IN ADDITION TO THE CLIMATIC CONDITIONS THE TYPE OF SOIL HAS AN IMPORTANT ROLE IN CONSTRUCTION AND MAINTENANCE OF THE ROAD STRUCTURE.

The useful life of the road depends upon the type of construction (road structure) , intensity and nature of the traffic, bearing capacity of the sub grade (soil) ,climatic conditions and the maintenance of the road .Road stability can only be maintained if the soil foundation remains in a relatively dry condition. As the moisture content increases the strength of the sub soil decreases .This effect of the moisture content is more marked in the case of the clay soils than with granular soils such as sand and the gravels ,which is considered relatively good.The control of the moisture content of the sub grade is an essential feature of a good road .A clay sub soil is subject to volume changes arising from changes in the moisture content and such a soil is apt to become water logged during the monsoon and dry out and crack in dry months .This can be reduced by the incorporation of granular materials such as gravel and sand or clinker .The sub grade also required protection against loss of moisture.Before designing the sub soil drainage of a road ,a survey should be conducted for soil and water conditions in the sub soil .Free water can be intercepted or controlled and a high water table can be lowered by way of the installation of sub soil drains .The position of the drains should be maintained at least four feet below the formation level.Size spacing and depth of the laterals (cross drains) below the formation level depend upon the nature of the soil ,it's moisture retentive capacity,rainfall and the longitudinal slopes of the ground.Drains should be installed well in advance of the road construction to allow as much time as possible for the establishment of equilibrium moisture conditions.In order to determine the suitability of a particular soil it is necessary to know the proportions of each material in the soil and also whether the particular material possesses the properties required of it .The field testing may be conducted to verify the contents existing in the natural soil of the road as it has an important role in the road construction and suitability under the merits and demerits of the soil mechanics.Clay soils in cuttings do not generally needs further compaction.In fact heavy compaction of such a soil may be harmful as it may destroy the internal structure of the soil , resulting in loss of the strength.