CONSTRUCTION OF SEWER final (1) gro.pptx

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1. CONSTRUCTION OF SEWERS By Your Name

2. All kinds of liquid waste produce by the community is known as sewage.it includes domestic waste water, industrial waste water and storm water What is sewage? What is sewer? A Conduit conveying sewage is known as sewage What is sewerage? Network of sewer required in collection, conveyance,and disposal is known as sewerage

3. Method of sewerage 1.COMBINED SYSTEM : In combined system only one sewerage is used to collect DWF& WWF. This system is best suitable for community which recieve average uniform rains and rainfall season spread for short period. Ex : Chennai 2.SEPARATE SYSTEM : In separate system employees two sewer, one sewer handle DWF and other one WWF. Best suitable for community which receives high intensity heavy rainfall seasons spread for long period. Ex : Mumbai 3,PARTIALLY COMBINED AND PARTIALLY SEPARATE SYSYTEM ; This system employs two sewer and opne is for WWF and other sewer occasionally accommodate WWF along with DWF. It is suitable for communities which receive uniform rains with Occasional Heavy rains,

4. Sewers, their Construction , Maintenance and Required Appurtenances

5. Introduction Sewer pipes , are generally circular pipes laid below the ground level, and generally sloping continuously towards the outfall. They are designed to flow under gravity, except for the outfall sewer which carries the treated and pumped sewage into the discharge source.

6. Shapes of Sewer Pipes These Sewer pipes are normally circular in section , although various other sections are used also. Here are some shapes about which we are going to discuss later.

7. Selection of Shape Self cleansing velocity in DWF Sufficient freeboard in max discharge Easy cleaning and maintenance Structurally safe and Stable

8. Classification Circular Sewer Non Circular Sewer Egg shaped (Ovoid Shape) Rectangular Shaped Horse shoe Shaped Parabolic Shaped Semi Elliptical Shaped U – Shaped Semi circular Shaped Basket handle Shaped

9. Circular Sewer

10. Easily Manufactured Gives maximum aea for a given perimeter Most efficient since it gives greatest hydraulic mean depth when running half or full Economical since it utilizes minimum quantities of material Less settlement of deposits due to uniform cuvature Self cleansing cannot be maintained at DWF conditions in combined system Suitable only when variation of discharge is not large Advantages Disadvantages Circular Sewers

11. Non-Circular Sewer

12. Egg Shaped Sewer (Ovoid Shape) Type of closed sewer. Depth is one and half times of their width. Smaller radius at bottom and Larger at the top. Mostly constructed with RCC.

13. Egg Shaped Sewer Provides slightly higher velocity for low flows over circular sewer of equal capacity Effective in combined system Unstable as small end of egg is down and has to support weight of upper broader section Difficult to construct Expensive as more material is required High construction cost Not self cleansing in absence of adequate gradient Advantages Disadvantages

14. Rectangular shaped Sewer Ease in construction and More stability Hydraulically inefficient

15. Horse shoe Shaped Sewer Used for large sewer with heavy discharges such as trunk and outfall sewers. Suitable when headroom for the construction of sewer is limited. Invert of the section may be flat , parabolic or circular. Crown is semicircular and can support extra external load without the aid of backfilling.

16. Parabolic Shaped Sewer Upper arch of sewer forms the shape of parabola. Used for carrying comparatively small quantity of sewage. Invert of the section may be elliptical or parabolic.

17. Semi Elliptical Shaped Sewer Used for soft soil as it is more stable. Useful only for carrying large amount of Sewage. Adopted when sewers have width > 2m.

18. U-Shaped Sewer Used for combined sewer having maximum flow of Storm water. Used for Long Sewers and Specially in open cuts. Invert is in the form of Semi circular arch.

19. Semi-Circular Shaped Sewer This section gives a wider base at bottom and hence it becomes suitable for constructing large sewers with less available headroom. It is outdated.

20. Basket handle Shaped Sewer Bottom portion is narrower in width than upper portion. Carries small discharge through bottom narrow portion and runs full during monsoon. Useful for maintaining self cleansing velocity in DWF. Outdated.

21. Forces Acting on Sewer Pipes Structural Design Considerations This slide introduces the topic of forces affecting sewer pipes and emphasizes the importance of structural design considerations in ensuring their integrity and functionality.

22. Types of forces acting on sewage: Internal Pressure of Sewage Pressure Due to External Load Temperature Stresses Flexural Stresses

23. Internal pressure arises from the flow of sewage within the pipes. Sewer systems primarily designed as gravity conduits experience minimal internal pressure, as they rely on the force of gravity for sewage flow. However, in situations where sewers must operate under pressure, such as outfall sewers or during overflow events, internal pressure can become significant. This internal pressure tends to exert force outward on the pipe walls, potentially leading to pipe bursting and inducing tensile stresses. Therefore, sewer pipes intended for pressurized applications must be constructed with materials capable of withstanding tension. Internal Pressure

24. The internal pressure exerted on a sewer by the sewage flowing through it induces circumferential tensile stress called hoop stress in the wall of the sewer which is given by the following expression- σ = pD / 2t In which σ = hoop stress or circumferential tensile stress; p = internal pressure of se wage; D = diameter of sewer; and t = thickness of sewer wall. Hence the pressure mains or pressure sewers must be designed to withstand the above stress.

25. Pressure due to external load Buried sewer pipes are subjected to external loads from various sources, including the weight of the pipe itself, backfill materials used during installation, and additional loads from traffic or nearby structures. These external loads result in compressive stresses within the pipe material, particularly significant for deep-seated sewer lines. Engineers use empirical formulas, such as those developed by Marston, to assess stresses from external loads and select suitable pipe materials and reinforcement techniques accordingly.

26. CASE1:-For pipes resting on or projecting above For pipes resting above or projecting above undisturbed ground in cohesionless soil and covered with fills such as in highway culvert the external load likely tto come per unit length of pipe(W) is given by:- W=C p ɡD²(1) where C p =a coefficient whose value depends upon the type of pipe and character of foundation backfill.Typical value of Cpare given in table ɡ= sp.wt of the fill material D= the external diameter of the pipe = (Internal diameter+2*Thickness)

27. For C p values for resting pipes following table can be used

28. CASE2:-For Flexible pipes For flexible pipe buried is narrow trenches and with thoroughly compacted side fills,the external load per unit length of the pipes is given by:- W=C.ɡ.B.D(2) Where C=a coefficient characterising the fill material and ratio H/B CASE3:-For Rigid Pipes For rigid pipes buried in narrow trenches and throughly compacted with cohemsionless fills the external load per unit length of the pipe is given by:- W=C.ɡ.B²(3)

29. for flexible pipes and rigid pipe in narrow trenches the C value given in the table can be used

30. CASE4:-For Superimposed loads the amount of super imposed loads which is transmitted to the pipe can be evaluated by using boussineq equation. Assumong the fill surface to be horizontal,the equation is:- p t =3.H 3 .p/2 π  Z 5 Where pt=unit pressure developed at any point is the fill at depth H below the surface due to traffic load P=superimposed load Z=the slant height of the considered point from the load P H=Distance of the top of pipe below the surface of the fill

31. Temperature variations in the environment can cause sewer pipes installed above ground to expand and contract. If expansion or contraction is restricted, longitudinal stresses may develop within the pipe material. To accommodate thermal movements and prevent potential damage or deformation, expansion joints are occasionally incorporated into the pipe design. However, for sewer pipes buried underground, temperature stresses are typically less significant and may not require specific design considerations related to thermal expansion and contraction. Temperature Stresses

32. The amount of temperature stresses in the pipe are calculated by following expression:- Elongation=L.ɑ.T But Strain=elongation per unit length =L.ɑ.T/L =ɑ.T Stress=E.Strain=E P .ɑ.T f=E P .ɑ.T Where E p =Modulus of elasticity of the pipe material ɑ=Co-efficient of expansion of the material T=Change in Temperature

33. Sewer pipes may encounter flexural stresses when supported between structures or exposed due to ground washout, resembling beams. These stresses arise from the distribution of loads, including the weight of the pipe itself, the weight of sewage flowing within the pipe, and any additional superimposed loads. Structural analysis techniques are employed to evaluate the bending behavior of sewer pipes under various loading conditions, ensuring their structural integrity over time. Flexural Stresses

34. Material Selection and Design Considerations The selection of appropriate materials and meticulous design considerations are critical for ensuring the durability and effectiveness of sewer pipes. Factors such as soil characteristics, anticipated loads, and environmental conditions guide the selection of materials for sewer pipe construction. Advanced design techniques are employed to ensure that sewer pipes can withstand the anticipated forces and environmental stresses over their service life. By meticulously assessing these factors and implementing robust design practices, engineers can optimize the performance and longevity of sewer systems while minimizing the risk of structural failures.

35. Brick • This Material is used particularly for construction of large diameter combined sewer or particularly for storm water drains. • Advantage- Can be constructed to any required shape and size • Disadvantage- higher cost Slow work progress Larger space requirement • To prevent ground water infiltration The pipe are plaster outside surface with mortar using sulphate resistant cement to Avoid entry of tree roots and ground water through brick joits. Inside surface mortar + high alumina cement or polyurea coating.

36. Concrete • Advantages – Relative ease with which required strength may be provided – Wide range of pipe sizes – Rapid trench backfill • Disadvantages – crown corrosion by sulphide gas – mid depth water line corrosion by sulphate – outside deterioration by sulphate from soil water

37. PLAIN CEMENT CONCRETE OR REINFORCED CEMENT CONCRETE Strong in tension as well as compression. Resistant to erosion and abrasion. They can be made of any desired strength. The carrying capacity of the pipe reduces with time because of corrosion.

38. Precast concrete • Commonly used for branch and main sewers • Two types – Non-pressure pipes • used for gravity flow (sewers running partially full) • Plain (I.D. 80mm-450mm; thickness: 25mm-35mm) • Reinforced (I.D. 80mm-2600mm; thickness: 25mm-215mm) – Pressure pipes • used for force mains, submerged outfalls, inverted siphons and for gravity sewers where absolute water-tight joints are required. • Reinforced (I.D. 80mm-1200mm; thickness: 25mm-120mm) • Length : 2-3m • These pipes have plain ends or spigot and socket ends.

39. Stoneware or Vitrified Clay • These pipes are used for house connection as well as lateral sewers. The size of the pipe available is 5cm to 30cm internal diameter with length 0.9 to 1.2m. • Skilled labour required for caulking the joints with yarn soaked in cement mortar and packing in the spigot and socket joints

40. ADVANTAGES Resistant to corrosion ,hence fit for carrying polluted water such as sewage. Interior surface is smooth and is hydraulically efficient. The pipes are highly Impervious. Strong in compression.

41. DISADVANTAGES Heavy,bulky and brittle and hence ,difficult to transport. These pipes cannot be used as pressure pipes,because they are weak in tension. These require large number of joints.

42. Asbestos Cement • usually used in sizes ranging from 80 mm to 1000 mm in diameter • Advantages – Non corrosiveness to most natural soil conditions, . – good flow characteristics, – light weight, – ease in cutting, drilling and fitting with specials, – ease of handling, – tight joints and – quick laying and backfilling are to be considered • Disadvantages – subject to corrosion by acids, highly septic sewage and by highly acidic or high sulphate soils – Cannot stand high super imposed loads and may be broken easily

43. Cast Iron • Available in diameters from 80 mm to 1050 mm and are covered with protective coatings • supplied in 3.66 m and 5.5 m length. • Classified as LA, A and B according to their thickness. • Application: Cast iron pipes with a variety of jointing methods are used for pressure sewers, sewers above ground surface, submerged outfalls, piping in sewage treatment plants and occasionally on gravity sewers where absolutely water-tight joints are essential or where special considerations require their use.

44. • Advantage: long laying lengths with tight joints, ability when properly designed to withstand relatively high internal pressure and external loads and corrosion resistance in most natural soils. • Disadvantage: – corrosion by acids or highly septic sewage and acidic soils • Inside coating shall be by Cement mortar and outer coating shall be coal tar

45. Steel • Pressure sewer mains, under water river crossings, bridge crossings, necessary connections for pumping stations, self-supporting spans, railway crossing and penstocks are some of the situations where steel pipes are preferred • Advantages – withstand internal pressure, impact load and vibrations much better than CI pipe. – more ductile and withstand water hammer better. • Disadvantage – cannot withstand high external load – main is likely to collapse when it is subjected to negative pressure – susceptible to various types of corrosion • Inside coating by high alumina cement mortar or polyurea and outside by epoxy.

46. Ductile Iron Pipes • made by a metallurgical process, which involves the addition of magnesium into molten iron of low sulphur content. • The ductile iron pipes are usually provided with cement mortar lining at the factory by centrifugal process to ensure a uniform thickness throughout its length • available in the range of 80 mm to 1000 mm diameter, in lengths of 5.5 to 6 m • Advantages – excellent properties of machinability, – impact resistance, – high wear and tear resistance, – high tensile strength and ductility – corrosion resistance. – strong, both inner and outer surfaces are smooth, – These pipes are approximately 30 % lighter than conventional cast iron pipes UPVC Pipe (unplasticized polyvinylchloride )

47. – resistance to corrosion, – light weight for transportation, – toughness, rigidity, – economical in laying, jointing, and maintenance and – easy to fabricate .

48. High Density Polyethylene (HDPE) Pipes • Advantages – Offer smooth interior surfaces and – relatively higher resistance to corrosion – they are available in solid wall – When laid in straight gradients without depressions, they can easily offer longer life cycle • Joints are usually fusion welded or flange jointed depending on straight runs fittings.

49. Plastic Sewers (PVC Pipes) Plastic is recent material used for sewer pipes. These are Used for internal drainage works in house. These are available in size 75 to 315 mm external diameter and used in drainage works. They offer smooth internal surface. The additional advantages they offer are resistant to corrosion, Light weight of pipe,economical in laying.

50. Glass Fiber Reinforced Plastic Pipes This martial is widely used where corrosion resistant pipes are required. Glass fiber reinforced plastic (GRP) can be used as a lining material for conventional pipes to protect from internal or external corrosion. It is made from the composite matrix of glass fiber, polyester resin and fillers. These pipes have better strength, durability, high tensile strength, low density and high corrosion resistance. These are manufactured up to 2.4 m diameter and up to 18 m length (IS:12709-1989).

51. Lead Sewers They are smooth, soft and can take odd shapes. This pipe has an ability to resist sulphide corrosion. However, these pipes are very costly. These are used in house connection.

52. LAYING OF SEWERS

53. LAYING OF SEWERS All the sewer pipes are generally laid starting from their outfall end, towards their starting ends. The advantage gained in starting from the tail end, , (i.e. out fall end) is the utilisation of the tail length even during the initial periods of its construction, thus ensuring that the functioning of the sewerage scheme has not to wait till the completion of the entire scheme.

54. LAYING Starting from outfall end to starting end. Manholes are required to be constructed. Laying is done between two man holes. Detailed map study Map showing all roads, cable lines, lanes etc., is prepared. Pipe line with size and length is marked. The position of existing pipe lines, curb lines, water lines will also be marked.

55. CENTER LINE AND OFFSET LINE MARKING: Centerline of the sewer is marked by drawing an offset line on the ground parallel to the center line at suitable fixed horizontal distance(2-3m). OFFSET DISTANCE = ( 0.5 x trench width) +0.6m. This offset line should be drawn on that side of the trench which is not likely to be disturbed by the piling of the excavated earth. Offset is used to find the center line by offsetting the fixed distance from offset line. Temporary bench marks along this offset line ,at intervals of 200-300m should also be established by carrying the levels from G.T.S bench mark.

56. TRENCHES: The Road pavements may have to be first of all ,removed . (Softer pavements –pick axes by manual labour ) and (hard concrete pavements –pneumatic drills or spades) The inner portion is remover either by pick axes or power shovels, boom and bucket excavator … Min. width of the trench = 15cm more than the dia of sewer For smaller dia pipes =60-75cm. Pipes in firm ground no need of embedding of concrete and the bottom half portion of the trench is excavated as the shape of the pipe . Pipes in softer or ordinary grounds need embedding of concrete. A recess is also excavated at required intervals for accommodation of joints and caulking at the bottom

57. TIMBERING: It is used when the excavation is more than 2m and It is used to resist soil from falling into the trench. The sheeting or sheeting boards are the wooden planks which are placed in actual contact with the trench sides, either horizontally or vertically. The braces are the cross wooden pieces extending from one side of the trench to the other side, and may also be called struts. The ranger are waless are the timber planks which transfer the transfer the loads from the sheeting boards to the cross braces by jointing the sheeting boards together. The softer soil require more elaborate and closer timbering than that of harder soils. In case of extremely soft and water logged soil or in places where timber is not available steel sheeting(water tight and durable) are used.

58. DEWATERING Dewatering is require if the w.t is high or if the sewer happens to be laid very deep. Its removed by open jointed drain constructed below the sewer trench ,which discharges into an independent water course either by gravity or pumping. Its removed by pumping out the water from the sump constructed in the trench. It should be operated for 24 hours if the water keeps on flowing int the trench. Dewatering in sandy soils become enormous, because if the water flow is large and the sub soil is sandy , quicksand is caused.so it is done by well pointing system. NO. of well points are installed by water jetting method along the sides of the trench at suitable intervals of 1.5m . All these well points are connecter to a common header pipe which is connected to a pump.

59. SHAPING THE TRENCH AT THE BOTTOM TO THE CORRECT LEVEL: It should be excavated equal to the bottom of the embedding concrete or to the invert level of the sewer. The bottom most portion of the trench should be excavated carefully so as not to dug more than require. Two vertical posts are first of all driven along the sides of the trench at the interval of 10m at all junction and change points. A horizontal board spanning the entire width is now fixed on that post by means of nails is called sight rail. A vertical pencil line is marked on the wooden board and a needle is fixed on that line. A string is stretched between the needle and produce a line in space and used to measure the depth of foundation ,invert level and embedded concrete. A boning rod or traveler is used to measure levels at the bottom of the trench

60. LAYING : After the bedding concrete has been laid in the required alignment and levels, the sewer pipes are lowered down into the trenches. Smaller pipes less than 40cm dia can be directly handled by masons and their helpers. Larger pipes can be lowered by passing ropes around each end of the pipe length.For very large pipes machines have to be used. The sewer pipe lengths are usually laid from the lowest point with their socket end facing upstream so that the spigot end can easily be jointed. After joints are jointed ,the jointing is done with lead caulking or cement mortar as required. During jointing care should be taken to see that the pipe lengths remains in true level and alignment, and do not get disturbed till the jointing material hardens.

61. TESTING TEST FOR LEAKAGE OR WATER TEST:- Sewers are tested to ensure no leakage in their joints. They are tested between manhole to manhole under a test pressure of 1.5m of water head. Lower end is plugged and the water is allowed to flow through the upper end . Depth of water in the manhole is maintained to the testing head of about 1.5m. The sewer line is watched by moving along the trench and the joints which leak or sweat are repaired.

62. TEST FOR STRAIGHTNESS OF ALIGNMENT OR OBSTRUCTION:- It is tested by placing a mirror at one end and the lamp at the other end. If the pipe line is straight full circle of light will be observed . If the pipe is not straight it would be apparent and the mirror will also indicate any obstruction in pipe. Obstruction in the pipe is tested by inserting a smooth ball of dia 13mm less than the internal dia of sewer at the upper end of the sewer . If no obstruction is their the ball rolled down to the lower end.

63. Backfilling: While backfilling the excavated earth it should be laid equally on either side of the sewer and it should filled up in layers of 15cm thick. Each layer should be properly watered, tamped and rammed. Earth filling above the crown of the sewer should be carefully carried out. After few months of exposure , when the top layer gets fully settled, the road pavements, may be constructed . This will prevent the subsidence and cracking.