The only true test of the fitness of any stone for use as a road metal is by an experimental trial upon a certain length of roadway; but in making the first selection for such trials it is well to make the following investigations:—

(1.) Ascertain from local persons, such as masons, quarrymen, and others, their opinion of the qualities of the stones in the neighbourhood.

(2.) Make a trial of the stone for toughness. This can be done by setting a good stone-breaker to work upon a heap of the stone as quarried and carefully watching how much he can break in an hour.[26]

(3.) Ascertain what power the stone has to resist abrasion. This is done in France by putting the broken metal into a revolving cylinder and then carefully noting by weight what the cubes lose by contact with each other. Another plan may be adopted by pressing the stone against a grindstone with a uniform pressure, and noting the loss caused by such contact.

(4.) The power to resist compression may be easily ascertained by placing small cubes in an hydraulic press and noting under what pressures each cube will crush.

(5.) The effect of weather is not easily ascertained artificially, although it is suggested that a good test may be made by soaking the stone in a saturated solution of sulphate of soda; and then on exposure to the air, if soft, it is said the stone will disintegrate as if under the action of thaw succeeding frost.[27]

The specific gravity of a stone is no criterion whatever as to its fitness. Clay-slate has a higher specific gravity than a tough flint, and yet the former is almost useless as a road metal; the latter, on the contrary, often making excellent roadways.

The qualities necessary for a really good road metal are hardness, toughness, not easily decomposed or affected by the weather, and at the same time the stone when broken ought to have some power of cohesion without the necessity of much binding material. The question of cost I put aside at once, as it is well known that the best road metal is always the cheapest where there is much or heavy traffic.

Local circumstances must to a great extent determine what stone to use upon a roadway, but the following list may be of use:—

Syenite.

—This is a granite in which hornblende takes the place of mica, and is an excellent road material; the darker the colour the more durable it is found to be.

Granite.

—This should have more felspar than quartz, and have as little mica as possible; the closer the grain the better. Coarse-grained granites soon decompose.

Trappean Rocks.

—Some of these are excellent for road metal. Basalts of dark colour and close grain should be selected. Greenstones with similar characteristics are good; as is also Whinstone.

Gneiss.

—Is inferior to granite; it has mica in layers and is not a good road metal.

Clay Slates.

—These are useless, as they crumble on exposure or degenerate into mud.

Limestone.

—The Metamorphic, Silurian, and Carboniferous limestones may be used if crystalline in appearance, but the Lias and Oolitic are of little use.[28]

Sandstones.

—Some of these, if cherty or containing a large percentage of iron, may be used; but as a rule they are quite unfitted for use as a road metal.

Flints.

—These, if tough, make excellent roadways; but unfortunately they are sometimes too brittle for heavy traffic. Surface-picked flints are better than those from a quarry.[29]

Pebbles.

—These are found on sea shores and river beds. They are composed of very various rocks, and are much water-worn and rounded; when broken they sometimes answer very well if mixed with gravel to bind them.

Gravel.

—This, if of a flinty character, and not too much mixed with earthy matter, makes good roads for light traffic, if carefully watched or well rolled during formation. Pit gravel should always be screened through wire screens of 1¹/2 to 1³/4 gauge, and the small can be used for footpaths.

In some places it is difficult to obtain any natural stone for the purposes of road metal; in these cases slag from blast furnaces or ordinary clinkers from furnaces are sometimes used. Oyster shells are used on the roadways near the Gulf coasts[30] and charcoal in Michigan, United States.[31] I have myself made a most excellent roadway with coral on the coast of Jamaica, and no doubt many strange materials have been, and still are, used for this purpose.

“I never mix” is an adage that should be followed by surveyors as regards road metal. Do not mix a soft material with one that is harder for either construction or maintenance of a roadway; the effect is what is known as a “bumpy” road, arising from the fact of the soft stone wearing faster than the hard. The hardest metal should be kept for the top or surface layer of the roadway.

As an instance of the extreme difficulty besetting the question of the best material for road metal, I will here give a table showing the comparative coefficients of quality assigned to them by the engineers of the French Department of the Ponts et ChaussÉes.[32]

Coefficients of Quality of Road Materials.

It will be seen by the above table how different are the results obtained from materials of the same character.

Breaking stone for the purpose of using it as a road metal was, until comparatively recent years, always effected by hand; now, as in other cases, machinery has stept in and somewhat supplanted manual labour. Hand-broken road metal, however, still finds favour with road surveyors; it is better broken, and in some districts, the occupation finds employment for persons who otherwise would be thrown on the rates for support.

In breaking stone by hand the breaker sits and strikes the stone with a small cast-steel chisel-faced hammer, weighing about one pound, at the end of a long, straight-grained but flexible ash stick.[33] The breaker also has another hammer, weighing about five pounds, with which he reduces the size of the large stones before breaking them into the proper size for road metal. This latter size is often a matter of choice, some engineers preferring it to be broken so small as will pass through a ring of only 1¹/2 inch in diameter; others are content with 3 inches, especially where the roads are steam rolled. An old method of gauging used to be “such a size as the stone breaker could put in his mouth,” but this was unsatisfactory to all persons concerned, and “to pass all ways through a ring of 2¹/2 inches internal diameter” is now the size most generally adopted.

Mr. Codrington says[34] “a good stone breaker will break 2 cubic yards of hard limestone to the ordinary gauge in a day, and some men will break more. Hard silicious stones and igneous rocks can only be broken at the rate of 1¹/2 or of 1 cube yard per day; of some of the toughest, such as Guernsey granite, a man can only break on an average half a cube yard per day. River gravel, field stones, or flints, which are already of a small size, can be broken at the rate of 3 or 4 cube yards per day.”

This may be taken as fairly representing a day’s work, the price for breaking however must vary considerably in different localities on account of the variety of the stones to be broken and the value of labour; in some districts the road metal does not cost more than 1s. per cube yard, in others 2s. 6d. and 2s. 8d. is not considered too high, and it was to meet and reduce this great expense that steam stone-breaking machines have been introduced. These machines are known as “Ellison’s,” “Newall and Archer’s,” “Hope’s,” and “Blake’s,” the latter being that which is best known and most generally used in this country.

The foregoing illustrations will give a general idea of the manner in which the stone is broken or crushed between strong iron jaws; in all cases a revolving perforated screen is necessary (not shown in the drawings) to separate the stone broken to proper gauge from that which is too large, and also from the spalls or chippings.

The Blake’s or “Blake Marsden’s” machines are of various sizes and weights; the following particulars with respect to them, as advertised, may be of use.[35]

Mr. Till, the Borough Engineer of Birmingham, speaking of the work done by one of Blake’s machines in 1874, says:[37] “The stone-breaking machine at Holliday Street will break on an average 40 tons of ragstone per day, at a cost, exclusive of wear and tear of machine, of 10¹/2d. per ton, but it produces 16 per cent. of dust or fine stone; of the remainder one-fifth has to be rebroken by hand, the whole is very irregular in size and very flaky in comparison with hand-broken stone. The machine is much more efficient in breaking granites or pebbles. It has, however, been found very useful during the last two years, in consequence of the difficulty of obtaining labour.”

Mr. Jacob, the Borough Engineer of Barrow in Furness, read an excellent paper on the subject of stone-breaking machinery to the members of the Association of Municipal and Sanitary Engineers, at their meeting in Manchester in 1875,[38] giving a full description of one of Blake’s machines, to which I will refer my readers.

Mr. Codrington[39] gives the result of breaking whinstone in a 16-inch by 9-inch Hope machine, from which it appears that the total cost, including wages, coal, oil, cottonwaste, etc., wear and tear of machinery, and, I presume, interest on first cost of machine, was about 1s. per cube yard. This effected a saving of 10d. per cube yard as compared with the same stone broken by hand, and the machine broke 40 tons of stone per diem.

To make a stone-breaking machine pay, it is necessary:

(1.) To give it nearly constant work.

(2.) That the stone to be broken shall be too tough to break economically by hand.

(3.) That the machine shall be at the quarry, so as to save the expense of much handling.

(4.) To exercise care in feeding, to give it a sufficient supply without allowing an undue quantity of stone to pass in at one time.

(5.) As about 20 per cent. of grit or dust is produced, this must be used for foot-paths, or as a binding material for roads, or in asphalte or tar paving.

In addition to the grit which is produced, a great many long and thin pieces of stone pass through the machine, which have to be again broken by it before they could be used as road metal; and having once taken this form, they will frequently pass several times through the machine before they get properly broken.

The wear and tear of a stone-breaking machine is very considerable, as can be easily imagined; it has been known to reach as high as 62·5 per cent.[40] of the first cost of the machine in one year. The objections to stone-breaking by machinery are principally:

(1.) In some districts labour can be successfully employed in this manner.

(2.) Hand-broken stone is sharper in fracture, as it is done by a blow and not by gradual pressure, whereas machine-broken stone is often flaky or with rounded edges, and frequently each stone may be cracked and shaken by the pressure.

(3.) Want of uniformity in the size of the stones.

The smaller the stone is broken the heavier a cubic yard of it will weigh, as the percentage of vacant space between each stone will be less. It has been found by experiment, however, that 55 per cent. of ordinary road metal is solid, so that the weight of a cubic yard of it can easily be ascertained in the following manner.[41]

Multiply the weight of a cubic foot of any stone by 27 to bring it to a cubic yard, and then multiply this by 0·55: the result will be the weight of a cubic yard of the same stone when broken for metalling.

A cubic yard of Guernsey granite broken to pass through a 2¹/2 inch ring has been weighed, and gives an average of 1 ton 3 cwt. 2 qrs.

A cubic yard of ordinary broken road metal will, when properly spread, cover an area of about 30 square yards of surface of a roadway.

The following specimen specification for the supply of stone either unbroken or broken may be of use.

Specification for the Supply of Road Metal.

(1.) The road metal must at all times be clean and free from clay or other dirt, and fully equal to the sample; if required to be broken, each cube must have a square face and sharp edges, and pass all ways through a 2¹/2 inch ring.

(2.) The metal must be delivered in (name of town) free of all charge to the corporation, either at a railway station or at one of the depÔts of the corporation, at the option of the contractor, such option to be declared in the tender.

(3.) The metal must be supplied on the order of the borough engineer in such quantities as he may specify, and must be delivered within the time specified in the order. The contractor shall not be required to supply and deliver more than tons in any one week; but the corporation will be at all times ready to take the metal in larger quantities.

(4.) The bill of lading or railway invoice shall be taken as prim facie evidence of the weight of metal supplied; but the corporation retain the right to test the accuracy of such bill of lading or railway invoice, by passing the metal over a weighbridge as it is received.

(5.) Metal delivered at a depÔt by carts shall be measured when broken and paid for at the rate of cwt. per cubic yard.

(6.) The corporation retain the right to reject all metal which shall not be equal to the sample, or at their option to pay a reduced price according to its value.

(7.) Quarterly payments will be made by the corporation on the certificate of the borough engineer, and within one month from the date of such certificate.

(8.) The borough engineer shall be the sole judge as to the fitness of the metal supplied, and his certificate, in writing, shall be conclusive evidence upon the point as between the corporation and the contractor.

(9.) If the contractor shall make default in the supply and delivery of road metal in accordance with the terms of this specification, and within the time specified for the purpose in the order of the borough engineer, the corporation shall be at liberty to obtain such road metal as they may deem fit and necessary from another source, and any excess in price or other loss they may consequently incur, shall be recoverable by them from the contractor as liquidated and ascertained damages.

(10.) Tenders must be sent in only on the prescribed form, and the person tendering must insert in his tender the name of two persons who will join him in a joint and several bond to the corporation in the sum of l. for the due performance of the contract.

(11.) Each person tendering must send to the office of the borough engineer a sample of the road metal he offers, accompanied by a full description, and the name and position of the quarry from which it is produced; such sample to be not less that one cwt. in weight, and to be retained by the corporation in the event of the tender being accepted.

(12.) The corporation do not bind themselves to accept the lowest or any tender; and they further retain the right to reject a contractor in the event of his failing to find sureties to their satisfaction in compliance with the 10th condition.

(13.) The word “corporation” shall mean the mayor, aldermen and burgesses, of in their capacity as the urban sanitary authority for . The word “contractor” shall mean the person whose tender is accepted, and who has signed these conditions; and the words “borough engineer” shall mean the engineer to the said corporation for the time being.

Since writing this chapter my attention has been directed to a stone-breaking machine which is said to substitute a “knapping” for that of the usual crushing motion which is so generally the great defect in these machines: I allude to that known as “Baxter’s patent knapping-motion stone breaker,” by which a rapid jerk or blow is given instead of the slow crushing movement, thus (it is contended) causing less waste from dust and chippings, and also less strain of the machinery and less power to drive it.