76. Iron pipes are, under certain conditions, a very useful substitute for earthenware pipes and conduits built of stone or brick.

When the foundation is in soft material, a bed of concrete is formed, varying in thickness from 9 inches to 3 feet, according to the size of pipes and the solidity necessary to ensure stability. The joints when of spigot and faucet should be caulked; those having flanges should be bolted together, and any open spaces filled in with cement. The filling or backing should be carefully rammed to prevent the water forming a channel along the outside; the ends should also be protected by wing-walls or pitched slopes similar to the practice adopted in the case of earthenware pipes. The space between the wing-walls at either end should be pitched where any scouring action of the stream when in flood is anticipated, as by so doing the possibility of the foundations of the wing-walls being undermined is prevented, especially at the inlet end.

77. Arched Culverts. It is more economical to build stone or brickarched culverts when the waterway of a stream necessitates a span of over 5 feet. The arch may take the form of a semicircle, segment of a curved ellipse, or compound curve having different radii. The latter two forms are seldom employed in culverts of small span, that of the segmental arch being generally made use of. By adopting a flat arch, the embankment forming the approach to and over the stream may be reduced in height below what would be required if a semicircular arch were constructed.

78. The small convexity or rise of arch in relation to the span adopted at times in practice presents a weakness, while the horizontal thrust on the abutments, inseparable from this form of arch, is very great.

79. The semicircle should always be adopted in small spans, and whenever possible, in those of moderate span, as this class of arch offers advantages in simplicity of form, great strength, and small lateral thrust.

80. The employment of steel trough decking for culverts has of late years been greatly advocated and applied in practice; they present a form of great strength, and can be erected expeditiously. The necessity for making an embankment in the case of arches is to a great extent, if not entirely, avoided by adopting this method of construction. Steel trough decking of a light section suitable for small spans, such as that adopted in the building of culverts, is manufactured for spans of from 4 to 10 feet, the weight of which is from 20 lbs. per square foot upwards.

81. The depth or thickness of arch stones for culverts depends upon the span, the form of the arch, and the class and quality of material employed in its construction. These remarks also apply in a measure to the thickness to be given to abutments, which will be treated under Bridges.

82. Figs. 34, 35, and 36 show the different forms of construction usually adopted and described in the preceding paragraphs. Fig. 34 represents a cross

section of a 6-feet culvert built of brickwork or concrete, with semicircular arch and an invert, the finished road level being shown 2 feet above the arch. When the bed of the stream has a considerable declivity, and there

FIG. 34.-Section of 6-feet culvert built with brickwork or concrete.

is consequently a risk of wearing away of the bed during freshets, an invert between the abutments should be provided. The space between the wingwalls, and for a distance along the bed of the stream beyond the newels, should also be pitched to prevent the undermining of the structure.

Fig. 35 shows a culvert built of brickwork, the span being 10 feet, and having a segmental form of arch with a rise or versed sine of one-fifth the span.

This arrangement admits of a reduction in the height of the embankment in forming a road compared with a semicircular arch. The stream is assumed to have a gentle current, and the bed not liable to be scoured or worn by freshets. It will also be observed that a springer course of stones is introduced, which affords a more solid and efficient means of starting the building of the arch than would be possible by forming the skewback with bricks alone.

Fig. 36 shows the principle of steel trough decking applied to culverts, where the raising of an embankment would be necessary if a semicircular arch were adopted. The general arrangement is shown in the illustration, the trough decking occupying a depth of only 8 inches, the space between the ridges being filled with concrete, and the metalling applied to a thickness of 12 inches or an inclusive depth of 20 inches from the under side of beams to the finished surface of the road. With a semicircular arch the finished surface of the metalling would be at least 10 feet 9 inches above the belt or springer course: whereas, using the trough decking, there is a saving of 9 feet of unnecessary embankment, at the site of the structure, and which, if carried out, might cost from £50 to £200.

83. It is advisable to construct catchpits at the entrance to all drains and culverts in localities where, during a heavy rainfall, there is any likelihood of débris or boulders being washed down the stream. They should be about 3 feet deep, and of the full width of the water-course, and be faced with stones built dry or in lime mortar.

84. Bridges.-Bridge designing and building is a very extensive subject,

FIG. 36.-Section of 15 ft. culvert and approaches, showing relative levels of road with semi-circular arch and with trough decking.

and embraces the selection and disposition of various materials under different circumstances.

In a work on road making the leading features necessary to be observed in the erection of ordinary structures can only be touched upon. The author refers his readers for further information to the many valuable works published specially on this subject.

85. Materials used for Building Bridges.-Bridges may be built of stone, brick, concrete, cast iron, or steel; or a combined structure of masonry and steel girders may be adopted. Wooden bridges are sometimes erected in localities where timber is available, but generally these are of a temporary nature, and do duty until permanent structures are substituted.

86. Selecting Sites for Bridges.-The selection of a site for a bridge, unless it is predetermined by a line of road which cannot be changed, should be one which affords the greatest security for the foundations, and is otherwise the fittest for economical construction. The centre-line of the bridge should be, if possible, set out perpendicularly to the direction of the flow, and on a straight part of the stream or river to be crossed. Where the locality does not admit of crossing at right angles, the bridge assumes an oblique direction, which, besides adding to the difficulties of construction, greatly increases the cost of erection. When the width of the river to be bridged is great, it may be advantageous to divide the opening into bays by the erection of piers. The position of these must be parallel to the flow of the stream, otherwise the faces of the piers would be exposed to the effects of the water striking against them obliquely; they should be further protected by cut-waters or starlings in order to ward off any floating material such as ice or timber.

87. Waterway of Rivers.-As the area of the waterway will be to a certain extent reduced if piers are erected in mid-stream, provision should be made for this by increasing the total width of opening between the abutments, the object aimed at being to maintain the original force and direction of the current. If the cross-section is diminished it will cause an amount of damming back of the water, which, when it rises to a certain height, will increase the velocity. This may cause damage by flooding the adjoining land, or the bed of the stream may suffer by being worn away. On the other hand, a greatly enlarged cross-section will have the effect of decreasing the current, and causing the bed of the stream to become silted up with deposits from the higher reaches.

What may naturally be expected under such circumstances is the formation of banks, and should these accumulate to any extent, a diminished area or cross-section is the result.

With a series of bays, however, it is possible that only a few of these may be passing the water of the stream in its normal condition. The increased currents induced by the alteration of the bed of the stream will

have, during floods, a tendency to scour the bottom, and may eventually prove destructive to the structure by undermining the foundations.

88. Nature of Material of Foundations.-Before deciding on the design of a bridge, it is necessary to thoroughly examine the nature of the material on which the foundation of the structure will rest. The usual method of ascertaining this is by sinking trial-pits to a suitable depth, or by boring, and from the results obtained the class of foundation necessary may be determined. The nature of the different soils in connection with foundations are generally divided into three classes.

The first, or incompressible material composing the strata, includes rock and such other material the stability of which is not impaired when saturated by water.

The second class embraces such material as compact stony soils, dry gravel and sharp sand.

The third class includes all materials which are compressible or spread laterally under pressure, such as ordinary clay, the common earths, soft sand and marshy soils, which are usually in a more or less compact state, and sometimes in a semi-fluid condition.

On a foundation of rock the bed is prepared by levelling its surface, and removing all loose and decayed pieces, reducing all projecting points and filling up all hollows with flat rubble, masonry, or concrete.

Where the stratum composing the rock dips, or inclines to the horizon, it should be stepped, all inequalities being filled in with concrete. The surfaces thus prepared should be perpendicular to the direction of the pressure which the foundation has to sustain, and the courses composing such foundation should have an area sufficient to bear that pressure with safety.

89. Safe Load on Foundations.—The safe load allowed on foundations of the first class ought not to exceed one-eighth of the pressure which would crush it. For general purposes this may be reckoned at 9 tons per square foot on rock of moderate hardness, and 2 tons per square foot on soft sandstone; while rock equal in strength to good cement concrete can safely sustain a load of 3 tons on the square foot.

Foundations on the second class of materials have considerable frictional stability when the stratum is not affected by being saturated with water. These must be excavated to such a depth that the structure will rest on ground not likely to be impaired by the disintegrating effects of frost which has been pointed out under the article on culverts. The safe load or greatest pressure permitted in practice on soils for foundations of this class is generally from 1 ton to 1.5 tons per square foot. The foundation courses, or footings, are usually spread out to a considerable extent beyond the ordinary thickness of the walls they sustain, such breadth being as a rule made 1 and 2 times that thickness in gravel and on hard clay and sand respectively.