Cross-sheathing is set at frequent intervals and the trench filled up to it to reduce the length of open trench which must be kept free of water. It is advisable not to cut off the sheathing and remove the embankment at any point until the construction is completed to mid-stream. It will usually be necessary to keep a pump going constantly during construction. If the stream is subject to freshets it may be well to set the pump upon a flat-boat anchored against the up-stream sheathing. The boiler may be kept upon this boat or upon the bank, the steam-pipe in the latter case being carried along the sheathing.

If the bed of the river is gravelly considerable trouble may be experienced from water leaking into the trench, the entering water having perhaps passed into the ground many feet from the sheathing. The embankment may in such a case be carried as far as possible from the sheathing on every side, or a thin layer of fine sand, sandy loam, or loamy clay may be spread over the bottom for 50 or 75 feet above and below the trench. Also manure, brewery-meal, etc., has been used to stop up the pores of the gravel. Heavy, closely woven

canvas is excellent for use in such a case, in large squares or strips tightly sewed together, one end being fastened above water against the outside of the sheathing, the other anchored by stones or other weights beyond the part of the bed which is giving trouble.

An excellent material for the embankments is a puddle of clay, sand, and gravel. Clay alone is almost useless. Fine and coarse sand mixed, with or without gravel, is better than clay alone. All sticks, roots, and large stones should be removed from this puddle, and anything which, reaching through the embankment, may offer a course for the water. If puddling material is scarce a double row of sheet-piling may be carried around the work, the two rows being from 2 to 5 feet apart, braced together only at the top, and the space

between them filled with puddle well worked and rammed. Experience in this class of work is almost essential to its proper prosecution, and written directions can give only the barest outlines for meeting but a few of the difficulties which may be encountered. Pluck, foresight, a fertility in expedients, and common sense are prime requisites for this work. The water must never for an instant be allowed to get the upper hand. If nothing else is at hand the very clothes off one's back should be taken to stop a leak temporarily, should

PUDDLE

FIG. 31.-COFFER-DAM PUddle-walls.

one unexpectedly develop in an embankment. Never permit a brace, stick, or any object to extend through an embankment or puddle-trench or -wall. If a trench surrounded by water shows signs of collapsing from outside pressure and no material for additional rangers and braces is at hand the trench can sometimes be saved by allowing water to fill it, and then, when the material has been obtained, the water can be pumped down and bracing put in as it lowers. But this is a somewhat desperate remedy.

An outlet for the Massachusetts Metropolitan Sewerage System at Deer Island, in 5 to 10 feet of water, was built in open trench, with double sheathing and puddle, as in Fig. 31. The sewer was 6 feet 3 inches inside diameter and the trench 10 feet wide on the bottom, concrete being carried from about I foot beneath the sewer to an average of 4 feet above it. The cost of the trench, including coffer-dam, sheeting left

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in place, and back-filling, was $44 per lineal foot." (Engineering News, vol. XXXI, page 121.) The material through which the trench was carried was sand and gravel. The work was done by day labor.

If the trench is in rock or a tight coffer-dam cannot be made except at great expense it may be cheaper and better to resort to divers. When not in rock, however, the excavation of the trench should be done by a dredge or similar appliance if possible, as divers' labor is very expensive.

The end of an outlet which discharges at some distance from the shore of a stream or other body of water should be so located and designed that currents, tides, or storms cannot wash it full of sand or mud, that it cannot settle down into the bottom, that it cannot be undermined by tides or currents, and that the sewage discharged will not settle in front of it and block the outlet. This may be accomplished by laying the sewer in a trench as described above, and at the very end placing a right-angled bend pointing upward and extending 1 to 3 feet above the bottom, this upright pipe being surrounded with a cone-shaped mass of concrete. Or the end of the sewer may be continued straight, but raised gradually until the outlet is 2 or 3 feet above the bottom, it being supported between two rows of piles back to where it has 2 or 3 feet of covering.

It is not so necessary that an outlet pipe be straight in line and grade provided the grade continually falls at a sufficient rate. The use of flexible-jointed iron pipe, such as is frequently employed for water-pipes at river-crossings, may often be used for sewer-outlets. They should be properly protected by concrete, riprap covering, or piling. furnishing and laying 2200 feet of 24-inch iron pipe with Ward flexible joints in a bottom consisting of sand, gravel, loose and solid rock, in a trench having an average depth of 4 feet, the depth of water at high tide being 11 to 30 feet,

For

from $16.85 per foot to almost double this amount was bid in 1898. In ordinary river-work the cost should be much less.

Whenever subaqueous work of any considerable extent is being done it will be well to have a diver's outfit on hand, as its immediate use may sometimes effect a saving of the work from serious damage.

ART. 80. CROSSING RAILROADS AND CANALS.

Railroads should be crossed with particular care, both that no accident may occur to either the workmen or to passing trains, and because of the difficulty of afterward repairing breaks or defects at such points. This applies also to sewers constructed in or close to the foot of railroad embankments.

It is not advisable to tunnel under a railroad unless the

sewer runs quite deep and the material is stable. A settling of the ground above the tunnel might prove disastrous to trains, and this settling is extremely probable, owing to the jarring of passing trains. If there is a culvert under the road through which the sewer can be passed it will often be well to take advantage of this, if only a slight detour be necessary in order to do so.

If the sewer is to pass under the railroad in open cut each rail should be first supported by bridge timbers, beams, or iron rails placed under the ties lengthwise of the track and extending 10 to 20 feet beyond each side of the proposed trench. For a trench 4 to 6 feet wide a 12 X 12 bridge timber 25 or 30 feet long may be used. A heavy steel rail may be used under the same conditions, but is not generally so stiff. Each beam or rail is placed in a trench dug under the ends of the railroad-ties, just sufficiently deep and wide to enable it to be placed under the track-rail. Hardwood plank are then driven between this and the ground and wedges driven between each tie and the beam. The trench

is then excavated, horizontal sheathing being used. The earth excavated cannot be thrown upon the surface unless the track is temporarily out of use. It may be handled by a cable-way excavator which swings sufficiently high to clear all trains. The buckets for this it will be well to have largethose holding a cubic yard will do that the number of trips may be lessened. The back-filling can be returned in the

same way and should be most thoroughly tamped.

Another method of handling the earth, particularly applicable where there are but two or three tracks, is to throw the excavated material beyond the outside track by one or two handlings, a space for this having been left clear of earth by previous management. If the trench is shallow and as short a length as practicable opened at a time it may even be possible to throw the excavated earth directly onto the completed sewer, but if this is done only a very few men can be worked at this point.

After the completion of the work with thoroughly tamped back-filling the trench should be wet down every two or three days for several weeks, the bridge timbers or rails being left under the ties meantime. Just before each wetting earth should be placed and tamped on the filled trench to 2 or 3 inches above the ties. When the trench shows no settlement after a wetting down the supporting timbers or rails may be removed.

For small sewers it will be well to use iron pipe with lead joints for railroad-crossings, and for large sewers the arch and side walls should be reinforced (see Plate VI, Fig. 8). In general it is better to place no manhole or other appurtenance between or within several feet of any tracks.

A trench in or near a railroad embankment is subject to the jarring of the trains and needs to be carefully sheathed. This is sometimes difficult if the trench be wholly or partly upon the slope of the embankment, since there is nothing