when back-filling is dumped from excavator-buckets, that no frozen lumps fall upon the sewer.

The back-filling of trenches has been sufficiently discussed in Art. 54. When this is thrown in without ramming particular care should be taken that all pipe be first well covered with earth, since stones and frozen lumps invariably roll to the foot of the face-slope of the back-filling and might crack unprotected pipe.

It is frequently necessary to cut a sewer-pipe to a certain length or to split one in two to obtain a channel for a manhole bottom. This can be done with a cold-chisel and hammer, a light cut being made first entirely around or along the pipe and this gradually deepened until the pipe of itself breaks in two. The pipe is sometimes filled with sand well packed before the cutting is begun, but this is not necessary if care be used.

ART. 74. BUILDING MASONRY SEWERS.

Circular or egg-shaped masonry sewers may consist of a ring of masonry of uniform thickness throughout, or this ring may be much thicker in the arch than in the invert, or there may be invert-backing masonry resting upon a platform foundation or filling the irregular spaces of a rock cut. If the sewer comes under either of the first two cases it is usually made entirely of either brick or concrete, owing to the expense of dressing stone to make tight work in comparatively thin rings and to give a smooth interior surface. For massive masonry, as in invert-backing or heavy arches, stone can be used and is in many cases cheaper than brick. In some instances concrete may be cheaper and better than either.

A simple ring invert can be used only where the soil is firm and compact enough to stand when given the shape of the outside of the invert; such as clay, pure or mixed with sand or loam. If it will not retain this shape while the sewer

is being built, but is solid enough to offer good foundation, as damp sand, the bottom of the trench may be given a flatter curve and lined with a board or plank cradle, upon which concrete or stone masonry is placed for the invert-backing, to be lined with 4 inches of brick-work. In rock cuts the same plan may be adopted, since it is usually impracticable to bring the rock to the exact shape of the sewer (see Plate VI, Fig. 10).

If artificial foundation is necessary this usually consists of a platform, upon which the masonry rests, and which is placed directly upon the trench bottom or supported upon piles.

If the arch is of such dimensions that the thrust is more than the banks can be trusted to sustain, and a shape similar to that shown in Plate VI, Fig. 5 or 9, is adopted, concrete or stone masonry may be used for the side walls, and a platform is generally necessary for foundation except in a rock trench.

Where no invert-backing is necessary the method usually employed is as follows: Templets, two for each gang of masons, are provided conforming to both the inside and outside shape of the sewer. A convenient form is shown in Fig. 22, which is for two rings of brick. This is made of A

FIG. 22.-TEMPLET FOR BRICK SEWERS.

boards or plank, 2-inch plank being sufficiently heavy for any but very large sewers. A templet for an egg-shaped sewer can of course be made in the same way. Each ring of brick

is represented in the templet by a layer of plank, its inside edge conforming to the inner surface of said ring. A number of fourpenny or fivepenny nails are driven along the edge of each plank at equal intervals, the space between them being the thickness of a brick plus that of the mortar-joint, usually about 2 inches. Each templet should be an exact duplicate of the other, including the position of the nails. At the exact centre A of the cross-piece a notch is cut or a nail driven.

When the bottom of the trench is about to grade one of these templets is set in a vertical position so that the centre of the cross-piece is exactly in the centre line of the sewer, the cross-piece level, and the inside of the templet at the proper grade for the sewer-invert. About 12 to 20 feet along the trench the other templet is similarly set, the sides of the templets containing the nails facing each other. If now a cord is stretched from any nail in one templet to a corresponding nail in the other the excavation should be exactly the same distance outside this as is the outside of the templet. If the excavation should be carried too far it must be filled with sand well rammed, or with good cement mortar.

The cord is now stretched between the lowest nails in the outer rings of the two templets, and the brick laid to this line from end to end. The cord is now shifted to the next nail in the same ring and the next row of brick laid. When two or three courses have been laid on one side of the centre the same number are laid on the other side, and both sides of the sewer are carried up simultaneously, for which reason the masons usually work in gangs of 2, 4, 6, or 8. Not more than the last number can work to advantage on one section of invert, but several sections may be under construction simultaneously.

When four or five courses have been laid a plank is placed on these for the masons to stand on, and the brick-work is continued row by row, each row being laid carefully to line.

The bricks of succeeding courses should break joints at least

3 inches.

After the outer ring has been completed to the springingline the next is laid in the same way. The bricks of each ring should be bedded in mortar at least inch thick, and every joint should be completely filled. Considerable difficulty will be found in getting any but experienced sewermasons to lay the brick radially, but smooth work cannot be obtained otherwise and this must be insisted upon. joints should be carefully struck. If they are not they should be afterward raked out and pointed.

All

If the brick do not absorb more than 2 or 3 per cent of water in the absorption test they should not be wet, as they cannot then be made to stay in place. But if they take more water than this, they should be wet just before using. A quick test for this on the ground is to drop a brick into mortar and remove it. If the mortar does not in two or three minutes grow dry where it touches the brick they probably do not need wetting.

The mortar is usually mixed in a box on the bank (it should never be mixed on the ground for any purpose) and lowered into the trench in a pail by a rope provided with a hook, where it is emptied onto the mortar-boards. These boards are usually 24 to 30 inches square. The brick is

7812

placed in hods on the bank and lowered to the masons. A convenient form of hod is shown by Fig. 23, which is made of sheet iron and can be quickly filled and emptied. The material is usually lowered by hand for small sewers, and the man who does this should have a heavy leather palm-piece for each hand. FIG. 23.-HOD FOR A leather glove or mitten would not last a LOWERING BRICK. day of hard usage at such work. To permit of lowering the material a platform is usually thrown across

-8"

th, trench above where the masons are working. If stone is being laid, or much material is to be used at one place, or the trench is quite deep, the material may be lowered by a windlass set in a portable frame or by a derrick. If excavatingmachinery is being used this may be utilized for lowering the material.

As the invert of the sewer rises it becomes difficult for the masons to lay the brick, and the material if in the bottom of the sewer is too far from the work. A platform is then necessary and can be made by sawing plank of such length as to be at the desired elevation when placed horizontally crosswise of the sewer. Three or four of these can be thus laid, with a few brick under the centre of each as additional support,

#

FIG. 24. MASONS' PLATFORM FOR BRICK SEWERS.

and a platform of loose plank placed over them. But this is apt to distort the green brick-work at the ends of the crossplank, and it is better to have a number of plank cut to the shape of the sewer-invert cross-section, which will distribute the load along their entire length, and to rest the platform on these (see Fig. 24).

When one section of invert is completed one of the templets is moved ahead the length of a section and set. The other will not be needed by the masons, since one end of the cord will be fastened to nails stuck into the joints of the invert just completed. The second templet can, however, be used to advantage for grading the trench ahead of the masons.

In bonding the new work with that previously laid (the