ROAD CROSS -DRAINAGE AND THE SUB -SOIL MOISTURE CONTENT OF THE SUB -GRADE MUST HAVE AN APPROPRIATE DESIGN AND SURVEY WORK

It is necessary to follow a survey work of the road before designing the sub -soil drainage system for the road, where water conditions in the sub -soil must be verified. Free water can be intercepted or controlled and a high water table can be lowered by the installation of sub -soil drains. The position of the drains should be such that the water table is maintained at least 1.2 metres below the formation levels in case of the hilly areas and in the case of concrete roads the base of the slab shall not be less than 30 cm above the anticipated water level in the said drains or the surrounding areas. The back fill used in the drainage trench should consist of a properly designed filter material, especially where the sub soil is liable to cause silting of back fill . It is very clear from the beginning that stability of the roads can only be maintained if the soil foundation remains in a relatively dry condition. As the moisture content increases the strength of sub -soil decreases. This effect of the moisture content is more marked in the clay soils than with granular soils such as sands and gravels. Further 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 previous road surface, by seepage 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 (movement of moisture) is stopped due to putting on top of an impervious layer of road surfacing. About one metre below the road surface moisture content stays about the same.Hence the main object, therefore is to maintain the sub soil under and adjacent to the carriage way in as uniform a condition as possible by preventing moisture entering the road bed rather than by drawing all water from it.It is not necessary to drain granular soils such as sand and gravel because they are not apprreciably affected by changes of moisture content, however water can't easily be drained from heavy clays because it is held in suspension by capillary forces as in blotting paper:-- Er Fateh chand Guleria, Director RTI welfare Association registered number HPCD, 3552 , Bilaspur HIMACHAL PRADESH phone number, 9459334377