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![Therefore, combined discharge (Q) = 0.5+4.25
= 4.75 m3/s
Now, 𝐴 =
𝑄
𝑉
=
4.75
3.2
= 1.48 m2
All velocities must lie between (0.75-3.2) m/s . Hence, 3.2 m/s is assumed.
Diameter of sewer , 𝐷 =
4𝐴
𝜋
=1.374 m
Adopting commercially available size 1.4 m,
𝐴 =
𝜋𝐷2
4
=
𝜋𝑥 (1.4)2
4
= 1.54 m2
𝑉 =
𝑄
𝐴
=
4.75
1.54
= 3.08 𝑚/𝑠<3.2 m/s. Hence,ok.
Check for cleansing velocity during dry weather flow
𝑞
𝑄
=
0.25
4.75
=
1
19
= 0.0526
𝑞
𝑄
=
𝜃
360
[1 −
360𝑠𝑖𝑛𝜃
2𝜋𝜃
]5/3
56](https://image.slidesharecdn.com/designandconstructionofsewer-3-g2-180813133018/75/Design-and-construction-of-sewer-56-2048.jpg)
![[18.94
𝜃
]3/5
+
360𝑠𝑖𝑛𝜃
2𝜋𝜃
-1 = 0
Solving, 𝜃 = 93o
𝜗
𝑉
= [1 −
360𝑠𝑖𝑛𝜃
2𝜋𝜃
]2/3
Or, v= 3.08 x 0.529 = 1.629 m/s
0.75 m/s < 1.629 m/s < 3.2 m/s Hence,ok.
Check for self cleansing velocity during minimum flow
Assume Qmin = 1/2 of DWF
𝑞
𝑄
= 0.0263
𝑞
𝑄
=
𝜃
360
[1 −
360𝑠𝑖𝑛𝜃
2𝜋𝜃
]5/3
Solving, 𝜃 = 78.077
o
𝜗
𝑉
= [1 −
360𝑠𝑖𝑛𝜃
2𝜋𝜃
]
2/3
Or, v= 3.08 x 0.43 = 1.32 m/s
0.75 m/s < 1.32 m/s < 3.2 m/s Hence , ok.
57](https://image.slidesharecdn.com/designandconstructionofsewer-3-g2-180813133018/75/Design-and-construction-of-sewer-57-2048.jpg)
![When flowing depth of pipe = 0.75m
𝑑
𝐷
= 0.75
Central angle (𝜃) is given by , cos
𝜃
2
= (1 −
2𝑑
𝐷
)
= (1- 2 x 0.75 )
𝜃= 240o
𝑞
𝑄
=
𝜃
360
[1 −
360𝑠𝑖𝑛𝜃
2𝜋𝜃
]5/3
Discharge(Q) = 1.05 m3/s
Therefore, q = 0.9575 m3/s
Now,
𝜗
𝑉
= [1 −
360𝑠𝑖𝑛𝜃
2𝜋𝜃
]
2/3
Velocity (v) = 2.42 m/s
Therefore , 𝜗 = 2.74 m/s
59](https://image.slidesharecdn.com/designandconstructionofsewer-3-g2-180813133018/75/Design-and-construction-of-sewer-59-2048.jpg)
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Design and construction of sewer
- 1. Group Members: Sharmistha, 151 Shikshita,152 Shrijan, 153 Shubham, 154 Siddhi, 155 Sital, 156 Tutor: Asst. Prof. Shukra Raj Paudel Department of Civil Engineering IOE, Tribhuvan University 2018-07-30 3.0 DESIGN AND CONSTRUCTION OF SEWERS
- 2. Objectives of thePresentation 2 To know and understand the design procedures of sewers To know about the different types of sewer available To know about the various types of material used in construction of sewers To have a detailed knowledge on construction of sewers
- 3. Presentation outlines Shapes ofSewers Design Criteria of Sewers Sewer Materials Design of Sewers of separate and combined systems Numerical on design of sewers Construction of sewers 3
- 4. 4 Types of Flow PressurizedFlow Open channel flow Vacuum Flow 3.1 Design criteria of sewers
- 5. 5 DESIGN Discharge Velocity 3.1 Design criteriaof sewers(cont.)
- 6. 3.1 Design criteriaof sewers(cont.) Design period • 20 to 30 years Specific gravity of sewage • Sewage consists of 99.9% water • Specific gravity is nearly equal to 1 Velocity of flow • Velocity of flow must lie between self cleansing velocity and limiting velocity. 6
- 7. Camp Shieldformula: V = 8β S − 1 gd f where, V=self cleansing velocity β=dimensionless constant whose value depends upon the characteristics of sediments present in the Weisbach friction factor; S=Specific gravity of sediments; g=acceleration due to gravity; d=diameter of solid particles 7
- 8. Minimum Velocity: Also known as self-cleansing velocity. It is the velocity at which solid particles will remain in suspension. Criteria for determining minimum velocity: For combined system, self cleansing velocity is 0.75m/s. According to Badwin Latham self-cleansing velocity depends upon the diameter of sewers. Table 3.1 Self cleansing velocities 8 Diameter(cm) Velocity (m/s) 15-25 1.0 30-6 0.7 >60 0.6
- 9. 9 Sewer materials Limitingvelocities Vetrified tiles 4.5-5.5 Cast iron 3.5-4.5 Stoneware 3.0-4.0 Cement concrete 2.5-3.0 Ordinary brick lined 1.5-2.5 Earthen channels 0.6-1.2 Maximum Velocity: Also known as Limiting Velocity Velocity above which scouring or erosion of inner surface will occur Scouring mainly due to abrasive action of harder materials like sand, girt, gravel etc Limiting velocity depends upon the material of sewer
- 10. Minimum sewersize: Not less than 15 cm. Recommended size is 20 cm. In hilly areas it may be 10 cm. Standard pipe sizes are: 20 cm, 30 cm, 35 cm, 40 cm with increase of 5 cm upto 2m. Sewer grades: It is the slope at which sewer is laid. Generally follows the natural slope of the ground. Not steeper than 1 in 20. Slope of 1 in 40 to 1 in 80 for house sewer connected to public sewer. Gradients for self cleansing velocity of at least 0.6 m/s is presented in the table 3.3. 10
- 11. Table 3.3 Minimumgradient Diameter of sewer(cm) Gradient 20 0.003 30 0.002 45 0.0015 60 0.0008 75 0.0006 100 0.0005 11
- 12. Hydraulic formulae: 1.Chezy’s formula: V = C RS where, V= velocity if flow; C= Chezy’s coefficient; R= Hydraulic mean depth; S= Slope or gradient of sewer Different formulae for the calculation of Chezy’s coefficient: a) Kutter’s expression: 𝐶 = 23+ 0.00155 𝑆 + 1 𝑛 1+ 23+ 0.00155 𝑆 𝑛 𝑅 where, n=roughness coefficient 12
- 13. b) Bazin’s formula: C= 157.6 1.81 + m R where, m is Bazin’s coefficient 2. Hazen-Williams Formula 𝑉 = 0.849 𝐶 𝐻 𝑅0.63 𝑆0.54 where, 𝐶 𝐻 is the Hazen- William’s coefficient 3. Manning’s formula: V = 1 N R2 3 S1 2 where, 13
- 14. Conduit material Condition ofinterior surface Good Fair Salt glazed stoneware 0.012 0.014 Cement Concrete 0.013 0.015 Cast iron 0.012 0.013 Brick unglazed 0.013 0.015 Asbestos cement 0.011 0.012 Plastic smooth 0.011 0.011 14 Table 3.4 Manning’s coefficient n 5. Crimp and Bruge’s formula: 𝑉 = 83.47𝑅2 3 𝑆1 2
- 15. Hydraulic elementsof sewers: Most commonly used sewers are circular. Hydraulic elements for different flow conditions are presented below: Source: Neupane, 2016 15 Source:Neupane,2016
- 16. i. Circular sewersrunning full: Area of flow section A = π 4 D2 Wetted perimeter 𝑃 = π𝐷 Hydraulic mean depth 𝑅 = 𝐴 𝑃 = 𝐷 4 Manning’s formula: Velocity of flow V = 1 N R2 3 S1 2 Discharge 𝑄 = 𝐴. 𝑉 = 𝐴 𝑁 R2 3S1 2 = 0.3116 𝑁 𝐷8/3 𝑆1 2 16
- 17. ii. Circular sewersrunning partially full: Central angle θ is given by 𝑐𝑜𝑠 θ 2 = 1 − 2𝑑 𝐷 1. Depth 𝑑 = 𝐷 2 − 𝐷 2 cos θ 2 = 𝐷 2 1 − cos θ 2 Proportional depth = 𝑑 𝐷 = 1 2 1 − cos θ 2 2. Area 𝑎 = π 4 𝐷2 × θ 360 − 𝐷 2 cos θ 2 𝐷 2 sin θ 2 = π 4 𝐷2 θ 360 − sin θ 2π Proportional area = 𝑎 𝐴 = θ 360 − sin θ 2π 17
- 18. 3. Wetted perimeterp= π𝐷 θ 360 Proportional perimeter= 𝑝 𝑃 = θ 360 4. Hydraulic mean depth 𝑟 = 𝑎 𝑝 = π 4 𝐷2 θ 360 − sin θ 2π π𝐷 θ 360 = 𝐷 4 1 − 360 sin θ 2πθ Proportional hydraulic mean depth = 𝑟 𝑅 = 1 − 360 sin θ 2πθ 18
- 19. 5. Velocity offlow v = 1 N R2 3 S1 2 Proportional velocity = 𝑣 𝑉 = 𝑁 𝑛 𝑟 𝑅 2 3 Taking 𝑁 𝑛 = 1, 𝑣 𝑉 = 𝑟 𝑅 2 3 = 1 − 360 sin θ 2πθ 2 3 19
- 20. 20 6. Discharge 𝑞= 𝑎 × 𝑣 = 1 n r2 3S1 2 Taking 𝑁 𝑛 = 1, Proportional discharge= 𝑞 𝑄 = 𝑎 𝐴 𝑣 𝑉 = 𝑎 𝐴 𝑟 𝑅 2 3 = θ 360 1 − 360 sin θ 2πθ 5 3
- 21. Partial flowDiagram: • Flow fluctuation occurs in sewer line so it is always designed for partial flow. • Crimp and Berges have developed the following diagram for the calculation of hydraulic elements of sewers known as partial flow diagram. Source:Modi, 2001 21
- 22.
- 23. 3.2 Shapes ofSewers 1 23 Source:en.Wikipedia.org
- 24. Selection of Shape Self cleansing velocity in DWF Sufficient freeboardin max. discharge Easy cleaning and maintenanc e Structurally safe and stable 24
- 25.
- 26. Advantages Easily manufactured Gives maximumarea 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 curvature 26 Disadvantages Self cleansing cannot be maintained at DWF conditions in combined system Suitable only when variation of discharge is not large Circular Sewer
- 27. 27 d/D a/A v/Vq/Q 1.0 1 1 1 0.9 0.949 1.124 1.066 0.8 0.858 1.140 0.988 0.7 0.748 1.120 0.838 0.5 0.5 1 0.5 0.4 0.373 0.902 0.337 0.3 0.252 0.776 0.197 0.2 0.143 0.615 0.088
- 28. Non-Circular Sewer 28 EggshapedParabolicRectangularHorse shoeSemi elliptical
- 29. Egg shaped sewer(OvoidShape) 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 29 Source:28dayslater.co.uk d/D v/V Ovoid Circular 0.25 0.7 0.698 0.20 0.62 0.61 0.1 0.44 0.4 0.05 0.29 0.25
- 30. Provides slightlyhigher 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 Egg Shaped Sewer 30Advantages Disadvantages
- 31. Hydraulically inefficient Ease in construction Morestability Rectangular Shaped 31 Source:en.Wikipedia.org
- 32. 32Horse shoe ShapedSewer 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 Source:www.corrugatedmetalc ulvert.com
- 33. 33Parabolic 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 Source:www.corrugatedmetalc ulvert.com
- 34. 34Semi Elliptical ShapedSewer Used for soft soil as it is more stable Useful only for carrying large amount of sewage Adopted when sewers have width greater than 2m Source:www.corrugatedmetalc ulvert.com
- 35. 35U 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 Source:www.slideplayer.com
- 36. 36Semi circular ShapedSewer This section gives a wider base at bottom and hence it becomes suitable for constructing large sewers with less available headroom It is outdated Source:www.slideplayer.com
- 37. 37Basket handle ShapedSewer 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 Source:www.slideplayer.com
- 38.
- 39. Resistance to corrosion Resistanceto abrasion Strength Durability Cost Hydraulic efficiency Imperviousness Points to be considered 39
- 40. Plain or Reinforced Cement concrete Steel Plastic Vitrified clayor stoneware Asbestos cement Cast Iron Brick 40
- 41. Asbestos cement Manufacturedfrom mixture of asbestos fibre,silica and cement Size 75 to 500 mm in diameter and length up to 4.0 m Light in weight , easy to cut and assemble without skilled manpower , quick laying and backfilling Structurally not very strong 41 Source:www.shutterstock.com
- 42. Vitrified clay orstoneware Manufactured from clay and shales of special qualities and grades Used mainly in house drainage and lateral sewers Size available 5 to 30 cm internal diameter with 0.9 to 1.2 m in length 42 Source:en.Wikipedia.org
- 43. Merits • Highly resistantto sulphide corrosion due to high velocity • Inner surface is smooth hence hydraulically efficient • Posses high compressive strength • Enough resistant to erosion due to grit and silt • Highly impervious • Cheap and easily available Demerits • Weak in tension • Brittle in nature • Quite bulky and heavy • Difficulty in laying and transportation 43
- 44. Brick Used forconstruction of large sized combined sewers Advantage: Can be constructed to any required shape and size Disadvantages: Higher cost Large space requirements Slow work progress 44 Source : www.teamipr.com
- 45. Steel Used formain , outfall and trunk sewers having large diameters where high external and internal pressure are encountered. Perfectly impervious , light in weight , easily welded Can absorb vibration and shock loads due to its flexibility Made corrosion resistant by heavy galvanization or bituminous coating Cost is high compared to cast iron pipes 45 Source :http://peoriapublicradio.org
- 46. Cast Iron Availablein diameter from 150mm to 750mm and 3 to 3.5 m length Stronger to withstand tensile,compressive and bending stress Can withstand vibration,high external and internal pressure Easy to join and watertight Brittle in nature and expensive Easily acted upon by acids Difficult to transport and handle 46 Source : www.indiamart.com
- 47. 47Used Under followingcircumstances Heavy external loads High internal pressure Crossing low level areas Under expensive road surface Protection against contamination Temperature variations Vibrations Wet ground conditions
- 48. Plastic Used forinternal drainage works in house Size 75 to 315 mm external diameter Smooth internal surface Offer resistance to corrosion,light in weight,economic in laying, jointing and maintenance Tough and rigid Ease in fabrication 48 Source:www.dreamstimecom
- 49. Plain or ReinforcedConcrete Used from 80mm to 450mm diameter with thickness varying from 25 to 35mm Equally strong under internal and external pressure Easily manufactured even at site Economical for medium and large sized installations Easily corroded by action of contents of sewage.Such corrosion is known as crown corrosion Precast concrete and Cast insitu concrete pipes 49 Source:www.kanapipeline.com
- 50. 50 Crown Corrosion With thegradual deposition of organic and inorganic matter at the bottom of sewer,the flow of sewage in the lower layer become stale leading to the anaerobic conditions favourable for the Sulphate Reducing Bacteria which convert sulphate to H2S as shown.Thiobacillus thioxidans convert H2S to H2SO4. The sulphuric acid deposited at the crown of the sewer reacts with concrete and forms CaSO4 which falls down as droplet making the crown of sewer uneven and thinner.
- 51.
- 52. 3.4 Design ofsewers of separate and combined systems Fig: Separate system 52Source:en.Wikipedia.org 53
- 53.
- 54. Design the sectionof combined circular sewer from the data given below: Area to be served is 150 hectares Population of locality is 1,00,000 Maximum permissible velocity 3.2 m/s Time of entry is 5 minutes Time of flow is 20 minutes Rate of water supply is 270 lpcd Runoff coefficient is 0.45 Assume suitable data if necessary. Solution: Assuming 80% of the water supplied will be reaching the sewers as sanitary sewage ,quantity of sanitary sewage produced; Average quantity of sanitary sewage flow (DWF) =1,00,000 x 270 x 0.8 lit/day =(100000x270x0.80) / (1000x24x60x60) m3/s =0.25 m3/s 54
- 55. Maximum or peakquantity of sanitary sewage =Peak factor x DWF (Assuming peak factor = 2 ) =2 x 0.25 m3/s =0.5 m3/s Tc = Te + Tf where Tc= time of concentration Te = time of entry Tf = time of travel or flow The quantity of storm water will be maximum when storm duration is equal to time of concentration. Thus, t = tc= 25 minutes 𝑖 = 1020 𝑡 + 20 𝑖 = 1020 25+20 = 22.67 mm/hr The storm water runoff is given by rational formula ,as 𝑄 = 𝐶𝑖𝐴 360 , where C=0.45, i=22.67 mm/hr , A=150 hectare =4.25 m3/s 55
- 56. Therefore, combined discharge(Q) = 0.5+4.25 = 4.75 m3/s Now, 𝐴 = 𝑄 𝑉 = 4.75 3.2 = 1.48 m2 All velocities must lie between (0.75-3.2) m/s . Hence, 3.2 m/s is assumed. Diameter of sewer , 𝐷 = 4𝐴 𝜋 =1.374 m Adopting commercially available size 1.4 m, 𝐴 = 𝜋𝐷2 4 = 𝜋𝑥 (1.4)2 4 = 1.54 m2 𝑉 = 𝑄 𝐴 = 4.75 1.54 = 3.08 𝑚/𝑠<3.2 m/s. Hence,ok. Check for cleansing velocity during dry weather flow 𝑞 𝑄 = 0.25 4.75 = 1 19 = 0.0526 𝑞 𝑄 = 𝜃 360 [1 − 360𝑠𝑖𝑛𝜃 2𝜋𝜃 ]5/3 56
- 57. [18.94 𝜃 ]3/5 + 360𝑠𝑖𝑛𝜃 2𝜋𝜃 -1 = 0 Solving,𝜃 = 93o 𝜗 𝑉 = [1 − 360𝑠𝑖𝑛𝜃 2𝜋𝜃 ]2/3 Or, v= 3.08 x 0.529 = 1.629 m/s 0.75 m/s < 1.629 m/s < 3.2 m/s Hence,ok. Check for self cleansing velocity during minimum flow Assume Qmin = 1/2 of DWF 𝑞 𝑄 = 0.0263 𝑞 𝑄 = 𝜃 360 [1 − 360𝑠𝑖𝑛𝜃 2𝜋𝜃 ]5/3 Solving, 𝜃 = 78.077 o 𝜗 𝑉 = [1 − 360𝑠𝑖𝑛𝜃 2𝜋𝜃 ] 2/3 Or, v= 3.08 x 0.43 = 1.32 m/s 0.75 m/s < 1.32 m/s < 3.2 m/s Hence , ok. 57
- 58. Calculate the diameterand velocity of a circular sewer at a slope of 1in 150 when it is running just full at a discharge of 1.05 m3/s. The value of n in Manning’s formula is 0.011. What will be the discharge and velocity when flowing 0.75 depth of pipe for the same slope. Solution: Using Manning’s equation,we have 𝑄 = 1 𝑛 𝐴 R2/3 S1/2 Q = 1.05 m3/s , n=0.11, s=1/150 Thus, by substitution , we get 1.05 = 1 0.11 𝑥 𝜋 4 𝐷2 𝑥 𝐷 4 2/3 𝑥 1 400 1/2 or, D= 0.7436 m Velocity (v) = 1 𝑛 R2/3 S1/2 = 1 0.011 0.7436 4 2/3 1 150 1/2 = 2.42 m/s 58
- 59. When flowing depthof pipe = 0.75m 𝑑 𝐷 = 0.75 Central angle (𝜃) is given by , cos 𝜃 2 = (1 − 2𝑑 𝐷 ) = (1- 2 x 0.75 ) 𝜃= 240o 𝑞 𝑄 = 𝜃 360 [1 − 360𝑠𝑖𝑛𝜃 2𝜋𝜃 ]5/3 Discharge(Q) = 1.05 m3/s Therefore, q = 0.9575 m3/s Now, 𝜗 𝑉 = [1 − 360𝑠𝑖𝑛𝜃 2𝜋𝜃 ] 2/3 Velocity (v) = 2.42 m/s Therefore , 𝜗 = 2.74 m/s 59
- 60. 1. Calculate thevelocity of flow in a sewer of circular section having diameter of1m, laid at gradient of 1 in 600. Use Manning’s equation taking n=0.012 60
- 61.
- 62. Solution: For sewer runninghalf full, A= 𝝅𝑫 𝟐 𝟖 ; P= 𝝅𝑫 𝟐 R= 𝑨 𝑷 = 𝑫 𝟒 Hydraulic Radius(R)=0.25m A= 𝜋𝐷2 8 =0.3627m2 Using Manning’s equation; Velocity(V)= 1 𝑛 R 𝟐 𝟑S 𝟏 𝟐=1.35 m/sec Discharge(Q)=AV=0.3927 * 1.35 =0.053 m3/sec 62
- 63. 2. Design asewer to serve a population of 120000; the daily per capita water supply allowance being 180 litres, of which 80% find its way into the sewer. The permissible sewer slope is 1 in 1000, peak factor=2 and take Manning’s n=0.012 63
- 64.
- 65.
- 66.
- 67. 3. Calculate thediameter of a sewer to serve an area of 20 square kilometer with a population density of 250 persons per hectare. The average rate of sewage flow is 350 lpcd. The maximum flow is 50% in excess of average together with the rainfall equivalent of 15 mm in 24 hrs, all of which are runoff, take the Vmax as 3m/sec. 67
- 68.
- 69.
- 70. 4.Calculate the diameterof combined circular sewer with the following data: Rate of water supply=100 lpcd Population density=100 persons/hec Peak factor=2.7 Area=35 hectares Rainfall intensity=15 mm/hr Slope=1 in 750 Rugosity coefficient=0.011 Runoff Coefficient=0.4 The sewer should run 0.6 full during peak flow 70
- 71.
- 72.
- 73. 5. A cityhas a population of 1 lakh with a per capita water supply of 200 lpcd. Design sewer running 0.7 times full at maximum discharge. Take n=0.013, slope=1 in 600 and peak factor=2.25. Assume 80% of w/s contibutes for sewage. 73
- 74.
- 75.
- 76. 3.6 Construction ofsewers 1.Sewers are expensive to construct and if not built correctly remedial works can be disruptive, time consuming, costly and, in some circumstances, have adverse effects on a company’s reputation. 2.In many instances, a lot of mistakes can be avoided by considering the specification and requirements before and during construction. 3.This is an on-site guide for contractors and operatives constructing sewers, with advice notes provided to avoid some of the common on site errors. 76
- 77. Trenching and bedding 1.Trenchesmust be adequately supported, free from boulders and tree roots must be taken out. 2.Muddy ground, water and soft areas in the trench base must be removed. 3.Materials, spoil and equipment must be stored safely and plant should be operated within a safe working distance. 4.The trench must be adequately protected from slips, trips, falls, site traffic, and have a safe means of access 77
- 78. Fig.1-Trenching and bedding 1.Trenches should be adequately dewatered to provide a firm base but not dug wider than necessary as excessive loading may be placed on the pipe. 2. Should ground conditions be unsuitable for pipe laying and manhole construction, an engineer must be consulted to design a solution. 78 Source: www.designingbuildings.co.uk Trenching and bedding
- 79. 1.In addition, caremust be taken to prevent site debris, sludge or silt from entering the sewer network which could ultimately cause flow restrictions, blockages ,flooding , pollution and also affect the receiving wastewater treatment works. 2.Costs associated with such incidents may be recovered from those responsible. 3.In addition, should an inappropriate discharge of site groundwater or construction material cause a pollution incident, this may lead to prosecution. 79Trenching and bedding
- 80. Installation of sewagepipes The installation of sewage pipe consist of the following steps. 1. Locate the positions of the manhole on the ground along the longitudinal section of the sewer line. It is common practice to lay sewer line between two manholes at a time. 2. The center line pegs of the sewer are driven at a distance of every 7.5 m or 15m. 3. The center line of the sewer line should be properly maintained by providing an off-set line usually marked at a distance of 2m to 3m. The off-setline helps in locating the sewer center line when excavation is carried out to laying of sewer pipe. 80
- 81. Pipe laying andbeddings 1.Pipes must be evenly bedded along the length of the pipe, usually full bed and surround for semi rigid and flexible pipe materials. 2.Rigid pipes may require less granular bedding material. 3.Sewers located within highway or areas of traffic should have 1.2 m of cover. In other areas, 0.9 m of cover is required. 4.Where this is not possible, a full protective concrete bed and surround must be provided, inclusive of flexible joints. 81 82
- 82. 82 1.Pipes should belaid in 3 m maximum lengths with the joints ‘pushed home’ into sockets. 2.Furthermore, care must be taken to ensure the pipe jointing seals are free from grit, silt etc. which will likely cause the pipe length to fail later air testing. 3.It is recommended that sewers are air tested at regular intervals as pipes are laid. 4.Pipes should be cleanly cut, be free from defects and laid without back fall and dips. Pipe laying and beddings
- 83. Jointing of pipe 1.Pipesshould be laid in 3 m maximum lengths with the joints ‘pushed home’ into sockets. 2.Furthermore, care must be taken to ensure the pipe jointing seals are free from grit, silt etc. which will likely cause the pipe length to fail later air testing. 3.It is recommended that sewers are air tested at regular intervals as pipes are laid. 4.Pipes should be cleanly cut, be free from defects and laid without back fall and dips. 83
- 84. Gradients and backfilling 1.Itis recommended that sewers are laid using pipe lasers to achieve a single consistent gradient. 2.Where there is little fall such as gradients up to 1:150 extra care should be taken to prevent dips. 3.Back laying of pipes should be avoided where possible as level errors and the positioning of unforeseen existing services may require corrective measures which can be either expensive or impossible to rectify. 4.Pipes should be backfilled and compacted in 150 mm layers to 300 mm above the pipe crown. 5. Care should be taken during compaction so that the sewer remains in good line and level, in particular adjacent to manhole chambers. 84
- 85. Testing of sewer Fig:Testing of sewer Source: www.ejprescott.com 1.The straightness of the sewer pipe can be tested by placing a mirror at one end of the sewer line and a lamp at the other end. 2.If the pipe line is straight, the full circle of light will be observed. 3.However, if the pipe line is non-straight, this would be apparent and the mirror will also indicate any obstruction in the pipe barrel. 85
- 86. 1.Any obstruction presentin the pipe can also be tested by inserting at the upper end of the sewer a smooth inserting at the upper end of the sewer a smooth ball of diameter 13mm less than internal diameter of the sewer pipe. 2.In the absence of any obstruction, such as yarn or mortar projecting through the joints etc. the ball shall roll down the invert of the sewer pipe and emerge at the lower end. 86Testing of sewer
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