Macadam Roads

A roauway made with small piceos ot stone iirrnly set in place is called a macadam road. ïhore are many niethods of ConatrüCtiöTJ, and the variations are chiefly in the foundation. A macadam top may be placed upon a coniinon tlirt foundation, but tlie best macadam roads are built with a stone foundation. The inethod of constructing such a road is given in the folloiving extract froni the t,reatise of Mr. Cleraens Ilerschel, printed ju the book on "Improveruent of Highways," published by the League oí American Wheelnien: "Thetrue principie of roadmaking consists in givingovery road two component parta; one - the foundation - to be solid, unyielding, porouaand of lárge material; the other - the top surface - to be made up of lighter material, bound compactly and evenly over the rough foundation. "The roads of this kind, with macadam for the top surface, are called Telfoi-d roads by English writers, from Telford, who first built them in England. The Central park 'gravel roads' belong under this head, gravel takiwj; llie place of the macadam of the Telford roads. These foundation roads are of fai'greater tnportance than any other Kind for state, cotinly or town roads, alao for p;irks and drivewa.ys. The top surfaco of all these roads must have a certain in lination to cause efficiënt surface drainage. Variousauthoritiea urivc vari0U8 rules for llic amounl öf l lis incüiiation or side slope. It would seein juat tliat il should depend on the nature of the top covertnjj, beinj; less or more solid than for looser or sof ter matei ials, and also on the RHktie of the road. "In Haden, one f the smaller Germán States, but which is worthy to be taken asa model in mattere of road building, and in France, the riso at the center is giveu as 1-40 to 1-60 of the wjdth of the road, according to the nature of the material; that is, inclinations of 1 in L'ü and 1 in 30. The rules in Prusaia prescribe inclinations of 1 in 84 for roads falling more than 4 in a hundred; and 1 in 18 for roads on a grade of between 2 and 4 in a hundred; and 1 in 13 for those in a grade of less than 2 in a hundred. When iirst built tho center should bo made some four inches too high to allow for after settling. HALF OF CROSS SECTION. "If the stone for the foundation - for which most anything will do, and that kind should be taken which is clieapcst to procure - liappens to be got outcheapest in larger pieces than the above dirnensions, it will do no harm. ïhis foundation course is sometimes set so as to present an inclination on top, and the cover then put on of a uniform thickness over the whole bread th. Tliis is perhaps best, but is somewhat inoro expensive. It will do, in nearly all casea, to set the foundation course on a level, or as near so as the stones will allow, and then make the top crowning, by making the covering say three-quarters of an inch or an inch less thick at the edges than in tho center. The stones forming the foundation should not be set in rows, nor ever laid on their Qat sides, but set up on edge and made to break joints as much as possible; that is, set up irregularly. Af ter they are set up, the points that project above the general level may bo broken off, and the interstices generally filled up with small stone. "More or less care and work are nccessary in this part of tho operation aceording to the importance of the road and tho depth and character of the material used for the top covering. To roll the road at this stage is to be recommonded; afterwards it becotnes a requisito. Tho point never to be lost sight of is that this foundation course must rcmain porous, must be pervious to water, so that all rain water that shall soak through the top covering will iind through it ineans of escape to the ground underneath; thence, according to the nature of tho subsoil, it is left either to soak into the ground or must bo further led away by appropriate drains." Another authority in the samo work, writing upon the top surface for a stone road, says: "Experience has shown that while six inches may be a sufficient depth for light travel, twelve to eighteen inches are necessary if heavy loads must be carried at all seasons of the year. For country macadam roads the limits would be betvveen six and twelve inches, according to the character of the subsoil. For the thorough construction of macadam roads the stones are nowcrushed by machinery and then rolled with steam road rollers of ten to fif teen tons weight. The weight of the roller per square inch on the bearing surface sliould be equal to the expected weight, per square inch, under the wheel8 of the heaviest loads. "Where thorough work of the best character is wanted, engineering skill and experience are requisite, but the general methods may be briefly stated, so that wherever bad sloughs neediillingup with stone, the macadam plan may be followed at least approximately. The trench or road bed should be dug out to the required width and depth, the largor stones should then be selected and firmly packed together across the foundation in irregular courses, in the general forra of a street pavement. They should all rest on their flattest or broadest surfaces, and the thin or jagged edges upward. The interstices or spaces should be closely lilled up with chips and the whole laver be well rainined or rolled into a solid mass. "On top of this place a layer of finely broken stone of the thickness requisite to bring the surface to grade, with a good crown in the center. This should be rolled until the whole mass is firmly bound together, and then covered with sharp gravel or fine screenings of broken stones of sufficient depth to bed and bind the layer of broken stone. In the future inaintenance of such a road the most important point is to prevent the forniation of ruts, by keeping the surface so uniformly even that there will be no inducement for travel to f ollow beaten tracks. The cost of the macadam structure precludes its use on country roads, except in occasional bad spots wherc stone is necessary."