Opinions from Others.
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What Causes Road Waves ? —Leading Road Engineers Address Letters to the Editor.*
From Mr. H. Percy Boulnois, M.I.C.E.
The Editor, THE COMMERCIAL MOTOR.
0.301F have read with considerable interest, the remarks in your issue of the 8th inst.: on Colonel Crompton's theory that the harmonic-percussive effect of traffic produces waves in road surfaces.
Road makers are to be congratulated on the fact that a man of Colonel Crompton's scientific; attainments and world-wide experience should have been brought into the arena of road construction and maintenance, and, personally. I am of opinion that there is a great. deal of truth in his theory.
Fewer Waves in Pre-roller Days.
I can well remember the days when a large number " macadamized " roads were constructed without any steam rolling, or, in fact, any rolling whatever, except. that which was applied by the wheels of the passing traffic. This, of course, took time and a great deal of manual labour, as men had to be constantly employed in .sweeping or raking the displaced metal into the ruts caused by the traffic. But the result, if the r..-iad had been properly looked after, was a road surface with very few waves, if any at all. Then very heavy steam rollers were introduced15 to 20 tons, and a road that had. previously taken three months to consolidate was completed in as many days. The results were often very unsatisfactory, the waves were prodigious, and a road, thus quickly constructed, very speedily became deformed and ilinost impassable. Then, as lighter rollers were ,iced, this deformation of the surface became much less, though it still remained.
The Upward Line of Least Resistance.
It. is evident that if a weight is applied to the surface of a material, which is not strictly rigid, it tends to press the material in a downward direction, but the movement of the material will take the line of least resistance, which, in the case of a road resting on a fairly solid foundation, is in an upward direction. such a movement may be frequently seen where an embankment is in process of formation on swampy ground. As the embankment is made, the ground on each side rises, displaced by the weight of the embankment, and special means have frequently to be (anployed in order to counteract this movement.
Why do the Waves Synchronize Across the Whole Width of the Road?
In the case of macadamized roads, the wavy forma
tion is undoubtedly started by the roller, and, having mice assumed this formation, the depressions of the -raves are increased by the bumping action of the harffic. It is, I think, evident. that these waves are ytot produced by the traffic alone, for, if. it were so, lhe waves would not synchronize on both sides of the oad, as the traffic is in opposite directions, and I do
not think there is any evidence to show that the waves are ever broken in their lines of synchronism across the lines of traffic. The roller, on the other hand„ moves backwards and forwards on the road whilst its construction is in progress, and thus the waves are started, and once there thetraffic does the rest.
It. may be said that there is a mass of evidence to show that roads, when finished by the roller, exhibit no signs of waves, which begin to appear after the traffic has been on the road for more or less time. This may be so, but is it not conceivable that, although these waves are not perceptible on the surface, they may, to a certain extent, exist in the sub-crust, and that the tendency to waviness has been initiated ?
I have rarely, if ever, seen a wavy surface on a wellmade compressed asphalt carriageway which has been consolidated with rammers and not by rolling.
Present Method of Roiling at Fault.
I am open to conviction, but my present view is that the present method of rolling is the cause of much of the waviness in the surface of our roads, and I shall be much interested to sec whether the threeaxle roller, recently invented by Colonel Crompton, will mitigate or overcome this trouble.
The subject is of very great. interest, and our thanks are, I think, due to Colonel Crompton for having investigated the matter and brought sonic light to bear on the subje.ct.—Yours faithfully,
H. PERCY BOULNOIS.
7, Victoria Street, S.W.
From the Borough Engineer of Fulham.
[1304 Sir,—While admiring the efforts of Colonel Crompton to investigate the problem of roads, as a road engineer I should have beer) more impressed by the theory he advances on road waves or road deformation if he had been IrlOre explicit as to : tho cause of the wheel harmonically beating the road surface ; at what speed there is a change as he suggests, from true rolling to rhythmical beating ; and if his illustrations could bear no other construction than that which he accorded to them.
Test Blows and Road Blows.
The theory is of considerable gravity for both road authorities and motor-vehicle (heavy type especially) manufacturers, if it be a true one., as it would nee.essarily involve. pavements if a totally-different character from those now being laid in considerable areas. Colonel Crompton must have overlooked the effect of his theory when he advocated a two-coursed pavement. He illustrated an instrument that was used in the testing of the material forming the surface course, one sample breaking after 42 blows and another after 1400 blows--the latter, however, he did not recommend: If the theory is sound, one would have thought that the sample which submits effectively to the greatest number of blows would have been the most satisfactory, and the material which would withstand only a moderate number would rapidly break up under the harmonic blows the wheel is supposed to give to the surface. But pavements do not break up in actual practice.
in my opinion, the effects are capable or explanation from other points of view. Roads ma.y be : (1) actually worn into depressions ; (2) deformed into waves an account of the use of unsuitable material ; and (3) slightly deformed to a liante.d extent through unequal compression. In every bituminous material used in roads, a capacity for movement in however slight a degree is looked for---a very stiff, unresisting composition in all conditions of weather would be regarded with suspicion.
The Consequence of Weak Places and Neglected Repairs. From the examples that have, conic under my notice, an old-established macadam road does not develop waves under heavy traffic, but rather holes at very irregular intervals : if the surface is tar-painted, the holes do not appear until the tar is ineffective. As these holes deepen and are. not repaired, other depressions are caused by the jumping of the vehicle (which is assisted by the action of the springs) out of the hole. on to an area that might have resisted the rolling of the wheels if the first weak place had not developed, and these depres.sions eventually appear about 2 ft. or 2 ft. 6 ins, from each other. The criginal
holes or depressions are due to local weaknesses, and are developed by actual wear or disintegration. In a newly-formed macadam road, under similar traffic, depressions appear almost immediately, but this, I contend, is due to the fact that the material has not been properly consolidated in the first instance, and the traffic continues the compression, the swaying of the body on the springs reflecting its effect on the road structure. In both cases, the general surface is retained.
• Waves are Preventable.
Bituminous and tarred materials are, in certain cases, deformed ; but this, in my view, is due to the binding agent being affected by hot atmospheric conditions, and thus lending itself to being pushed out of position, and just as a roller affects it when the material is first laid. This can be eliminated by including in the surface a larger aggregate, as in the case of asphalt macadam, where wave formation is least in evidence, even where a bitumen of low melting point is used, or by adopting a bitumen of high (comparatively speaking) melting point, where a fine aggregate is used. After three years of concentrated traffic of hundreds of motor vehicles daily, on a corner of a road laid with asphalt macadam, no waves have appeared ; yet, at the same place. in ordinary macadam, the depressions appeared within as many weeks.
The surface of the Thames Embankment is not strictly even ; it has been laid three years, and in parts seven years ; the traffic is heavy, and mainly of one class. If anything can be said of it, it is actually becoming more even, and this is due to the traffic distributing its pressure, so that the whole surface is being fully compressed.
No Harm Done by Motorbuses to Ten-year-old Asphalt.
Observation has been made on a slight depression in asphalt traversed frequently by motorbuses ; the springs were deflected from the normal about half-aninch (much more than the actual depth of the depression), yet no development has occurred in six months, and the pavement has been laid ten years, and not, repaired at this point, indicating that the material has sufficient resistance to counteract the intensity of the fall of the load into the hollow.
Spring Action on Curves.
But hollows are actually worn in wood pavements, especially at curves, and particularly so where the contour of the road is against. the curve ; the hollows are several feet apart, but., as the curve is negotiated, they are nearer to each other ; they are on the inner track of the curve only, and gradually disappear as the straight is reached. These hollows are a reflection of the swaying of the vehicle acting through the springs.
There are other illustrations which are suggestive or spring action, and it would be of considerable interest if some other method were employed to keep the body of a vehicle on an even keel,. and which would more effectively keep the wheels to the surface of the road. This will probably be found by a pneumatic arrangement. It is much snore comfortable to ride on a vehicle fitted with partially-deflated tires, if the road is somewhat uneven, than to ride on one that has its tires fully inflated.
Pavements Made in Situ: Inherent Defects.
In all pavements made on the road itself, there are bound to be inequalities, especially in the compression of the material, whether it be rolled while the material is cold or substantially hot, and it must necessarily be the case that the road engineer will rely on the ductility and cohesiveness of the binding agents to resist the ordinary effects of traffic.
If waviness in roads is strikingly in evidence. I consider it is only of importance in demonstrating that' the material has in some part. an inherent defect,
which can be overcome, in large degree, by a proper selection of material. If the springing of the vehicle does take an important part, then the mitigation of wave formation will he obtained by assistance from motor-vehicle builders, and between the two thE trouble will be removed altogether.
On the other hand, if Colonel Crompton's theory is upheld by future examination, then it. will be a problem of extreme difficulty for both the road engineer and the vehicle builder.—Yours faithfully,
Faawers Wools, M.Inst.C.E.,
From the County Surveyor of Surrey.
 Sir,—I have read with interest your comments on Col. Crompton's paper on the road problem. It is unfortunately a fact that all road surfacuags consolidated by rollers have a slightly-wavy surface„ and that those, corrugations are quickly aggravated on the incidence of heavy motor vehicles travelling at considerable speeds. It is also true that these corrugations are at very regular periods, and particularly so if there is a great deal of traffic by similar vehicles such as motorbuses. One would suppose that, the great variety of other descriptions of vehicles would tend to counteract this regular corrugation, but it seems to be of such a pronounced and dominating character as to set other vehicles with springs on a similar bouncing progress, as if they were compelled to dance to the same tune.
In my opinion, the process of corrugation of roads is not entirely confined to roller-consolidated roads, as a less marked but still very similar effect is shown eventually en wood paving. inasmuch as the concrete bed for this is floated off level, and the blocks themselves are very true, it is difficult to understand how this is started, My own idea is that it proceeds from the propulsion of the vehicles, which I cannot help ;thinking is by a series of jerks, modified no doubt to a great extent by the flywheels, but still far removed from a smooth rolling contact. If I am right in this, I confess I have no cure for the complaint in .view, but can only suggest in the first place the provision of surfaces as true and resistant to deformation as practicable, and on the part of the vehicle a larger wheel and the reduction of the oscillatory action due to springing.—Yours faithfully,
A. DRYLAIN D, M.Inst. C.E.
The Editor THE COMMERCIAL MOTOR.
 Sir,—I am sure that a large number of your readers will be glad to learn that a telephone is on the point of being installed at the Floating Light Inn, Standcdge Moors, Saddleworth, Yorks. 'this will be a blessing to drivers of commercial motors, especially on foggy nights, or in rough and wintry weather. It will also help when there are breakdowns. I am sure, too, that members of pleasure parties will be interested Co learn about it.
This installation would have been too costly for the tenant of the inn to bear the expense himself, as it is at least 6s. per week. A well-known Yorkshire owner, Mr. Allen Knight, of Linthwaite, was the originator of the proposal, and helped to get together sufficient money for the matter to proceed. He was well supported by several haulage contractors in Huddersfield and Bradford.
Although the provision of this facility is assured, there may be some other users who woulel care to subscribe towards the scheme, and 1 believe Mr. Knight would be quite pleased to hear from them, at TTe-dmefield, Manchester Road, Linthwaite, Yorks. Ile will no doubt furnish a list of subscribers to date, and give all particulars.—Yours faithfully,
[Whilst we are happy to acknowledge the enterprise and public spirit with which Mr. Allen Knight has acted, in getting together the necessary subscriptions, we think that similar instances should be reported to and dealt with by one or other of the local branches of the C.M.U.A, Provision of telephonic facilities on the road wilt, of course, form an integral part of the C.M.U.A. night-sheltersehezne.-ELL]