Part 8 (1/2)
The gravel will coenerally does not becohly soaked by rains Then it will begin to pack, but will beco this period the surfacedoes not suffice for this purpose, tending to accentuate the unevenness rather than to correct it
If gravel is placed in a trench in dense soil and rainy weather ensues, sufficient water will be held in the trench to cause unevenness froravel will become mixed with the soil to some extent and be thereby wasted Trenches cut froravel is placed, to the side ditches, will relieve this condition by affording an outlet for the surplus water Nevertheless some difficulty may be expected if the trench method is used and eather prevails If it is possible to close the road against traffic until the road is dry the -continued dry weather, the dispersion and loss of considerable gravel froravel is held between substantial earth berer when constructed by the trench method than otherwise
=Surface Method=--The surface raded earth road surface without earth shoulders to hold the gravel in place It is also soeas just eneral ravel does not compact as quickly as in the trench method and a considerable loss of material is likely to result froravel is loose But it has the advantage of being free from difficulties in eather and in some locations is therefore preferable to the trench method It is particularly applicable to those projects on which the placing of gravel continues throughout the winter, the gravel being dumped and spread, to be finally s=--Where gravels deficient in binder are utilized, clay for binder is soravel is placed on the road Thisthe clay on top of the lower course of gravel, placing the upper layer and sprinkling and rolling until the clay squeezes up through the surface layer Itdry clay on the upper course before it is harrowed and then harrowing to ravel Both methods are practiced, but the former is believed to be preferable A third method is to separate the sand and pebbles and to mix the clay binder with the sand and then spread the sand on top of the pebbles and
=Maintenance=--Gravel surfaces require carefulthe first season the road is used The gravel will co the process will be rutted and otherwise disturbed by traffic It is i this period to restore the shape once a week or at least twice a rader is usually eravel is so of the type known as the planer will prove to be thethat is very satisfactory for use on gravel roads
[Illustration: Fig 16--Road Planer]
CHAPTER VII
BROKEN STONE ROAD SURFACES
The broken stone road surface, or macadam road as it is usually termed, consists of a layer of broken stone, bonded or ceether by means of stone dust and water The surface may or n=--It has been an accepted assumption that the ravel road surface of equal thickness, and since this is probably the consensus of opinion of engineers familiar with both types, it may be accepted until experimental data are available on the subject
The thickness of the layer of macadam required for a road will depend upon the same factors that were considered in connection with the thickness of the gravel surface, ie, kind of stone used, character of earth foundation and nature of the traffic
The standard ood earth foundation is to be had and where the loads do not exceed about four tons has for years been eight inches thick For heavier loads or inferior foundation, a soreater thickness would be employed, but the best practice would probably provide a foundation course of the Telford type for doubtful foundation conditions, especially for the extreth such as very sandy loaions where stable foundation is always assured the thickness of the ht be reduced to six inches It should be borne in mind that the broken stone road is not adapted to the traffic carried by trunk line highways in populous districts, but is rather a type permissible on secondary roads and usually adequate for local roads It should never be e any considerable voluer automobile traffic or motor truck traffic If surfaced with a bituer automobiles per day, but not to exceed fifty trucks
=Properties of the Stone=--The stone employed for the broken stone road should possess the qualities of hardness and toughness and should be capable of resisting abrasion sufficiently well to have reasonable life under the traffic to which it is subjected Since the traffic ht on some roads to far beyond the limit of the economical capacity of this type of pavement on others, it follows that any particular deposit of stone h for soht be entirely inadequate As a general rule it has been found that stone that wears away at a moderate rate will, when used for water-bound macadam surface, result in a smoother trackway than one that ear very slowly It is not therefore altogether certain that the most durable stone to be had should be selected for a particular road This is especially true now that the water-bound hways and other heavily traveled roads, and is employed in locations where service conditions are not severe
The stone eood ce properties, because the surface depends for stability pri action of the dust froravel road, where little dependence is placed upon the bonding effect of the rock dust In preparing the stone for e rock is crushed and screened, and in that way a supply of the finer particles, which are a part of the output of the crusher, is obtained for use in bonding the surface This finely broken s, is essential to the construction of the water-bound type of surface
Rocks vary considerably in the ce properties of the dust, but usually the rocks classed as ”trap,” such as andesite, gabbro and rhyolite, and schist and basalt possess good ceood ce properties, but so value Quartz, sandstone and the granites are of low ce value
=Kinds of Rocks Used for Macadam=--Limestone and chert are the two sedimentary rocks, employed most extensively for broken stone roads
These rocks are found in widely distributed areas and vary in physical characteristics from very softconsiderable durability It is desirable to carefully test out the deposits of theseto ascertain the probable value of the rock, for the construction of the road surface
Of the igneous rocks, those classed as trap are best known to the road builder and many of the deposits of trap rock afford an excellent material for broken stone roads where the severest conditions of traffic are encountered The trap rocks are tough and durable and generally possess excellent ce properties
Granite and sandstone are seldo properties and a binder of soether For this purpose clay or the screenings from some other variety of stone may be utilized
Some other materials are occasionally employed for the construction of macadam surfaces Of these, oyster orareare of rather low durability but possess good ce properties Shale is a ht traffic roads
=Sizes of Stone=--The stone for the wearing course of a e as practicable, because the larger the pieces the more durable the surface If the individual stones are too large it is difficult to secure a se stones will be readily loosened by tipping as the wheels roll over them These considerations limit the size to a maximum of that which will pass a 2-1/2-inch screen Stone of excellent wearing qualities may be somewhat smaller, but never less than that which will just pass a 1-1/2-inch screen
For the lower course, the size is not particularly important except where the earth foundation is such as to require special construction