MOISTURE CONTROL AND THE SUB -SOIL DRAINAGE MAY PROTECT THE SOIL FOUNDATION, REQUIRED TO BE MAINTAINED RELATIVELY IN THE DRY CONDITIONS.

Road stability can only be maintained if the soil under road/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 dangerous than the granular soils such as sands and the gravels .As such the control of the moisture content of the sub grade is an essential feature of a good road.Water moving freely under the action of gravity can effect the moisture content of a sub grade by  reaching it through seepage and and by rise in Level of the water table.Moisture can also enter or leave the sub grade,either as a liquid or as vapour ,under the action of forces entirely inherent in the soil itself.Moisture content also increases when the above evaporation,ie movement of the moisture,is stopped due to putting on top of an imprevious layer of the road surfacing.About one metre below the road surface moisture content stays .So the main object is to maintain the sub soil under and adjacent to the carriage way in uniform condition as it may be possible by way of preventing the moisture entering the road foundation/bed rather than by drawing all water from it .As for as the sand and the gravel soils are concerned they are appreciably less effected than heavy clays because it is held in suspension by capillary forces .A clay 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 the dry months and this can be reduced by incorporating granular material such as gravel and sand or clinker and too by rolling the materials into the clay or by consolidating to a thickness of about 75 mm ,however care must be taken to prevent the direct contact between the clay sub soil and the foundation of the road surface for which the soling and the wearing coat,concrete or the ballast of lean mix may be used and the road surface should be well sealed to entering into the water from below soil and particularly in the cutting, intercepting drains should be laid along with the edges of the road.Before designing the sub -soil drainage system for road a survey should be made of the soil and the water conditions in the sub soil ,free water can be intercepted or controlled and a high water table can be lowered by the installation of a sub soil drainage and the cross drainage as per requirement at the site of work,maintained at least four feet below the formation level of the road .In the case of the concrete roads the base of the slab shall not be less than one feet above the anticipated water level in the said drains or the sorroundings.