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PREFACE:

Soil is the natural material over which the pavement is going to be constructed. According to CEN EN ISO 14688-1 (2013), soil is defined as ‘assemblage of mineral particles and/ or organic matter in the form of a deposit but sometimes of organic origin, which can be separated by gentle mechanical means and which includes variable amount of water and air (and sometimes other gases)’. The term also applies to ground consisting of replaced soil or man-made materials exhibiting similar behaviour, for example, crushed rock, blast furnace slag and fly ash. All types of soil derive from the disintegration of rocks and decomposition of vegetation.

 Disintegration and decomposition were caused by physical and chemical action. The major ones are wind, water, temperature variations and chemical reactions. Soils are characterised by the way they were created. They are defined as residual, sedimentary, aeolianand glacial. Residual soilsare those formed by rocks located just below them. Climatologic conditions (temperature and rainfall) were the main reasons of disintegration of the parent materials. 

Those soil types consist of inorganic grainy materials, in the form of fine particles in upper layers and in the form of more coarse particles in lower layers. These soils can be used as pavement foundation layer, provided that no drastic chemical disintegration has occurred (tropical climatologic conditions). Sedimentary soilsare those formed by the deposition of materials that were in suspension in aqueous environments, such as lakes, rivers and oceans. 

The sedimentary soils vary from clean sand to flocculent clay of marine origin. From the sedimentary soils, the alluvial soils are generally suitable as pavement foundation material.

 The marine soils, those created by ocean erosion of the materials transferred from rivers to the sea, should be treated with care in pavement engineering, particularly when they contain a high percentage of fine particles.

 Aeolian soilsare those formed by aeolian action, that is, materials transported, eroded and deposited by winds. These soils appear as sand dunes or calcitic silt. Pavement construction on sand dunes, which are not protected by topsoil, appears to be problematic.

 A cut on calcitic silt may have very high gradient (slope) owing to the cohesive properties of calcium. The usage, however, of disturbed calcitic silt on embankment is problematic because cohesion has been lost. Glacial soilsare soils formed during the glacier era. These soils may extend to a depth of many kilometres; they consist of boulders, cobbles, gravel, sand, silt and clay, and they are widely found in the Northern Hemisphere.

Glacial soils may be characterised with respect to their content in inorganic materials. Soils in which the inorganic ingredients of mineral materials outclass the organic substances are called inorganic soils. Otherwise, they are called organic soils, which are characterised by their dark brown colour and their characteristic smell.

 Regardless of the way they have been formed, the lack of homogeneity is a feature of soils. Soils appear to vary from loose to very well compacted, with or without cohesion, with continuous or non-continuous particle size distribution. The above heterogeneity appears in both horizontal and vertical levels. 

The highway engineer has to deal with many kilometres and in most cases has to use the existing soil without any adjustment. This fact makes the determination of its representative mechanical behaviour more difficult and tricky.