The Making of Roads.
Page 7
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An Extract from Mr. Hounsfield' s Appendix to Colonel Crompton's Recent Paper Before the I.A.E., Together with Excerpts from the Ensuing Discussion.
We published a page. of extracts from and some comments upon Col. Cronwton's recent paper on "The Wheel and the Road " in our last issue.
An appendix, by Mr. L. H. Hounsfield, includes a mathematical consideration of the relationship between road and wheel. In introducing this subsidiary matter, Mr. Hounsfield writes interestingly of the action of road rollers upon road surfaces, lie makes certain novel suggestions which we fear, however, are not within the limits of practical application.
"Some members of the Institution have submitted that certain kinds of vehicles improve the roads they pass over, and have laid stress on the beneficial rolling effects, apparently assuming that roads require rolling for their own benefit.
" The author understands that pot-holes are indirectly attributable to the steam rollers used in making the roads. The roller passing over the loose surface raises a wave of unconsolidated road material immediately in front of it and pushes it along till the resistance offered is such that it is easier for the roller to mount the wave than to push it along. This has the effect of giving a non-homogeneous road surface, and it seems possible that the ordinary traffic would do the work better if it were not for the fact that each driver endeavours to drive in the existing tracks, and thus, even were the use of steam rollers eliminated and ordinary traffic compelled to consolidate the roads, they would still be far from perfect, and thus the next best thing would be to roll with as many rollers as possible, all of different weights, wheels diameters amt wheel bases.
" As it appears to be a mistake to endeavour to consolidate the surface ab initio with a heavy roller, the author would suggest as a more practical solution that a very light roller should first pass several times over the newly laid metal. This would better assist the road metal to move about, and enable the faces to fit themselves together compactly ; the effect which it is desirable to obtain is that which would be secured by shaking the road, and may be illustrated by pouring coarse sand into a tin and then subjecting it to vibration ; the surface is lowered considerably in the tin and the sand becomes comparatively rigid to the touch.
The Discussion.
Sir GEORGE GIRD was the first member of the audience to be called upon by the president to speak. After complimenting Colonel Crompton, he aaid that he presumed that the proper object of the paper had been " constructive suggestion." With regard to the author's analogy between a roadway and railway, Sir George Gibb said that it must be remembered that on the railway the whole cost of maintaining the track. a.s well as of maintaining the carriage, is paid for out of the charges imposed on passengers and goods, so that he was not quite sure that the analogy in question would stand the test of examination, With regard to the author's constructional suggestions for roads, he thought the phrase "strength crust" to be a very useful one. Of the two strata into which modern roads should be divided, the one is the strength crust, the function of which is to bear, and the other is the carpet, the function of which is to wear.
He thought that it was unquestionable that motor traffic had introduced new factors in connection with the maintenance of roads, and that those meant a reconsideration of many of the laws and customs in connection with roads, and, moreover, they entitled ratepayers to equitable consideration by assistance to some extent from national funds.
Mr. F. W. LANCHESTER emphasized the limitations in regard to wheel diameter .which were imposed on all nractical designers. ," I think," said he, "it is up to the road maker to-adapt himself to the possible size of the wheels." The speaker then proceeded to draw attention to the importance of the question of frost on all roads in relation to their waterproofness. .
In connection with wood-paved roads, he said that he had seen, when suclaa road was beginning to wear, art extra effect upon the wood, which may be considered to have moved, presumably owing to the traffic which it bore. He wanted to know whether such movement was due to horsed or motor traffic.
Sir JOHN MACDONALD gave his listeners the results of much careful observation, which he has evidently made upon road surfaces on the Embankment. m London, in Edinburgh, and elsewhere. At one time the first-named thoroughfare, he said, was probably the worst road in Great Britain, and it was laid down by the authorities of London: It was difficult, then, to get any sane man to use it. At the present moment, he continued, the traffic on the Embankment amounts, at, the busiest parts of a day, to no fewer than 1600 vehicles per hour. It is nowadays a very good specimen of the strength crust and carpet formation of construction. Sir John Macdonald had much to say that was interesting concerning potholes due to horses, which showed his close power of observation.
Dr. W. C. INwiN criticised Mr. Hounsfield's suggested data with regard to the relative actions of road and wheel. They appeared to be based on eases of wheels of carts drawn by horses or other means, and not on, what seemed to be much more important in discussing the wear on the road, viz., the action of the driving wheels of mechanically-propelled vehicles.
Mr. E. S. SHRAPNELL-SM1TH challenged the author in regard to the practicability of large-diameter wheels. He thought that this was a problem to which designers of commercial vehicles had given a great deal of attention. He asked Colonel Crompton if, and when he gets the perfectly-carpeted road, he will still hold the same brief for the large-diameter wheel, or whether, with a perfect road, the small-diameter wheel would then become permissible. He asked also if any experiments were likely to ensue in connection with the testing rig which Colonel Crompton had told them was to be used to settle the co-efficients of friction between rubber tires and asphalt and other similar modern surfaces, in view of the fact that the available reliable data on the subject were small.
Colonel CROMPTON, in his reply, made a further plea for large wheels, and he instanced the early Fowler 10 ft. wheels on their big traction engines. " Eight ft. wheels," he said, "are the proper things for new countries or for soft ground." In regard to Mr. Lanehester's query concerning the curvature of wood blocks, he thought that such evidence really did show the push of the road and the permanent distortions, in fact. "They have gone as far as they can go," he added, "and they are probably unable to stand pushing a little more." Mr. Shrapnell-Smith had touched on what was an old controversy between
them, viz., the question of wheel-diameters. He must remind that gentleman that he was responsible for the introduction of 3 ft. diameter wheels in Lancashire, with results which were (mite well known to him. Colonel Crompton chaffingly asserted that the whole of Lancashire had had to be repaved in conaequenee of those 3 ft. wheels. He did not think that it was a great advantage to be able to side load in the country, although it was of great facility for town
Mr. 1.a!ox THOMSON, in proposing a vote of thanks to Colonel Crompton for his paper, eoncluded "We have had a great many phrases hurled at us this evening, and one comes to me with new force, and
that is the phrase wear and tear.' T have discovered what they mean. ' Wear ' means what a motor wheel does, 'tear ' means what a horse does."