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Show GOOD EARTH ROADS Huch Depends on Soil, Construction Construc-tion and Maintenance. 1EAD1KG QUALITIES OF SOILS From Standpoint of Road Building They Depend on Texture and Structure, Permeability and Capillary Power. Prepared by the United States Department Depart-ment of AgricuUure.) Koads constructed by grading the uutural soil to the required shape, grade aud alignment, without special surfacing of any kind, usually are designated des-ignated as earth, or dirt, roads. The efficiency of such roads depends (1) on the quality of the soil composing the roadbed, (2) on proper 'construction, ad (3) on adequate maintenance. Soils. The principal qualities of soils from the standpoint of road construction depend de-pend upon texture and structure, per-aieability per-aieability and capillary power. Soils usually are classified as clay, sand, Joam, grovel, marl or gumbo, according to the material predominating in their composition. Soils composed of two different materials mixed in such proportions pro-portions that the character of the mix-, mix-, tare is decidedly intermediate may be designated conveniently by naming koth components, as sand-clay, sand-xumbo, sand-xumbo, gravelly clay, etc. Clay. Clay is a soil of very fine texture which results from the complete decomposition de-composition of rocks or minerals. Ture elay is very retentive of moisture, and Esuully becomes plastic and unstable when wet, but when mixed with other materials, such as sand or gravel, its stability may be increased greatly. In general, a clay road, to be passable, vmst be kept dry, and since clay is very difficult to drain, it happens seldom that roads composed entirely of- clay can be kept in good condition at all seasons of the year. No matter how well such a road may be graded and crowned, the surface absorbs water in wet seasons and subsequent trafllc will produce mud. But when the road is shaped and drained properly it will dry oat quickly when the weather becomes favorable and may soon be restored to its original shape. On the other Sand, clay roads, when very dry, usually usu-ally produce considerable dust under traffic, and for this reason may become Tery insanitary in extended dry seasons. sea-sons. The extent to which clay roads are subject to the objections mentioned alwve usually varies according to the so-called "slaking" qualities of the clay. Some varieties of clay, when compressed com-pressed into a ball and placed in water, will continue in the original shape for considerable time, even if thoroughly thorough-ly saturated and softened, while compressed com-pressed balls from other varieties will slake or break down almost immediately immedi-ately upon being placed in water. Clays f the first-mentioned variety are called "nonslaking clays," and of the latter, "slaking clays." It has been observed lliat slaking clays produce more mud fn wet weather aJl more dust in dry weather than those of the nonslaking rariety. This characteristic of clays is further considered in, the discussion of sand-clay roads appearing in another part of this bulletin, where methods for testing clay are suggested. Most clays, as they occur in nature, contain more or less sand or gravel, which, as pointed out above, usually has a stabilizing effect by making the clay more easily drained and increasing Its bearing power when wet. Sand. Sand is composed of granular particles par-ticles of mineral or stone which occur 5n nature and which will pass a one-fonrth-rnch-mesh screen. The one-fourfn-inch-mesh screen is fixed arbi- trarily as the dividing Hue between sand and gravel and Is generally, though not universally, accepted as such. Nearly all sand consists essentially essen-tially of quartz grains that are very hard and durable. But there Is no coherence co-herence between the different grains, and therefore soils composed principally princi-pally of sand are unstable, except when confined in some way. If properly confined con-fined and protected from undermining, sand foundations may, under favorable climatic conditions, prove entirely satisfactory sat-isfactory for almost any type of road crust, but it is practically impossible, under any circumstances to make a satisfactory sat-isfactory road surface of sand alone. Sand roads are at their best when they are kept moist, and for this reason rea-son they should be designed with a view to retaining moisture in the sand rather than to effective drainage, as is the case with clay roads. Such roads sometimes are improved temporarily by mixing sawdust, straw, pine leaves or other similar material into the surface, but, in general, the sand-clay crust is the most satisfactory improvement for roads of .this character. Loam. Loam Is a soil composed of clay and sand, mixed with a considerable percentage per-centage of finely divided vegetable matter mat-ter or humus. The quality of loam from the standpoint of road building depends very largely upon the proportions propor-tions in which sand and clay are present pres-ent and on the character of these materials. ma-terials. Loam that contains about 60 per cent of moderately coarse sand and from 20 to 30 per cent of good cementing cement-ing clay usually will make an excellent road surface for light traffic. Material of this kind is drained easily and Is fairly stable, even when wet. Another advantage is that it will not become very dusty under traffic in dry weather and frequently will cement together into a very hard, compact surface. Roads surfaced with such material are commonly called "topsoll" roads, because be-cause the topsoll of cultivated fields possesses the desired characteristics more frequently than that to be obtained ob-tained from any other source. Gravel. Gravel is made up of small rounded particles of stone, which occur in nature na-ture and are sufficiently large to be retained on a one-fourth-inch-mesh screen. Gravelly soils are distributed widely in the United States, and .vary widely in quality. In general, when a soil contains as much as 40 to 50 per cent of gravel and sufficient clay or other cementing material to bond the gravel particles together, it proves a very satisfactory material for construction con-struction of roads, because It is drained easily and is very stable when compacted. com-pacted. Marl. Marl is clay containing a relatively , large percentage of carbonate of lime. It grades into calcareous clay and argillaceous ar-gillaceous limestone, and Its value for road building is variable. In general this material has few advantages over the best varieties of ordinary clay for use in constructing a road, unless the percentage of lime carbonate is sufficiently suffi-ciently high to give it, when compacted, approximately the character of limestone. lime-stone. The best varieties of marl become be-come very hard and compact when placed in a road surface, and in some localities this material Is used extensively exten-sively for road surfacing. Gumbo. Gumbo is essentially loam containing contain-ing a high percentage each of clay and decayed vegetable matter and a very low percentage of sand. Gumbo has a strong tendency to absorb and hold water, wa-ter, and therefore is hard to drain. It is harder to handle in a road surface than clay, because the vegetable matter is an added weakness, but if protected from surface water by a proper system sys-tem of drainage, It may be used to construct con-struct a fair subgrade for supporting some type of impervious road crust. Gumbo has a relatively high capillary power, and for this reason its occurrence occur-rence in a roadbed usually will necessitate necessi-tate the use of subdrainage, even under conditions favorable for surface drainage. |