This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
These subjects may be brought together under a single head as constituting the means of approach to a building.
Ordinary roads may be divided into 2 classes, - temporary and permanent. Attention will here be confined to the former.
The first idea of a road is a path or track on which a foot-passenger can travel. In the American forests the trees are blazed or marked to show the direction. On the prairies men travel by compass or by the stars; or by watching their own shadows, or noting the direction of the wind. Successive travellers following the same route will tread down a forest path, which is the first step towards road-making. On such a road, rivers will be crossed by swimming or wading, or by rafts; or felled trees might be used on very narrow streams; while ranges of hills would be passed by following the beds of mountain torrents. The employment of animals necessitates the improvement of the roads. The footpaths are widened, the forest is cleared, rude bridges of logs are formed, or rafts made of wood, of empty vessels, or of inflated skins.
Suppose it is required to make a temporary road from one settlement to another in a wild unmapped country. If a traverse were run by compass and chain between the 2 places, and plotted on paper, the magnetic bearing of the one place from the other would be ascertained, and a straight line could be run between them by means of the compass. If 2 flags are set up in the proper direction at some distance apart, then, by means of a third flag brought into line with the 2 former, a straight line could be run for many miles with a very slight deviation from accuracy. Where a compass is not available, a fire lighted at one place may, by its smoke, enable its direction to be seen from the other.
This line so run, and marked by a trench cut in the ground, will often be a practicable line for the road in a new country; if not, it will at any rate he a valuable guiding line towards which all deviations caused by various obstacles should return. The line so marked out should be cleared for a width of 10, 20, or 30 ft.; a ditch cu1 on either side to serve as a drain, and the earth excavated thrown in the centre of the road to assist the rain-water to run into the ditches. Inequalities of surface can then be levelled as far as possible. Small streams may be crossed by temporary bridges if wood is available; if not, their banks must be cut down, if necessary, to a gentle slope, so as to enable (arts to pass where the stream is dry or nearly so, and such slopes, as well as the bottom of the stream, may be paved, if material is available.
The following is a description of a temporary road of this kind made over the dry bed of the Chenab river in the Punjab, and may be taken as a general example.
The total length for the roadway across the Chenab measures 10,600 running ft., of which 1350 ft. consist of a metalled road; 3500 ft. rest on firm soil, extending from the road embankment to within 1000 ft. of the south side of river, and the remaining 5800 ft. extend across entire sand.
The roadway consists of one layer of grass fascines, each fascine being 24 ft. long, 6 in. in diameter, and tightly bound with grass, packed closely together and covered with 6 in. of clay. On the surface of the clay, and to prevent its cutting into grooves, a very thin layer of loose grass is constantly maintained. An inch of clay is first laid down on the sand, all hollows are filled in and low points somewhat raised, that the foundation may not suffer from the lodgment of water. In other places the finished road is 1 or 2 in. above the sand.
Whatever improvements are made in such roads should be directed towards the most formidable obstacles at first; this is, indeed, self-evident, the strength of a road, as of a beam, being only that of its weakest part; but it is not always easy to determine what are the most formidable obstacles, nor whether it will be more economical to lay out a given sum in raising a portion of embankment, cutting down a hill, improving the surface, or building a bridge, but much of course will depend on the peculiar circumstances of each case.
Similarly to the trellis road used on the early railways in the United States, ordinary roads of a temporary character are sometimes constructed exclusively of timber, and are termed plank roads.
The method most generally adopted in constructing plank roads consists in laying a flooring, or track, 8 ft. wide, composed of boards 9-12 in. in width, and 3 in. thick, which rest upon 2 parallel rows of sleepers, or sills, laid lengthwise in the road, and having their centre lines about 4 ft. apart, or 2 ft. from the axis of the road. Sills of various-sized scantling have been used, but experience seems in favour of scantling about 12 in. in width, 4 in. in thickness, and in lengths of not less than 15-20 ft. Sills of these dimensions, laid flatwise, and firmly imbedded, present a firm and uniform bearing to the boards, and distribute the pressure they receive over so great a surface, that, if the soil upon which they rest is compact and kept well drained, there can be but little settling and displacement of the road surface, from the usual loads passing over it. The better to secure this uniform distribution of the pressure, the sills of one row are so laid as to break joints with the other, and to prevent the ends of the sills from yielding, the usual precaution is taken to place short sills at the joints, either beneath the main sills or on the same level with them.
The boards are laid perpendicular to the axis of the road, experience having shown that this position is more favourable to their wear and tear than any other, and is beside the most economical. Their ends are not in an unbroken line, but so arranged that the ends of every 3 or 4 project alternately, on each side of the axis of the road, 3 or 4 in. beyond those next to them, for the purpose of presenting a short shoulder to the wheels of vehicles, to facilitate their coming upon the plank surface, when from any cause they may have turned aside. On some roads, the boards have been spiked to the sills, but this is unnecessary, the stability of the boards being best secured by well packing the earth between and around the sills, so as to present, with them, a uniform bearing surface to the boards, and by adopting the usual precautions for keeping the subsoil well drained, and preventing any accumulation of rain-water on the surface. The boards for plank roads should be selected from timber free from the usual defects, such as knots and shakes, which would render it unsuitable for ordinary building purposes, as durability is an essential element in the economy of this class of structures.
Boards 3 in. thick offer all the requisites of strength and durability that can be obtained from timber in its ordinary state, in which it is used for plank roads.
Besides the wooden track of 8 ft., an earthen track of 12 ft. in width is made, which serves as a summer road for light vehicles, and as a turn-out for loaded ones; this, with the wooden track, gives a clear road surface of 20 ft., the least that can be well allowed for a frequented road. It is recommended to lay the wooden track on the right-hand side of the approach of a road to a town or village, for the proper convenience of the rural traffic, as the heavy trade is to the town. The surface of this track receives a cross slope from the side towards the axis of the road outwards of 1 in 32. The surface of the summer road receives a cross slope in the opposite direction of 1 in 16. These slopes are given for the purpose of facilitating a rapid surface drainage. The side drains are placed for this purpose parallel to the axis of the road, and connected with the road surface in a suitable slope.
Where, from the character of the soil, good summer roads cannot be had, it will be necessary to make wooden turn-outs, from space to space, to prevent the inconvenience and delay of miry roads. This can be effected by laying at these points a wooden track of double width to enable vehicles meeting, to pass each other. It is recommended to lay these turn-outs on 4 or 5 sills, to spring the boards slightly at the centre, and spike their ends to the exterior sills.
The angle of repose, by which the grade of plank roads should be regulated, has not. yet been determined by experiment, but as the wooden surface is covered with a layer of clean sand, fine gravel, or tan bark, before it is thrown open to vehicles, and as it in time becomes covered with a permanent stratum of dust, it is probable that this angle will not materially differ from that on a road with a broken-stone surface, like that of M'Adam or of Telford, when kept in a thorough state of repair.
In some of the earlier plank roads made in Canada, a width of 16 ft. was given to the wooden track, the boards of which were laid upon 4 or 5 rows of sills. But experience soon demonstrated that this was not an economical plan, as it was found that vehicles kept the centre of the wooden surface, which was soon worn into a beaten track, whilst the remainder was only slightly impaired. This led to the abandonment of the wide track for the one now usually employed which answers all the purposes of the traffic, and is much more economical, both in the first outlay and for subsequent renewals and repairs. The plank roads possess great advantages in a densely-wooded country, and will bo found superior to every other kind as a temporary expedient.