he Effect of Traction Engine and Heavy Motor Traffic upon
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Foundations of Roads.* .laving had occasion to examine-the foundations and sub1 of roads in various English counties, the Author has !n iouch impressed with the fact that in almost every emit instance these investigations have revealed the fact it the subsoil is concave in cross section, with deeper kings under wheel tracks, and that, as a necessary 4:onILleflee, the foundation stones or metalling are in the shape an invert rather than a fiat arch; thus, not only losing cohesion and power of -resisting pressure, but ever sink:, with wider-gaping joints, through which the water ely percolates to the subsoil. This characteristic has :ome very general, during the past to or 12 .,,ears, in glish country roads, and is due to the rapid growth of iew class and weight of traffic, in the shape of traction ;ines and heavy motors. It may be taken that very few glish country roads have large rubble or hand-packed ne foundations, the vast majority being composed of ers of small stone, in varying thickness, upon the natural 'soil; following, in fact, the Macadam rather than the ford principle. It is, therefore, proposed to confine eonn-atiou to foundations of the former description. Although -ays referred to as maintaining the absolute necessity for land-packed foundation of large stones, it may interest p,-lish readers to know that Telford (like a great many d construelors have to be when dealing with public funds) a an oppormnist, and he does not appear to have been always to practise what he preached. As an instance, Author some years ago had charge of a portion of the .ewsburv and Holyhead road, which was constructed by ford. The turnpike trustees so starved this road, prior their extinction, that Parliament granted the county of marvoii an extension of time before abolishing the system oll-laking, in order that funds might be provided w11(=UPLi to restore this road. In effecting this, the Author more than surprised to find, that for miles there was hand-parked foundation in the road !
No Foundations: A Concave Subsoil.
leferring to notes taken of the thickness of metallingn main roads examined in various counties, the average .kness of hard material, whether hand-packed foundai and metalling, or metalling alone. upon English country :n roads does not exceed 8 or 9 inches. In many counties,
kbridgment of a Report, presented to the ?Ark Congress by Mr. R. J.
nas, County Surveyor of Buckinghamshire, the principal roads have generally not more than 6 inches of metalling (with no hard foundation) upon them, and only in a very few instances does the writer know of material exceeding 12 inches. The subsoil being in the majority of eases concave, as already stated, it follows that, where it is composed of clay, stiff loam, chalk, or other water-holding material, it is of the utmost importance to prevent ingress of water from the road surface, for, if the concentrated weight of traffic is such as to fracture the surface met-tiling, water readily obtains access to the subsoil through the crevices, and lies imprisoned in the invert, where it softens the soil, thereby seriously reducing its weight-resisting qualities. Had not the exceptional weights of recent years transformed the cross section of subsoil from convex to concave, and further depressed it under line of wheel tracks, it is obvious that water finding access to it would flow away to the sides, and thus do but little permanent injury. VT0111 numerous sections cut out of roads affected by tractiKinengine traffic, it is found that the actual displacement or fracture under the line of wheel tracks is generally prismoidal in form, with a gradual extension of base line, transverse and longitudinal.
Effects Below the Surface.
Assuming that the metalling of a road is composed of several layers of equal-sized stone, it is obvious that such stones must overlap each other, or " break joint "as it is described in building. If, therefore, one stone on the upper surface has to withstand a given weight, it follows that, allowing for lateral support and slight cohesion, such downward weight is distributed over four stones in the second layer, and so on. When this is worked out, it will be seen that the fan downward pressure borne br the surface of the road is felt in the lower layers over smaller areas to a vanishing point, and that the line of this maximum pressure would represent an inverted pyramid with sides at an angle of from 40 to so degrees with the horizon. This can be verified, approximately, by observing the pressure on layers of balls or other circular articles of equal size piled unon each other, and working out mathematically its vertical distribution if applied equally upon a given number on the upper surface. Given equal pressure of say nine units upon a square measuring three spheres each way on the surface. we have tin unit of pressure upon each sphere. The fun
vertical pressure of these nine transmitted to but four in the second layer, and again to but one in the third, whilst in the fourth layer only some iths of an unit falls upon any one sphere. (The Author hopes to produce diagrams of tests made with models of road sections.)
'The Author does not seem to us to make his meaning clear. We believe, however, that his intention is to demonstrate the fact that any vertical pressure which comes upon a road surtace is "coned out" over the foundation (Or hubsoil) by reason of the interlocking and bonding al the roadstone. If this be his meaning, it is obvious that the thicker the road the smaller is the reaction which the foundation (or subsoil) is called upon to exert in order to sustain the load, and this view does not aplear to us to be open to challenge.—En.;
A two-wheeled car with its load would weigh, say, 30 cwt. If with three-inch wheels the weight upon road would be 5 cwt. per inch of width, that maximum vertical pressure upon the road surface would be reduced to a point at a depth of about 4 inches.
The longitudinal bearing of an ordinary cart wheel on a good hard road is about 3 inches, so that the pressure of a wheel base would extend over 9 square inches, giving 186 lb. per square inch. The subsoil of a road having 6 inches of tnetallingas cover would appear, therefore, to escape the maximum pressure entirely under ordinary cart traffic, and have less than i cwt. per square inch to sustain. Where traction-engine traffic is encountered, we have, in the case of a 14-ton engine, a weight of approximately 41 tons upon each driving wheel, which would probably have tires 18 inches wide. This is allowing the same pressure per inch of width of tire (5 cwt.) on the road surface as that of the ordinary cart already referred to, even if the entire width of the 18-inch wheel bore equally upon the road—which is almost an impossibility upon roads of the least convexity practicable. The maximum vertical pressure of 5 cwt. per inch width of tire commences, in this case, on a base of 18 inches, compared with the 3 inches of the ordinary cart, and with this base the point of the inverted pyramid carries such maximum vertical pressure about 18 inches below the surface of the road, so that the subsoil of a road having a 6-inch crust must be affected to a depth of 12 inches by the full vertical weight. It thus follows that the subsoil of a road having 6 inches of metalling upon it is as well able to withstand ordinary horse-drawn traffic as it can support traction-engine traffic with 18 inches.
License or No License?
The Author must draw attention to a serious anomaly in English Highway Law, i.e., that as drivers of heavy traction engines require no license, owners have not complete control over their men or drivers, and these drivers can be guilty of actions entailing dismissal without seriously endangerin.g their prospects of another engagement, which might be the case had they to produce a license, probably endorsed. This lack of responsibility on the part of drivers cannot have conduced to a rational use of public roads, or to due regard for the rights or convenience of other traffic. Drivers of small traction engines, i.e,, heavy motors weighing, with their load, up to 12 tons, must hold a license which can be endorsed or revoked, and this restriction has undoubtedly produced much more reasonable and considerate driving. If a driver is debarred from taking a small engine along a highway unless he is licensed annually, it appears wrong that he can drive a heavy engine without any license or restriet7on.
Ten "Collars' equal One Engine.
In the foregoing observations, attention has not been drawn to the increased stresses and friction to which a road foundation is subject under heavy self-propelled or "live axle " vehicles, when compared with those which are borne by a road traversed by passive or horse-drawn vehicles, but it is obvious that they must he very much more severe. I laving kept close observation for five years upon a portion of a 2I-foot main road leading from one country town to another, with ordinary country traffic to sustain, which would include agricultural and other traction engines, the Author found that the foundation—exceptionally good lime-stone pitching—had not worn or sunk in the slightest, but that the granite surface had worn away an inch per 100,000 " collars," taking a ton to a " collar " and ten collars to a traction engine, and that the central seven feet of the road had worn away three times as much as the side seven feet. It would appear, therefore, that there had been an user of the centre of road by three vehicles out of every five, the other two utilising the sides.
A Desire to Bar the Cross Bar.
The references made to traction engines also apply " heavy motors," i.e., traction engines %veighing under f tons, or, if self-contained, ix tons when loaded—with, he
ever, the important difference that projecting cross bars. tires are prohibited on the smaller engines! This prohl Lion of projecting cross-bars is undoubtedly a step in direction of minimising the injury done to road foundatio by increasing the actual longitudinal roadway borne upon the wheels. There is still, however, the concentration weight by the inner portion of tire when traversing crown of roads, and, as these heavy motors travel at spel far in excess of the legal five miles per hour, they aff
road foundations very seriously. There is, in favour heavy motors, the schedule of weights permitted on whe of varying width and diameter, which tends toward reduci the injury to roads, but generally makers have confir themselves to small-diameter wheels, for, out of the lat number of heavyrnotors registered by the Author in Buckii hamshire, only one has driving wheels exceeding four f in diameter, the majority being 3.1 feet, thus being s much smaller than ordinary' cart wheels. Heavy mot. should, without doubt, pay an annual license like tracti engines, and the drivers of the latter should, as alrez stated, be licensed.
How to Tackle the Problem.
The Author is confident that it is of vital necessity to seci
a well-rounded cross section in the natural ground bet the fabric of the road, to keep it thoroughly well drab of subsoil water, and to prevent at almost all cost the mission of storm or other road water to it from the surfa The fabric of roads used by traction engines should the fore be of such solidity and depth that fractures and fissu through to the subsoil are highly improbable, and the rn dense and impervious to water such fabric is the less p liability of injury to such subsoil--the weakest link in chain of road construction. In order to make English rts of sufficient strength to sustain all heavy traffic, it will necessary to : (1), remove the road coating, lower the s soil wherever necessary to restore the essential cross fall contour, rectify any defects in under drainage, replace roll the old metalling, adding sufficient new to produci thickness well beyond the minimum necessary for sustain' all traffic without fracture or distortion, and to finish with a coating of hot tar or similar material to close inr stices and form an impervious face; or (a), remove as ab and, after producing satisfactory cross section and corn ing drainage, lay foundation of large stones or other h material carefully hand-packed and tightly wedged up, ca pitting with steam-rolled metalling, tar painted; or where the surface of the road at the sides is too high, rem sufficient to form the required cross section, and coat wl of old road in sufficient thickness of good metalling, ro and tar painted; or (4), after contouring old surface cc roll in sound wear-resisting tarred material.
In the first method, there would be much additional lab
in screening the old surface to obtain clean metalling, also the risk of finding a mixture of stones varying lati in quality ; but there would be the unquestionable adjustm of subsoil contour and sub-drainage, points of vital imp. ance. A very large quantity of material would be necess; on most English roads, to provide a covering capable taking without fracture heavy-traction traffic. The sec, method would no doubt effect the greatest improvement, the cost would be prohibitive, if any lengths were done, unless substantial Government aid was forthcoming. ' third suggestion, whilst being much cheaper, would h the inherent weakness of a concave subsoil liable to wa lodging whenever its cover was injured. On some roE where the sides were sufficiently mobile in very wet sease and it was obvious that the subsoil was higher than at centre, the Author has effected considerable improvemen. cross section by the systematic use of steam rollers up down these sides during wet weather. The fourth and method would also entail the perpetuation of the sub weakness, although the risk of access of water should reduced to a minimum.
Were ample funds available, no doubt a combination Nos. a and 4 would produce the most-satisfactory and mately-economical road for the bearing of all forms of her traffic.