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New set of design curves relating the cumulative standard axles and the CBR value to the total pavement thickness.Ģ. The salient features of IRC guidelines revised in 1984 are:ġ.
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‘Traffic’ denotes the total number of vehicles in both directions (irrespective of whether the design is for a two-lane or a dual carriageway.) For estimation of future traffic, the growth rate is assumed as 7.5 per cent.įor single-lane roads, the traffic intensity is taken to be twice that for two-lane roads (because of the concentration of traffic on one lane only). The thickness of construction is given by a set of 7 curves, as shown in Fig. IRC Guidelines for Design of Flexible Pavements : (iii) The curves are applicable only under the conditions for which they were method is not reliable for high values of CBR. The advantage of the CBR method is that it can be used to find the total thickness of the pavement and that of the individual courses in addition to the thickness of the subgrade soil (provided the CBR-values of the materials of the courses are also known). The California bearing ratio (CBR) value of the subgrade soil was the basis for the method of design of flexible pavements, developed originally by the California State Highway Department, and adopted by The Road Research Laboratory, London, for developing their own design procedure and design charts. (This method is obsolete and is of historical importance.) The higher the group index, the weaker is the subgrade soil.
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The group-index of a soil, an arbitrary index assigned to different soil types based on the percent fines, liquid limit and plasticity index, is defined by the following equation – Highway Engineers (Highway Research Board) in 1945. The group-index method was devised by the U.S. IRC Guidelines for Low Traffic Volume Roads: (IRC: SP: 72-2007 and IRC: SP: 77-2008) 5. Hence, only the following methods will be presented here: 1.