CHAPTER XI

BROKEN-STONE PAVEMENTS.

As has been seen in the study of stone-block pavements, the developments led to a general reduction in the size of blocks. So with the irregular stone pavements, they, too, decreased in size as their use increased. While probably small broken stone were used in roads for many years previous, it was not until 1764 that what is known at the present time as macadam roads were first built systematically by M. Tresaguet, a French engineer, who was the first to adopt this plan, and it came into general use about ten years later. His method of construction as described by himself is as follows:

"The bottom of the foundation is to be made parallel to the surface of the road. The first bed of the foundation is to be placed on edge, and not on the flat, in the form of the rough pavement and consolidated by beating with a large hammer, but it is unnecessary that the stones should be even with one another.

"The second bed is to be likewise arranged by hand, layer by layer, and beaten and broken coarsely with a large hammer, so that the stones may wedge together and no empty space may remain.

"The last bed of 3 inches in thickness to be broken about to the size of a small walnut with a hammer on one side of a sort of anvil, and thrown upon the road with a shovel to form a curved surface. Great care must be taken to choose the hardest stone for the last bed, even if one is obliged to go to more distant quarries than those which furnish the stone for the body of the road. The solidity of the road depending on this latter bed, one cannot be too scrupulous as to the quality of the materials which are used for it."

The object of this lower course of large stone was to separate the wearing surface from the subgrade, rather than to form a foundation for the road.

This method as just described was practically that adted by Telford some forty years later in England, the difference being principally in making the subgrade level and forming the crown with the stone itself, rather than making the base parallel to the finished surface of the road as Tresaguet did. The following is taken from Parnell's treatise on Roads, which gives Telford's specifications in detail:

"Upon the level bed prepared for the road materials the bottom course, or layer of stone, is to be set by hand in the form of a close, firm pavement. They are to be set on the broadest edges, lengthwise across the road, and the breadth of the upper beds is not to exceed 4 inches in any case. All the irregularities of the upper part of the said pavement are to be broken off by a hammer, and all the interstices to be filled with stone chips, firmly wedged together by hand with a light hammer. The middle 18 feet of pavement is to be coated with hard stone as nearly cubical as possible, broken to go through a 24-inch ring, to a depth of 6 nches; 4 of these 6 inches to be first put on and worked by traffic, after which the remaining 2 inches can be put on. The work of setting the paving-stones must be executed with the greatest care and strictly according to the foregoing directions, or otherwise the stone will become loose and in time may work up to the surface of the road. When the work is properly executed, no stone can move; the whole of the material to be covered with 11 inches of good gravel, free from clay or earth."

Parnell, in commenting on this last clause of covering the road with gravel, says: "The binding which is required to be laid on a new-made road is by no means of use to the road, but, on the contrary, is injurious to it. This binding by sinking between the stone diminishes absolute solidity to the surface of the road, lets in water and frost, and contributes to preventing complete consolidation of the mass of the broken stone."

A contemporary of Telford and a man whose name has been given to this class of roads, in the English-speaking world at least, was Macadam. He worked on very different principles, in that

he not only did not require the foundation-course, but stated that he considered it positively injurious. He enunciated the following principles as fundamental: "That it is the native soil which. really supports the weight of traffic; that while it is preserved in a dry state, it will carry any weight without sinking, and that it does in fact carry the road and carriages also; that this native soil must be previously made quite dry and a covering impenetrable to rain must then be placed over it in that dry state; that the thickness of the road should only be regulated by the quantity of material necessary to form such impervious covering and never by any reference to its own power of carrying weight."

In some evidence given before a Parliamentray commission upon the subject of roads, soon after Macadam had taken up their reconstruction, he stated in answer to a question by one of the committee that he considered that 10 inches of well-consolidated material was sufficient to carry any load, and that without any reference whatever to the foundation. He also added that he would prefer a soft foundation to a hard one, going so far as to say that he would prefer a bog if it were sufficiently hard to allow a man to walk over it. It must be remembered that all of these roads were very different when first built from those of the socalled macadam roads of to-day, as they received no rolling whatever, but were consolidated wholly by traffic.

The question as to which is the better system, Telford's or Macadam's, is one that has been discussed for a good many years. It is hardly necessary to say that at the present time Macadam's idea of having a soft, yielding foundation for his road is not considered good practice. On the other hand, the foundation as described by Telford is expensive, and in roads of light traffic, with a good natural foundation, it would seem to be unnecessary. Where a particularly solid roadbed is required it is the custom of many engineers to build what is called the telford-macadam road, that is, it has a telford base with a macadam wearing surface. Macadam's own particular work, when he took it up, consisted of repairing old roads rather than constructing new, and it is said that he was so successful that in many instances the cost of reconstruction per mile was but little, if any, more than had been the previous cost per annum for maintenance, and it is also true.

that the condition of the roads was very much improved. The roads built previous to his time were very crude, although containing an immense amount of material, but laid with an entire lack of scientific knowledge. Macadam describes this old process in vogue at that time as follows:

"The practice common in England and universal in Scotland. in the formation of a new road is to dig a trench below the surface of the ground adjoining, and in this trench deposit a quantity of large stones; after this a second quantity of stones broken smaller, generally to about 7 or 8 lbs. weight. These previous pieces of stone are called the bottoming of the road, and are of various thicknesses, according to the caprice of the maker, and generally in proportion to the sum of money placed at his disposal. On some new roads made in Scotland in the summer of 1819 the thickness exceeds 3 feet.

"That which is properly called the road is then placed on the bottoming by putting large quantities of broken stone or gravel, generally a foot or 18 inches thick, at once upon it, and from the careless way in which this is done the road is as open as a sieve to receive the water which is retained in the trench."

This description allows one to understand the radical change made by Macadam when he inaugurated his system. It can be readily seen how, under any material amount of traffic, a road constructed in such a manner might soon become very rough and uneven and very disagreeable for traffic. When one thinks of building to take the place of these roads others with a maximum thickness of 10 inches and made up of stones which, according to Macadam's standard, could be easily placed in one's mouth (and which should not weigh more than 6 ounces), it can be readily understood that a Parliamentary investigation was quite in order before any great amount of money was expended in this work.

The trite saying that "Nothing succeeds like success," however, was just as true then as it is now, and it required but a short time, and very little testimony, to satisfy even the Parliamentary committee that the money was well expended. Macadam utilized the old material already in place, and by breaking it as he did, by hand, gave employment to a large number of people, many of whom, as he says, were old men and women.

In speaking of the relative merits of macadam and telford roads, Mr. A. J. Cassatt, in a letter to the Commissioner of Public Roads in New Jersey, says that as a result of his experience with both systems, commencing with the telford, he is very strongly in favor of the macadam under any circumstance and for any kind of subgrade. He says that during the long periods of dry weather in the summer the roads are apt to disintegrate and the surface become covered with loose stones, and that this occurs more frequently, and to a greater extent, in the telford than in the macadam. Another objection he makes is that the surface stones wear off much more rapidly with the solid telford base than with the macadam, which always wear smoothly and uniformly except when the bond is broken in the early spring, when the frost is coming from the ground.

Although the macadam and telford roads are taken up under the head of Pavements, they should not, strictly speaking, be classed as such, as they are not suitable for city streets, although used to a considerable extent in many large cities on account of their cheapness. Their principal objection is their extreme dustiness, requiring almost constant sprinkling, which causes a great amount of mud, and this is entirely out of place in a city street. Any one doubting the advisability of the macadam for an urban district should visit the Back Bay section of Boston on a windy day in winter, when the ground is free from snow and the weather too cold to permit sprinkling, and he will be thoroughly convinced. As long, however, as a law remains on the statute-books allowing a property owner to pay for the first cost of a pavement and compelling the city to pay for all future pavements, so long will macadam streets be laid, as thrifty taxpayers will be willing to undergo the discomfort of the dust for the sake of avoiding a heavy assessment for a good pavement.

The question involved in the construction of a macadam street in a city is very different from that governing the construction of a suburban road; and while the general principle of the construction is necessarily the same in both cases, what would be proper for one might be decidedly improper for the other.