set up; the foundation may be constructed without handling all of the various materials on the street, which sometimes is a difficult and costly proposition; any repairs necessary to subsurface structures may be more readily made since the slabs can be removed as a whole and later replaced.

FOUNDATIONS OVER MARSHES. Sometimes it becomes necessary to carry a road across marsh land which is extremely unstable, so much so that a crow-bar will sink in it of its own weight. When the topography of the land is such that there is no opportunity for subdrainage, unless a very heavy fill is put in which will be several feet above the level of the marsh, some support will have to be given to the fill. In any event there will be more or less settlement and no improved form of surfacing should be placed on the fill until all settlement has ceased.

Paul D. Sargent,* M. Am. Soc. C. E., describes the construction of a brush mattress under a new fill for improving the bearing power of a bog on two roads as follows: "For this work, boughs of pine, spruce, fir, or hemlock were used, care being taken to exclude those having stems more than 2 inches in diameter. These boughs were laid shingle fashion in courses, first crosswise and then lengthwise of the road, and were four courses deep, that is, two transverse and two longitudinal layers, the total depth being 16 inches. On the boughs was placed a 2-foot fill of gravel. One of these roads was built in 1908 and the other in 1910, and to the writer's knowledge, not the slightest settlement of the foundation has occurred. Both bogs were so bad that it was impossible to drive a horse across them, and even a man jumping would shake them for a radius of 50 feet."

OLD PAVEMENTS AS FOUNDATIONS. In New York City old stone block pavements have been used as foundations for asphalt pavements. There are many places on Broadway and in other parts of the city where this experiment has been tried. In some cases, in order to provide room for the asphalt surface, the blocks were taken up and relaid on their sides, and some engineers maintain that a great deal of the repair work on the asphalt pavements in these places is due to the fact that there is more. * See Proceedings, Am. Soc. C. E., April, 1912, page 550.

or less movement to the blocks. On the other hand, some good results have been secured where the old block pavement was not disturbed. As a general rule, however, it is not advisable to use any other than a well-constructed concrete foundation under any type of block, brick, or bituminous pavement, with the exception of certain types of bituminous pavements constructed by the penetration and mixing methods, which will be discussed in later chapters.

BITUMINOUS CONCRETE FOUNDATIONS. A bituminous concrete foundation has been used to some extent as a substitute for a hydraulic cement concrete foundation for asphalt and other bituminous pavements. Probably the first construction of this kind was used in Washington, D. C., from 1872 to 1887, when coal-tar pavements were laid quite extensively in that city. In Omaha, in 1891, a 6-inch bituminous foundation for an asphalt pavement was built of broken stone and gravel thoroughly mixed with asphalt. Foundations of a similar character are sometimes used today. They are not as strong, however, as those of hydraulic cement concrete and have the further objection that the wearing surface cannot be removed without disturbing the foundation.

Foundations for bridges and sidewalks will be considered under the chapters especially devoted to these subjects.

CHAPTER VII

EARTH AND SAND-CLAY ROADS

OCCURRENCE. According to statistics obtained in 1909 by the Office of Public Roads, earth roads comprise about 90 percent of the total road mileage in the United States or 2,000,000 out of 2,200,000 miles. Although many of the States are spending large sums of money in constructing highways with a broken stone or gravel surface, it is obvious that the improvement of the total mileage in this manner is a stupendous task. The construction and maintenance of earth roads is therefore of great importance, particularly in those sections of the country where money or good road-building materials such as stone and gravel are wanting. In some States, in fact, the solution of the good-roads problem at the present time is based mainly upon the successful construction and maintenance of their earth roads. The labors of those who have made the improvement of earth roads a serious study have led to the construction of sand-clay roads in certain sections. This type has been built with varying degrees of success in different parts of the country so that in 1909 the mileage in the United States was about 24,500, distributed as follows: about 50 percent in the Southern States, about 20 percent in the States of Michigan, Minnesota, and Wisconsin, and 10 percent in the States of California and Washington.

SOILS

The formation of soils and their classification has previously been given in Chapter VI, hence in this chapter only their relation to the construction and maintenance of earth roads. will be considered. The soil conditions in different parts of the country and even in restricted localities are so variable that what may be an advisable method of construction in one place

will not serve at all in another. Therefore a careful examination of the soils found is essential.

SAND. Sand is practically the only soil that makes a better road surface in a wet than a dry condition. This fact is well illustrated by the wet sand on a beach between the low and high water marks. It is possible to draw heavy loads over this surface without difficulty. A sandy road when dry, however, offers about as much objection as a road of clayey soil when it is wet.

CLAY. Clays are of two kinds, ball clay and slaking clay. The ball clay is extremely plastic, and, as its name implies, tends to ball or lump up. It will keep its shape even if immersed in water for some time. A slaking clay, on the other hand, does not have this same power of plasticity and is more crumbly in its nature, and more readily miscible with water. Although this property is an advantage from the standpoint of its use in road construction, it is apparent that the slaking clays do not have the binding powers contained in the ball clay. A soil which is largely composed of clay acts just the opposite from sand under different climatic conditions, as might be inferred from what has been stated relative to sand. In dry weather the surface becomes hard, and if kept in proper shape makes a good surface. In continued wet weather, however, the water soaks into the clay and softens it, with the result that the surface no longer can support the traffic.

SAND-CLAY. An earth road in which the surface is composed of a soil that is a mixture of sand and clay will be much more satisfactory, under most conditions, than one composed of either of these soils alone. There are many places throughout the country where a top soil is found which is a mixture of these two materials and serves to make an excellent road of this kind. A suitable top soil is described by Professor C. M. Strahan,* M. Am. Soc. C. E., as follows:

"It is the top-layer of soil on a cultivated field (or one formerly in cultivation) which has been intermixed and pulverized and

*See Municipal Engineering, December, 1910.

exposed to the action of the weather and of plant life. Usually it shows the results of this action in being bleached out and is of a different consistency from the subsoil below.

"It must be a mixture of clay and sand or gravel. Rocks more than 2 inches in diameter should be shovelled aside in loading, or discarded when dressing the road. Very fine sand is objectionable. Samples separated in a nest of six sieves (graded from 10 to 100 mesh) which show much fine sand below the 40-mesh sieve should be rejected, as they will usually be deficient in clay. The coarse sand and gravel should be at least 50 percent of the mass, and of hard, tough consistency, not readily crushed by traffic. Mica in any considerable quantity damages the soil for road purposes. Soft, black soils with large amounts of humus are entirely unsuitable. When separated by pulverizing and sifting, the contents of the finer sieves, when mixed with water, should give a sticky, smooth ball of nearly pure clay."

Where a suitable soil is not found ready to use, it is possible to mix a sand and clay together on the road. The success of a sand-clay road depends largely upon the selection of the clay. Practically all clays have a tendency to shrink when dry and to expand when wet, and if climatic conditions are such that the road is alternately wet and dry, its effect will be detrimental to the road, particularly if too much clay has been used in the construction. The theory of the sand-clay road is that the voids between the grains of sand in the wearing surface should be entirely filled with clay. This involves mixing the two materials together, a process which is carried out on the road. Although a clay which will readily mix is desirable, the binding power should not be sacrificed to too great an extent in seeking the former quality. The best clay to use will be one which slakes sufficiently to enable the lumps to be broken up, but which is plastic enough to cement the sand together and form a smooth and impervious surface. To find the approximate percentage of clay to be added to the sand, a rough determination of the voids in the sand can be made by taking a known volume of sand and finding how much water can be poured into the same