Gravel path

“ To what particular practice do you allude, when you inform the Committee that gravel is unskilfully applied to the roads in general?—I see that on gravel roads, the gravel is put on after being very imperfectly sifted, and the huge pieces not being broken, and the gravel is laid on the middle of the road and allowed to find its own may to the sides. ”

“ The Ideal Road Gravel.—The ideal road gravel is a mixture of pebbles, sand and earthy material, the pieces varying from coarse to fine in such a manner that when the gravel is compacted into a road surface the spaces [Pg 81] between the larger pebbles are filled with the finer material. The pebbles are of a variety of rock that is highly resistant to wear so that the road surface made from the gravel will have the quality of durability. The gravel possesses good cementing properties, insuring that the pieces will hold together in the road surface. ”

“ A gravelly soil is usually the best material for an earth-road, and when practicable every such road should be covered with a coating of it. The larger gravel, however, should never be placed at the bottom and the smaller at the top, as the frost and the vehicles will cause the large gravel to rise and the small to descend, like the materials in a shaken sieve, and the road will never become smooth and hard. ”

“ Thickness of Layer.—The thickness of the layer of gravel required depends both upon the type of soil upon which it is placed and the nature of the traffic to which the road will be subjected. Gravel surfaces should not ordinarily be constructed on highways carrying heavy truck traffic, but if gross loads of three or four tons are the heaviest anticipated, the gravel will be reasonably stable. On such roads, a layer of well compacted gravel ten inches thick will support the loads if a well drained earth foundation is provided. ”

“ As it invariably happens that subaerial erosion acts most powerfully along an anticlinal line, the gravels in the hollows and channels would be the longest-preserved from destruction. And we find here and there on the surface of the Boulder-clay patches of this clayey gravel or loam generally capping the higher grounds, frequently many of them in a curved or straight line, showing the direction of the old channel in which they were originally formed. These so-called "Denudation gravels" are never of any great extent or thickness, nearly all having been removed by erosion. ”

“ In the formation of such roads, and before they become bound or firth, a considerable portion of the sub-soil mixes with the stone or gravel in consequence of the necessity of putting the gravel on in thin layers; this mixture of earth or clay, in dry warm seasons, expands by the heat, and makes the road loose and open, the consequence is, that the stones are thrown out, and many of them are crushed and ground into dust, producing considerable wear and diminution of the materials ”

“ The best gravel for road-building stands perpendicular in the bank; that is, when the pit has been opened up the remainder stands compact and firm and cannot be dislodged except by use of the pick, and when it gives way falls in great chunks or solid masses. Such material usually contains tough angular gravel with just enough cementing properties to enable it to readily pack and consolidate, and requires no further treatment than to place it properly on the prepared roadbed. ”

“ The tendency of material to spread under the roller and work toward the sides can be resisted by rolling that portion nearest the gutters first. To give the surface the required form and to secure uniform density, it is necessary at times to employ men with rakes to fill any depressions which may form.
[Pg 135] In order to maintain a gravel road in good condition, it is well to keep piles of gravel alongside at frequent intervals, so that the person who repairs the road can get the material without going too far for it. ”

“ But it is well known to geologists that a sheet of coarse angular gravel such as this, notwithstanding its looseness, is much less readily attacked by a small stream than is a surface of hard clay or even chalk. Thus plains of Tertiary deposits capped by gravel, under the action of rain or rivers develop into gravel-capped plateaus and hills, which fall abruptly into open flat-bottomed valleys. The denudation takes place at the edge, where the gravel rests on the Tertiary strata and numerous springs are given out ”

“ A broken piece of granite used for macadamising a road is a more complex instrument, about the toolishness of which no doubt can be entertained. It will, however, I think, be held that even a piece of gravel found in situ and left there untouched, provided it is so left because it was deemed suitable for a road which was designed to pass over the spot, would become a tool in virtue of the recognition of its utility, while a similar piece of gravel a yard off on either side the proposed road would not be a tool. ”

“ There are other cases besides that of gravel, in which I should think it unprofitable to lift a road. The road between —— and —— is made of very soft stone, and is of so brittle a nature, that if it were lifted it would rise in sand, and there would be nothing to lay down again that would be useful. ”

“ A layer of broken stone, 4 inches thick, is laid over this pavement. The road-covering thus prepared should be rolled with the steam roller until the upper layer has become perfectly compact and consolidated. The second layer, about 3 inches in depth, is then laid on; a coating of clean coarse gravel, 11⁄2 inch thick, termed ‘binding,’ is spread over the surface, and the whole well rolled as before, and you have the requisites of a good road—viz., clean, hard, and even at all seasons. No road should be considered made [66] until it is completely rolled. ”

“ Where rock is scarce and gravel plentiful, the lower layer may be made of the heavier or coarser gravel, leaving the finer gravel for the upper layer, or boxing; but there is no doubt that the broken stone pitching makes the most efficient bottom layer. No gravel ballast should be used which is not free from clay or earthy sand. ”

“ Where gravel is the material to complete the road with, I have already mentioned, that it ought to be completely cleansed of every particle of clay or earthy substance, and its different sizes ought to be selected and arranged by means of riddling or washing; in the use of the riddle, the particles of earth or clay adhere so much to the stones that it frequently requires to be exposed to the sun, air, and frost, for several months, and then riddled over again. ”

“ From this tunnel vertical holes, or ‘jump-ups,’ are made upwards into the soft gravels, which are then taken out in blocks. As these are removed the roof is allowed to fall in. On the surface the gravel is put into a circular iron tank or buddle in which there are revolving forks. Here it is washed with water, and the big stones thrown away. The clear gravel is then drawn off into long troughs or sluices, down which water is flowing. On the bottom of the trough there are small ledges of wood or iron, behind which the gold collects, while the lighter gravel is washed away. ”

“ The roadbed should be first graded, then carefully surface-drained. The earth should then be excavated to the depth to which material is to be spread on and the foundation properly shaped and sloped each way from the center so as to discharge any water which may percolate through. This curvature should conform to the curvature of the finished road. A shouldering of firm earth or gravel should be left or made on each side to hold the material in place, and should extend to the gutters at the same curvature as the finished road. ”

“ Judging from the section given by Sir Joseph Prestwich, [36] the highest point which the gravel attains is about 59 feet above the river, and its surface in the pit, where the implements are found, is 40 feet above it. The gravel rests upon the Cornbrash, or upper member of the Lower Oolite; but the valley itself, though partly in the limestone rock, has been cut through a considerable thickness of Oxford Clay and of Boulder Clay, which here overlies it. The gravel consists of subangular stones in an ochreous matrix, interspersed with irregular seams of sand and clay. ”

“ These gravels are constantly in movement, being urged forward by the shore currents especially during storms. Large banks of gravel may be swept away in a single night, and in this way the coast is laid bare to the erosive action of the sea. Moreover, the movement of the gravel itself wears down the subjacent rocks. Hence in many places barriers have been erected to prevent the drift of the pebbles and preserve the land, while often it has been found necessary to protect the shores by masonry or cement work. ”

“ Unquestionably, walks near the house should be graveled; they naturally have too [175] hard usage to keep turf in good condition. Graveled walks should be dug out a foot or more in depth, filled in with broken stone, this covered well with coarse gravel, and finished with a coating of a couple of inches of whatever fine gravel is chosen. ”

“ Where the solid rock has been reached, it has been cut into deep and narrow gorges. Still higher up the valleys, the terraces gradually become more and more broken, narrower, and less thick, until, at a height of from seven to nine thousand feet, they become lost, and blended with the piles of fallen detritus.
I carefully examined in many places the state of the gravel, and almost everywhere found the pebbles equally and perfectly rounded, occasionally with great blocks of rock, and generally distinctly stratified, often with parting seams of sand. ”

“ The system of a platform of poles, hurdles, or fascine-work is well known, and has been proved to be effectual in preventing any dangerous lateral or vertical movement. Sand, fine gravel, ashes, or other binding material, spread over the surface of peat consolidates it and assists in preventing movement, and it is well if such a covering extends for some distance in front and at the sides of an embankment to keep the ground from rising. ”

“ And when we consider the vastness of the drift-deposits, extending, as they do, over continents, with a depth of hundreds of feet, it would puzzle us to say where were the sea-beaches or rivers on the globe that could produce such inconceivable quantities of gravel, sand, and clay. The production of gravel is limited to a small marge of the ocean, not usually more than a mile wide, where the waves and the rocks meet. ”

“ I learn more, studying over and over again the same Bagshot sand and gravel heaps, than I should by roaming all Europe in search of new geologic wonders. Fifteen years have I been puzzling at the same questions and have only guessed at a few of the answers. What sawed out the edges of the moors into long narrow banks of gravel? What cut them off all flat atop? What makes Erica Tetralix grow in one soil, and the bracken in another? How did three species of Club-moss—one of them quite an Alpine one—get down here, all the way from Wales perhaps, upon this isolated patch of gravel? ”

“ It goes fast enough at first to carry still coarse gravel; but the big angular blocks of rock have been dropped, and the stones it now leaves in its bed are -55- smaller, and become rounded and smoothed as it goes farther and farther across the plain. At many places it deposits gravel or sand, more especially at the inner side of the curves which the stream makes as it winds down the valley. When the stream runs low in summer, strips of bare sand and shingle are seen at each of these bends; and the stones are always well smoothed and lie on the whole regularly. ”

“ The implements in each are substantially of the same age and character; the matrix of red gravel, in which they rest, is of the same composition; the beds rest directly upon the eroded surface of the chalk or the gault, and are more or less overlain by sands of the same description. But it is incredible that such deposits (if of the same age) , should owe their origin to one and the same river; for if so, in order to reach the higher level, it must have been swollen to the height of 100 feet above the level at Broomhill and Shrub, and extended three miles to the south. ”

“ Through the center, where the lower disc of the open circles touched the ground, ran a deep bed of coarse gravel, covered with a thick layer of smooth round pebbles, forming a perfectly drained pathway about three feet in width which extended uniformly from one end of the archway to the other. Conforming to the contour of the arches, rising and receding in unison, this pathway was bordered on either side by what appeared to be a continuous terrace of three stone benches, each one foot high and of the same width. ”

“ This country having no high mountainous part in the middle, so as to give it a considerable declivity towards the shores and rivers, the gravel has remained in many places, and in some parts of a considerable thickness. But in other parts of the island, where the declivity of the surface favours the transportation of gravel by the currents of water, there is less of the gravel to be found in the soil, and more of the fragments of stone not formed into gravel. ”

“ In deltas and the lower portions of large rivers gravel deposits are comparatively rare and indicate periods when the volume of the stream was temporarily greatly increased. In the higher torrents also, gravels are rare because transport is so effective that no considerable accumulations can form. In most countries where the drainage is of a mature type, river gravels occur in the lower parts of the courses of the rivers as banks or terraces which lie some distance above the stream level. ”