HIGHWAY MATERIALS AND THE SUB GRADE SOIL

HIGHWAY MATERIALS

Highway materials are those materials that are used for the construction of roads. One should be well informed about the materials that are used for the construction of highways, quality of the materials, proportions of the materials, strength etc in order to construct a good highway.

Highway materials can be classified on three categories. They are:

1)    Mineral materials: subgrade soil, sand, aggregates, brick pebbles etc are the mineral materials.

2)    Binding materials: cohesive soil, Cement, lime, tar, asphalt etc are the binding materials.

3)    Other miscellaneous materials: reinforcing steel, timber, bricks, gabion wires etc are also used while constructing the highway.

SUBGRADE SOIL

Sub grade soil refers to the soil below the paved road or simply the gravel type road. Thus, it is one of the integral parts of the road pavement structure.  For a road to be stable and durable, subgrade soil is needed to be stable. Due to the unstable subgrade soil or due to the poor condition of the subgrade soil various problems arise on the road pavement like formation of waves, corrugation, cracking of the paved surface, rutting etc. So, the basic function of the subgrade soil is to provide adequate support to the paved road. Compacted soil and stabilized soil are generally used as sub base soil for highway pavements.

WHAT ARE THE CHARACTERISTICS OF THE SOIL?

Soil is formed by the disintegration of rocks over large period of time and soil contains void and solids. There are various characteristics of the soil. But one should know about the desirable properties of soil that can affect the quality of the highway.

The desirable qualities of soil are as follows:

1)    Stability

2)    Incompressibility

3)    Permanency of strength

4)    Good drainage

5)    Ease of compaction

6)    Minimum change in the density and stability of the soil under the adverse condition and due to the impact of the water table

INDEX PROPERTIES OF THE SOIL

In order to make the study of the soil, soil are classified to various groups and types. These classification makes it simple for any highway Engineer much more easy to choose and identify the type of soil at sites.

So, when the identification and classification of soil is based upon the soil properties, then it is referred as the index properties of the soil.

Index properties which are usually used are grain size distribution, liquid limit and plasticity index and also sometimes shrinkage limit, field moisture equivalent, centrifuge moisture equivalent and compacted dry density are also used.

Grain size analysis:

·        Analysis of the size of the grain and their distribution accordingly is referred as grain size analysis. The grain size distribution is done by mechanical analysis. The soils which are coarse grained are analyzed by sieve analysis and the fine grained soils are analyzed by sedimentation analysis. So, the mechanical analysis is carried out to determine the percentage of individual grain size present in sample soil.

Consistency limits and indices:

·        Clay, which falls under the fine grained soil, shows different nature than other types of soil. It may be in liquid state or may be stiffed according to the moisture condition available. Especially the water content present within the clay is responsible for the different behaviors of the clayey soil. Clay exhibits plastic property. Plasticity is the property by virtue of which a material is capable of extending to the limit without any rupture on its surface and cannot return to its original shape by itself but may achieve its original shape and size if the needed amount of external force is applied.

·        There are test like Atterberg limit and indices proposed by Atterberg for determining the consistency and plastic properties of the soil.

·        Liquid limit is defined as the minimum water content of any substance (soil in this case) that allows the substance to flow freely within any change in the state of the substance itself when even the small shearing force is present.

·        Plastic limit is defined as the minimum water content present in the soil at which the soil remains in the plastic state. Plastic state is that state in which soil can freely exhibit or show its plastic properties or plasticity. Above we already have discussed what is plasticity.

·        Plasticity index actually shows the range of moisture content above which the soil remains in plastic condition. So, it is the difference between the liquid and plastic limits.

·        Shrinkage limit is the maximum moisture content at which further reduction in water content does not cause any reduction in the volume of the soil.

While dealing with the cohesive types of soil like clay soil. Cohesive types of soil are regarded as the poor type of engineering materials. Liquid and plastic limit depends upon the type and amount of the clay present in the soil. And we know the soil having the high values of the liquid limit and plastic limit have poor engineering values. Also the studying of the values of the liquid limit and plasticity index helps to classify the cohesive soils.

1)    SOIL CLASSIFICATION BASED ON GRAIN SIZE:

There are several classification based on the grain size. The mostly accepted grain size classification system is the M.I.T classification system and Indian Standard Classification System of the soils. The difference in these two types of classification system is that the limits of the grain size for each of them is different to fall under the category of gravel, sand, silt and clay.

GRVAEL	SAND			SILT			CLAY
	COARS	MEDIUM	FINE	COARSE	MEDIUM	FINE	COARSE	MEDIUM	FINE

          2.0        0.6           0.2      0.06        0.02        0.006   0.002    0.0006    0.0002

                   CLASSIFICATION BASED ON GRAIN SIZE (M.I.T)

2)    TEXTURAL CLASSIFICATION

SOIL CLASSIFICATION SYSTEM:

The various soil classification systems that are use in the field of highway engineering are:

(i)                Burmister descriptive classification

(ii)              Casagrande  soil classification

(iii)            Unified soil classification of revised casagrande soil classification

(iv)            U.S. Public Roads Administration classification

(v)              Highway Research Board (HRB) or American Association of State Highway Officials classification or Revised PRA classification

(vi)            Federal aviation agency classification

(vii)          Civil Aeronautics Administration classification

(viii)        Compaction classification

Unified soil classification of revised cassagrande soil classification has been widely accepted for the classification of soils for civil engineering purposes.

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