BROKEN-STONE ROADS. I2I Upon this pavement the layers of broken stone are placed, and the road-surface completed in the same manner as for a macadam road.
The practice of Telford was to grade the road-bed flat, and then construct his pavement deeper in the middle than at the sides, using for a roadway 16 feet wide stones about 8 inches deep at the middle and 5 inches at the sides. This practice is still followed by some engineers,but it is now more common and usually considered preferable to make the surface of the road bed parallel to the finished surface and the pavement of uniform thickness. Fig. 22 shows a section of tel ford road as now commonly constructed.
The following extract from the specifications of Mr. James Owen, for telford roads in Essex County, New Jersey, may be regarded as representing the best prac tice in such construction.
"After the road-bed has been formed and rolled, as above specified, and has passed the inspection of the Engineer and Supervisor, a bottom course of stone, of an average depth of inches, is to be set by hand as a close, firm pavement, the stones to be placed on their broadest edges lengthwise across the road in such manner as to break joints as much as possible, the breadth of the upper edge not to exceed four (4) inches. The interstices are then to be filled with stone chips, firmly wedged by hand with a hammer, and projecting points broken off. No stone of greater length than ten (to) inches or width of four (4) inches shall be used, except each alternate stone on outer edge, which shall be double the length of the others and well tied into the bed of the road; all stones with a flat smooth surface must be broken, the whole sur face of this pavement to be subject to a thorough settling or ramming with heavy sledge hammers, and thoroughly rolled with a ton roller. No,stone larger than one and one-half (ti) inches to be left loose on top of telford.' The proper foundation to be used for a broken-stone road depends upon the nature and condition of the road-bed upon which it is to be constructed and the nature of the traffic to pass over it. If a firm, well
compacted, and thoroughly drained road-bed may be obtained, of material which will not readily soften under the action of moisture, there will usually be no need for a special foundation, but the first layer of the macadam may be placed directly upon the surface of the road-bed. If, however, the road-bed is of a ma terial retentive of moisture, not thoroughly drained, and likely to become soft in wet weather, and the broken stone be laid immediately in contact with it, the stones of the lower layer of macadam may be grad ually worked down by the weight of the traffic into the soft earth, and the soil at the same time work up into the voids in the stone, causing a gradual disintegration of the road. It may thus also become retentive of moisture and subject to the disrupting action of frost. In this case some foundation must be provided which is capable of resisting the penetrating action of the soft material of the road-bed and of distributing the load over it.
It is not intended in the above to imply that the use of a foundation of this character should take the place of proper drainage. The advisability of artificial drainage should always be carefully considered, and where the road is threatened by water which may be removed by the construction of drains they should be used, but frequently thorough drainage is difficult or doubtful, and it is desirable to adopt heavy construc tion such as the telford foundation gives.
It is commonly claimed by the advocates of the macadam system of construction that on any well drained and well-compacted road-bed there will be no tendency on the part of the stone to work down or of the soil to work up, and hence that the Telford foun dation is an unnecessary expense. The difficulty of procuring a perfectly stable and reliable road-bed in many localities is, however, very generally recognized, and telford pavements are largely used.