length of 24-inch hose with no coupling on one end can be placed at the end of a line leading from a fire-hydrant to and down a manhole, and this end pushed into the drain; when the water is turned on the hose can be pushed forward as if it were a flexible rod, and the water from the hose will wash the obstruction loose and bring it back to the manhole, where it can be removed from the sub-drain well by hand, the water rising up and overflowing into the sewer. Sand, gravel, and even brick-bats have been washed out of drains by this hydraulic process.

When laying a pipe sewer the manhole is not usually constructed until the sewer has been laid on each side of it. In quicksand if the trench is opened through where the manhole is to be it will immediately fill up above the sewer. The pipe must therefore be plugged at the end. It is, for this and other reasons, often desirable in quicksand to build the manhole before the sewer reaches it, openings for the sewer being left in the manhole-wall at the proper points. The excavation for the manhole must be made in a well-hole close-sheathed for at least a foot lower than the sewer-invert. It will be found difficult to get the bottom in with the ordinary methods, particularly if there is a sub-drain well to be put in. In such a case the following plan has been used with success: A 12- or 15-inch pipe with two T branches of the size of the sub-drain, temporarily plugged, is lowered into position to act as the sub-drain well, the bell being up and the branches being placed at the grade of the sub-drain, which connects into them. The manhole excavation having first been carried to the depth necessary for the foundation, this pipe may be lowered by resting upon it with the knees and digging the sand from the inside, care being taken to keep it vertical and in the proper position. When it has reached the required depth the sand is scooped out a little below its lower end and one or two bucketfuls of concrete placed there and rammed

(see Plate X, Fig. 9). It is well to place a board bottom inside the pipe on top of the concrete and to place brick on this to keep the concrete from being forced up, the brick being removed after the concrete sets. If necessary another 12- or 15-inch pipe is placed upright in the hub of this one. A length or two of drain-pipe is fixed in each branch of the sub-drain well in a horizontal position and in the proper line to connect with the sub-drain when laid, and the manhole bottom is then dug out to the grade of the bottom of the foundation and concrete placed there, before the sand rises, in small areas of 8 or 10 feet at a time. This is done rapidly and the concrete loaded with brick, if necessary, to hold it down. The concrete is placed last where the channel comes and a split-pipe invert is at once forced down in it to the proper grade, and a straight-edged plank placed on edge in the invert bottom and braced down from the sheathing to hold it in position. The formation of the manhole bottom is then completed and the walls built in the usual way, sewer-pipe being built into the manhole-walls where the sewers are to enter it, but loosely enough to permit of sliding the pipe out. The sewer already laid or to be laid is carried through this opening by a pipe cut to the necessary length, the sheathing having been cut away here to permit this.

Another plan is to lay a plank foundation for the concrete, one plank at a time being put in place and fastened to the sheathing, thus forming of the whole a tight box, in which the manhole is built. Flush-tanks, inlets, and other appurtenances can of course be built in the same way.

ART. 79.

RIVER-CROSSINGS AND OUTLETS.

For convenience of inspection and as permitting easier maintenance it is best to carry a sewer across a stream on a bridge or trestle, keeping its invert at the hydraulic gradient;

unless, of course, this is below the river-bed, when the sewer will occupy that position. Very often the use of bridge or trestle is impossible or prohibitively expensive, and then an inverted siphon is necessary. In either case the pipe will probably be of iron or wood, although a combination of these with masonry is sometimes used. In some instances it may be better to build the siphon in tunnel, when it should be lined with brick or concrete; or, as is usually better, two or more iron or wood siphon pipes may be laid in the tunnel, easy access to them being thus afforded.

A bridge or trestle for supporting a sewer should seldom be built of wood, owing to the difficulty of providing for the sewage when necessary renewals are being made. It may in some instances be unsafe to support a sewer by an existing

FIG. 30.-SEWER CROSSING CREEK ABOve Water.

bridge, owing to the great increase of load thus brought upon it. (An 18-inch cast-iron pipe flowing full of sewage will weigh about 225 pounds per lineal foot.) The bridge has in some cases been relieved of this weight by constructing the pipe in the form of an arch, but this is not generally advis

able. A simple design for a short span, as over a creek, is shown in Fig. 30; or the pipe could be supported inside an iron or steel box girder of suitable size and strength. There is no danger of the sewage freezing unless the pipe is exposed for a stretch of many hundred feet.

If the distance to be crossed is more than 200 or 300 feet the inverted siphon will in most cases be found advisable. Its construction under water will be similar to that of rivercrossings laid to grade, except that in the latter the most advantageous depth cannot be chosen.

The joints and pipe of subaqueous siphons and other sewers should be perfectly water-tight, as it will be necessary at times to empty them of sewage for inspection. They should, if of small pipe, be laid to as straight a line and grade as any part of the system. If they are sufficiently large to be entered this is not so important. They should never be laid on the bed of the river, but always beneath it.

For a sewer up to 30 or 36 inches diameter cast-iron pipe with lead or hardwood joints may be used. The trench is excavated at least 18 to 24 inches wider than the pipe and 6 to 12 inches below its grade. Inside this trench the pipe is placed and suspended to grade or blocked up at intervals. The joints are made and concrete is placed under the pipe at all points, completely filling the trench for a distance of 2 or 3 feet above the pipe, or to the surface of the river-bed, it all being thoroughly rammed. If the concrete does not reach. the bed of the river it is well to throw loose stone over and along each side of it.

If the river is not very deep, or a time can be chosen when such is the case, it is in many instances practicable to confine it to half the width of the bed, at the point of crossing, by an earthen embankment or timber coffer-dam, or combination of both, carried, just up stream from the line of sewer, from above the water-line out to mid-channel, across the line of

sewer, and back again to the bank a few feet lower down. The enclosed space is then pumped out, the trench dug and sheathed, the pipe and concrete put in position and covered, and the dam removed and a similar one placed upon the opposite side of the river, which then flows over the pipe already laid. In many cases the best form of dam for sewer-crossings is made by permitting the close sheathing of the pipe-trench to serve also as a dam, extending above the water-surface and backed by earth embankment. A brief statement of the details of carrying out this plan, which must, however, be varied under different conditions, is given.

The sewer having been laid up to the river-bank, a stout stake is driven into the river-bed about 10 feet from the end of this and in line with the down-stream side of the trench. If necessary another is driven a few feet lower down and a brace set from the foot of this to the top of the former. A frame of rangers and braces is built upon the bank, of dimensions proportioned for the proposed trench, and floated to place in line with the trench already dug, the inner end being fastened in position against the end of this trench and the outer being held by the stake just mentioned. Sheathing is then driven on both sides and the end of this frame (the end braces are flush with the ends of the rangers) as deep as is possible before excavating is begun, and earth banked against the outside of it. The water is then pumped out and the trench excavated, the sheathing being kept driven as low as possible, additional rangers and braces being added, and the excavated material thrown just outside of it. When this trench is at grade the pipe is laid, concrete put in, and trench back-filled ahead to cross-sheathing which has been set just back of the end of the pipe. Another frame has meantime been started just ahead, sheathing driven, and outer embankment made. The cross-sheathing between the new and the completed trench is drawn and the excavation continued.