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Part A: Engineering CONSTRUCTION OF SEWERS CHAPTER 3: DESIGN AND CONSTRUCTION OF SEWERS 3.1 GENERAL The major roles of a sewer system can be listed as follows:
- Improvement in the environment by removing the sewage as it originates
- Preventing inundation of low lying areas that may be otherwise caused by not providing sewers
- Prevention of vector propagation by sewage stagnations
- Avoiding cross connections with freshwater sources by seepage In addition, there is a strong emphasis on: a) Avoiding sewer impacts on groundwater quality by infiltration of soil water into sewers and exfiltration of sewage into soil water, occurring rather as a cycle depending on the flow conditions in leaky sewers, and b) Moving away from the mind-set that a sewer system shall necessarily be an underground sewer right in the middle of the road with costly construction, upkeep and remediation and making the objective realizable if necessary in an incremental sewerage commensurate with optimizing the area coverage in the available financial and human resources to create and sustain the system. This chapter presents the following: Part - 1 Estimation of Design Flows Part - 2 Types and Hydraulics of Sewers Part - 3 Design of Sewer Networks Part - 4 Types and Construction of Manholes Part - 5 Laying, Jointing and Construction of Sewers PART - 1 ESTIMATION OF DESIGN FLOWS 3.2 DESIGN PERIOD The length of time up to which the capacity of a sewer will be adequate is referred to as the design period. In fixing a design period, consideration must be given for the useful life of structures and equipment employed, taking into account obsolescence as well as wear and tear. The flow is largely a function of the population served, population density, water consumption, lateral and sub main sewers are usually designed for peak flows of the population at saturation density as set forth in the master plan. Trunk sewers, interceptors, and outfalls are difficult and uneconomical to be enlarged or duplicated and hence are designed for longer design periods. In the case of trunk sewers serving relatively undeveloped areas adjacent to metropolitan areas, it is advisable to construct initial facilities for more than a limited period. Nevertheless, right of way for future larger trunk sewers can be acquired or reserved. The recommended design period for various components shall be as in Table 2-1.
Part A: Engineering CONSTRUCTION OF SEWERS 3.3 POPULATION FORECAST Methods of estimation of population for arriving at the design population have been discussed in Section 2.6. When a master plan containing land use pattern and zoning regulations is available for the town, the anticipated population can be based on the ultimate densities and permitted floor space index provided for in the master plan. In the absence of such information on population, the following densities are suggested for adoption as in Table 3.1. Table 3.1 Densities of Population vs. Populated areas Source: CPHEEO, 1993 In cities where Floor Space Index (FSI) or Floor Area Ratio (FAR) limits are fixed by the local authority this approach may be used for working out the population density. The FSI or FAR is the ratio of total floor area (of all the floors) to the plot area. The densities of population on this concept may be worked out as in the following example for an area of one hectare (ha) Roads 20% Gardens 15% Schools (including playgrounds) 5% Markets 2% Hospital and Dispensary 2% Total 44% Area available for Residential Development = 100 - 44 = 56% or 0. Actual total floor area = Area for residential development x FSI Assuming an FSI of 0.5 and floor area of 9 m^2 /person Number of persons or density per hectare
Part A: Engineering CONSTRUCTION OF SEWERS The peak factor also depends upon the density of population, topography of the site, hours of water supply and hence, individual cases may be further analyzed if required. The minimum flow may vary from 1/3 to 1/2 of average flow. 3.6 INFILTRATION Estimate of flow in sanitary sewers may include certain flows due to infiltration of groundwater through joints. Since sewers are designed for peak discharges, allowances for groundwater infiltration for the worst condition in the area should be made as in Table 3. Table 3.3 Ground water infiltration Source: CPHEEO, 1993 Once the flow is estimated as per Table 3.3, the design infiltration value shall be limited to a maximum of 10% of the design value of sewage flow. Care shall be taken that in high ground water locations and coastal locations, the sewer pipes shall not be stoneware or vitrified clay pipes and instead shall be cast iron / ductile iron pipes or other non-metallic pipes with safeguards against floatation as discussed later in the section on laying of sewers. 3.7 SEWAGE FROM COMMERCIAL INSTITUTIONS The industries and commercial buildings often use water other than the municipal supply and may discharge their liquid wastes into the sanitary sewers. Estimates of such flows have to be made separately as in Table 3.4 (overleaf) for their potable water supply. 3.8 INDUSTRIAL EFFLUENTS TO BE DISCOURAGED The mixing of industrial effluents through discharge into public sewers is undesirable due to the possible detrimental effects of such effluent on the operation of biological sewage treatment process. This aspect has been well recognized in recent times and industrial areas having polluting industries are generally located such as to avoid mixing with sewage. However, in cities that have undergone unregulated growth in the past, polluting industries may exist in pockets of mixed land use. In such cases, those industries are required to implement zero liquid discharge (ZLD) by reusing the effluents after appropriate treatment in house. Of all the industries, this shall strictly apply to the automobile service stations and machine shops from where the spent metal plating baths and oil & grease shall be prevented from entering the sewers.
Part A: Engineering CONSTRUCTION OF SEWERS Table 3.4 Institutional needs for potable water 3.9 STORM RUNOFF The sanitary sewers are not expected to receive storm water. Strict inspection, vigilance, and proper design and construction of sewers and manholes should eliminate this flow or bring it down to a very insignificant quantity. However, in small habitations where rainfall is almost a continuous affair, it may be necessary to include storm water in the design of sewers as under. 3.9.1 Estimation of Storm Runoff The storm runoff is that portion of the precipitation, which drains over the ground. Estimation of such runoff reaching the storm sewers therefore is dependent on the intensity, duration of precipitation, characteristics of the tributary area, and the time required for such flow to reach the sewer. The design of storm water sewers begins with an estimate of the rate and volume of surface runoff. When rain falls on a given catchment, a portion of the precipitation is intercepted by the vegetation cover that mostly evaporates, a portion hits the soil and some of it percolates down below and the rest flows over the ground. The higher the intensity of rain, the higher will be the peak runoff.
Part A: Engineering CONSTRUCTION OF SEWERS Table 3.5 Percentage of Imperviousness of Areas Source: CPHEEO, 1993 (3.2) When several different surface types or land use comprise the drainage area, a composite or weighted average value of the imperviousness runoff coefficient can be computed, such as: where, I : Weighted average imperviousness of the total drainage basin A 1 , A 2 , An : Sub drainage areas I 1 ,I 2 ,In : Imperviousness of the respective sub-areas. The weighted average runoff coefficients for rectangular areas, of length four times the width as well as for sector shaped areas with varying percentages of impervious surface for different time of concentration are given in Table 3.6 (overleaf). Although these are applicable to particular shape areas, they also apply in a general way to the areas, which are usually encountered in practice. Errors due to difference in shape of drainage are within the limits of accuracy of the rational method and of the assumptions on which it is based. 3.9.2 Rational Method 3.9.2.1 Runoff-Rainfall Intensity Relationship The entire precipitation over the drainage district does not reach the sewer. The characteristics of the drainage district, such as, imperviousness, topography including depressions and water pockets, shape of the drainage basin and duration of the precipitation determine the fraction of the total precipitation, which will reach the sewer. This fraction known as the coefficient of runoff needs to be determined for each drainage district. The runoff reaching the sewer is given by Equation (3.1). 3.9.2.2 Storm Frequency The frequency of storm for which the sewers are to be designed depends on the importance of the area to be drained. Commercial and industrial areas have to be subjected to less frequent flooding. The suggested frequency of flooding in the different areas is as follows -:
Part A: Engineering CONSTRUCTION OF SEWERS 3 - 8 Table 3.6 Runoff Coefficients for Times of Concentration Source: CPHEEO, 1993
Part A: Engineering CONSTRUCTION OF SEWERS The relationship may be expressed by a suitable mathematical formula, several forms of which are available. The following two equations are commonly used: (3.3)
where, i : Intensity of rainfall (mm/hr) t : Duration of storm (minutes) a, b and n : Constants The available data on i and t are plotted and the values of the intensity (i) can then be determined for any given time of concentration, (tc). 3.9.2.4 Time of Concentration It is the time required for the rain-water to flow over the ground surface from the extreme point of the drainage basin and reach the point under consideration. It is equal to inlet time (t) plus the time of flow in the sewer (tf). The inlet time is dependent on the distance of the farthest point in the drainage basin to the inlet manhole, the shape, characteristics and topography of the basin and may generally vary from 5 to 30 minutes. In highly developed sections, the inlet time may be as low as 3 minutes. The time of flow is determined by the length of the sewer and the velocity of flow in the sewer. It is to be computed for each length of sewer to be designed. a) Tributary Area For each length of storm sewer, the drainage area should be indicated clearly on the map and measured. The boundaries of each tributary are dependent on topography, land use, nature of development and shape of the drainage basins. The incremental area may be indicated separately on the compilation sheet and the total area computed. b) Duration of Storm Continuously long, light rain saturates the soil and produces higher coefficient than that due to, heavy but intermittent, rain in the same area because of the lesser saturation in the latter case. The runoff from an area is significantly influenced by the saturation of the surface to nearest the point of concentration, rather than the flow from the distant area. The runoff coefficient of a larger area has to be adjusted by dividing the area into zones of concentration and by suitably decreasing the coefficient with the distance of the zones. A typical example of the computation of storm runoff is given in Appendix A.3.1.
Part A: Engineering CONSTRUCTION OF SEWERS 3.10 MEASUREMENT OF FLOWS IN EXISTING DRAINS/SEWERS Quite often, the measurement of flows in existing drains or sewers will provide valuable data for a more realistic assessment of the design flows. In general, non-sewered areas will most certainly be having a set of drains where the generated sewage will be flowing out. The assessment of the flows in drains can be done by a variety of methods right from the rudimentary crude method to the most sophisticated dye tracer method. The choice of methods presented hereunder is considered to be appropriate to the conditions in the country particularly, away from metropolitan centres. a) The Float Method At the very outset, a non-intrusive method is called for. This can be done by finding out the time taken for a float like an empty match-box or a plastic box to travel for about 3 m in a straight reach and measuring the width and depth of flow in the drain. If we assume the respective values as 20 seconds, the width as 0.9 m and depth of flow as 0.6 m, the flow can be assessed as 0.8 × 0.9 × 0.6 × (3/20) × 1000 = 65 lps. The factor of 0.8 is the average velocity in such drains for the depth of flow. b) The V notch method This requires the insertion of a V notch plate in the drain at a location where the downstream discharge can be a free fall. These plates can be cut out from stainless steel (SS) or Teflon sheets of nominal thickness of about 2 mm and inserted tightly into the drain and the gaps can be closed by a mixture of clay and cement in equal proportion mixed to a thick consistency and smeared on the downstream side. The V notch is best chosen such that the angle subtended is 90 degrees. The clearances to be ensured are shown in Figure 3. Figure 3.1 Typical mounting of a V Notch in a drain The depth of flow is measured over the lower tip of the V bottom and the discharge is Q = 1.42 × tan of angle of V notch × H power 2.5 (3.5)
Part A: Engineering CONSTRUCTION OF SEWERS Figure 3.3 Typical mounting of a rectangular weir with end constrictions in a drain Figure 3.4 Palmer-Bowlus flume installation in drains e) The Palmer-Bowlus Flume This can be used in case of both the drains and pipes flowing under gravity. Its major advantages are (i) less energy loss; (ii) minimal restriction to flow and (iii) Easy installation in existing conduits. It is a readymade piece for various widths and diameters. The placement in a drain will be as in Figure 3. and that in a sewer pipe will be as in Figure 3.5 overleaf.
Part A: Engineering CONSTRUCTION OF SEWERS Figure 3.5 Palmer-Bowlus flume installation in circular sewer This has the specific advantage of its ability to be placed in a manhole to measure the sewage flow in the gravity sewer as long as the flow is not exceeding the diameter of the sewer. Typical installation details are seen in Figure 3.6 overleaf. The depth of flow needs to be measured in only one location and thus it is a lot easier. In addition, it can be easily removed after measurement. The only disadvantage is it cannot be used when the depth of flow exceeds the diameter of the sewer and to this extent, it may have limitations in the surcharged condition of sewers in historical cities. This also has the advantage of facilitating a flow measurement in large diameter sewers, which flow under gravity and the flume itself is much simpler as in Figure 3.7. The chart for getting at the flow once the depth is measured is obtained by relating to a standard curve supplied by the plume manufacturer depending on the shape of the plume. This is also available as software linked to a personal computer. The combination of the Palmer-Bowlus and Tracer dye techniques have been reported as early as 1974 as illustrated in Figure 3.7. It is a system worth inducting in large trunk sewers near the outfalls to have an integrated measurement of the flows and key quality parameters or at least for the flow details and variation patterns.
Part A: Engineering CONSTRUCTION OF SEWERS f) The Venturi Pipe or the Dall Tube While dealing with old pumping mains, there is a chance of detecting a venturi pipe fitting in the pipeline, as was the standard practice in those years. The flow through it is a function of the difference in head of the fluid at the mouth and the throat and the formula for a given venturi metre is very simple as Q = K × (a1 x a2) (factor) (3.9)
factor = SQRT (2gh/(a1^2 - a2^2 )) (3.10)
Where K = 0.95 to 0. a1 = area in sqm at mouth a2 = area in sqm at throat h = h1-h h1 = piezometric water level in m at mouth h2 = piezometric water level in m at throat It is thus clear that once the difference in head is measured between sewage pressure head at mouth and at throat, the square root of the same is directly proportional to the flow. It is possible to connect a differential Mercury manometer to the sampling ports in the metre and open the quarter turn-cock when flow needs to be measured and to note the reading. A simple wall chart relating the difference to the flow will be more than needed. Of course, instrumentation is possible by connecting the two pressures to a differential pressure transmitter and taking its output to a square root extractor and then to a multiplier for the constant for the metre and thereby get a continuous reading of the flow without any interventional systems. Suffice to say that so far as estimation of flows for design of sewer systems or augmentation of sewer systems are concerned, where an existing pumping station with a venturi meter in the delivery main is available, a simple mercury manometer U tube, connected to the ports of the venturi meter may help in ascertaining the variation of the flow pattern and arrive at peak flow factors etc. more realistically. A Dall tube is nothing but a venturi pipe-fitting of a reduced length and as otherwise all other properties of flow measurements are the same. In fact, if possible this can be inserted into an existing pumping main for the evaluation of the above flow patterns.
Part A: Engineering CONSTRUCTION OF SEWERS PART - 2 TYPES AND HYDRAULICS OF SEWERS 3.11 TYPES OF COLLECTION SYSTEM These are separate sewers, combined sewers, pressurized sewers and vacuum sewers. 3.11.1 Separate Sewers These sewers receive domestic sewage and such industrial wastes pre-treated to the discharge standards as per the Environment Protection Act 1986. The consent to discharge into sewers are given by the local pollution control administration. 3.11.2 Combined Sewers These sewers receive storm water in addition and have some advantages in locations of intermittent rainfall almost throughout the year and with a terrain permitting gravitated collection and obviously being confined to a very small region as a whole. As otherwise, in regions of seasonal rainfall like in monsoons, the combined system will have serious problems in achieving self cleansing velocities during dry seasons and necessitating complicated egg shaped sewers etc. to sustain velocities at such times, plus the treatment plant to be designed to manage strong sewage in dry season and dilute sewage in monsoon season as also the hydraulics. These sewers are also ideally suited for resorts and private development. 3.11.3 Pressurized Sewers Pressurized sewers are for collecting sewage from multiple sources to deliver to an existing collection sewer, and/or to the STP and are not dependent on gravity and thus topography is not a challenge. Typically, sewage from establishments in the vicinity is collected in a basin fitted with submersible pump to lift and inject the sewage to a sewer on the shoulder of the roadway, thus sparing the riding surface from the infamous digging for initial repairs and often for repairs. The principle advantages are the ability to sewer areas with undulating terrain, rocky soil conditions and high groundwater tables as pressurized sewers can be laid close to the ground and anchored well besides there cannot be infiltration, and exfiltration is quickly detected and set right and essentially smaller diameter pipes and, above all, obviating the cumbersome deep manholes as also road crossings by CI or DI pipes with trenchless technology laid inside a casing pipe and installation without disrupting traffic, opening trenches across paved roadways, or moving existing utilities etc. An important issue is for each plot to have a grinder pump set and each commercial plot to have its own grease interceptors to remove excessive fats, oils & grease before the grinder pump. Obviously, this system is not suitable for continuous building area. A disadvantage is the need to ensure unfailing power supply to the grinder pump and hence this is perhaps limited to high profile condominiums and not the public sewer systems in India. A typical profile is shown in Figure 3.8 overleaf.
Part A: Engineering CONSTRUCTION OF SEWERS 3.12 MATERIALS, SHAPES AND SIZES OF SEWERS 3.12.1 Introduction Factors influencing the selection of materials for sewers are flow characteristics, availability in the sizes required including fittings and ease of handling and installation, water tightness and simplicity of assembly, physical strength, resistance to acids, alkalies, gases, solvents, etc., resistance to scour, durability and cost including handling and installation. No single material will meet all the conditions that may be encountered in sewer design. Selection should be made for the particular application and different materials may be selected for parts of a single project. The determination of the suitability in all respects of the pipes and specials for any work is a matter of decision by the engineer concerned on the basis of requirements for the scheme and guided by Appendix A.3-10 on relative limitations on use of pipe materials in specific locations. 3.12.2 Brick Brickwork is used for construction of sewers, particularly in larger diameters. Many old brick sewers are still in use and the failures are mainly due to the disintegration of the bricks or the mortar joints. Because of the comparatively higher cost, larger space requirement, slower progress of work and other factors, brick is now used for sewer construction only in special cases. The advantage of brick sewers is that these could be constructed to any required shape and size. Brick sewers shall have cement concrete or stone for invert and 12.5 mm thick cement plaster with neat finish for the remaining surface. To prevent ground water infiltration, it is desirable to plaster the outside surface. Inside plaster can be with mortar using high alumina cement conforming to IS 6452 or polyurea coating and the outer surface shall be plastered with mortar using sulphate resistant cement. 3.12.3 Concrete The advantages of concrete pipes are the relative ease with which the required strength may be provided, feasibility of adopting a wide range of pipe sizes and the rapidity with which the trench may be backfilled. However, these pipes are subject to crown corrosion by sulphide gas, mid depth water line corrosion by sulphate and outside deterioration by sulphate from soil water. These shall be manufactured with sulphate resistant cement and with high alumina coating on the inside at the manufacturers works itself. Protective measures as outlined in corrosion protection in sewers shall be provided where excessive corrosion is likely to occur. 3.12.3.1 Precast concrete Plain cement concrete pipes are used in sewer systems on a limited scale only and generally, reinforced concrete pipes are used. Non-pressure pipes are used for gravity flow and pressure pipes are used for force mains, submerged outfalls, inverted siphons and for gravity sewers where absolute water-tight joints are required. Non-pressure pipes used for construction of sewers and culverts shall confirm to the IS 458. Certain heavy-duty pipes that are not specified in IS 458 should conform to other approved standards.
Part A: Engineering CONSTRUCTION OF SEWERS 3.12.3.2 Cast-in-situ Reinforced Concrete Cast-in-situ reinforced concrete sewers are constructed where they are more economical, or when non-standard sections are required, or when a special shape is required or when the headroom and working space are limited. The sewer shape shall be of an economic design, easy to construct and maintain and shall have good hydraulic characteristics. Wide flat culvert bottoms shall be provided with “Vee” of at least 15 cm cuvettes in the centre. All formwork for concrete sewers shall be unyielding and tight and shall produce a smooth sewer interior. Collapsible steel forms will produce the desirable sewer surface, and may be used when the sewer size and length justify the expense. It is desirable to specify a minimum clear cover of 50 mm over reinforcement steel and a minimum slump consistent with workability shall be used for obtaining a dense concrete structure free of voids. The distance for cutting concrete shall be kept to a minimum to avoid segregation and the vibrating of concrete done by approved mechanical vibrators. Air entraining cement or plasticizing agents may be used to improve workability and ensure a denser concrete. Concrete shall conform to IS 456. 3.12.4 Stoneware or Vitrified Clay These pipes are normally available in lengths of 90 cm and the joints need caulking with yarn soaked in cement mortar and packing in the spigot and socket joints, which requires skilled labour. Specifications for the AA class and A class are identical except that in the case of class AA pipes, one hundred percent hydraulic testing has to be carried out at the manufacturing stage, whereas in the case of Class A only five percent of the pipes are tested hydraulically by following IS 651. The resistance of vitrified clay pipes to corrosion from most acids and to erosion due to grit and high velocities gives it an advantage over other pipe materials in handling acid concentrations. A mini- mum crushing strength of 1,600 kg/m is usually adopted for all sizes manufactured presently. The strength of vitrified clay pipes often necessitates special bedding or concrete cradling to improve field supportive strength. 3.12.5 Asbestos Cement For sewerage works, asbestos cement pipes are usually used in sizes ranging from 80 mm to 1000 mm in diameter. Standard specifications have been framed by the BIS in IS 6908. Non corrosiveness to most natural soil conditions, freedom from electrolytic corrosion, good flow characteristics, light weight, ease in cutting, drilling, threading and fitting with specials, allowance of greater deflection up to 12 degrees with mechanical joints, ease of handling, tight joints and quick laying and backfilling are to be considered. These pipes cannot however stand high super imposed loads and may be broken easily. They are subject to corrosion by acids, highly septic sewage and by highly acidic or high sulphate soils. Protective measures as outlined in corrosion protection in sewers shall be provided in such cases. While using AC pipes strict enforcement of approved bedding-practices will reduce possibility of flexible failure. Where grit is present, high velocities such as those encountered on steep grades may cause erosion. It is stated that in a recent process of manufacture titled Maaza, high forming pressures of up to 80 kg / sqcm, leading to very smooth surface and very few air pores are possible. However, the relevant BIS standard or code of practice is awaited.
