In some of the Southern states where other materials for surfacing roads are not available, a mixture of sand and clay in proper proportions has been found a desir able material for this purpose. In some instances the mixing of sand and clay for road work has received considerable attention, and good results have been obtained. The relative amounts of sand and clay to be used depend upon the character of the materials and can only be determined by experiment in each case. Where good materials are available, a fairly hard surface, well adapted to light traffic, may be obtained. It is claimed that these roads are less noisy and less dusty than macadam. They require the same main tenance as an ordinary earth road, but form a harder surface than earth usually found in a natural state. Mr. William L. Spoon, of the United States Office of Public Roads, has made an investigation and report upon this method of construction.
* "The best sand-clay road is one in which the wear ing surface is composed of grains of sand in contact in such a way that the voids or angular spaces between the grains are entirely filled with clay, which acts as a binder. Any excess of clay above the amount necessary to fill the voids in the sand is detrimental. If a small section taken from the surface of any well constructed sand-clay road is examined with a magnifying glass, the condition of contact which exists between the grains of sand and the small proportion of clay which is required to fill the voids may be seen. Wherever this proper condition of contact exists for a few inches in thickness upon the surface of the road, it will bear comparatively heavy traffic for a long time, even when the subsoil is sand or clay.
"All the experiments which have been made by this Office indicate that the materials should not be mixed in a dry state, but that they should be thoroughly mixed and puddled with water. It makes little difference by what method the stirring or mixing is done, so long as it is thorough and proper proportions of the materials are obtained. If an excess of clay is used in the mixture, the grains of sand which are not in contact are free to move among and upon each other, so that no particle exerts more resistance to pressure than if the entire mass consisted of clay alone. On the other hand, if an insufficient amount of clay is used, the mix ture will lack binding power and will soon disintegrate.
" It has been pointed out that thorough stirring and puddling are absolutely essential to successful sand clay construction. This is most easily brought about immediately after a hard or prolonged rain, the clay having been previously spread and the large lumps broken up as completely as possible. The surface should then be covered with a few inches of sand and plowed and harrowed thoroughly by means of a turning plow and a cutaway or disk harrow. This stage of the work will of course be found somewhat disagree able, leading, as it does, to the formation of a thick, pasty mud; but it is the only practicable way in which the necessary mixing can be accomplished. Many experiments have been tried with dry mixing of the clay and sand, but all have been more or less unsuccess ful. In cases where the plowing and harrowing are considered too expensive, the mixing may be left to traffic. This, however, inevitably leads to a muddy
road surface, for a long time, although finally it is possible, by a proper distribution of the sand upon the to bring about a fairly good result, even by this simple method." " It has already been shown that the best mixture for sand-clay construction is one in which there is just enough clay to fill the void in the sand, thus producing the proper cementing bond in the road surface. No exact rules can be laid down for calculating in advance the best mixture. It must be remembered that the relation of weight and volume will vary widely in different clays, according to the amount of water which they contain. Some clays, especially the more plastic varieties, even after they are as thoroughly dried as they can be by the hottest summer sun, will still hold as much as 20 per cent of water. This water is known to chemists as water of combination,' because it seems to be either combined with or held in the structure of the clay particles in such a way that it can only be driven out at a high temperature. It is apparent from this that in handling a clay of this kind, even when it seems quite dry, each ton will contain 400 pounds of water which does not enter into the consideration of volume. The amount of clay necessary to fill the voids in any given sand will therefore be found to vary." "Practical experience has shown that the tendency is to calculate too little rather than too much sand for given amounts of clay, and almost invariably a second and even a third application of sand is necessary over and above the calculated amount. It often happens that clay will work up to the surface under the action of traffic,' in which case an extra top dressing of sand should be added when required. " Upon a clay subsoil "the foundation having been properly prepared, the surface should be plowed and harrowed to a depth of about 4 inches until it is pul verized as completely as possible. It is then covered with 6 to 8 inches of clean angular sand. The sand should be spread so that the layer is thickest at the center of the road, following in general the same method as was outlined for spreading clay upon a sandy founda tion. The first mixing by plow and harrow is now done while the materials are still in a comparatively dry state. It has been found that the clay founda tion can be more evenly disintegrated when in that condition. After this first mixing has been finished the road is finally puddled with a harrow after a rain. In case an excess of clay works to the surface and tends to make the mixture sticky, sand is applied until this trouble is overcome.
" Upon the completion of the mixing and puddling, the road should be shaped while.it is still soft enough to be properly finished with a scraper and at the same time stiff enough to pack well under the roller or under the action o traffic. In case it is impossible to obtain the proper consistency of the surface material, it is better to shape the road when somewhat too wet than when it is too dry, even if it is necessary to stop traffic upon it for a few days. The ,road should be opened to traffic as soon as practicable after completion, as this will be found to have a beneficial effect upon it."