end of which should be toothed or racked back) all loose brick and mortar should be removed, and the brick cleaned and wetted before applying fresh mortar.

The arch of the sewer is built upon a centre," which is removed when the arch is completed and the mortar suffi

consists of lagging supported Probably the most common To templets similar in form

FIG. 25.-CENTRE FOR BRICK SEWERS. ciently set. The centre usually by curved ribs of wood or iron. form is that shown in Fig. 25. and general construction to those for the invert are nailed lagging-strips about 1 inch thick and 1 inches wide, spaced 2 inches between centres, there being a templet at each end and intermediate ones spaced 3 or 4 feet apart. The laggingpieces should be perfectly parallel, as their edges are used for lining the brick-work. If the radius of the arch exceeds 2 or 3 feet the lagging may be of 1- or 2-inch by 3-inch strips, spaced 4 inches between centres; but the 24-inch spacing will give a better surface whatever the radius. The templets should lack 3 or 4 inches of being complete semicircles, so that when in position the bottom of the centre may be about 1 or 2 inches above the springing-line of the arch. The centre may be held in position by a triangular frame under each templet, supporting a plank along each side of the sewer, upon which the centre rests, it being raised to exact position by wedges, as shown in Fig. 25. When the arch is

completed the wedges are knocked out and the centre drops onto the two planks and can be pulled forward, sliding upon these. It is sometimes difficult, particularly with large and heavy centres, to draw them out, and to facilitate this a light temporary track has in some instances been built under the centre, which was placed upon wheels which rose 2 or 3 inches above the track when the centre was wedged up into position. By knocking out the wedges the centre drops onto the track and can be readily rolled forward. The use of rings of angle-iron to support the lagging gave good satisfaction in Denver, Col. (see Transactions of Am. Soc. Civil Engineers, vol. XXXV, page 113). For very large sewers it may be better to build each centre in place and take it apart in order to move it.

The arch should be built up at a uniform rate on both sides at once, and the last row of brick to be laid in each ring should be at the crown and should be driven tightly into place as a key. It may be necessary to split brick for this purpose, but it is better to have on hand a number of thin arch-brick (of wedge-shaped section), hard and tough, which will stand driving. The outside of the arch is usually plastered with to inch of mortar. The centre should be left under until the mortar is so set that there is no danger of the arch becoming deformed if it is drawn, the time varying with the character of cement, shape and thickness of the arch, and other details of construction. It is probably well in most cases to back-fill to the crown of the arch as soon as it is completed. But if the soil is wet, like muck, or if, when excavating-machinery is used, the buckets usually contain considerable water, no back-filling should be done until the mortar is thoroughly set.

If the arch is of stone or concrete masonry lined with brick the brick ring is laid as described above and the stone or concrete built on top of it. The arch is sometimes built

of concrete without a lining, in which case the lagging-strips must be set close together. In the Wachusett (concrete) Aqueduct, I feet 6 inches in diameter, sheets of galvanized iron and zinc greased with black-oil were fastened over the lagging on the centres with good results.

After the removal of the centre the arch masonry will ordinarily be found somewhat uneven, with mortar adhering in flat lumps to a large part of its surface. These should be

removed and the joints so pointed as to render the surface. more even, or the whole inside of the arch may be plastered.

If there is masonry backing to the invert this is usually laid as uncoursed rubble or concrete up to within 4 inches of the invert-surface, the templet having been set to indicate this, and the brick lining is then laid as above described. If concrete is used and is not carried to the sides of the trench (see Plate VI, Fig. 9) a form of plank is used, inside which the concrete is rammed, and the plank removed when this is set. If the trench is sheathed and the concrete is built against the sheathing this cannot be pulled, but must be left in or cut off above the concrete. If stone masonry is used for invert-backing it is better to lay the course of stone next to the brick lining with radial beds.

If concrete is used for the entire sewer special forms must be made for each size of sewer, at least two sets being in use by each gang. The form for the invert may be made similar to an arch-centre, except that the lagging must make tight joints (its edges being bevelled to permit of this) and only the two or three on the bottom be fastened to the templets. This form is fixed in position, concrete is placed in the bottom, between the lagging and the earth, and rammed; one or two strips of lagging are then slipped into position on each side and concrete placed and rammed behind these; more strips are added and concrete rammed behind them, and so on until the concrete is brought to the springing-line of the

arch. The forms should not be removed until the concrete is set. There is much danger that in this invert construction dirt and stones will get into the concrete, to its detriment, and great care must be taken to avoid this. The forms must be strongly braced down from the bank, to resist their tendency to rise when the concrete is rammed. should be just wet enough to permit water to be brought to the surface by light ramming. No heavy rammers should be used.

The concrete.

For making a concrete arch, if there is no brick lining, a centre is used with close lagging, or an open-lagged centre may be covered with sheet metal, as on the Wachusett

FIG. 26.-FORM FOR CONCRETE ARCH.

Aqueduct mentioned above. The outside form may be con. structed as shown in Fig. 26, the forms being placed 3 to 5 feet apart, the lagging being loose and put in one strip at a time.

66

Concrete sewers have been built in a travelling mould (Ransome method), by use of which the entire sewer is constructed continuously, foot by foot. A core in the shape of a ribbon which can be readily withdrawn after use (Chenoweth system) has been used for small concrete sewers, to which the use of the ordinary centre and form is not adapted.

A 42-inch sewer was built in Coldwater, Mich., in 1901, with a monolithic concrete invert, the arch being of concrete voussoir blocks 5 inches by 24 inches on the intrados, which construction was found to be cheaper than brick, and satisfactory otherwise.

ART. 75. BUILDING MANHOLES AND OTHER

APPURTENANCES.

These can most conveniently be, and usually are, built of brick. The foundation is sometimes of brick, but concrete. is better in most cases. A stone slab set on concrete makes a good bottom for catch-basins.

The channel through a pipe-sewer manhole is sometimes built of brick, but a split pipe is better. If brick be used, the inside of the channel should be plastered with a coat of neat Portland cement. If any branch channel in a manhole is not to be used at once it should be temporarily closed to prevent deposits forming in it. The bench may be built up of brick plastered on top with cement, or of concrete. Or the whole manhole bottom may be of concrete, a wooden core being slipped into the opposite pipes and spanning the manhole to give the shape to the channel.

In leaving the manhole-opening in a brick sewer the end brick in every alternate course of the outside ring may be laid radially, thus presenting toothing protruding at right angles to the sewer-barrel. In this steps with horizontal treads can be built of brick trimmed to the necessary shape, from which the manhole can be carried up without danger of its sliding off the sewer.

To insure having the manhole of the proper size and shape a board templet may be used, being laid, in pipe-sewer manholes, upon the concrete foundation when this has set, and the brick started by it and carried vertically to the proper