OCR Text |
Show GPuWELROADTYPES Constit Jte About One-Third of Surfaced Roads in the United States. EEST SELECTION CF GRAVEL Quality of Material Is Next in Importance Impor-tance to Proper Drainage Clay Is Poorest Cementing Material We Have. Iloads that are artificially surfaced with gnvel, or with .earthy material In Uie composition of which gravel pie-dominates, pie-dominates, are called gravel roads. This designation covers a variation in type which ranges from roads surfaced sur-faced with a natural sand-clay mixture containing a considerable percentage of gravel to those having so-called "gravel macadam" surfaces, composed almost wholly of pebbles and constructed con-structed in practically the same manner man-ner as first-class macadam roads. Gravel Grav-el roads constitute about one-third the total mileage of surfaced roads In the United States and are confined to no especial section of the country. In the following discussion of gravel roads it is convenient to consider the subject under the two general heads, "Selection of Gravel" and "Construction "Construc-tion Me;hods." Selection of Gravel. Ordinarily the selection of gravel for use in road surfacing must be confined to local materials which are or can be made suitable for that purpose. Because of high freight costs it seldom hnppens that road gravel of even the very best quality is transported by rail a greater distance than about 100 mill's from the source of supply, and in the vast majority of cases it is hauled directly from pit to road in wagons. Since the quality of gravel varies to a considerable extent, sometimes some-times in different parts of the same pit, it is essential that local road officials, offi-cials, who frequently are charged with the selection of such material, should have some knowledge of the relation . between the physical characteristics of gravel and its wearing qualities when placed in a road surface. While such knowledge can best be gained by actual ac-tual experience with different kinds of gravel, there are certain general requirements re-quirements which may be stated for the guidance of the inexperienced. For example, the following excerpt from Bulletin No. 2 of the Michigan state highway department presents in very concise form the characteristics which gravel for road surfacing should possess, in the Judgment of the state highway commissioner, in order to meet the Michigan conditions. Gravels that come from the pit with the proles cemented together, even though they contain no clay, will re-cement re-cement in the road aod become harder than they were in the pit. Tests of specimens of this kind always show that there is much lime present and usually some iron, both of which are excellent cementing materials. Briefly, Brief-ly, the experience of the state highway high-way department warrants the statement state-ment that there are few, if any, bank gravels in Michigan that do not contain con-tain enough limestone and other soft pebbles which grind up under traffic to furnish sufficient binder to cause them to consolidate In a few months' time, if separated from the surplus sand and earth, and properly treated after applying to the road. Iu accordance with these suggestions, sugges-tions, gravels are considered valuable for road purposes in the following order : 1. Almost in direct proportion to the percentage of pebbles constituting the mass. 2. In direct proportion to the value as road metal of the rock fragments constituting the pebbles. 3. In direct proportion to the value as a cementing material under all conditions con-ditions of weather, of the liner particles par-ticles of earthy matter constituting the filler or binder. In g'lu.Tal, the physical characteristics characteris-tics ol' gravel which determine Its suitability for use in surfacing a road are : (1) The durability of the pebbles or rock fragments, () the quality of the binder, (,T) the grading of the pebbles, and (4) the proportion in which the binder material Is present. While the inlluence each of these factors should exert in fixing the selection of gravel for a particular road depends upon local lo-cal conditions and necessarily is a question for individual judgment to decide, nevertheless a few points iu connection with each factor that may aid materially In reaching a decision will he summarized in the following paragraphs. Durability of Pebbles. The principal qualities which determine deter-mine tlie durability of pebbles or stone of any kind when placed In a road surface are hardness, toughness, and resistance to wear. The extent to which pebbles possess these qualities quali-ties depends very largely on the character char-acter of the parent stone from which they were originally produced and accordingly ac-cordingly varies over a wide range. Since nearly all gravel deposits contain con-tain pebbles which have been formed from many different kinds of stone, it has not been considered practicable to apply the ordinary laboratory tests for determining hardness, toughness, and per cent of wear to gravel, nnd the matter of comparing these qualities, quali-ties, as possessed by the pebbles from different gravel deposits, usually depends de-pends on visual inspection. Not infrequently infre-quently a very casual inspection will reveal which deposit, among a great number, contains the largest percentage percen-tage of hard, durable pebbles. There are comparatively few cases where this point cannot be determined with sufficient accuracy by sorting out the pebbles contained in representative samples from the. different deposits, testing the various kinds with a hand hammer, and determining the relative proportions in which the more durable materials are present in the sample by means of a weighing device. The Binder. No matter how durable may be the pebbles contained in a given gravel deposit, they cannot be used successfully success-fully in a road surface unless they can be well bonded together so as to present pre-sent a combined resistance to the disturbing dis-turbing action of traffic. To accomplish accom-plish this bond requires that the gravel contain some cementing or binding agent such as iron oxide, carbonate of lime, or clay. As already explained, certain pebbles, such as those composed com-posed of limestone, possess the property prop-erty of becoming firmly bonded together togeth-er by virtue of their own cementing value. Grading and Proportions. For gravel to make a satisfactory road surface, the stone particles should be graded in size so that the amount of binder required will be reduced to a minimum. The reason for this is that the binder usually is much less resistant to wear than the stone particles, and therefore it is desirable de-sirable that the latter form as large a part of the wearing surface as is practicable. Most gravel deposits as they occur in nature satisfy this requirement re-quirement in so far as grading of the pebbles is concerned, but they nearly always contain pebbles of a size larger larg-er than it is desirable to incorporate in a road surface. Natural deposits also not infrequently contain too large a proportion of sand or clay to produce pro-duce satisfactory results. It is desirable, desira-ble, therefore, that specifications covering cov-ering gravel for use in road construction construc-tion should limit the proportions in which the fine and coarse materials shall be present and a maximum limiting limit-ing size for the pebbles, as well as definite requirements regarding the quality of the pebbles and of the binder. |