Imágenes de páginas
PDF
EPUB

opening being ordinarily tightly closed by a cap or plug. When the sub-drain is under a small sewer, the vertical branch should lead into a manhole, opening either in the sewer-invert or, better, in the bench. In either case the opening should be plugged so that absolutely no sewage can

Iron
Cover

SECTION ON A-B.

-B

FIG. 45.-MANHOLE WITH SUB-
DRAIN INSPECTION HOLE.

enter it (see Fig. 45).

Manholes of special design will be required by unusual conditions, but in every design the three principal requirements of a manhole should be met: it should offer easy access for inspection and for cleaning of the sewer, and should assist in ventilation of the same; it should also be impervious to seepage of ground water, and be so proportioned as to resist the pressure of the surrounding earth. For this last purpose the curved form is better than the polygonal.

Manholes for sewers larger than 30 to 36 inches are usually built up from the sewer-arch and have no special bottom construction. The sewer-invert under the manhole should be reinforced, however, and a large foundation area provided, if the ground is at all yielding. The manhole shaft is sometimes placed on one side of the sewer, both for strength and for facility of access (see Fig. 46). The foundation of a manhole should be perfectly solid. If the soil is soft, a plank platform may be used. Owing to the irregular shape of the bottom, concrete usually gives better results as to strength, shape, and imperviousness than does brick-work. The bore of each sewer should be continued through the bottom by a smooth channel of uniform section

and slope, either straight or with a continuous curve.

This channel can be plastered with Portland cement, lined with brick or with split vitrified pipe. The last method gives the smoothest surface and is the

[blocks in formation]
[ocr errors]

TOP PLAN

[blocks in formation]

least as high above the invert as the top of the sewer-pipe, and benches should slope up to the sides of the manhole at an angle of at least 10 or 15 degrees with the horizontal.

The manhole walls are usually built of brick, 8 inches thick from the top to a point 10 or 12 feet below the surface, and increasing in thickness with the depth. If the bottom is a circle or a well-designed oval with no radius greater than 6 feet, a 12-inch wall should be strong enough at any depth, unless the ground is a quicksand or similar material, or is very wet. The outside of the manhole should be plastered with cement mortar to keep out ground water or water used in settling the trenches, and to prevent the lifting of the top foot or two by freezing ground.

In several cities manholes have been built entirely of concrete. These are generally more watertight than brick ones, and stronger. Special forms are required for their construction, and the providing, placing and removing of them add to the cost and inconvenience of concrete manhole construction.

The top of the manhole is generally capped with an iron casting sufficiently deep to permit the laying close to it of brick or stone paving materials. This will be about 8 or 10 inches, except where the pavement is made for heavy or city traffic, where it may need to be 12 to 16 inches.

Where the street is not paved, each manhole-head should be surrounded for a distance of at least 2 feet by cobble, rubble or stone block pavement, to protect both the head and passing vehicles. In some cases the head is provided with inside flange or lugs to hold it in place on the manhole; but this is undesirable, for it is better that the head should slide out of place than that it should rupture the top of the manhole masonry, as would probably occur if the pavement around the head should move. Where the pavement has a concrete base, this holds the head more firmly than a flange could.

The cover should be sufficiently strong to support the heaviest wheel pressure. It should be provided with ventilation-holes giving as much area of opening as possible. Its upper surface should be roughened to provide foothold for horses. It should offer as little obstruction as possible to traffic, and be practically noiseless. The ventilation-holes should be through the elevated rather than through the depressed parts of the cover,

since by this construction the stoppage of the holes by dirt and snow and the entrance of dirt into the sewer are considerably lessened. Such a manhole-head and cover, as used in Syracuse, N. Y., is shown in Fig. 47. Covers are sometimes provided with locks to prevent the opening of the manhole by unauthorized persons, but these locks cause much trouble in some instances, particularly in freezing weather. A better plan probably is to make the covers so heavy that they cannot readily be raised without the use of some strong implement adapted to this purpose.

On roads and streets not paved with hard, permanent pavement, more or less dirt will be sure to enter through the ventilation-holes and if allowed to reach the bottom of the manhole may sometimes form stoppages, particularly in small sewers. To prevent this some cities provide a bucket of some kind suspended under the holes, smaller than the manhole-opening that the air may pass up between the bucket and the walls; or a special construction of some kind is designed for this purpose. (see Fig. 47). These receptacles should be cleaned before they become filled with dirt. The bucket supports must be so strong that the bucket cannot drop into the sewer, even when filled with dirt or ice.

A flush-tank or flushing-manhole should be watertight. It should be so proportioned as to hold the required amount of water without increasing the head on the sewer beyond the limit set (Art. 17). The flush-tank is usually set at the upper end of a sewer line, toward which much sewer air rises, and the sewer should therefore be provided at that point with ample ventilation. In spite of this, many automatic flush-tanks are so built as to afford the sewer absolutely no ventilation, forcing the adjacent houses to unwillingly, and usually unknowingly, provide it. Since flushing-siphons cannot permit of ventilation through their passages, a vent should be furnished the sewer just below the flush-tank. In some cases a manhole is placed just below, even in contact with, the flush-tank, which serves for both ventilation and for inspection and cleaning. Or the sewer may be continued straight into the flush-tank and provided with a removable

stopper, while the flushing water enters the side of the sewer just below the tank.

Flush-tanks may be built of brick with concrete bottoms, the whole being made watertight, but concrete reinforced with

[merged small][subsumed][merged small][merged small][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][merged small][merged small][merged small][graphic][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][ocr errors][subsumed][subsumed][subsumed][merged small][merged small][merged small]

steel rods is preferable in most cases, as this construction is more watertight and stronger than brick.

The automatic flushing appliances in common use act on the principle of the siphon, the variations being in the method of starting the flow. Most of those now used have no moving

« AnteriorContinuar »