Where, t g = thickness of granular layer in mm. (v) The following empirical relationship are used to estimate the elastic modules of subgrade In addition to these, vertical compressive stresses and strains within the bituminous course, granular base, and granular sub-base courses are also considered.
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Horizontal tensile strain at the bottom of the bituminous binder course under the centre of a wheel of the dual wheels is correlated with ‘fatigue failure’ or cracking, with the limit of fatigue being 20% cracked surface area.Ĭ. Vertical compressive strain on top of the subgrade under the centre of the dual wheels is correlated with ‘rutting’ or the vertical permanent strain of the pavement, the allowable rut depth being 20 mm.ī. (iv) Pavement failure criteria or the critical strains considered are:Ī. (iii) Traffic volumes up to 150 million standard axles (msa) are considered in two ranges, 1 to 10 msa and 10 to 150 msa. (ii) Soils having CBR-values in the range of 2 to 10% are covered. (i) The flexible pavement is designed as a four- layer structure – bituminous surfacing consisting of two courses -wearing course and binder course, granular base and granular sub-base over the soil subgrade. Salient features of the revised guidelines are:
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The software PAVE was developed based on mechanistic -empirical (M-E) principles. The design catalogue is based on the results of the Ministry of Surface Transport (MOST) study: “Analytical Design Approach for Flexible Pavements” implemented by IIT Kharagpur. This is based on the revised guidelines incorporated in “IRC: 37-2001 – Guidelines for the Design of Flexible Pavements”. The total thickness obtained from the chart is distributed into surface course of thickness x, base of thickness y, and sub-base of thickness z, as shown in Table 7.3. The pavement design chart extrapolated up to 200 million standard axles is shown in Fig. The method of computing the cumulative standard axles the number of standard axles per commercial vehicle is designated as ‘vehicle damage factor (VDF)’. Recommendations on the types of pavement materials suitable for various courses. 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. ‘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. 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.