FORMS OF CONSTRUCTION - BROKEN-STONE With reference to the method of preparing the subgrade to receive the stone, there are two forms of construction—surface construction and trench construction. The surface construction consists simply in placing a layer of broken stone upon the former earth road and leaving it to be com pacted by traffic. In the West many miles of road are constructed on this plan with limestone. As a rule this material readily pul verizes under the traffic, and the powder cements well; conse quently the road soon binds together. Such roads are not first class but give good returns on their cost. On account of the sim plicity of the construction, this form of road will not be considered further.
The trench construction consists in excavating a trench of the required width and depth, and depositing the broken stone in it.
302. With reference to the lower course of stone there are two systems of construction,—the macadam and the telford, so called after two noted English road builders (§ 274). The macadam road consists of two or more layers of crushed stone, its distin guishing characteristic being that the lower course of crushed stone is placed directly upon the earth road-bed. The telford road consists of a pavement of stone blocks set upon the road-bed covered with one or more layers of crushed stone, its distinguish ing feature being the paved foundation.
The most important claims of the advocates of the telford construction are: (1) that the open foundation is necessary for drainage; (2) that the sub-pavement is necessary on soft or poorly drained soil to prevent the small fragments of broken stone from working down into the soil and the soil from working up into the stone; and (3) that the telford is the cheaper, since the expense of crushing is saved.
The most important claims of the advocates of the macadam system are: (1) that the drainage afforded by the telford con struction is no better than that with the macadam construction; (2) that on any well drained soil there is no tendency of the stone to work down or of the soil to work up; (3) that tile drainage and macadam construction are cheaper than the telford system; and (4) that since the introduction of the machine rock-breaker, it is cheaper to crush the stone and lay the macadam foundation than to place the telford.
The view taken by different road builders in this matter is probably largely due to the necessities of the vicinity in which they have worked and to the skill with which the two systems have been applied in work which has come under their observation. The foundation which is proper in a given case is determined by the nature and condition of the soil upon which it is constructed. If the road-bed is thoroughly drained and is composed of material which will not readily soften, there will be no need of a telford foundation. If, on the other hand, the soil is retentive of moisture and can not be thoroughly drained, it may be necessary to provide a foundation which will prevent the soil from working up into the stone and the road metal from working down into the soil. This foundation may be, according to the intensity of the difficulty to be met, a layer of sand or gravel, or a telford foundation laid directly on the soil, or a telford foundation upon a layer of sand or gravel. The choice between these two forms of construction, however, often depends upon the kind and accessibility of materials. For example, a soft laminated local stone may be used for a telford foundation, while a more expensive and harder stone is imported for the macadam top.
MacAdam insisted upon a foundation of small fragments under all circumstances, but Telford used the paved foundation only as circumstances seemed to require it. To MacAdam is due the credit of discovering the supporting power of a layer of comparatively small angular fragments of stone.