Show constitute A bout about one third of surfaced roads in the united states slates w BEST SELECTION OF GRAVEL quality of material Is next in importance to proper drainage clay la Is poorest cementing material we have H ave r I 1 prepared by the united states ment ot of agriculture beads that are artificially surfaced with gravel or with earthy material la in the composition of which gravel predominates tire re called balled gravel roods roads this designation covers a varin tion in typo which velch ranges froidl roada sur bur faced with a natural sand band clay mixture i containing a considerable pe percentage re of gravel to those having buying so po called gravel macadam surfaces composed almost wholly of pebbles and constructed ted in practically the akimo inan der as aa first class macadam mac adain raa gravel roads constitute about one third the total mileage of surfaced roads in tho united states and tiro are confined to no especial section of the country 1 in the following discussion of gravel roads it Is convenient to consid erthe subject under the two g general fiern heads selection of gravel arid and construe lion methods Select selection loh of gravel ordinarily the selection of gravel for use in road surfacing mastbe must be confined to local materials which are orean be made suitable for that purpose because of high freight costs colts it seldom happens that road gravel of even evea the very best quality la is transported fi rail a greater distance than about miles from the source of supply and in tho the vast majority of cases it ff la is hauled directly from pit to roads road in 3 since the quality of gravel snaps to a considerable extent some t r c t f L 9 Y L t I 1 it A r 9 t 9 P d f b tf t I 1 it e t 0 t c it S g t t f C t 1 tr ut jj stone of any idne when placed lu A road surface are ba hard nesa toughness arid resistance to wear tile extent to which pebbles pebble possess the quail ties depends very largely on tile the elmr acter of atthe the parent stone kono from which they were originally produced andaca and accordingly caries over a wide range sanco nearly all g ravel gravel deposits contain pebbles which hava hamoi been formed from many different kinds of stone it if has not be been considered to apply the ordinary laboratory tests testa for determining hardness toughness and per cent ot of wear to gravel arid and the ille maiter or of comparing these qualities its as possessed by the pebbles fro from 1 in dIt different Terent gravel deposits usually depends on visual inspection not infrequently a very cadun casual I 1 inspection will which deposit among a great liu number niber contains conjoins the largest percentage I 1 ago of hard durable pebbles there at aie e comparatively few cases basea where this point cannot be bd ened with sufficient accuracy by sorting out the pebbles contal contained tied in representative samples from the different deposits ie testing the various kinds with a hand ha hammer miner and determining the relative proportions in which the more durable materials ore are present in the sample by means of a weighing device ordinarily the most durable pebbles which occur la in gravel deposits n ara m those composed of either quartzite or chert though many deposits conta contain ltv fragments of f limestone 0 or r trap rock ick either of which may make an excellent load surfacing sili facing material for exam pit pie the so called blue gravel which occurs in certain sections of th the econn coun kiy ry usually Is composed largely of trap rock lock fragments and generally ranks high no as a road material while limes limestone tono pebbles usually are bess re rc sisti tnt to wear than those of trap rock they possess the advantage of a relatively high cementing value and their presence in gravel may aid greatly in Be securing curing it a well bonded surface without an excess of orly clay the binder wo no matter matier how durable may be 0 the pebbles contained in a given gravel deij deposit they cannot be used successfully in a road surface unless they can call bo be well ban ded together so as to pre sent a resistance to the als vilia action of traffic to accoe elsh this bond requires that the gravel some cementing or binding gent such as iron oxide carbonate C f lime or clay As already explained pebbles such as aa those com coia ased of possess assess the prop ty of f becoming firmly bonded r by by virt virtue ue of their own cementing due the principal cementing agent or indor pres present presentia entin in most gravel grael depos s Is eldy clay and in case of deposits aich do not carry binder ny la Is tile the material added to meet the deficiency whala there are an any exceptions to this bulej ru leit as Is bur auti suti 1 ea ly general to warrant a rr a treating i special ca cases ses those deposits in litch the binder Is not clay the suitability of clay for use no as a ader for a gravel road raad surface do de ands on exactly the same character ties as ai its suitability for use in sanday construction that la Is a quality C clay which could be used ally in a sand clay surface also bould make a satisfactory binder for gravel grave I 1 surface provided it ja la used i proper proportion and la Is properly fixed with ith sand grading and proportions per for gravel toni to make ake a satisfactory ad surface the stone particles auld be graded in size so that the mount of binder required will be buceta to a minimum the reason r this Is that the binder usually Is uch less resi resistant siant to wear tha than a t alie to one particles and therefore it lodo Is de rable that the latter form as 19 large part of the wearing surface as to la ti cable most gravel deposits as an icy occur in nature satisfy this re il rement in so far as grading of the abbles Is concerned burthey but they nearly ways contain pebbles of a size larg than it Is desirable to incorporate i a road surface natural deposits so not no infrequently contain contal atoo too larke large proportion of sand or clay to pro ice c e satisfactory results it la Is desera ie 0 therefore that specifications coving gravel for use in road on MI I 1 should limit the proportions in it itch the fine and coarse materials hallbe all be present and a maximum limit ig g size for the pebbles ns esvell weli a aa finite requirements regarding fh ha el lallay of the pebbles and of the v in id geri general eral it ass that itts factory gravel will conf conform oiin tho llomong limits as to percentages of tie ie and coarse mner material tal n 1 material retained rethen on a va inch pyre iv 55 to 75 per cent 2 1 material retained on a inch eve not loss less than 15 percent per cent 8 3 material clay passing a 20 sh stove sieve for the surface io course arse fedt 8 to 31 per cent 4 clay passing 1 a 6 20 iesh sh sieve for the foundation coarse course 3 to 15 per cent the sand should be at least tricel 8 griat great h aslie as sa lie alij content and thir and nd and clay when thoroughly mixed be sufficient to flit fill the voids be axeen een the larger gravel pa particles i he ie percentages given above usually it 11 conform with this requirement A A satisfactory factory mech mechanical anical analysis ana lysta ai a gravel ravel sample along the lines in bated abed above frequently w may P be roade mad a the field by first separating the are material from the fine by means a JA inch mesh screen t L nd kid then the clay out of the fine flue ma bial lal as described in the discussion sand clay roads boehr Both the tho coarse I 1 id d fine materials obtained from the mple ample wa may y be further separated into ferent efferent sizes by means of sulta suitable bli r beus diffor if for any reason this in tad able ablo in making the test 7 f |