Clay Pipe Engineering Manual

Chapter 8, Construction of Special Structures

Special structures and appurtenances are essential to the proper functioning of any complete system of sanitary sewers. These may include manholes, terminal cleanouts, service connections, inverted siphons, and other structures or devices of special design.

Many states have established criteria through their regulatory agencies gov-erning the design and construction of appurtenances to sanitary sewer sys-tems. In addition, each private and pub-lic engineering office usually has its own design standards which have developed during years of experience. Therefore, many variations will be found in the design of these structures. The following discussion is limited to a general description of each of the various appurtenances, with special emphasis upon the features considered essential to good design.

Manholes

Manholes are among the most common appurtenances found in sewer system. Their principal purpose is to permit the inspection and cleaning of the sewers.

Most manholes are circular in shape, with the inside dimension sufficient to perform inspecting and cleaning opera-tions without difficulty. A minimum inside diameter of 4 feet for circular manholes has been widely adopted for sanitary sewers.

The manhole is usually constructed directly over the center line of the sewer. The manhole may be constructed tan-gent to the side of the sewer for better accessibility. Consideration must be given to the need for introduction of cleaning and test equipment into the sewer.

The opening into the manhole must provide accessibility to the interior without difficulty. A minimum clear opening of 24 inches is recommended. The opening may be centered over the manhole, or constructed off-center in such a way as to provide a vertical side for the entire depth.

A typical manhole of the type used by many engineers and municipalities is shown in the following illustration.

NOTE: Pipe may be laid through the manhole. Cut out top half when manhole base or ledge is finished.

A base slab of concrete, preferably at least 8 inches thick, should be provided to support the weight of the manhole and to prevent the entrance of ground water. The flow should be carried in smoothly constructed U-shaped channels which may be formed integrally with the con-crete base. The height of the channel should be adequate to contain the flow. Adjacent areas should be sloped to drain to the channel. Where more than one sewer enters the manhole, the channels should be curved smoothly and have sufficient capacity to carry the maxi-mum flow. Where the sewer changes direction or size in a manhole, or a branch sewer enters a manhole, the invert should be sloped to allow for the sewer height increase. Effective invert shaping may be accomplished by plac- ing special channel sections of clay pipe through the bottom of the manhole.

Extreme caution should be exercised in the placement of manholes to assure an unyielding foundation. Settlement of the manhole may cause shearing of the pipe adjacent to the manhole. Short stubs (24 inch length maximum) with flexible compression joints and or flexible boots should be used at the manhole walls to help absorb minor movement.

Manhole Frames, Covers and Steps

Manhole frames and covers are normal-ly made of cast or ductile iron. All metal-bearing surfaces between the frame and cover where subject to traffic should be fabricated to insure good seating. Solid covers are preferable to the perforated type on sanitary sewers to restrict objectionable odors and to limit the entrance of surface waters. Adequate ventilation can usually be obtained through the house connections. Locked or special bolted down covers may be used to prevent theft, vandalism or unauthorized entrance.

Steps and ladder rungs are provided in some manholes as a means of access. To the greatest possible extent, steps should be made of corrosion resistant materials. Firm anchorage in the wall and provision in the design to prevent slipping are desirable objectives. Since there have been many serious failures of manhole steps, some engineers omit all steps, preferring the use of other confined space entry equipment.

Drop Manholes

Differences in elevation of incoming and outgoing sewers, which would result in deposition of solids or nuisance to main-tenance personnel, should be avoided. When it is necessary to drop the eleva-tion of the sewer at a manhole, the drop should be made by means of an outside connection similar to that shown. Fitting dimensions govern the minimum verti-cal outside drop that can be made. The designer's judgment will determine, where the difference in elevation war-rants using an outside drop instead of lowering the upstream or branch sewer. Concrete encasement of the entire out-side drop is desirable to protect it against damage during backfilling of the trench. Vertical curves may also be used to accomplish the change in elevation.

Terminal Cleanout Structures

Terminal cleanouts are sometimes used at the ends of branch or lateral sewers. Their purpose is to provide means for inserting cleaning tools, for flushing or for inserting inspection equipment into the sewer.

A terminal cleanout amounts to an upturned pipe coming to the surface of the ground. The turn should be made with bends to allow flexible cleaning rods to pass through it. The diameter should be the same as that for the sewer.

Terminal cleanouts are limited in use-fulness and should never be used as a substitute for a manhole. They are permitted under some state regulations only at the ends of branch sewers which may never be extended and must be within approximately 150 ft. of a man- hole.

Aboveground Sewers

Occasionally in rolling or hilly terrain it has been desirable and economical to build sewers above the surface of the ground, across gullies or valleys. The sewer and its supporting structures should not interfere with storm drainage.

Sewers may be constructed above the natural ground in carefully compacted fills or berms of adequate height and width to support and protect the sewer. Usual trenching methods are employed. Surface drainage may require culverts through the fill.

Structural design of overhead sewers is similar to that of comparable structures with supporting members of tim-ber, steel or reinforced concrete. Foundation piers or abutments should be conservatively designed to prevent settlement and rupture of the sewer. The impact of flood waters and debris must also be considered in some locations. Very high stream or valley cross-ings may require trestles, trusses or suspension structures.

Protection against vandalism, freezing and prevention of leakage are import- ant design and construction considerations. Insulation around the sewer may be required. Use of the ASTM C 425 Compression Joints for Vitrified Clay Pipe and Fittings is recommended.

Measuring and Sampling Flow in Sewers

Flows in sewers may be estimated by a number of available methods. Approx-imate flows may be computed using the Manning open-channel flow equation if the slope of the sewer is known. Flows may be roughly determined from mea-surement of the wetted cross-sectional area of the sewer, supplemented with velocity measurements.

Today, most flow measurement in sew-ers make use of Palmer-Bowlus flumes, weirs, or velocity. The latter is obtained by the use of meters capable of measur-ing velocity by magnetic or doppler sen-sors. Velocity measurements in combi-nation with head measurements make it possible to calculate flows by Q = V x H. This replaces the Manning Equation both in accuracy and convenience. The Parshall Flume, unlike the Palmer-Bowlus Flume, requires a positive head loss at the downstream end and is not utilized in in-line systems.

No one method of measuring flows applies to all situations. Accurate open channel flow measurement depends on existing line conditions and the compo-sition of the sewage. Even in optimal conditions, plus or minus 10% accuracy system wide is all that can be expected. Flows approximating 40,000 gallons per day and below are measured by means of weirs as better resolution can be obtained. Flows above these levels can usually be measured satisfactorily by Palmer-Bowlus flumes or velocity.

The weir, as generally used for sewage flow measurement (gaging) is simply a vertical, watertight bulkhead placed at right angles to the sewage flow with a sharp-edged opening cut in the bulk-head. The shape of the opening varies, but rectangular, triangular and trapezoidal patterns are most frequently used.

The depth of the flow over a weir (head) varies as the rate of flow (discharge) over the weir. Head-discharge relationships for a commonly used triangular weir are given on page 112. A head-measuring device should be located at a distance from the weir plate equal to at least 2-1/2 times the maximum head likely to be encountered so that head measurements will be free from disturbances at the weir.

Continuous head-recording devices provide a means for ascertaining instantaneous flow rates and total flows through sewers over an extended period of time.

When manual observation of head are made, sufficient head readings should be taken to insure that all major changes in flow rate are recorded. Readings made from 1 to 4 times an hour will normally be sufficient to accomplish this purpose. More head observations may be required to obtain the accuracy desired for sewers where the flow fluc-tuates widely.

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