the Public Roads Classification System that was developed from the results of extensive research conducted by the Bureau of Public Roads, now known as the Federal Highway Administration of the United States. Several revisions have been made to the system since it was first published. The system has been described by AASHTO as a means for determining the relative quality of soils for use in embankments, subgrades, subbases, and bases. In this system of classification, soils are categorized into seven groups, A-1 through A-7, with several subgroups, as shown in Error! Not a valid bookmark self-reference.. The classification of a given soil is based on its particle size distribution, LL, and PI. Soils are evaluated within each group by using an empirical formula to determine the groupindex (GI) of the soils, given as GI = (F - 35)[0.2 + 0.005(LL - 40)] + 0.01(F - 15)(PI - 10)
where, GI = group index
F = % of soil particles passing 0.075 mm (No. 200) sieve in whole number based on material passing 75 mm (3 in.) sieve,
LL = liquid limit expressed in whole number, and
PI = plasticity index expressed in whole number.
Soils containing clay exhibit a property called plasticity. Plasticity is the ability of a material to be moulded (irreversibly deformed) with out fracturing.This behavior is unique to clays and arises due to the electrochemical behavior of clay minerals. The stiffness or consistency of fine grained soils depends on their moisture content, and varies with variations in the amount of moisture present. Depending on its moisture content, a soil can exist in one of the following states: viscous liquid, plastic solid, semi solid and solid. Atterberg in 1911 proposed a series of tests, mostly empirical, for the determination of the consistency properties/states of fine
grained soils. Atterberg limits define the moisture contents at which the soil changes from one state to another. These include the liquid limit (LL), the plastic limit (PL), shrinkage limit (SL). They are determined by tests carried out on the fine soil fraction passing the 425μm (No. 40) sieve. Liquid limit(AASHTO T89) may be defined as the minimum water content at which the soil will start to flow under the application of a standard shearing force (dynamic loading). Plastic limit(AASHTO T90) – measure of toughness – the moisture content at which the soil begins to fracture when rolled into a 3mm diameter thread. Shrinkage limit (AASHTO T92) is the maximum moisture content after which further reduction in water content does not cause reduction in volume. It is the lowest water content at which a clayey soil can occur in a saturated state. Plasticity index (PI=LL-PL) is the numerical difference between the liquid and plastic limits. Thus, it indicates the range of moisture content over which the soil remains deformable (in plastic state).Consistency limits and the plasticity index are used in the identification and classification of soils. Generally, soils having high values of liquid limit and plasticity index are poor as subgrades/engineering materials. Both the liquid limit and plastic limit depend on the type and amount of clay in the soils. In soils having same values of liquid limit, but with different values of plasticity index; it is generally found that rate of volume change and dry strength increases and permeability decreases with increase in plasticity index. On the other hand, in soils having same values of plasticity index but different values of liquid limit, it is seen that compressibility and permeability increase, and dry strength decreases with increase in liquid limit. Soils that cannot be rolled to a thread at any water content are termed as Non-Plastic (NP).
Liquid limit: Minimum water content at which the soil will flow under the application of very small shearing force.
Plastic limit: Minimum moisture content at which the soil remains in a plastic state.
Plasticity index: It is numerical difference between the Liquid limit and Plastic limit.
Shrinkage limit: Maximum moisture content at which further reduction in water content does not cause reduction in volume.
Subgrade Soil Strength
he factor on which the strength characteristics of soil depend on Soil type, moisture content, dry density, internal structural of the soil, type and mode of stress application.
Evaluation of soil strength
1. Shear test
2. Bearing test (Plate load test)
3. Penetration test. (CBR)