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(July, 77)for laying, and a rigid pavement would thus "be preferable* Onthe other hand, where settlement is expected, a flexible pavementwould be preferable because it could accept small deformationswithout damage and would be easier to repair if larger deformationsoccurred* Streets serving developing areas are more suitably pavedwith flexible materials f which are easier to dig up and repair thanrigid pavements* For such areas f construction in flexible materialsin two stages is often suitable f with the first stage beingconstructed for a relatively short design life covering the periodwhen construction traffic is heavy and excavation for utilitiesfrequent f and the secoi3d stage being designed for a longer life andconstructed when the pace of development slows, using the firststage as roadbase or basecourse* In designing two-stage flexiblepavements, careful consideration should be given to the extent bywhich compaction and consolidation during the first stage life willaffect the design parameters for the second stage, so that undueextravagance is avoided*If there is no practical reason for choosing one type inpreference to the other, it is sometimes advantageous to preparedesigns and call tenders for both types, so that a choice can bemade on economic grounds* In such cases s overall cost, includingestimated recurrent costs discounted back over the design life,must be considered rather than first cost,Other factors to be taken into account in selecting the typeof pavement to be used include speed of construction, riding qualityand the availability and relative costs of different materials*The thickness of sub-base to be provided for a flexiblepavement may be found from the strength of the subgrade, and thecumulative number of standard axles to be carried, using FigureV*7«5* Sub-bases provided in Hong Kong will generally need tohave a CBR value of at least 3Q« The most suitable materialavailable meeting this requirement is all-in crushed rock*Partially decomposed rock may also be suitable. Traffic cox*~siderations will, usually require a sub-base thickness of at least150 mnu No sub-base is required if the CHS value of the subgradeis in excess of the minimum CBR value shown on Figure V»7*5 a sbeing necessary for the sub-base,Suitable roadbase materials available in Hong Kong comprise!lean concrete and! dense bitumen macadam* The thicknesses requiredfor either of these materials are given in terms of the cumulativenumber of standard axles to be carried on Figures ?»7*6 and ¥.7*7*1dhen traffic heavy enough to require a surfacing thickness of morethan 100 mm is expected, the additional thickness may be added tothe roadbase in the form of an extra layer of dense bitumenmacadam*