unit 6 sewer appurtenances and materials

24 Pages·2008·1.28 MB·English

UNIT 6 SEWER APPURTENANCES AND
MATERIALS
Structure
6.1 Introduction
Objectives
6.2 Manholes
6.2.1 Manhole Frames, Cover, and Steps
6.2.2 Drop Manholes
6.3 Terminal Cleanout Structures
6.4 Junction Chambers
6.5 Flush Tanks
6.6 House Sewers
6.7 Lampholes
6.8 ~trkeItn lets
Catch Basins
Silt Basins
Stormwater Standby Tank
Grease and Oil Traps
Regulators
Measurement of Flow Rate
Siphons
Outfall Sewers
Materials
Sewer Construction
Sewer Management
Summary
Answers to SAQs
6.1 INTRODUCTION
'
Certain appurtenances are essential to the proper functioning of any complete system of
sanitary, storm or combined sewers. These appurtenances include manholes, tenninal
cleanouts (often called lampholes), flush tanks, service connections, underdrains, inverted
siphons, inlets, catch basins, junction chambers, diversion chanibers, tide gates, and other
structures or devices of special design.
Many concerned authorities have established criteria through their regulatory agencies,
governing to some extent the design and construction of appurtenances to sanitary
sewerage systems. Some public bodies exercise similar general control over the design
and construction of storm water facilities including the usual appurtenances thereto. In
addition, each private and public engineering office usually has its own design standards
which have developed out of years of experience. It is to be expected, therefore. that
many variants will be found in the design sf even the simplest of structures. The
following discussion is limited to a gefieral description of 'each of the various
appurtenances, with special emphasis upon the features considered essential to good
design.
Many of the components q ep ertinent to the design of an appurtenances whether it be for
a sanitary, combined or storm sewer. In general, refinements in design to achieve no
deposition of putrescible matter are applicable only to sanitary or combined sewers.
Wastewater Collection Manholes are located over the pipe centerline under the following circumstances :
and Characteristics
(i) when there is a change in the pipeline diameter,
(ii) when there is a change in pipeline slope,
(iii) when there is a change in pipeline direction,
(iv) at all pipe intersections,
(v) at the uppermost end of each lateral, and
(vi) at intervals not exceeding about 150 m (400 ft).
Objectives
This unit is aimed at enabling the student to understand the basics regarding the use of
following items in a given sewerage system :
manholes,
terminal cleanout structures,
junction chambers, flush tanks, and house sewers,
lampholes and street inlets,
catch and silt basins,
stormwater stand by tanks,
traps,
regulators and weirs, etc., aid
sewer materials, construction, and managment.
6.2 MANHOLES
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Manholes are among the most common appurtenances found in sewerage systems. Their
principal purpose is to permit inspection and cleaning as well as repair of sewers, and
removal of any obstructions; and, also they allow sampling and flow measurement.
Most manholes are circular in shape, with the inside dimension sufficient to perform the
necessary operations regarding inspection and cleaning without difficulty. A minimum
inside diameter of 4 ft (1.25 m) for circular manholes has been widely adopted. A typical
manhole for a lateral sewer is shown in Figure 6.1. However, 3 ft 6 in is used in some
localities; and a diameter of 3 ft has been felt convenient for access only into large sewers.
When the width of the sewer does not exceed the width of the manhole, the manhole is
usually constructed directly over the centre line of the sewer. For better accessibility the
manhole, for very large sewers, may be centered over the sewer with a landing platform
offset from an opening into the sewer itself. Consideration must be given to the need for
the introduction of cleaning equipment into the sewer.
The opening into the manhole must enable a man to gain access to the interior without
difficulty. A minimum clear opening of 21 inches is recommended; it may be centered
over the manhole or as is frequently done, it may be positioned off the center in such a
way as to provide a vertical side for the entire depth.
lfipical manholes of the types used by many municipalities are shown in
Figures 6.2 to 6.5.
A base slab of concrete preferably at least 8 inches thick should be provided on smaller
sewers to support the walls of the manhole and to prevent the entrance of pund water.
All sanitary flow should be camed in smoothly constructed U-shaped channels which
may be formed integrally with the concrete base or may be constructed separately of
concrete or brick. The side height of the channel should be one-half to three fourths of the
diameter of the sewer with the higher side height used where conservation of energy is
important. Adjacent floor areas should be sloped to drain to the channel with a slope of
1 inch per ft. Where more than one sewer enters the manhole, the flowing-through
channels should be curved smoothly and should have sufficient capacity to cany the
maximum flow. Wherever the sewer changes direction or its size, or a branch sewer is
allowed to join the system a manhole needs to be provided.
Sewer Appurlcnances
N,anhoie cover
and Materials
Flow
Outlet sevier pipe
Q
(b) Section A-A
(d Vertical Section
Figure 6.1 : Sectional View of a Lateral Sewer Manhole
;kwork
Cement Plaster
31%'' min
' 1
b
Section A-A
Section C-C
.. ,
Figuri 6.3.: A Typical Manhole
Wastewater Collection
and Characteristics
1" Cement Plaster
Section B-B
B -J
Section CC
Figure 6.3 :T ypical Manhole with a Vertical Side
.
.
Ground Surface Ground Surface ,
ID" Cemmt Plaser
Cover No1 Brickwork
Section B-B
Section A-A
Figure 6.4 : A Shallow Manhole
Sewer Appurtenances
and Materials
Minimum 2'-10.7' for Paved Street
Minimum 3'-6.5" for Unpaved Streets
Manhole Frame
Street Grade
Grade Points \
iij\ti.?rl.il Etr:tcm
I
ScctlCIil H-13
Sectrm A-A
Omit this step in
Street Grade
Paved Streets
/
Max
for Unpaved Streels 16"
5-in. by I-in. Ring Seat
for
for
Figure 6.5 : A Concrete Pipe Manhole
Wastewater Collection Effective invert shaping for small pipes has been accomplished by placing fittings
and Characteristics
through the bottom of the manhole and later breaking out (covering) the upper half of the
fittings.
It is very desirable to place the first pipe joint outside a manhole as close as practical to
the manhole, because thorough compaction of the manhole backfill is not always
achieved. In many instances, a flexible-type joint will provide the best solution to a
possible differential settlement.
The materials commonly used for the construction of manhole walls include brick,
poured concrete, precast rings and segmental block. The choice of the material 'is
influenced by the following :
(a) Cost in place, including material, labor and equipment.
(b) Durability under all reasonably expected conditions of service.
(c) Adaptability of the material to meet field conditions with particular reference to
changes in location, grade or alignment made during construction.
(d) Depth of inanhole and characteristics of surrounding material.
Brick walls are normally constructed 8 in thick. Bricks are laid radially with horizontal
joints not greater than 112 inch thick. Joints should be completely filled with portland
cement mortar, struck and pointed on the inside of the manhole. The outside of the walls
of brick or segmental block manholes should be plastered with at least a 112 inch
thickness of cement mortar.
Manhole walls of plain poured concrete may be constructed to about the same or
somewhat less thickness than would be adopted for brick masonry. Standard
prefabricated steel forms are available for this type of construction; however, sometimes
reinforced concrete walls are desirable.
Precast concrete walls are available in circular sections of various heights, and are
practically always reinforced in order to reduce the wall thickness and weight. Special
transition sections are furnished to reduce the diameter of the manhole at the top as
required in order to seat the frame and cover. Special care should be taken to secure tight
joints. The wall sections can be obtained or cast in place to the desired specification.
Height adjustment should always be made in the full diameter part of the manhole.
6.2.1 Manhole Frames, Cover, and Steps
Manhole frames and covers are almost invariably of close-grained gray cast iron with a
combined weight for the frame and the cover varying form 400 to 600 lb. All
metal-bearing surfaces between the frame and cover, wherever subject to traffic, should
be fabricated to insure good seating. Solid covers are preferable to the perforated type on
sanitary sewers because they diminish the spread of objectionable odors and the entrance
of surface waters. Adequate ventilation can usually be obtained through the house
connections, Open-type covers are most common on storm and combined sewers. Locked
or specially bolted-down covers may be used to prevent theft, vandalism or unauthorised
entrance.
Steps or ladder rungs are provided in most of the manholes as a means of access down to
the bottom, but in some cases portable ladders are used. Steps should consist of heavy
cast iron, wrought iron or stainless steel in order to resist, as much as possible the
weakening due to corrosion. The steps should be spaced at about 12 to 15 in. There have
been many serious failures of manhole steps and a minimum thickness of 1 in is,
therefore recommended. Some engineers deliberately omit all steps, preferring the use of
portable safety ladders.
6.2.2 Drop Manholes
Differences in elevation of incoming and oct~oingse wers, which would result in holding
up of solids or in nuisance to the maintenanw -sunnel, should be avoided. When it is
necessary to drop the elevation of the sewe: at a n~vholet,h e drop should be made by
means of an outside connection- in this regarci k e3 imensions of the related fittings
govern the minimum vertical outside drop that can be made. The designer's judgement
will determine, in each case, where the difference in elevatio~*~,:a nts using an outside
.
drop instead of lowering the upstream or branch sewer. The outside connection is
provided for the protection of a man entering the manhole and the resulting structure is
known as a drop manhole. Therefore, sometimes when a lateral sewer joins a deeper,
submain sewer, the use of a drop manhole will reduce the amount of excavation needed Sewer Appurtenances
and Materials
by allowing the lateral to maintain a shallow slope (Figure 6.6). The waste water drops
into the lower sewer through the vertical pipe at the manhole.
Figure 6.6 :A Typical Drop Manhole Structure
Encasement of the entire outside drop in concrete or brick masonry is needed to protect it
against damage during the backfilling of the trench. Maintenance may be facilitated by
providirig a cross instead of a tee at the top of the vertical drop, with a cast-iron riser from
the cross to the surface of the ground where a cast-iron lamphole frame and cover are
installed. When such a drop gets plugged, a ball or a chain is dropped down to break any
sticks, thereby permitting the plugging material to get washed out.
Manholes should be built so as to cause minimum head loss and interference with the
hydraulics of the sewer line. One way to maintain a relatively smooth flow transition
through the manhole, when a small sewer joins one of a larger diameter, i,r tc match the
pipe crown elevations at the manhole (Figure 6.7).
(1
Match crown
Sewer invert
Figure 6.7 : Intersecting Pipes of Different Diameters at a Manhole
: SAQ P
(a) What are ~nanlioles?
(b) DISCUSthSe various types of:manhc ' . !:,lint oul the~rre spective
advantages.
6.3 TERMINAL CLEANOUT STRUCTURES
Terminal cleanouts are sometimes used at the ends of branch or lateral sewers. Their
purpose is to provide a means for inserting cleaning tools for flushing or for inserting an
inspection light into the sewer. In fact, a terminal cleanout amounts to an upturned pipe
coming to the surface of the ground. The turn is made with bends's0 that flexible cleaning
rods can be passed through them. The diameter of a bend should be the same as that of
the sewer. The cleanout is capped with a cast iron frame and cover. Care should be taken
to rxiaintain proper alignment of the pipe while encasing it with concrete. The frame and
cover of the terminal cleanout structure should be made of gray cast &on.
Tees were often used, instead of pipe bends, in older engineering practice, and the
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Wastewater Collection ' structures were called "lampholes". Modern sewer cleaning equipment cannot be passed
and Charactehtics from the surface through such structures, so their use is not recommended nowadays.
Terminal cleanouts are limited in usefulness 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 lie within 150 ft of a manhole.
6.4 JUNCTION CHAMBERS
Inspection of large sewers is usually made by means of a brick or concrete structure
known as a junction chamber. The junction chamber provides access to the sewers and
also allows the flow from the sewers entering the chamber to be combined without
excessive turbulence and loss of head.
Each junction chamber presents a special design problem necessitating a careful
structural and hydraulic consideration. The principal objective in the design is to provide
a safe and economical structure which will combine the flow smoothly without
decreasing the velocities appreciably and without causing backwater conditions in the
sewers which enter the chamber. A typical junction chamber is shown in Figure 6.8.
(a) Plan @) Section 2'-2"-2"'
Figure 6.8 :A 'Qpical Junction Chamber lor Manhole)
6.5 FLUSH TANKS
Flush tanks are rarely used at the present time. An occasional flushing with a fire hose or
the use of other modern sewer cleaning methods at the upper ends of laterals where some
deposition of solids may occur will usually serve the purpose equally well.
6.6 MOUSE SEWERS
House sewers should not be less than 6 inches in diameter; and should preferably have a
minimum longitudinal slope of 114 in per ft, laid to straight line and grade. Slopes of as
little as equal to those of the street help minimize infiltration and root penetration.
Connections to the main street sewer should not be made with Y or T-branches. The use
of T is preferred in sanitary sewers where simultaneous discharge of house sewers into
the street sewer is not common, since experience has indicated frequent breakage of the
eye where rodding of the house connections has become necessary. The use of Y-branches
and curves would be indicated only where hydraulics can be significantly improved.
6.7 LAMPMOLE
A lamphole is an opening, constructed in a -- :-, slightly larger than necessary to permit
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the insertion of a light into the sewer. L-mpho. .s are sometimes used as a makeshift
substitute for manholes to permit the insptction or he flushing of sewers, or for
ventilating purposes.
Lampholes should be constructed of 8 in to 12 in tiles or casi-I.," pipes. The lower
section should be cast iron tee on a firm foundation, but if constructed of tiles it should be
reinforced with concrete to take up the wlight'of the shaft. Lampholes are not comnionly
used on stiver systems on account of their lack.
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6.8 STREET INLETS Sewer Appurtenances
and Materials
A street inlet is an opening in the street surface (as discussed in an earlier unit), usually in
the gutter, designed to permit the passage of water from the street surface into the sewer.
7.5'
Normal S.3"
Deprosrcd
Gumr
Figure 6.9 (a) : Street Jnlets at Dayton, Ohio
Figure 6.9 (b) :S treet Inlet and Catch Basin
Many types of street inlets have been designed, some of which are illustrated in
Figure 6.9 (a) and (b). In the design of inlets, the features that influence the location,
,performance, convenience and capacity include :
(1) height of kerb;
(2) traffic density etc.;
(3) position of inlet - whether vertical in the face of the kerb, flat on the bottom of
the gutter, or a combination of these two;
(4) overall dimensions; height, width and length;
(5) direction of major axis of the opening;
(6) roughness of gutter surface;
(7) shape and slope of gutter at inlet - whether a continuous grade or depressed in
front of inlet, and depth and character of depression;
Wastewater Collection (8) slope of gutter above, at and immediately below the inlet;
1
and Characteristics
(9) use of deflectors or other devices at the bottom of the gutter to direct the flow
into the inlet;
(10) location, size and shape of bars or grill;
(11 ) cross-section of the street, and crown of street surface; and
(1 2) characteristics of street surface.
It is evident from all of these factors that a useful formula applicable to all street inlets
cannot be devised in simple form.
Placing of Street Inlets
Inlets are usually placed near street corners to avoid unsightly breaks in pavement
grade between the street intersections. Where the blocks are so long that the rate of
flow of storm water will overtake the capacity of the gutter, an intermediate inlet
may be placed in the block.
The placing of an inlet at the intersection of the two kerb lines at a comer results in
a lower first cost, but on a heavily traveled street the inlet may be damaged or
displaced. This position of the inlet forces pedestrians to cross the flowing stream
of the gutter in wet weather. These objections can be overcome by having two
inlets at each comer, set back far enough from the kerb intersection to avoid
interference with the cross walk.
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6.9 CATCH BASINS
Catch basins are used to interrupt the flow of sewage before it enters the sewer, causing
deposition of suspended grit and sludge and the detention of floating rubbish which could
enter and clog the sewer. A separate catch basin may be used for each inlet or, to save
expense, the pipes from several inlets at a comer may discharge into the same catch basin.
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Various types of catch basins are successfully used some holding water in a trap and
others discharging directly into the sewer. Catch basins are constructed like manholes
with diameter varying from 2.5 to 4 ft. Catch basin in this size range will care for the
inlets at four comers of an intersection, each draining a city block. The construction of
the catch and its cover follows the principles given for the construction of manholes. In
unusual situations it may be necessary to install a larger basin, but too large a catch basin
is undesirable because of the probable production of bad odour. The outlet from the catch
basin may be submerged to prevent the odours escaping from the sewer into the street,
but odours are often created in the catch basin. Some engineers arrange the trap so that it
can be opened for observation down the sewer, thus combining the advantages of a
manhole with the catch basin.
6.10 SILT BASINS
Basins to interrupt the flow of sewage may be installed on storm sewers to provide a
place for the deposition of grit particularly on sewers where threre are no catch basins. In
order to promote the deposition of grit the velocity of flow should not be greater than
2 fps, and to prevent the deposition of organic matter it should not be less than 1 fps. Silt
basins are undesirable on separate or on combined sewers because of the inevitable
collection of organic matter.
il
6.11 STORMWATER STANDBY TANK
I
The function of a stormwater standby tank is to serve as a storage basin (as discussed
earlier) to diminish the magnitude or the fate of storm flow; or to serve as an equalizing
basin to retain excess storm flow during a temporary restriction of the outlet such as
during high tide to minimize the pollution of receiving waters by diminishing the