DESIGN AND DESCRIPTION OF SUB GRADES MUST HAVE GENERAL FOUNDATION PROPERTIES, AND NO MATERIAL WITH CBR VALUE LOWER THAN 80 PERCENT SHOULD BE USED IN BASE COURSE CONSTRUCTION WORKS.

The sub grades must have the property of recovery if the stresses are not of a repetitive nature and do not exceed the safe bearing capacity of the sub grade. A certain amount of occasional and infrequent overstressing the pavement is not harmful. Therefore it will be uneconomical to design pavements for unusual heavy loads passing once in a way. For the design, the sub grade strength must always be assessed at the maximum moisture condition likely to occur during the entire service life of the pavement and the CBR value determined accordingly. It is better to improve the sub grade rather than to provide a thicker pavement. If the sub grade can be made strong enough to carry the full load, it may only be necessary to superimpose upon it a skin to keep it waterproof and to prevent abrasion. Where the traffic intensity consists of heavy vehicles in excess of 1500 per day, it may be worthwhile to go in for bound layer to atleast 5 to 7.5 CM thickness in the top of the base course prior to laying of the wearing surface. (Well consolidated bituminous macadam, lean cement concrete, soil cement, lime puzzolana concrete etc are bound layers) Where a 15 CM thick water bound macadam is indicated, 10 CM thick base could be considered as equivalent. When the wearing surface consists of thin surface dressing or open graded premix carpet 2.5 CM thickness, the thickness of surfacing should not be counted towards the total thickness of the pavement. For poor soils with CBR values of 4to5 the following approximate thickness of pavement may be taken :- State Highways 47 to 52 CM Major District Roads:--38 to 46 CM Other district roads :--20 to 30 CM and the village Roads :-- 10 to 15 CM.