2. Sand and clay placed on a soil foundation and mixed. This is necessary where the old road has neither a sand nor clay foundation and it is impossible to find the two ingredients ready mixed, but possible to get both in separate state near at hand. The clay should first be placed on the road to a depth of 4 inches and the required width. It is not wise to place more than a few hundred lineal feet of clay before the sand is hauled, as the clay rapidly hardens and makes the mixing process difficult. After, say, 400 feet of clay has been placed, the clay should be broken by means of a plow and harrow, if it has become hard, and sand to a depth of 6 inches placed on it. This should be plowed and harrowed in thoroughly. This is best done immediately following a rain, as the two can be more satisfactorily mixed. The traffic aids the mixing and should be encouraged on the road. After the mass appears to be well mixed, the road should be properly shaped, as previously explained. The road should be given watchful attention and should sand or mud holes appear, a second plowing and mixing should be given it.

3. Clay hauled on a sand foundation and mixed with the sand. The mixing process is similar to that described under second head. It is only necessary to add that as the foundation is sand, a little more clay will be necessary than where the foundation is of clay or soil.

4. Sand hauled on a clay foundation and mixed with clay. The clay foundation should be plowed to a depth of 4 inches and harrowed with a disk or tooth harrow until the lumps are thoroughly broken or pulverized. Sand should then be added to a depth of 6 inches and mixed as before described.

Sand and clay can be mixed best when wet, but as most road construction is done in the summer months, it is necessary to do most of the mixing dry and keep the road in shape after the first two or three rains, while the passing wagons and vehicles give the road a final wet mixing. A sandclay road is the cheapest road to maintain, for the reason that it can be repaired with its own material. With a drag or grading machine ruts can be filled with material scraped from the edges, whereas on gravel or macadam roads, this is not possible. The repairing of these roads can be done almost exclusively with the drag, only enough hand work being required to keep the gutters open and the growth of weeds cut on the shoulders. Holes are repaired by adding more sand-clay, and when many of them appear fresh sand-clay should be spread over the surface of the road. If the road gets into really bad condition, the roadbed should be plowed up, reshaped and fresh sand-clay added. This is unnecessary where the road is maintained properly and the travel is not too heavy for the type of construction.

GRAVEL ROADS1

At the close of 1914, 45 per cent. of the surfaced roads in the United States were gravel roads, as shown in detail in a table in Part III of this volume. The presence of good gravel in many parts of the country and the low cost of constructing and maintaining gravel roads will make them a leading type for many years to come.

Some gravels are much better for road construction than others. In Michigan, where three-fifths of the surfaced roads are built of gravel, the value of this material for the purpose is held to vary with the percentage of pebbles in it, the roadbuilding value of the rock of which the pebbles are composed, and the cementing properties of the fine material mixed with the pebbles. In this State at least 60 per cent by weight of the gravel for state reward roads must be pebbles larger than -inch. No pebbles larger than 2 inches are used in the bottom of the road and none larger than 1 inches in the top. The binder required for holding the pebbles together is clay, uniformly mixed with the pebbles, free from lumps, and amounting to not over 10 per cent of the total weight of the gravel.

There is a large mileage of gravel roads in New Jersey, and as a result of experience with them, the State highway department rejects gravel with over 5 per cent retained on a 14-inch circular opening and over 35 per cent retained on a 3-inch circular opening. Three grades are recognized. Grade A is a pebble gravel with a clay binder with not less than 25 nor more than 35 per cent retained on a 1-inch circular opening, not less than 40 nor more than 60 per cent retained on a 10-mesh sieve, not less than 8 nor more than 20 per cent passing a 200-mesh sieve, and the balance a fairly well graded sand. Grade B is a sandy gravel depending upon oxide of iron for its cementing properties, with 20 to 40 per cent retained on a 10-mesh sieve and 10 to 25 per cent passing a 200-mesh sieve. Of this material passing a 200-mesh sieve, at least 40 per cent must be soluble in a 1:3 dilution of hydrochloric acid. Grade C is gravel which does not fall under either of the previously mentioned grades but is approved by the engineer for the bottom part of gravel roads.

1 Revised by Frederic E. Everett, State Highway Commissioner of New Hampshire.

In Illinois, the State highway department requires the gravel to be rather uniformly graded in size from fine material to pebbles that will just pass a 3-inch ring, and not over 15 per cent of the mass (exclusive of clay) passing a 3-inch ring. It must not contain over 5 per cent of loam but it must have 15 to 25 per cent of clay by dry measure. If a local gravel does not form a good bond, the contractor must supply a bonding gravel for the top -inch of the road. All of this material must pass a 1-inch screen and contain 40 per cent of pebbles retained on a 1-inch screen and from 20 to 30 per cent of clay and loam, not more than 5 per cent being loam.

The variations in these specifications show the range of properties of the materials found useful by experience. Few attempts have been made to prepare a general specification for road gravel on this account. The following requirements were adopted by the American Society of Municipal Improvements in 1916 and recommended by the Committee on Materials for Road Construction of the American Society of Civil Engineers:

Two mixtures of gravel, sand and clay shall be used, hereinafter designated in these specifications as No. 1 product (for top course) and No. 2 product (for middle and bottom courses.)

No. 1 product shall consist of a mixture of gravel, sand and clay, with the proportions of the various sizes as follows: All to pass a 13-inch screen and to have at least 60 and not more than 75 per cent retained on a 1-inch screen; at least 25 and not more than 75 per cent of the total coarse aggregate (material over 1-inch in size) to be retained on a 1-inch screen; at least 65 and not more than 85 per cent of the total fine aggregate (material under inch in size) to be retained on a 200-mesh sieve.

No. 2 product shall consist of a mixture of gravel, sand and clay, with the proportions of the various sizes as follows: All to pass a 23-inch screen and to have at least 60 and not more than 75 per cent retained on a 4-inch screen; at least 25 and not more than 75 per cent of the total coarse aggregate to be retained on a 1-inch screen; at least 65 and not more than 85 per cent of the total fine aggregate to be retained on a 200-mesh sieve.

It is evident that the most useful information concerning the value of any gravel for road work is obtained by examining a road built of it. If there is a good gravel road and the source of this gravel is not known, a sample of the gravel can be analyzed mechanically by a portable sand tester, and the gravel deposits in the vicinity tested by the same instrument until one is found showing about the same properties. An exact agreement should not be expected. Tests of the gravel in a satisfactory road in the State of Washington and of the material in the pit from which it was obtained gave the following variations:

Mechanical Analyses of Identical Gravel Sampled at Pit and in the Road

Where coarse gravel is composed of rock pebbles giving a cementitious powder some engineers consider it unwise to use enough clay binder to fill the voids. If roads of coarse gravel bound with a large amount of clay are used by many automobiles the pebbles become dislodged and the road does not become hard, it is claimed. Consequently these engineers prefer to use a smaller amount of clay and to allow the traffic to wear down the road and produce the necessary binder by attrition and internal disintegration of the mass of gravel. This process makes it necessary to maintain the road carefully for some time after its completion, but is stated to give a better road eventually with some classes of gravel.

In New England, where gravel roads have been built extensively, it is generally considered safe to use on roads for light traffic the gravel from any pit where the face stands vertical and has to be loosened before it can be shoveled. Other gravels usually have to be supplied with a binder. It is always desirable to make a careful search for all deposits of gravel and an examination of the quality of each before deciding upon the deposit to use. In Dubuque County, Iowa, for instance several months were spent in such an investigation because the local limestone was too soft for road use. Finally a satisfactory pit was found 13 miles from the road to be improved, and by transporting it on a light narrow-gauge railway to the road and then distributing it by branches of this railway and by motor trucks and dump wagons, its cost on the road was kept down to a satisfactory figure.

Preparing the Gravel

The management of the gravel pit should receive enough study and attention to make sure that the material is delivered to the wagons or cars at the lowest cost. The organization for the purpose will depend upon the location of the pit, the quality of the gravel and the quantity of material to be taken out. Where there is only a small percentage of the gravel which is over size,

and the remainder runs a uniformly good mixture, the large stones can be removed by a flat gravel screen, or, on small works, can be forked out during loading. It is not always necessary to go to the expense of screening. With a good foreman in the pit it may be possible to get a proper mixture of the material from a pit where the gravel lies in strata of different sized pebbles, provided there is also a good foreman on the road, so that the strippings, if any, will be placed on the shoulders and the overlarge material will be used for foundations in low places.

Where there is a considerable proportion of overlarge stone in the gravel it is customary to set up a crushing and screening plant at the pit. For example, Kane County, Illinois, has an outfit consisting of a jaw crusher, screen, elevator and storage bin holding 15 cubic yards. The gravel is first screened, because by taking out the material of suitable size for road work only the large stone is fed to the crusher and its capacity is thereby much increased. The presence of the small stone in the crusher tends to clog it and retard the breaking of the large stone. The screened and crushed material is discharged by gravity from the bins into the 5-yard motor trucks which are used for delivering it. The pit material is delivered to the screen by a belt conveyor, 18 inches wide and 40 feet long. One end of the belt is under a platform having a hopper over the belt. The gravel is brought by slip scrapers to the platform and dumped through the hopper onto the conveyor.

In some plants of this character the gravel is run over a bar screen or "grizzly" which holds back all oversize stone and delivers it to the crusher. This keeps the large stone entirely out of the screen. In Wisconsin work the screen has -inch perforations for the first half of its length and 12-inch perforations for the second half, giving three sizes of gravel. The jaws of the crusher are set to give about equal parts of the two coarser sizes separated by the screen.

As the pebbles composing gravel are rounded and do not lock together as well as broken stone, it is customary to use somewhat smaller sizes of gravel than of crushed stone. Gravel obtained from beaches and rivers is usually more rounded than that from pits and consequently may not be so good for roads, unless suitable binding gravel can be used for a wearing surface or limestone screenings or other good binding material can be used with it.

Pit-run Gravel Roads

Many miles of gravel roads have been built by dumping the gravel on the roadbed, spreading it roughly and allowing traffic to consolidate it. The consolidation is a tedious process,