Show I GOOD EARTH ROADS Much Depends on Soil Construction Construe Construe- tion and Maintenance LEADING QUALITIES OF SOILS F From m Standpoint of f Road Building They Depend on Texture and Structure Permeability and Capillary Power Prepared by the United States Department Department Depart Depart- m ment nt of Agriculture noa Roads s constructed u by grading the natural soil to the required shape grade and ami without special surfacing of any any kind kind usually are des des- es- es as earth or dirt roads The efficiency of such roads depends s 1 on the quality of the soil composing the roadbed a 2 on proper construction and 3 on adequate maintenance Soils The principal qualities of soils from the standpoint of road construction depend de de- de- de pend upon texture nn and structure per permeability and capillary power Soils usually are lre classified as clay sand loam gravel marl or gumbo according to the material predominating In their composition Soils composed of two different materials mixed in such proportions proportions pro pro- portions that the character of the ture is decide decidedly ly intermediate may be designated conveniently b by naming namin both components as san sand clay clay san sand sand- gumbo gravelly gravell clay etc Clay Clay Is a soil of very fine tine texture e which results from the complete decomposition decomposition de de- de composition of rocks or minerals Pure Pur e clay Is very ery retentive of moisture an and d usually becomes plastic an and unstable e when wet but when mixed with other materials such as sand or gravel It Its s stability may be Increased greatly I In n general a fi clay road to be passable must be he kept dry and since C clay fir Is vcr very difficult to drain It happens seldom that roads composed entirely of clay clar can be kept In good condition at fit all seasons of the year ear No matter how well weir such a road may be graded ra e and crowned the surface absorbs water in wet seasons and subsequent traffic will produce e mud But when the road is shaped and drained properly it will dry out quickly when the weather becomes favorable fa and may soon be bo restored to its original shape On the other hand clay roads when very dry usually usually ally produce consi considerable erable dust un under er traffic and for this reason may become er very In exten extended ed dry s spa spa- a- a sons sous The extent to which cla clay roads roas are subject to the objections mentioned above usually varies ac according ol to the so called slaking sInking qualities of the clay Some varieties of cla clay when compressed compressed compressed com com- pressed into a ball and placed In water will continue in the original shape for forn fora n a considerable time even If thoroughly thorough thorough- ly saturated an and softened while compressed compressed com coin pressed balls from other varieties will slake or break down own almost Immediately Immedi upon being placed place In water Clays Clas of the mentioned first-mentioned variety are called clays and of the latter s slaking clays cias It has been observed that slaking clays produce more mud In wet weather a l more dust In dr dry weather t than an those of the g variety This characteristic of cla clays s 's is i s further considered In the discussion o of f clay Fand roads appearing appearing- In another r part of this bulletin where metho methods method s for testing clay are suggested Most clays as they occur in nature i contain more or less sand or gravel 1 i which as pointed out above usually y has hasa a stabilizing effect by making th the thecla e cla clay more inure easily drained nn and Increasing b its bearing power when wet Sand Sand San Is composed of granular par titles tides of mineral or stone which occur r In nature an and which will a pass one one- mesh Inch screen The one one- Inch fourth screen Is fixed aibl- aibl as the dividing line between sand and amI gravel and Is generally though not universally accepted as such Nearly all sand consists essentially essentially essen essen- of ot quartz grains that are very hard and durable But Hut there is no coherence coherence co co- between the different grains and therefore soils composed compose principally principally pally of sand are unstable except when confined In some way If properly confined confined con cony fined lined and protected from undermining sun sand foundations may under favorable climatic conditions prove lOve entirely S satisfactory satisfactory sat St t for almost any t type pe o of road crust but hut It Is practically impossible under an any circumstances to make u a satisfactory satisfactory sat sat- road rond surface of sand alone Sand roads are at their best when they are kept moist and for this reason reason reason rea rea- son they hould be designed with a th aview aview view to retaining moisture In the sand rather than to effective drainage as is the thc case with clay roads Such roads sometimes are improved temporarily b by mixing sawdust straw pine leaves or other similar material Into the surface but In general the clay snarl crust Is the most satisfactory Improvement for roads roas of this character Loam Loam Loans Is a soil composed of clay and sand mixed with n a considerable percentage percentage per per- of t finely nel divided vegetable matter mattel matter mat mat- ter tel or humus The quality of loam from the standpoint of road building depends very verj largely gel upon the pro proportions proportions pOl In which sand an and clay are present pres pres- present ent and on the character of these ma ma- Loam that contains about GO 60 GOper GOper per cent of moderately coarse san sand and from 20 to 30 per cent of good cementIng cementing cement cement- ing lug clay usually will make an nn excellent road surface for light traffic Material l of this kind Is drained easily and and andIs Is fairly stable even when wet Another advantage e Is that It will not become ver very dust dusty under traffic in dry ry weather and frequently will cement together into a very hard compact surface Roads surfaced surface with such material are commonly called topsoil roads h because bethe be be- cause the topsoil of cultivated fields possesses the de desired characteristics more frequently than that to be obtained obtained ob ob- from nn any other source Gravel Gravel Gra is ma made e up of small roun rounded e particles of stone which occur in nature nature na na- ture and are sufficiently large to be retained one-fourth-inch-mesh one on a n screen screen Gravelly soils are distributed widely In the United States and and- vary wi widely el In la quality In general when a n soil soil soi contains as much as 40 to 50 per percent percent percent cent of gravel and sufficient clay or other cementing material to bond bon the gravel particles together it proves avery a aver avery ver very satisfactory material for tor construction construction con con- construction of roads because It is drained easily an and Is very stable when compacted com com- Marl Marl is clay containing a relatively large o percentage of carbonate of lime It gra grades es Into calcareous clay an and argillaceous argillaceous argillaceous ar ar- limestone an and its value for tor road building Is variable In general this material has few n advantages over the best varieties of ordinary clay foru for forus u us use C- C Cin In constructing a roa road unless the percentage of lime carbonate Is sufficiently sufficiently sum hl high h to give It It when compacted approximately the character of limestone lime lime- stone The be best t varieties of marl become become become be be- come very hard and compact when placed In a road surface and In some localities this material Is used extensively extensively exten exten- for road surfacing Gumbo Gumbo is essentially loam containing contain ing inn a u high percentage each ench of clay and decayed vegetable matter and a very low percentage of sun sand Gumbo has n a strong ten tendency ency to absorb an and hold water wa wit- Vatel ter tel and therefore Is hard to drain Itis It ItIs ItIs Is har harder er to handle In a road surface than cia tiny clay because the tine vegetable matter is an n added ed weakness but If protected from surface water by a proper system tem tern of drainage it may be use used to construct con cony a fair e for supporting some type of Impervious road crust Gumbo has a relatively high capillary power and for this reason Its occurrence occur occur- rence In n a n roadbed usually will tate tote the use of SUb rl e even under conditions favorable for surface drain drain- age |