CHARACTERISTICS OF DIFFERENT GRAVELS. Any gravel which stands vertical in the bank, showing no signs of slipping when thawing out in the spring. requiring the use of the pick to dislodge it, and falling in large chunks or solid masses, is suffi ciently clean and free from clay for use on the road, and usually contains just enough cementing material to cause it to pack well.
Pit gravel usually contains too much earthy material, and can be greatly improved by screening. Gravel is still being deposited in drifts and bars by streams, and this will be found to partake of the character of the pit gravel of the locality, except that it gen erally contains less clay, and may have an excess of sand. This is often called river gravel, and is one of the best sources of road material. Lake gravel varies greatly in character. It is usually free from earth and contains sufficient sharp sand to pack well; but is liable to be slaty—an undesirable quality.
Table 17, page 156, shows the sieve analysis of these gravels. Each sample was first washed in successive waters until the water remained clear, and then the wash water was allowed to stand until the matter in suspension was precipitated. The precipitate was dried in an oven to dryness and then weighed; and the washed gravel was air-dried, and then sifted and weighed. The per cent of voids in the washed gravel was obtained by comparatively gently ramming the gravel under water in a small metal cylinder, the ramming not being severe enough to crush any of the pebbles or fragments.
Table 18, page 157, shows the results of a mineralogical analysis of such of these gravels as had passed a screen having finch meshes. The matter recorded in Table 17 as being in suspension is called clay in Table 18, although part of it was doubtless organic matter and part fine sand, but the error is not material.
To study these gravels further, each will be considered in order.
This is a screened drift gravel obtained near Ur bana, Champaign Co., Ill., which has been used in a few instances
on private driveways. Table 18 shows only 3.8 per cent of clay present, which will have only a small binding effect. There is 7.6 per cent of iron oxide (Fe, 0) in the clay, but there is so small a proportion of clay in the gravel that the iron contained in it will have an inappreciable binding effect. The principal source of binder is, then, the 65 per cent of ferruginous limestone. Limestone it self when pulverized makes an excellent binding material, and a small part of the limestone is in the form of flat chips that may be easily crushed under the wheels, but the most of the frag ments are rounded and not easily crushed except by comparatively heavily loaded wheels. There is only a small per cent of crys talline rocks present, and these are hard and not readily crushed. and consequently can not materially affect the binding qualities of the mass. The gravel also contains 22.2 per cent of quartz; but this material is very hard and not easily crushed, and besides its dust is almost wholly devoid of cementing properties. Both the quartz and the crystalline rocks are quite sharp and angular, which is a very desirable condition, and aids the binding action of the clay and the limestone dust. This gravel packs only slowly in the road, particularly under the light traffic of a private driveway: but under moderate traffic makes a fairly good road, and is not much affected by freezing and thawing.
Decatur.This is a gravel much used on the country roads near Decatur, Macon County, with satisfactory results. This gravel has a comparatively large amount of fine sand. An exam ination of Table 18 shows that it contains more than twice as much clay as the Urbana gravel, but only about one third enough to fill the voids. A considerable portion of the limestone both of the pure and the ferruginous—a total of 30 per cent,—is in thin friable chips, and is easily crushed by the traffic, thus making an excellent binder. The ferruginous limestone contains an unim portant proportion of iron; but the ferruginous sandstone is heavily charged with iron oxide, which makes a good cementing material. This gravel makes a smooth, hard surface, reasonably free from dust in the summer and mud in the winter.