AGENTS OF DESTRUCTION BROKEN-STONE ROAD. A broken-stone road is a delicately balanced construction, and is peculiarly open to the destructive action of the traffic and the weather. A careful study of the various agents of destruction is necessary to a thorough understanding of the best methods of construction and maintenance.
The effect of narrow tires, equal-length axles, small wheels, and hitching die horses between the wheels, was considered in the chapter on Earth Roads ace § 187-93.
This effect of the wheel varies greatly with the condition and material of the roadway. If the surface is perfectly smooth and the load per unit of area is not beyond the crushing resistance of the stone, the amount of wear is probably insignificant, and possibly is beneficial to a certain extent, since a certain amount of dust is necessary to replace that inevitably swept away by wind and water; but the moment irregularities of any kind occur on the surface, deterioration begins, since the wheels then immediately begin to pound. When the pressure of the tire is greater than either the crushing strength of the stone or the cohesive strength of the binder, the damage is very great.
stones and to loosen the binding material; and the dislodgement of one fragment makes it easier to displace. others. The surface irregularities produced in this way are continually increased by the impact of the wheels, and the binding material thus loosened is more readily carried away by wind and water. The breaking of the binding material also permits water to penetrate more easily into the road-bed. The effect of the horses' feet is particularly de structive on a steep grade—both in ascending and in descending.
Tracking. The tendency of a team to follow the track of the preceding vehicle leads to the formation of ruts. If drivers would vary their track only a few inches, one set of wheels would counteract the effect of the others, and the road would remain comparatively uninjured. The advantage of this is proved by the fact that wherever there is a turn in the road the ruts disap pear, however deep they may be on the straight part, because the horses naturally vary their course round the corner, and one wheel obliterates the track of the preceding one. Tracking is easily remedied by a little attention on the part of the driver; and to secure this, sign-boards calling the driver's attention to the disadvantage of tracking, are sometimes set up Legible signs at each half mile and at prominent points have proven very effective in preventing tracking and the consequent ruts. Sometimes piles of stone or barricades are placed upon the road to direct travel temporarily. The barriers are sometimes so arranged as to keep the travel in parallel straight lines, and sometimes so as to force the traffic to move in gentle serpentines. The barriers are changed from time to time so as to distribute the travel over every part of the road. This method of limiting the travel is applicable only on comparatively wide roads; and even then it is not defensible, owing to the obstruction of traffic.