walls with brick, viz., the thickness at the top of the wall is made 1 foot 10 inches, and at every fifth course downwards the thickness is increased by half a brick, the face of the wall being built with a batter of 1 in 6 to 1 in 8, which is generally curved as shown at fig. 26. In retaining walls constructed of stone without mortar, the thickness may vary from one-third to one-half the height, the face being built perpendicular or with but a slight batter, to avoid as much as possible rain-water being carried inwards to the earth backing, which would occur if the joints were inclined by giving the face an excessive batter. Should it be found necessary to batter the face of a wall built dry, the difficulty alluded to may be obviated by pointing the outside joints of the wall. Trautwine's rule for a retaining wall, which sustains a backing of sand, gravel, or earth, level with its top, is, that the thickness of the wall should be not less than that stated in the following table, when the foundations are about 3 feet deep, the backing being deposited loosely, as when tipped from carts, wagons, etc., and the batter not exceeding 1 inches to a foot.

TABLE VIII.

Wall of cut stone or first class large-ranged rubble in
mortar,
Wall of good common scabbled mortar rubble or brick, ⚫4
Wall of well-scabbled dry rubble,

in}

35 of its entire vertical height.

•5

The effect of stepping the back of a wall increases the friction of the earth against it, giving increased stability, so that when the thickness has been proportioned with a vertical back by Trautwine's rule, it may be converted into one having offsets, which has the advantage of greater resistance to overturning, and yet contains no more material. The method of arriving at this is as follows:-From the point c, fig. 27, half the height of a b, draw a line from d one-half of the thickness of the wall at the top a a, through c to e, and from the latter draw the vertical line eƒ; divide d f into any number of equal parts, for instance three, and produce vertical lines to the base be; then divide ƒ e into one more equal part than ƒ d contains, and draw horizontal lines crossing the vertical ones. From the points of division draw the line forming the offsets as shown in the figure.

67. Surcharged Retaining Walls.-When a retaining wall is surcharged, that is, retains a bank which slopes backward to a higher level than the top of the wall, the thickness, according to the foregoing rules, must be increased.

This class of wall, fig. 28, is principally made use of in cuttings, but in some cases walls of this description are built to retain the toe of an embankment. The following table, given by Molesworth, shows how the thickness of a surcharged wall of rectangular shape may be arrived at.

The height Y is substituted for H, in fig. 25, or height of the wall from ground level, and is the perpendicular at the end of a line L equal to H, measured along the slope to be retained. The value of Y for various slopes is as follows:

68. Counterforts.-Counterforts are projections of masonry at the back of a wall, built at intervals, and of a rectangular shape; they are intro

duced to increase by their weight the stability of retaining walls, but this depends on the efficient manner in which these are bonded to the retaining wall at their junction.

69. The result of the experience of Sir B. Baker in connection with the building of retaining walls may be given. "In ground of an average character the thickness of retaining walls should be one-third of the height from the top of the footings. A wall quarter of the height in thickness. and battering 1 to 2 inches per foot on the face possesses sufficient stability when the backing and foundations are both favourable." He also states that "under no ordinary conditions of surcharge or heavy backing is it necessary

to make a retaining wall on a solid foundation more than double the above or half of the height in thickness."

70. Breast Walls. Breast or revetment walls are very useful in cuttings. They are generally employed to act as a covering for earth which has sufficient stability to stand vertically alone. They need not be of great thickness, provided they are of sufficient strength to resist their own weight; it is advisable to raise them above the level of the ground where the slope joins the wall, in order to prevent stones and earth, which may become detached by the action of the weather, from falling on the roadway.

71. Culverts.-Culverts are necessary to carry a roadway over a watercourse, and for conveying the surface water from the road, as also that from the drains in cuttings and open ditches running parallel to the axis. of the road, which are conveyed and discharged into the stream.

72. Waterway of Culverts.-Care should be exercised in determining the waterway necessary for culverts under varying conditions, so that the opening may be built slightly in excess of the actual requirements. If a culvert is built too small, it is liable, during floods, to be washed away, along with possibly a considerable portion of the embankment, causing interruption of traffic and heavy cost for repairs.

Culverts of unnecessarily large dimensions involve a needless increase in the cost of construction. The area of waterway required depends principally on the following considerations :

The maximum rainfall during a severe storm.

The nature and condition of the soil, and the character and inclination of the surface.

The area to be drained, with special regard to the number and position of the branch streams which join that under consideration.

The annual recorded rainfall for any particular district does not aid the engineer to any extent. An average fall for a given period does not necessarily mean that a fixed quantity has fallen each day; it is possible that the total recorded amount in a certain time may have fallen on one particular day, and probably only during a few hours of that day. A thaw and heavy rain succeeding a fall of snow when the ground is frozen, and a heavy rainfall during a thunderstorm, are certainly the worst possible conditions to be provided for in practice.

The hard nature of the surface of the ground under the former conditions precludes any moisture being absorbed by the soil, and this circumstance, combined with the falling rain and melting snow, increase the difficulty of calculating the area of waterway necessary in any particular case. Data of a more or less valuable nature may be obtained by observing the flood-mark of a stream where it is confined within its banks; and by taking a crosssection of the stream the actual area may be computed, from which it is possible to determine with a reasonable degree of accuracy the necessary area

of waterway. Should any other culvert be built over the same stream within a reasonable distance of the proposed crossing, the area of the existing opening should be measured, which will guide the engineer, to a certain extent, in determining what waterway will be necessary. The evidence also of local parties when available may likewise be of great assistance.

73. Materials used in Building Culverts.-Culverts may be constructed of stone, brick, concrete, iron pipes, trough decking on stone or brick abutments, or of salt-glazed or earthenware pipes. The simplest form in which a conduit built of stone can be made is shown in fig. 29. When suitable material is plentiful, this form of conduit is extensively employed. This is, however, an expensive method of construction, when high embankments are formed over the conduit, as, owing to the great weight, large covers of considerable strength are necessary, varying in thickness up to 12 inches. The side walls also require to be built of dressed stone, as the water flowing in the conduit

20 2.0 20

Stone Cover
S"to 12" thick

FIG. 29.-Section of conduit built with stone.

would eventually find its way through the joints, if inferior masonry was used, and possibly wash away the earth backing by being thus softened. This may result in the failure of the structure and consequent stoppage of the traffic, as well as cause accidents, involving the authorities in considerable expense. When of a small size, and therefore difficult to get at, repairs cannot be carried out without stripping the material off the road. Experience of the maintenance of roads clearly indicates that conduits built of ordinary rubble, and with covers of insufficient strength, combined with the inferior quality of the stones often employed, cause great trouble and expense for repairs.

74. Salt-glazed or earthenware pipes are now generally made use of for conveying small streams under a road, and are a reliable substitute for the class of conduit just described, providing they are properly laid and jointed and the sides and top of the trench are filled in with suitable material. These drain pipes vary in size up to 24 inches in diameter. They can be laid in position quickly, and are comparatively less expensive than masonry conduits of a similar capacity. Two or more lines of earthenware pipes may be laid alongside of each other when the capacity of one is insufficient. The inlet and outlet ends of the smaller-sized pipes should be protected at the toe of the embankment by building round the outside of pipe flat-bedded stones finished with a sod cope on top. The large-sized class of earthenware pipes are generally protected by forming a pitched slope or wingwall to retain the embankment in position, and likewise to assist in directing the current of water into the culvert on the up-stream side of road.

In laying these pipes care should be taken to ensure a straight line being preserved, and that the outside joints are properly filled and finished with cement; the inside joints also should be pointed when possible, and thoroughly wiped to remove any projecting particles of cement, and lessen the chance of obstruction. The trench should be filled up under and round the body of the pipes with material free from stones, such as sand, furnace ashes, or similar material, the whole being well punned. When earthenware pipes are situated under a high embankment, it is necessary to strengthen them by placing concrete under and around the outside

FIGS. 30, 31.-15-inch stoneware pipe drain or conduit.

of the pipes. The upper portion of the pipes should, as a rule, never be less than 2 feet below the surface of the road. Instances occur, however, where the conduit pipe has to be laid only one foot below the road surface in order to gain a sufficient fall, and in such cases the pipes must be bedded in and covered with fine concrete. This method should be avoided, if at all possible; but if it cannot be so, the expedient of raising the road level should be adopted, which is an expensive alternative. The outlet or tail-end of conduits should have a sufficient fall, so as to discharge the water quickly, and prevent back

water reaching the pipes. The freezing of water in the pipes, when such a condition exists, tends to destroy them, so that a proper inclination for both the pipes and the outlet of the stream is imperative.

75. The diagrams, figs. 30, 31, 32, and 33, show the method of construction adopted with earthenware pipes described in the preceding paragraphs.