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which the ground should be formed for side embankments, by cutting the level steps a a a, and shows where the drains should be made.
The slopes at which cuttings and embankments can be safely made depend entirely upon the nature of the soil. In the London and plastic clay formations, it will not be safe to make the slopes of embankments or cuttings, that exceed four feet in height, with a steeper slope than three feet horizontal for one foot perpendicular. In cuttings in chalk or chalk marl, the slopes will stand at one to one. In sandstone, if it be solid, hard, and uniform, the slopes will stand at a quarter to one, or nearly perpendicular.
If a sandstone stratum alternates with one of clay or marl, as represented in Plate II. fig. 6., it is difficult to say at what rate of inclination the slopes will stand; this will, in fact, depend upon the inclination of the strata. If the line of the road is parallel to the line of the bearing of the strata*, in such cases, large masses of the stone become detached, and slip down over the smooth and glassy surface of the subjacent bed. There are many instances of slips in sandstone and marl strata, under such circumstances as those now described, where the slopes are as much as four to one. If the road is across such strata, or at right angles to
* The line of intersection of any inclined stratum with the horizontal plane is called the line of bearing of that stratum, or the drift line. The dip, or inclination, of the stratum is the angle formed between a horizontal plane and a line drawn at right angles to the drift-line on the bed of the stratum.
the line of bearing, then the slopes may be made one and a half to one, as represented in Plate II. fig. 7; but if the strata are horizontal, even should there be thin layers of marl between the beds of stone, as in Plate II. fig. 8., the slopes will stand at a quarter to one. But it will be necessary, if the beds of marl exceed twelve inches in thickness, to face them with stone.
In the Oxford clay, which covers so great a portion of the midland counties of England, the slopes should not be less in any instance than two to one, and even in some parts of this formation they should be made three to one, if the cuttings are deep. In all such cases, if any beds of gravel or sand are found intermixed with the clay, as shown in Plate II. fig. 9., drains should be cut along the top, and even in the sides of the cuttings; for, if this precaution be not taken, the water, which will find its way into the gravel, will, by its hydrostatic pressure, force the body of clay down before it, and slips will take place even when the inclinations are as much as four to one; and when this occurs it is extremely difficult to re-establish them.
In limestone strata, if solid, slopes will stand at a quarter to one; but in most cases limestone is found mixed with clay beds, and in such cases the slopes should be one and a half or two to one. In the primitive strata, such as granite, slate, or gneiss, slopes will stand at a quarter to one.
Before quitting this subject, it is proper to remark, that in every instance of deep cutting, the greatest pains should be bestowed in examining the character of the material to be removed; much difficulty will be avoided by proceeding in this way: but, on the whole, the best general rule to follow is, to lay out a line of road so as to avoid as much as possible deep cuttings and high embankments; they are always attended with great expense, and are unavoidably liable to many objections.
The footpaths should be formed at the same time as the bed of a road; also the fences, if they consist of mounds of earth or ditches: but these will be more particularly described in a subsequent chapter.
The following directions are taken from specifications according to which parts of the Holyhead road have been made.
"The black line on the section represents the natural surface of the ground, in the longitudinal direction of the new line, at about the middle of the space to be occupied by the road. The red line represents the proposed finished longitudinal surface of the bed, or what the road materials are to be laid upon; the red figures denote the depths of cuttings and the heights of the embankments, and also the rates of the inclinations: these rates of acclivity are to be strictly adhered to, and it is expressly stipulated that the contractor is to satisfy himself by his own measurement, or in any way he may think proper, as to the heights and depths, or any irregularities, of other parts of the surface of the ground to be cut down or embanked, or where there is to be side-cutting and forming, as no future claim on any pretence whatever will be allowed.
"The breadth of the finished road is to be thirty-six feet; viz. thirty feet for the carriage way, and six feet for the footpath. The slopes of all embankments from the outside of the quick borders are to be two horizontal to one perpendicular, neatly dressed and covered with green sod at least four inches thick, evenly laid, and closely jointed.
"The slopes of the cuttings on the southern sides are to be three horizontal to one perpendicular, and those of the northern side to be two horizontal to one perpendicular; these slopes are to be correctly formed, neatly dressed, and covered with a good vegetable sod, the green side placed uppermost, and neatly jointed, and evenly laid, and to be at least four inches thick.
"The surface of the bed for the materials of the carriage way is to be formed level from side to side, the breadth between the bottom of the side slopes in the cuttings at the level of the bottom of the road materials is to be thirty-one feet.
"The surface of the bed for the hard materials of the footpath is also to be level, and to be seven inches above that of the carriage road. The necessary breadth will be gained by the road materials resting on the sides of the slopes.
"Where there is to be cutting in the side of a hill, the slope of the bank is to be two horizontal to one perpendicular: the embankment is to be secured by cutting the slope of the hill below the line of road into level steps to receive the earth, and the road materials are not to be laid on the embankment until the inspector is satisfied it will stand."
"In each of these lots the contractors are to make the line of the road agreeably to the plans and longitudinal sections made and signed by Mr. Telford, as laid out upon the ground by him, or such person as he shall appoint. The breadth, shape, and construction shall be according to the particulars, and the cross sections for construction made out by Mr. Telford; that is to say, on level ground the bed shall be formed by removing the vegetable and other soft matters, and brought to a perfect level and consolidated state. If the ground is soft bog or morass, and less than four feet in depth, with hard ground below, the soft matter shall be removed; but if a greater depth, the whole surface shall be covered with two rows of swarded turf, the one laid with its swarded face down, and the other upwards. Where the road is. formed on sloping, it shall either be cut for the whole breadth into the solid bank, with as much more as to afford a solid foundation for a fence