POOR SOILS WITH CBR VALUES (4 to 5) MAY BE IMPROVED WITH THE FOLLOWING THICKNESS OF THE DESIGN PAVEMENT.

The CBR values are based on the laboratory tests according to IS 2720 .The thickness given is the total thickness of the pavement,over any sub grade or sub base of the known CBR . Normally no material with CBR value lower than eighty percent should be used in the base course construction of the road and the minimum thickness recommended for base course is 15 cm even on the village roads .It is suggested to improve the sub grade rather than to provide a thicker pavement.For the design of the pavement,the sub grade strength must always be assessed at the maximum moisture conditions likely to occur during the entire service life of the pavement and the CBR values determined accordingly.(GENERAL FOUNDATION PROPERTIES) :-- Very stable sub grades are well compacted and undisturbed foundation of the old roads ,with negligible settlement if properly compacted ,solid rock ,No base is needed on such subgrades except where a levelling course is required.(11) Stable sub grades are,gravel sand clay , soils which can be well compacted and where no base is needed .Other sandy and stable soils inferior to the above may be provided with 7.5 cm base .(111) Poor subgrades are soils very susceptible to non uniform movement,loams, peat and the plastic clays ,silty soils generally give trouble.It is better to remove silt to a depth about 45 cm from the road surface and fill to sub grade level with coarser materials.Peaty soils and the black cotton soils ,these are very compressible and are subject to seasonal volume changes .They should be avoided whenever possible or replaced with sound course materials .(1V) Sub grade where the water table may rise to with in 60 cm of the foundation:No base is needed on soil which can be thoroughly compacted, otherwise 7.5 to 15 cm base is desirable.For poor soils with CBR values of 4to 5 ,the following approximate thickness of the pavement may be taken:--!State High Way --47 to 52 cm Major District Road --38 to 46 cm Other roads --20to 30 cm and the village roads --10 to 15 cm .