« AnteriorContinuar »
should be built in the same way as retaining walls, and should increase, from one foot six inches in breadth at top, at the same rate as has been described for retaining walls.
These walls should have a strong coping of large stones, set on edge in mortar of the best description.
The following is a specification of a breast wall built across a very deep hollow in North Wales, on the Holyhead Road (Plate IV. fig. 4.) : —
"Across the hollow there is to be a breast wall built, in good lime and sand mortar, along the foot of the lower slope of the present road, or thirty feet distant from the retaining wall. This breast wall is to be two feet and a half thick at top, and to increase in thickness downwards at the rate of two inches and a half for every foot of depth, by a regular batter on the outside. There is to be a four-feet parapet wall on the top, two feet thick at the bottom, and eighteen inches at the top."
These walls may be be built without mortar, if the stones are flat bedded.
As their stability depends upon the workmanship, great care should be taken to have the stones properly selected, and laid in a correct and regular manner.
A coping should be made on the top of these walls; it should be of large stones set on edge, and laid in good mortar.
When walls are used for fences on embankments, they should always be built with mortar, or otherwise the shaking of the road will in most cases loosen and throw them down.
The following is a specification for stone fences on the Holyhead Road: —
"The stone dykes are to be four feet six inches high above the side drain; they are to be, when placed on breast walls, two feet thick at the bottom, and sixteen inches at the top; and where there is no breast wall below them, they are to be two feet six inches thick at the foundation, and one foot six inches at the top."
Cross drains should be built of good masonry eighteen inches in the clear. (Plate IV. fig. 5.)
They should be continued under the fences into the ditches on each side of the road. When made of stone masonry the side walls should be sixteen inches thick, faced on both sides, eighteen inches high at the upper end, and twenty-three inches at the lower end. The top of the walls to be level, and the bottom of the drain to have an inclination of one inch in every ten feet. The stones at top on which the covers are to be laid are to project about two inches and a half into the open space on each side, leaving about thirteen inches clear between them: the covers to be stone not less than four inches thick and twenty-seven inches long; they should be neatly jointed and closely laid together, and properly bonded on the side walls: a concave pavement of stones, not less than five inches deep, should be laid between the side walls. The body of the building should be placed so low as to admit of six inches of earth to be laid between the cover of the drain and the bottom course of the road materials, without elevating the surface of the road.
The ends of the cross drain must be secured with a strong pavement, four feet three inches by two feet three inches; the paving-stones below the discharging end should be of large stones, sunk so deep as to secure the whole from being injured by the current of water.
When a cross drain is connected with a watercourse, the upper end should be secured with wing walls, at least five feet in length, and there should be similar walls at the lower end. These wing walls should be covered with two rows of swarded turf, the lower one with the swarded side down, and the upper one with the swarded side up.
The following is a specification of a cross drain, five feet diameter, built on the Holyhead Road:—
"The arch to be hammer-dressed coursed work, and the rest of good sound rubble-work. It is to be in length the full breadth of the road and dykes. The faces to range with the faces of the breast walls, and the dykes to be continued over them.
"Water wings are to be built into and extended from each abutment for eight feet in length, and to splay back to eight feet apart at their extremities. They are to be founded at the same depth as the abutments, and be carried up to the level of natural ground. A stone pitching to be set between the abutments and water wings; to be set endwise to the streams, and be firmly secured at each extremity. Except the stone pitching, the whole is to be built in good lime and sand mortar. The thickness of the water wing walls to be the same as specified for the breast walls."
Specification for a Three-feet Stone Drain.
"The arch to be hammer-dressed, and the rest of the masonry good sound rubble-work. The abutments must be continued as water wings above and below the arch, for five feet in length, and be splayed back at their extremities; to be founded as low as the abutments, and rise to the springing of the arch. A dry stone pitching to extend under the arch and between the water wings.
"Except the pitching, the whole to be set in good lime and sand mortar up to the level of the roadway; to be the full length of the breadth of the road and dykes. The faces to range with the faces of the breast walls, and the stone dykes to be continued across the arch in the usual manner."
The water from the side channels of a road should be introduced into the cross drains by side openings or inlets; these should be built with stone masonry, and be ten inches by sixteen inches, and covered with sound flags, at least twenty-six inches long and sixteen inches broad, and two inches and a half in thickness. The top of these covers should be six inches above the level of the sides of the channels, and the whole of the inlet should be built outside of the side channels, as shown in Plate IV. fig. 6.
Inlets may be made along the side channels, and covered with iron grates eighteen inches or two feet square: the bars of the grates should be three quarters of an inch broad, two inches deep, and one inch apart, if made of cast iron: if the grates are made of wrought iron, it is usual to set the bars in an oak curb; but the cast-metal grates are found to answer better, particularly if bedded in stone or on brick curbing. In some situations it is found necessary to leave an opening or inlet under the footpath, as first described, as well as the grate, to allow the water to get off in thunder-storms: a provision is also sometimes made in the casting, to allow the grates to be turned up on a hinge, in case of sudden and large runs of water.
Outlets are necessary to receive and carry off the water from the side channels of the road. These outlets may be built of brick or stone: in most cases they should be about one foot square; when they are for the purpose of carrying the water from the channels into the side drains, on grounds nearly level, they may be made of large six-inch diameter tiles or iron pipes. There should be an outlet at the end of every cutting, to allow the water that