environmental engineering - weebly
TRANSCRIPT
Week -4
Environmental Engineering – II
Design of Sanitary Sewers
Dr.Amir Farooq
PhD. Environment Management
MSc. Environmental Engineering
BSc. Civil Engineering
Procedure for designing of Sanitary Sewer
i) Draw line to present proposed sewer in each street to be
served with an arrow head to indicate the direction of
flow of sewage
ii) Locate the manholes by giving Identification Numbers to
ease to detect fault in system
iii) Sketch limits of the service area for each lateral.
iv) Measure the service areas for each lateral
v) Prepare tabulation (Hydraulic Statement)
PARTIAL FLOW IN SEWERS
It is necessary to determine velocity and depth
of sewage in a pipe when it is flowing partially
full.
Various graphs for determining hydraulic
elements for partial flow are available; they are
used to determine depths of flow and velocities
A 915-mm (36 inches) circular sewer pipe is laid on a slope of
0.003. n = 0.013 when the sewer is full. What will the velocity of
the flow be when a sewer is carrying 8.5 m3/min?
Problem
Slope = 0.003
Dia = 36”
n = 0.013
INVERT LEVEL
INVERT LEVEL:
The lowest inside level at any cross-section of a sewer
is known as the INVERT LEVEL at that cross section.
SIGNIFICANCE:
Sewers must be laid at a particular slope to attain self
cleansing velocities. The required slope (while laying the
sewers) is achieved through calculations of invert levels.
INVERT LEVEL CALCULATIONS
IL = NGSL/RL – Depth of Sewer – Thickness of Sewer – Dia of Sewer
IL = Invert level
Minimum Cover = 3 feet or 1 meter
Know Upper & Lower Inver level of Sewer, gradient can be calculated as
given under
S = (IL Upper – IL Lower) / Span of Sewer
INVERT LEVEL CALCULATIONS
U/S IL = NGSL/RL – Depth of Sewer – Thickness of Sewer – Sewer Dia
D/S IL = U/S Invert Level – Drop (Length x slope)
SINGLE SEWER
TWO OR MORE SEWERS OF SAME SIZE
When equal dia sewers discharge in a manhole and the same
dia sewers receives the total discharge, LOWEST D/S I.L.
among the discharging sewers will be carried as U/S I.L. for the
receiving sewer.
SEWERS OF DIFFERENT SIZE
When receiving sewer dia is greater than the discharging sewer;
Keep the crowns at the same level
Drop the U/S I.L. of the receiving sewer by the difference in the
dia of the two sewers.
Cutting
It is the depth of the trench to be dug for the sewer line &
Cutting = IL – thickness of Pipe – Bedding provided
Variation in Sewage Discharge Results in Variation depth of
flow
Thus, leads to variation in hydraulic mean depth (r)
Changes in hydraulic mean depth, affects flow velocity (which
depends directly on r2/3)
Sewer must be checked and maintain a minimum velocity of
about 0.6 m/s at the time of minimum flow (assumed to be
1/3rd of average flow).
Design should also ensure development of a maximum
velocity of 2.6 m/s at least at the time of maximum flow and
preferably during the average flow periods also.
However, velocity generated must not exceed the scouring
value at time of maximum flow.
Effects of Flow Variation on Velocity in a Sewer
Assignment
Design of Sanitary Sewer
Q-1 A circular sewer is to carry 2.5 m3/min of sanitary
sewage when flowing full. Conditions are such that minimum
allowable grade must be adopted. Taking n=0.015 determine
the commercial pipe size
Q-2 A 300 mm sewer is laid at minimum slope. What is
maximum population that can be served by the sewer if the
average water consumption is 300 lpcd.
Q-3 Design a sanitary sewer to serve a population of
15,000 person supplied with a water at a rate of 300 lpcd.
Assume necessary data to arrive at design flow.
Q-4 Calculate the size and slope of a trunk sewer serving
a population of 0.5 million. Water consumption is estimated to
be 350 lpcd. A value of 0.013 may used for n. Assume other
necessary data.
Q-5 A sewer length of 100 m and 300 diameter has its
two ends at levels of 200m and 199.7m respectively.
Calculate the capacity of the sewer when running full.
Sewer Material
Materials used for transport of water can be used for carriage
of sewage
However, use of less expensive material is common
Types of Pipes based on material
PVC, AC, PCC, RCC, C.I., Steel, Clay
Cast Iron and Steel Pipes
Used when sewage line is under pressure
PCC Pipes
Normally used for small storm drains and sanitary
sewers.
Made in three classes with three wall thicknesses.
Sizes: 100 mm – 610 mm.
Concrete pipes > 610 mm in size are reinforced.
RCC Pipes
RCC pipes are most commonly sued pipes for
conveyance of sewage.
These are normally used for combined, large storm
drains and sanitary sewers in urban areas.
These pipes are made in five classes with two wall
thicknesses in class-I and three wall thicknesses in
other four classes.
Dimensions available for RCC pipes are shown in
Table.
Strength of RCC Pipes
Three edge-bearing test is used to determine the strength of
RCC pipes.
Load is applied on the pipe to produce 0.25 mm crack.
It defines the load that can be safely supported by the sewer.
Sewer Shapes
Sewer Pipes can be Circular, rectangular and Square
Circular Shapes are Preferred following reasons
It gives maximum Cross-Sectional area for amount of
material in the walls
Pre-cast circular pipes can be casted more conveniently
Circular pipes are more stable in trenches
Posses good hydraulic properties
Strongest shape to withstand the external pressure,
hence circular shape is more important in case of deep
sewers
LOADS ON BURIED PIPES
Sewer design requires prior knowledge of soil
and site conditions to determine overburden
loads that will be placed on buries pipes.
Total load on buried pipes is the sum of live load
and backfill load.
Live loads on the surfaces rarely influences the
design of sanitary sewers because of their
greater depths.
Backfill load is of more concern.
Backfill Load on Sewers:
Backfill load on buries pipes can be calculated using
Marston’s Equation.
W = CωB2
Where;
W = Load on the pipe per unit length, Kg/m
ω = Weight of the backfill material per unit volume, Kg/m3
B = Width of the trench, m
C = Coefficient that depend on depth of trench, character of
construction and fill material.
Backfill Load on Sewers:
Backfill load on buries pipes can be calculated using
Marston’s Equation.
W = CωB2
Where;
W = Load on the pipe per unit length, Kg/m
ω = Weight of the backfill material per unit volume, Kg/m3
B = Width of the trench, m
C = Coefficient that depend on depth of trench, character of
construction and fill material.
For ordinary trench construction, C may be calculated from
Where;
H = depth of fill above pipe
B = Width of trench just below top of pipe
K = ratio of active lateral pressure to vertical pressure
μ’ = coefficient of sliding friction between fill material and sides
of trench
The product Kμ’ ranges from 0.1 to 0.16 for most soils as
shown in Table.
in developing the strength of the pipe,
assuring it is laid to the proper grade, and
preventing subsequent settlement.
In unfavorable soil conditions, bedding is particularly
important.
SEWER BEDDINGS
Provision of proper bedding is very important;
Load Factor:
Load Factor expresses the increase in strength of sewer
by provision of proper bedding
Depending upon the load factor, following four types of beddings are
provided for concrete pipes
Fig: Method of bedding concrete pipes and load factors applicable to strength
SEWER BEDDINGS IN LAHORE (WASA)
Brick Ballast Crushed Stone
Load Factor = 1.7
Used under poor subsoil
conditions, above the water table
Load Factor = 1.9
Used under poor subsoil
conditions, below water table
Concrete Cradle
Load Factor = 3.0
Used under increased strength
requirements
Sewer Construction
Construction of Sewer comprises of following steps
1) Excavation
2) Bracing
3) Dewatering
4) Pipe Installation/Laying
5) Backfilling
6) Construction of Appurtenances
1) Excavation
Locate the centre line of the trench.
Excavate according to size of the trench and required gradient.
Start Excavation and construction preferably from end point.
Excavation is relatively easy from the starting
Depth increases as work proceeds (Deep excavation &
Dewatering)
Increases probabilities of run off of contractors.
Required, If GWT is above the bottom of the trench.
Sheeting, bracing and pumping for de-watering) .
2) Bracing
Sheeting & Bracing for trenches in unstable material
prevent caving or collapse of the walls.
3) Dewatering
Pipes are inspected to ensure that they
have no crack or defects.
Chain and pulley arrangement (large &
deep trenches) or cranes
Placing of pipes on line and grade in
trenches (excavated and dewatered)
Joined & pressed together with a winch.
4) Pipe laying Laying of pipes Old method
a) Offset line is located (Avoid disturbance and covering)
b) Measure Lay out of the trench from offset line and excavate.
c) Batten boards are placed across the trench at 10-15m intervals.
They are supported and fixed with ground as shown in figure.
d) The Centre line of the sewer is shown on the batten boards by a
nail, by the edge of upright cleat.
e) At the cleats, nails are fixed at the given gradient.
f) A cord is stretched along with these nails. This cord will be
according to required grade.
g) The centre line from the batten boards is transferred on the bottom
of the trench by means of a plumb rod. The grade is transferred
from the cord to the bed of trench by means of a stick marked in
even increments and having a short piece fastened at right angle to
its lower end.
h) Grade is checked by placing the short piece on the invert of each
length of sewer pipe and noting whether the proper mark touches
the cord.
Sewer Construction (Laying of Pipes)
JOINTS IN RCC SEWERS
“Bell and Spigot” and Tongue and Groove joints
Portland cement mortar or
bituminous material.
Entrance is reduced by
wrapping spigot with an seal
or pieces of old ropes cord of
appropriate thickness.
The gasket is driven into the
bell (calking Tool)
Joint is filled with mortar or
bitumen.
The inside of the pipe of
smoothen with a swab or
drag.
i) Bell and Spigot” joint
For 310-760 mm either joint can be used
For > 760 mm only tongue and groove is
used.
ii) Tongue and Groove Joint
v) Backfilling
Immediate Backfilling after laying and jointing of pipes
Delayed backfilling in case of class A bedding to
permit setting up of Concrete to support the backfill.
No water should be permitted to rise in backfilled
trenches.
Fill material (free of brush, debris, frozen material,
large rock and junks.
Tamping in layers ( 6” thickness to a depth of 2’)
Careful dropping upto 2’
Thereafter rapid backfilling .
Steps for Partially Combined Sewer for Urban areas
Find the present population of project area.
Find design population from given design period.
Find out average sewage flow for the design population. Using
this average sewage flow select Peak factor for the project
area (From Table No.1)
Draw layout of the sewer system keeping in view the layout of
the roads and streets. Represent sewer with line and
Manhole with dot.
Number the manholes and identify each sewer line e.g. M1M2,
M2M3 etc.
Allocate plots or area to each sewer line (Col 5,6,7,8)
Measure length of each sewer line according to the scale of
map. Indicate the direction of flow of sewer with help of arrow
head.
Find of sewage flow as 80 – 85 % of water consumption.
Calculate average sewage flow (Col. 10) and infiltration (Col.
11) for each sewer line.(For this particular design problem take
infiltration as 5% of average sewage flow).
Calculate Peak Sewage Flow (Col.12) and finally the design
flow (Col.14 & Col.15) for the sewer lines.
Using the method of back calculations, find appropriate
diameter (Col.17) and slope (Col.18) for your sewer assuming
that sewer is running full. For back calculation choose a
suitable design table with a suitable slef-cleansing velocity (0.6
m/sec). Refer Table-2.
Use graph from book by “Steel and “ to find the depth of flow
(Col.22) and actual velocity (Col.24 at “Design Flow”.
If actual velocity and depth of flow are satisfactory, then
diameter and slope of pipe are considered final.
If velocity is less than Self-Cleansing velocity, then increase the
slope of the sewer. For the new slope find Qf and then Qa/Qf
and d/D and finally Va/Vf and Va.
Find the Invert Levels (Col.28 & 29) for all the sewers and
complete the table of calculations called “ Hydraulic Statement”
(Note: a lot of care and vigilance should be exercised in
calculating the Invert Levels otherwise the whole scheme
may fail due to incorrect levels)
Draw profile or L-Section of all the sewer lines
Assignment
For ordinary trench construction, C may be calculated from
Where;
H = depth of fill above pipe
B = Width of trench just below top of pipe
K = ratio of active lateral pressure to vertical pressure
μ’ = coefficient of sliding friction between fill material and sides
of trench
The product Kμ’ ranges from 0.1 to 0.16 for most soils as
shown in Table.
in developing the strength of the pipe,
assuring it is laid to the proper grade, and
preventing subsequent settlement.
In unfavorable soil conditions, bedding is particularly
important.
SEWER BEDDINGS
Provision of proper bedding is very important;
Load Factor:
Load Factor expresses the increase in strength of sewer
by provision of proper bedding
Depending upon the load factor, following four types of beddings are
provided for concrete pipes
Fig: Method of bedding concrete pipes and load factors applicable to strength
SEWER BEDDINGS IN LAHORE (WASA)
Brick Ballast Crushed Stone
Load Factor = 1.7
Used under poor subsoil
conditions, above the water table
Load Factor = 1.9
Used under poor subsoil
conditions, below water table
Concrete Cradle
Load Factor = 3.0
Used under increased strength
requirements
Sewer Construction
Construction of Sewer comprises of following steps
1) Excavation
2) Bracing
3) Dewatering
4) Pipe Installation/Laying
5) Backfilling
6) Construction of Appurtenances
1) Excavation
Locate the centre line of the trench.
Excavate according to size of the trench and required gradient.
Start Excavation and construction preferably from end point.
Excavation is relatively easy from the starting
Depth increases as work proceeds (Deep excavation &
Dewatering)
Increases probabilities of run off of contractors.
Required, If GWT is above the bottom of the trench.
Sheeting, bracing and pumping for de-watering) .
2) Bracing
Sheeting & Bracing for trenches in unstable material
prevent caving or collapse of the walls.
3) Dewatering
Pipes are inspected to ensure that they
have no crack or defects.
Chain and pulley arrangement (large &
deep trenches) or cranes
Placing of pipes on line and grade in
trenches (excavated and dewatered)
Joined & pressed together with a winch.
4) Pipe laying Laying of pipes Old method
a) Offset line is located (Avoid disturbance and covering)
b) Measure Lay out of the trench from offset line and excavate.
c) Batten boards are placed across the trench at 10-15m intervals.
They are supported and fixed with ground as shown in figure.
d) The Centre line of the sewer is shown on the batten boards by a
nail, by the edge of upright cleat.
e) At the cleats, nails are fixed at the given gradient.
f) A cord is stretched along with these nails. This cord will be
according to required grade.
g) The centre line from the batten boards is transferred on the bottom
of the trench by means of a plumb rod. The grade is transferred
from the cord to the bed of trench by means of a stick marked in
even increments and having a short piece fastened at right angle to
its lower end.
h) Grade is checked by placing the short piece on the invert of each
length of sewer pipe and noting whether the proper mark touches
the cord.
Sewer Construction (Laying of Pipes)
JOINTS IN RCC SEWERS
“Bell and Spigot” and Tongue and Groove joints
Portland cement mortar or
bituminous material.
Entrance is reduced by
wrapping spigot with an seal
or pieces of old ropes cord of
appropriate thickness.
The gasket is driven into the
bell (calking Tool)
Joint is filled with mortar or
bitumen.
The inside of the pipe of
smoothen with a swab or
drag.
i) Bell and Spigot” joint
For 310-760 mm either joint can be used
For > 760 mm only tongue and groove is
used.
ii) Tongue and Groove Joint
v) Backfilling
Immediate Backfilling after laying and jointing of pipes
Delayed backfilling in case of class A bedding to
permit setting up of Concrete to support the backfill.
No water should be permitted to rise in backfilled
trenches.
Fill material (free of brush, debris, frozen material,
large rock and junks.
Tamping in layers ( 6” thickness to a depth of 2’)
Careful dropping upto 2’
Thereafter rapid backfilling .
Steps for Partially Combined Sewer for Urban areas
Find the present population of project area.
Find design population from given design period.
Find out average sewage flow for the design population. Using
this average sewage flow select Peak factor for the project
area (From Table No.1)
Draw layout of the sewer system keeping in view the layout of
the roads and streets. Represent sewer with line and
Manhole with dot.
Number the manholes and identify each sewer line e.g. M1M2,
M2M3 etc.
Allocate plots or area to each sewer line (Col 5,6,7,8)
Measure length of each sewer line according to the scale of
map. Indicate the direction of flow of sewer with help of arrow
head.
Find of sewage flow as 80 – 85 % of water consumption.
Calculate average sewage flow (Col. 10) and infiltration (Col.
11) for each sewer line.(For this particular design problem take
infiltration as 5% of average sewage flow).
Calculate Peak Sewage Flow (Col.12) and finally the design
flow (Col.14 & Col.15) for the sewer lines.
Using the method of back calculations, find appropriate
diameter (Col.17) and slope (Col.18) for your sewer assuming
that sewer is running full. For back calculation choose a
suitable design table with a suitable slef-cleansing velocity (0.6
m/sec). Refer Table-2.
Use graph from book by “Steel and “ to find the depth of flow
(Col.22) and actual velocity (Col.24 at “Design Flow”.
If actual velocity and depth of flow are satisfactory, then
diameter and slope of pipe are considered final.
If velocity is less than Self-Cleansing velocity, then increase the
slope of the sewer. For the new slope find Qf and then Qa/Qf
and d/D and finally Va/Vf and Va.
Find the Invert Levels (Col.28 & 29) for all the sewers and
complete the table of calculations called “ Hydraulic Statement”
(Note: a lot of care and vigilance should be exercised in
calculating the Invert Levels otherwise the whole scheme
may fail due to incorrect levels)
Draw profile or L-Section of all the sewer lines
Assignment