These drains should consist of salt-glazed plain pipes 2 to 3 inches in diameter, laid about 15 to 18 inches below the formation level at centre, and properly connected to the side drains. The junction is preferably made at the highest point of the pipe in the side drain, and an opening

FIG. 18.-Diagram showing arrangement of subsoil drains in retentive soils. formed of dry stone built, over where they join, up to formation level for inspection from time to time. Ordinary field or agricultural drain pipes are sometimes used, but being porous are liable to injury by frost, and cause great inconvenience and damage by breaking, as well as a temporary derangement of the drainage of the subsoil, which is sometimes difficult to locate. The trenches should be filled up with clean gravel or small stones, free from all clay or other soft matter, care being taken to hand-pack the material round the joints of the pipes.

The diagram at fig. 19 shows a cross-section of a road with the mitre

Formation

level

drains at a, and joining the side drains at b, and a cross-section of the mitre or transverse drain is shown at fig. 20. It is not essential that these cross drains should have a great fall; the purpose may be properly served by giving them an inclination of 1 in 50. The nature of the soil will afford the means of determining at what distance apart these cross or mitre drains should be placed; this may vary from 15 feet in very wet situations, increasing in distance to 30 or even to 50 feet where the subsoil is of a drier and less retentive nature. The surface water from the roadway is collected in the side channels and conveyed, where the level of the roadway is at or near the natural surface of the ground, to the ditches, by outlets cut at certain intervals on either side of the roadway. Where footpaths exist alongside of a road, drain pipes are laid across and underneath the footpath at convenient points, the outer edge of

FIG. 20.-Cross-section of mitre drain.

these pipes at the roadside being protected from damage by passing vehicles, by fixing a stone at each side of the drain pipe well into the ground, and covering them with a flat stone projecting 2 inches beyond the conduit, but in line with the edge of the footpath.

62. On embankments, especially in sidelong ground, the surface water, as already mentioned, is conveyed to the open ditches in wooden troughs, creosoted, or tarred and sanded, and fixed in the slopes. This method, although satisfactory in many cases, is not to be recommended as a permanent arrangement; a drain constructed of salt-glazed faucet and spigot pipes laid in clay puddle, or covered with concrete and finished at the toe of the slope with masonry, is required to keep it in position.

It is necessary to exercise great care in carrying out the work by placing puddle around, or otherwise protecting the joints, so as to ensure that no water will be permitted to percolate through the earthwork, which, if allowed, would in all probability result in considerable damage being done. to the embankments.

FIG. 22.-Diagram showing sand trap.

63. Drains in Cuttings and Different Arrangements of Sand Traps or Gullies. In cuttings, the system followed is somewhat different, being generally effected in the following manner. The side drains are utilized as a means for the disposal of the surface water which, in the first instance, enters a gully or sand trap, the overflow from the latter being connected to the side drain. The use of a sand trap, especially on a road having a considerable fall longitudinally, is to intercept any heavy matter or detritus which would otherwise gain entrance to the drain, and in course of time, by repeated accumulations, cause the drain to become choked. The simplest and cheapest form of sand trap is represented at fig. 21, built of brickwork with suitable covers, and at fig. 22, which is made either of earthenware or cast iron. Stoneware gullies are, as a rule, liable to damage by the passing traffic, and should be placed in the least exposed situations, or clear of the water channels. This form of trap is also made in detached portions, the basin or stoneware part forming the trap, with a cast iron cover and grating

fixed on the top. It is claimed that this form of gully trap is unbreakable under the heaviest class of traffic. Sand or gully traps are also made of cast iron in one piece and in different sizes to suit different localities. The gratings are detachable, and, on the whole, this class of trap is the best possible to adopt, and serves most effectively the purpose intended. This class of gully or sand trap is shown in fig. 23.

When one road crosses another, it often happens that two or more drains have to be connected to one gully or sand trap. This is most conveniently accomplished by building it of stone or brick

work, with a pavement or concrete floor of a sufficient area to embrace the side walls and form a foundation. The floor should be at least 18 inches below the level of the outlet pipe, to collect the detritus or heavy matter. If the trap be built of brick the walls need not exceed 9 inches in thickness, unless the drain is exceptionally deep, when it will be advisable to increase the thickness of the walls. A convenient size of trap for such a situation, with three inlet pipes, is shown in fig. 24. The brickwork is brought up to within a few inches of the level of the water table, the covering and grating being either of cast iron fitted properly into the brickwork, or it may be finished with a stone cover, and a wrought

FIG. 23.-Cast iron sand trap or gully.

iron hinged grating dished out to admit of the water from the channels passing freely into the sand trap. A space nearly equal in size to the iron grating requires to be cut out in the centre of the stone cover, provision being made for properly fixing the grating to it by means of batts.

These sand traps should be cleaned out periodically, and also immediately after a heavy fall of rain, so that the road detritus and sand may be prevented from entering the outlet pipe. The inlet

pipe or pipes should be placed at a higher level than the outlet pipe, to ensure a rapid discharge; 2 to 3 inches being quite sufficient for this purpose.

64. When it is necessary to lay a drain under a road, the work should be of a substantial nature, so as to avoid any possible derangement of the drainage, and probable damage to those using the road, by its failure.

Drains for conveying water under a road should be built of good stone and bedded in cement, with paved floor and substantial covers. Salt-glazed or earthenware spigot and faucet pipes are also employed; these should be cement-jointed, and if necessary protected with a layer of concrete under and round the outside of pipe.

65. Retaining Walls.-Retaining walls are structures in masonry, laid dry or in mortar, designed in various forms to support the sides of roads on hillsides, or when land is not available for forming side slopes beyond the formation level of the road. Where building materials of sufficient size can be procured, the retaining walls may be built of stones laid dry; when, however, the materials for this purpose are not large, flat-bedded stones, it is advisable to use mortar in building the walls.

With suitable material, drystone retaining walls are well adapted for road work, on account of their self-draining properties and their cheapness. The outside face is generally built with a batter, either straight or curved, varying from 1 in 8 inches to 1 in 12 inches; any excess of batter greater than 1 in 6 should be avoided where mortar is used, as the action of the rain tends to destroy the joints by washing it out, and affords lodgment for seeds of vegetation, the roots of which penetrate the wall and injuriously affect the adhesion between the mortar and stone. Retaining walls built with stones and mortar are rendered more secure from the effects of water by having drystone packing, usually 12 inches thick, set by hand at the back of and between the wall and earthwork. Weeping holes, formed of 2- or 3inch pipes, should be built into the wall at different levels; but especially at the base of the wall, and at a distance of from two to four yards apart, according to the nature of the soil and the dip of the strata.

The top of the retaining walls should be properly finished and protected by a coping of stone, close jointed, to prevent rain finding its way into the body of the wall.

Drystone retaining walls are generally finished on the top with a sod coping laid in two rows; when properly carried out, this method of protecting the walls suits the purpose exceedingly well. The profile or crosssection, and the thickness to which retaining walls are built, vary according to their position relative to the adjoining work and the nature of the material to be retained.

The foundations of retaining walls are subject to considerable and unequally-distributed pressures. They should be of a width or area sufficient to bear safely the pressure which they have to sustain according to the nature of the ground on which they are placed. A suitable drain should be laid at the base in front of a retaining wall to collect and convey any water escaping from behind the wall. Ordinary foundations may be classed under the following heads: (1) On material, the stability of which is not impaired by saturation with water, such as rock; (2) on firm earth, sand, gravel, and

hard clay; and (3) on soft earth. The area of the foundations may be increased to any desired extent by forming steps beyond the thickness of the wall proper, so that the weight of the structure may

Rock of a modesafely 9 tons per tons; soft earth,

be distributed over a large surface. rately hard description will bear square foot; firm earth 1 ton to 1 on the other hand, is only capable of bearing a very limited load; and to ensure safety the ground should be well drained, the soft earth removed by cutting a trench and substituting stable material, such as sand or concrete. In extreme cases it is necessary to drive short piles into the ground to compress and consolidate the soil, on which a platform is fixed, the heads of the piles being surrounded with concrete. The depth to which a foundation should be excavated must be such that it will be below the disintegrating influence of frost: 3 to 4 feet is considered a safe depth in Britain, while in countries subject to severe and long-continued frost a depth of from 4 to 6 feet is necessary.

14' 0"

Radius 80 feet

FIG. 26.-Section of retaining wall (built of brickwork).

66. Dimensions of Retaining Walls. -In determining the proportions to be given to retaining walls, the engineer is guided by the experiments, and from the successful results attained in existing structures which have withstood for a considerable period of time all the usual causes of destructibility. From these examples many practical rules have been deduced for the design of stable and economical retaining walls, as well as for the thickness of abutments and arches of bridges and culverts.

For retaining walls 6 to 10 feet high a thickness of one-fourth of the height of the wall above the natural ground is generally allowed. In large structures built of masonry, the height of the wall above the ground level is divided into offsets, and the thickness is varied from one-third to one-eighth of the total height, as shown in fig. 25.

The following dimensions are extensively used in building retaining

E