ROAD CORRUGATIONS EXPLANATIONS OF CAUSE.

Hawera Star, Volume L, 20 December 1930, Page 14

ROAD CORRUGATIONS EXPLANATIONS OF CAUSE.

What are the causes of corrugations on roads? This is a question which has been asked for years, and as yet ( there does not appear to have been any definite reply to it. The question was discussed at the annual meeting of the New Zealand Society of Civil Engineers, at Dunedin, in February, 1924, when a paper on the subject was read by Mr. Cl. P. Anderson. The paper stated that if a succession of cars all of the one weight and running in the same direction at the same speed were driven over a water-bound macadam road they would run smoothly until some inequality in the surface became manifest in the shape of a bump or a hole. The wheel of the car would drop under spring pressure into the hole, and as it passed it would be forced up again. Going at high speed inertia would carry it past its normal position until the spring sent it back to the road with an impact. Each successive wheel would repeat the process, each wheel would leap out of the hole and land on the same spot till a second hole was formed with a ridge between, after which a third and a fourth hole would be formed. Like a stream, these undulations would chase each other along the road in the direction of the traffic. In the case of a road, however, the traffic was not all in the same direction, and therefore the corrugations became stationary. It would be noticed, continued the paper, that corrugations did not occur on all roads and were not present on grades where the engine accelerated or where brakes were habitually used, but at places where a car ran freely with little engine action. With respect to the objection that heavy lorries with low speeds would not co-operate with cars in producing corrugations, the paper stated that on roads subject to heavy or predominant lorry traffic corrugations did not develop. Potholes might develop, but these were due to oscillations in the whole mass of a loaded vehicle. Though somewhat similar, they lacked the characteristic regularity of corrugations. The opinion was expressed by Mr. A. J. Paterson that corrugations were caused by the impact of a vehicle and the spring on the axle. Another engineer thought that corrugations were caused by the synchronous vibration of the springs. The pressux-e , on the backs of the corrugations at every vibration tended to form waves, the same as waves formed on the water, with the difference that waves on the solid road, when once formed, were waiting for the next action to take place, which might happen two hours later. Tho springs on cars were designed lor certain periodical vibrations. If a car travelled at a rate equal to 50 per cent, beyond the synchronous speed, it struck the back of one 'corrugation and landed on the middle of the next. Regarding corrugations being found on r flat roads and not on hilfs, he thought that this was due to the fact that on hills the driving wheel: tended to move back rather than drive forward the hump of the corrugations. Some interesting conclusion regarding corrugations have been arrived at by Mr. George E. Ladd, Economic Geologist to the TJ.S. Bureau of Public Roads. He states, inter aha, that corrugations are from 25 to 35 inches apart and average 31 inches. The maximum height from the bottom of the trough to the crest is li inches, any greater height indicating the beginning of pitting or ravelling. The

corrugations in the first method of development are due to the “kick back of sui’face materials arising from the spin of the rear wheels as they descend after bouncing over some obstacle, and in the second method of development to the squeezing of the gravel, rendered more or less plasitc bv the presence of clay, and consequently of moisture. The corrugations do not migrate. Except under special conditions thev are normal to the axis of tho road. ' Cases have occurred where they appear to swing on an axis near the centre of the road. Some time ago measurements were taken on corrugations of eleven roads in the Hawke's Bay district, where the conditions were suitable for speeding In all cases one chain was measuerd off and the number of corrugations counted. In ail twenty places were tried. The extreme distances from crest to crest varied from 26 to 32 inches, the average of the whole beino- 29£ inches. It was found that, in general, the corrugations were moie pronounced and more regular when on slight curves. Corrugation measurements taken in the Manawatu Countj showed the average width to be 29 inches. It is noteworthy that the average diameter of the rear wheel (including the pneumatic tyre) oi cars also measures 29 inches.