The system of pitching or paving the bottom of a road has the advantage of preventing the subsoil from working up among the road materials, and, when well executed, of distributing the pressure of carriages over a larger base; while the size of the paving-stones them selves prevents their sinking into the earth, as small atones are liable to do. The pavement also acts as a drain to the surface materials. In addition to these, the plan has, in many situations, the advantage of economy, as the cost of a pavement is considerably less than that of an equal depth of well-broken stone. In meet, if not all, of the cases in which the paving system has failed, the want of success may be attributed to very imperfect execution; as, if the stones are very irregular in size or badly set, or the thickness of metal is insufficient to protect the pavement from the shake of passing carriages, the stones become deranged, and the subsoil, working up among them, quickly spoils the road.
The case of the Highgate Archway road, which has been before alluded to, is a remarkable illustration of the absolute necessity of a firm bottoming under some circumstances. This road is over a subsoil of sand, clay, and gravel, and, being partly in a deep cutting (originally intended fur a tunnel), is much exposed to the influx of water. The road, which is rather more than a mile and a half long, was originally made of a quantity of gravel and sand laid on the natural soil, and covered with broken flints and gravel; but this plan not succeeding, the road was taken up, and pieces of waste tin were laid on the sub soil, over which were spread gravel, flints, and broken stone. This expedient did not produce the desired effect, and at length, in 1829, the road was placed under the management of the Holyhead-road Commissioners, its proprietors having failed, notwithstanding an enor mous outlay, and the application of 1200 cubic yards of gravel annually, to bring it into a satisfactory state. A thick coat of broken granite was spread on a portion of the road; but, owing to the unsoundness of the foundation, it never consolidated, the atones wearing into smooth pebbles by their attrition against one another, oven down to the bottom of the mass. The commissioners therefore determined, as paving-atones could not be procured without great expense, to lay a coating of Roman cement and gravel as a bed for the read-metal, an experiment which was attended with complete success The work was executed by Macneill, and consists of a composition of Roman cement with eight times its quantity of washed gravel and sand, which, after being mixed in a box, was laid on the bed of the road to a thickness of six inches and a width of about eighteen feet. A few minutes after being laid, the upper surface was indented, by means of a triangular piece of wood sheeted with iron, with numerous channels or groove., sloping about three inches from the centre to the aides, these channels serving for the stones to lie and fasten in, and fur conducting any water that might percolate through them into the side drains. This measure,
combined with an extraordinary extent of drainage, amounting in the whole to a length of 12,303 yards, proved so completes remedy that, in the first winter after the cement was laid, coaches were able to go up with four horses at a trot with the heaviest loads, though before the improvement six horses had mounted with difficulty at a walking pace. The effect of the alteration on the wear of the road was equally sstisfactory, four inches of quartz being worn away on the old bottom while only half an inch of the same stone was worn where laid on the cement foundation. The expenses of laying the cement composition, incluebng the formation of the bed of the road, wan about ten shillings per lineal yard, part of the gravel used being old. Macneill estimated the cost at from twelve to fifteen shillings per yard if new gravel were purchased.
The effect of a pared or concrete foundation in diminishing the draught appears, from the subjoined statement, founded on experi ments; with Mr. Jlacneill's road indicator, to be very great; but a more extensive series of trials is desirable for a comparison of different sys tems under various circumstances. The draught of a waggon weighing 21 cwt. was found to be as follows : On a well-made pavement . . .... . . 33 lbs.
On a road with mix inches of hard broken atone on rough pavement . . ..... . 46 On a similar road, with a foundation of Roman cement and gravel in lieu of pavement . . 44 On a road with a thick coating of broken atone on earth . . 63 On a road with a thick coating of grave on earth . . . 141 For the formation of the pavement of a metalled road, almost any hard stone that may be easily dressed with the hammer may be used. The stones should be tolerably regular in size, and laid In rows with their broadest face downwards, the interstices being carefully filled up with stonschippiogs, so as to pin the whole pavement together, and effectually prevent the earth from working up through the joints. In ono of Telford's specifications for the Holyhead roach, the dimeneions of the stones for a pavement 30 feet wide are given as 7 Inches deep in the middle of the road, 5 Inches at 9 feet from the centre, 4 inches at 12 feet, and 3 inches at 15 fifteen feet, the stones to be laid lengthwise seeress the road, and the upper edge in no case to exceed 4 Inches wide. All irregularities are to be broken off by the hammer, and the stone chips used in packing the )(Ante are directed to be wedged in by hand or with light hammers. so ramming is necessary, and it Is desirable to prevent carts which are used in the conveyance of the road materials from being drawn upon the pavement before it is covered with broken stone.