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Home>Economic
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Design Standards>Part 8 Sanitary Sewer
Municipal Design Standards Part 8 - Sanitary Sewer
Revised July 2004
8.01 APPROVALS, PERMITS, AS-BUILTS, AND MAINTENANCE BONDS
A. Plans and specifications for public sanitary sewer facilities
must be certified by a professional engineer registered in the State of Iowa and
utilize the NGVD of 1929.
B. Plans and specifications for public sanitary sewer facilities must be
reviewed and approved by the City Engineer prior to construction.
C. Plans and specifications for public sanitary sewer facilities must be
reviewed and approved by the Iowa Department of Natural Resources prior to
construction. Other local, state and federal permits may be required, depending
on the circumstances. It shall be the responsibility of the Engineer of Record
to acquire all applicable permits. A copy of all permits shall be provided to
the City Engineer before construction.
D. The Engineer of Record is responsible to submit "Record of Construction"
drawings to the City Engineer on reproducible vellum or mylar, and shall include
the horizontal locations of services.
E. A two-year maintenance bond covering defective materials and workmanship is
required for all sanitary sewer facilities.
8.02 DESIGN RESOURCES AND REFERENCES
The design for sanitary facilities shall be in conformance
with the following:
A. Requirements and Standards of the Iowa Department of
Natural Resources.
8.03 PERMITTED FLOWS AND CONNECTIONS
A. No combined sewers shall be constructed. Sanitary and storm
sewers shall be kept separate.
B. Only sewage shall be permitted in the sanitary sewers. Footing drains,
downspouts, sump pumps, etc., conveying clear water will not be allowed to
discharge into the sewer system. Air conditioning condensation water may be
allowed in the sanitary sewer.
C. Flows from commercial car washes must be discharged to the sanitary sewer
after passing through approved oil and sediment traps.
D. Each platted lot shall have a direct connection to a public sewer.
8.04 DESIGN FLOWS AND CAPACITY
A. Peak rates will be taken as approximately 2.5 times the normal
flow. Infiltration will be calculated by the addition of 100 gallons per capita
per day.
B. The peak wet weather flows for various land usage are as follows; adjust the
peaking factor for communities less than 10,000:
1. Single Family Dwellings
(100 gal/cap/day) x 2.5 (peak factor) +
1200 gal/acre day (infiltration)
= 250 gpcd + infiltration
Assume 3.5 people/home
2. Mobile Homes
(50 gal/cap/day) x 2.5 (peak factor) +
1200 gal/acre day (infiltration)
= 125 gpcd + infiltration
Assume 2.5 people/home
3. Multi Family Dwellings
(75 gal/cap/day) x 2.5 (peak factor) +
1200 gal/acre day (infiltration)
= 187 gpcd + infiltration
Assume 1.5 people/bedroom
4. Motels and Hotels
(50 gal/cap/day) x 2.5 (peak factor) +
1200 gal/acre day (infiltration)
= 125 gpcd + infiltration
Assume 1.5 people/room
5. Schools, without cafeteria or showers
(10 gal/cap/day) x 2.5 (peak factor) +
1200 gal/acre day (infiltration)
= 25 gpcd + infiltration
6. Schools, with cafeteria or showers
(20 gal/cap/day) x 2.5 (peak factor) +
1200 gal/acre day (infiltration)
= 50 gpcd + infiltration
7. Office Buildings
(10 gal/cap/day) x 2.5 (peak factor) +
1200 gal/acre day (infiltration)
= 25 gpcd + infiltration
Assume 1 person/200 sq. ft.
8. Light Industrial
(14,000 gal/acre/day) x 2.5 (peak factor) +
1200 gal/acre day (infiltration)
= 36,200 gpad
gpcpd = gallons per capita per day
gpapd = gallons per acre per day
C. If a proposed sewer is to serve a predominantly wet area or an area prone to
excessive infiltration and inflow, special design information should be obtained
from the City Engineer. If no information is available, the designer should use
a minimum of 1000 gpcpd for infiltration.
D. All sanitary sewers shall be a minimum of 8 inches in diameter.
E. Pipes will be sized to carry peak rates with the pipe flowing at no more than
0.67 of the pipe diameter for pipes 15" and smaller and 0.75 of the pipe
diameter for pipes larger than 15" in diameter.
F. All sewers shall have a slope which will give a mean velocity when flowing
full of not less than 2.0 feet per second based on Manning's formula using an
"n" value of 0.013.
G. Where velocities greater than 15 feet per second are calculated, special
provisions shall be made to protect against displacement, erosion or shock.
H. Sanitary sewers shall be sufficiently deep so as to receive sewage by gravity
from basements and to prevent freezing.
8.05 SERVICE CONNECTIONS:
A. A sanitary service pipe will be provided for every platted
lot or location where construction of a building is likely. A 4-inch sanitary
service pipe will be used for single-family residential. Pipes for multi-family
residential, commercial, or industrial, will be sized as required.
B. Sanitary service pipes will be extended from the main to the right-of-way
line or outer utility easement line, whichever is further.
C. Two sanitary services may be located in the same trench provided they
terminate at the right-of-way line adjacent to the parcel they will serve
(services crossing lot lines are not permitted).
D. The end of all sanitary services shall be marked with a treated 2x4 or 2”
plastic pipe extending from the service line to 3 feet above finished grade.
E. Service taps in manholes are allowed only in extreme conditions and with the
approval of the City Engineer. If permitted, service connections to manholes
must be between 6" and 12" above the invert elevation of the outlet. Sewer flow
channels in the manhole bottom must be provided for all services. Internal drops
for service connections may be permitted on manholes deeper than 12 feet upon
approval of the City Engineer. Internal drops shall be constructed of SDR 23.5
PVC with stainless steel bands and fasteners spaced at a maximum of 4 feet.
8.06 PIPE STANDARDS AND STRENGTH DESIGN:
A. Reinforced concrete pipe manufactured in accordance with ASTM
C-76 and meeting the following standards may be used for pipe 12 inches in
diameter or larger. Reinforced concrete pipe shall be subject to approval of the
City Engineer.
1. Joints: All joints will be confined O-ring gasket meeting ASTM C443. All pipe
36-inch diameter and smaller will have bell and spigot joints. Pipe larger than
36-inch diameter may have tongue and groove joints.
2. Wall Thickness: Minimum wall thickness will be B-wall as defined in ASTM C
76.
3. The required pipe strength and bedding requirements shall be calculated on a
case-by-case basis. The minimum pipe strength shall be Class III as defined in
ASTM C 76.
4. Pipe Markings: All pipe shall be marked with the date of manufacture and ASTM
class. If quadrant reinforcement is used, the top shall be marked on the outside
of the pipe.
5. No lift holes.
6. Hydrogen sulfide shall be considered in the design of concrete pipe sewers
downstream from lift stations.
B. Vitrified clay pipe manufactured in accordance with ASTM C700 and meeting the
following standards may be used for pipe 8 inches through 36 inches in diameter.
VCP is not allowed in North Liberty.
1. Joints: Shop drawing of gasket installation and joint assembly, subject to
approval of the City Engineer.
2. All pipe shall be "extra strength" as defined by ASTM C700.
C. Ductile iron pipe manufactured in accordance with ANSI/AWWA-C150/A21.50 and
ANSI/AWWA-C151/A21.5A and meeting the following standards may be used.
1. Joints: Use push on gasketed joints.
2. Thickness: Use Class 52 ANSI standard unless a thicker wall is required
because of depth.
3. Lining and Coating:
a. Inside of pipe and fittings: Double thickness cement lining and bituminous
seal coat conforming to ANSI/AWWA-C104/A21.4.
b. Outside of other pipe and fittings: Standard bituminous coating conforming to
appropriate ANSI Standard.
D. PVC truss pipe manufactured in accordance with ASTM D2680 and meeting the
following standards may be used for pipe 8 inches through 12 inches in diameter.
1. All PVC truss pipe shall be made of PVC compound having a minimum cell
classification of 11432 as defined in ASTM D4396.
2. Joints: Gaskets shall comply with requirements in ASTM F477 and F913. Joint
shall meet the requirements of ASTM D3212.
3. Pipe shall have a minimum pipe stiffness of 200 lb./inch/inch at 5%
deflection.
E. Poly vinyl chloride (PVC) manufactured in accordance with ASTM D3034-88 and
meeting the following standards may be used for 4 inch and 6-inch services.
1. Joints: ASTM D3212 gasketed.
2. All 4" and 6" services shall be SDR 23.5 or 26
F. The designer shall specify pipe material, bedding and trench width to
withstand anticipated dead and live loads. Minimum pipe and bedding standards
are listed in Table 8.1.
G. PVC A-2000 manufactured in accordance with ASTM F949 and meeting the
following standards may be used for pipe diameters 15 inches and up.
1. All PVC A-2000 pipe shall be made of extruded PVC with resin cell class 1245
as defined in ASTM D 1784.
2. Joints: Gaskets shall comply with requirements in ASTM F477. Joints shall
meet the requirements of ASTM D3212.
3. Pipe shall have a minimum pipe stiffness of 50 psi in accordance with ASTM
D2412 at 60 percent flattening.
H. PVC Vylon Profile Wall Sewer pipe manufactured in accordance with ASTM F794
and meeting the following standards may be used for pipe diameters 24 inches and
up.
1. All Vylon Profile wall pipe shall be made from PVC compounds which comply
with the requirements for a minimum cell classification of 12364A as defined by
ASTM D1784.
2. Joints: Gaskets shall comply with requirements in ASTM F477. Joints shall
meet the requirements of ASTM D3212.
3. Pipe shall have a minimum pipe stiffness of 46 psi in accordance with ASTM
D2412.
TABLE 8.1
|
PIPE MATERIAL |
SIZE |
JOINT |
MINIMUM STRENGTH |
MINIMUM BEDDING* |
|
Sanitary Sewers |
|
Reinforced Concrete |
12" & up |
See A |
Class III, B wall |
Class B |
|
Vitrified Clay |
8"-36"
|
Gasket |
Extra strength
|
Class B |
|
PVC Truss |
8"-12" |
Gasket |
|
Crushed stone encasement |
|
Ductile Iron
|
8" & up |
Gasket, push |
Class 52
|
Encasement
Class B |
|
Sanitary Service |
|
PVC |
4" & 6" |
Gasket |
SDR 23.5 |
Crushed stone |
|
DIP |
4" & 6" |
Gasket |
Class 52 |
Encasement
Class B |
*See
Figure 8.4.
8.07 MANHOLE STANDARDS (Figure
8.1 and 8.2):
A. Manholes shall be located as follows:
1. At the end of each line.
2. At all changes in grade, size, or alignment.
3. At all intersections of pipes.
4. At distances not greater than 400 feet for sewers 15 inches or less in
diameter and 500 feet for sewers 18 inches to 30 inches in diameter. Greater
spacing may be permitted in larger sewers.
B. Drop Connections: An external drop pipe shall be provided for a sewer
entering a manhole at an elevation of 24 inches or more above the manhole
invert. Where the difference in elevation between the incoming sewer and the
manhole invert is less than 24 inches, the invert shall be filleted to prevent
solids deposition.
C. Minimum Drop Across Manholes
1. For the same size pipe with a change in alignment of 45 or less, no drop is
required.
2. For the same size pipe with a change in alignment of greater than 45, or
junction of two inflow pipes, a 0.2' drop is required.
3. For the same size pipe with a junction of three or more inflow pipes, a 0.3'
drop is required.
4. When a smaller sewer joins a larger one, the invert of the larger sewer shall
be lowered sufficiently to match the 0.8 depth point of both sewers at the same
elevation.
D. Bedding: All manholes shall be placed on a minimum of 6 inches of crushed
stone bedding.
E. Materials:
1. Joints: All joints will be confined O-ring gasket meeting ASTM C443.
2. No lift holes through the entire wall.
3. Mark date of manufacture.
4. Inverts: Precast and cast-in-place inverts must provide a channel at least
one-half the depth of the pipe and match the full cross-sectional area of the
pipe. All junctions and changes in directions of inverts shall be smooth and
rounded to the maximum extent possible to supplement flow through the manholes.
5. Diameter: The minimum diameter for manholes is 48 inches for pipe 21 inches
in diameter and smaller, and 60 inches for pipe greater than 21 inches in
diameter.
6. Castings shall be Neenah R-1670 non-rocking self-sealing or approved equal
and “sanitary sewer” shall be cast in the cover.
F. Standard manholes, step details, drop connections and risers are shown in
Figures 8.1,
8.2 and
8.3.
G. All manholes shall be marked with two metal fence posts to remain in place
until landscaping is complete. In subdivisions, the posts shall remain in place
until landscaping has been completed by the lot owner.
H. If a manhole is in an area subject to surface water inundation, the casting
shall be bolted to the cone section with stainless steel anchors. In addition,
an exterior Cretex manhole chimney seal (or approved equal) shall be installed
that spans from the casting ring to the cone section.
I. Manholes in paving shall have an interior Cretex manhole chimney seal (or
approved equal) that spans from the casting ring to the cone section.
J. Concrete spacer rings, metal shims and bricks shall be the only materials
utilized to adjust manhole frame elevations. Mastic or grout must be used to
bond and seal materials used for adjustment.
8.08 PROTECTION OF WATER SUPPLIES:
A. There shall be no physical connection between a public or private potable
water supply system and a sewer appurtenance which would permit the passage of
any sewage or polluted water in the potable supply.
B. Under normal conditions, water mains parallel to sewers shall be placed at
least 10 feet horizontally from any sanitary sewer, storm sewer or manhole.
Where local conditions prevent this separation, the water main may be laid
closer provided the bottom of the water main is at least 18 inches above the top
of the sewer and the water main is placed in a separate trench or in the same
trench on a bench of undisturbed earth at a minimum horizontal separation of 3
feet from the sewer.
C. Water mains crossing sewer services, storm sewers or sanitary sewers shall be
laid to provide a separation of at least 18 inches between the bottom of the
water main and the top of the sewer. Where local conditions prevent this
vertical separation, the water main shall not be placed closer than 6 inches
above a sewer or 18 inches below a sewer under any circumstances. Additionally,
one full length of water pipe crossing the sewer shall be centered at the point
of crossing so that the water pipe joints will be equal distance as far as
possible from the sewer. The water and sewer pipes must be adequately supported
and have pressure tight joints. A low permeability soil shall be used for
backfill material within 10 feet of the point of crossing.
D. No water pipe shall pass through or come in contact with any part of a sewer
manhole. A minimum horizontal separation of 3 feet shall be maintained.
8.09 CREEK CROSSINGS:
A. Sanitary sewers crossing creeks shall be Class 52 D.I.P. encased in
reinforced concrete or 3/8" thick steel carrier pipe.
B. Rip rap all disturbed creek banks and bottom after construction.
8.10 BACKFILL AND BEDDING:
A. Within right-of-way - backfill shall consist of Class A crushed stone placed
in one foot lifts and compacted to 90% modified proctor density or suitable job
excavated material placed in one foot lifts and compacted to 90% Modified
Proctor Density. If Class A crushed stone is used, the top 12 inches of backfill
shall consist of suitable job excavated materials. Flowable mortar may be used
upon approval of mix design by the City Engineer. Sand backfill is not
permitted.
B. In all other areas backfill shall consist of suitable job excavated material
placed in one foot lifts and compacted to 85% Modified Proctor Density.
C. The gradation of bedding material shall be compatible with surrounding soils
to prevent migration of fines.
D. See Table 8.1 and
Figure 8.4 for pipe
bedding requirements.
8.11 PERFORMANCE & TESTING:
A. All sewers and manholes will be lamped and visually checked for leakage prior
to acceptance.
B. All sanitary sewers, including service pipes, will be air-tested using
current ASTM standards prior to acceptance.
C. All tests will be completed after backfill is complete.
D. Polyvinyl Chloride Truss Pipe and PVC Pipe shall have a deflection test
conducted after the final backfill has been in place at least 30 days. No pipe
shall exceed a deflection of 5%. If the deflection test is to be run using a
rigid ball or mandrel, it shall have a diameter equal to 95% of the inside
diameter of the pipe and the tests shall be performed without mechanical pulling
devices.
8.12 SEWER LOCATION AND EASEMENTS:
A. Manholes in street right-of-way must be located in areas which allow direct
access by maintenance vehicles. Manholes in areas outside the street
right-of-way shall be subject to the approval of the City Engineer in which case
access to the manhole shall be along a route in which the transverse slope does
not exceed 4% and longitudinal slope does not exceed 12%. Placement of sewers in
front yards outside of the right-of-way is discouraged.
B. Sewers shall be placed a minimum horizontal distance of 1.5 times the depth
from potential or existing building sites. Greater separations are desirable.
C. All sanitary sewers outside public right-of-way shall be placed in an
easement for operation and maintenance. Easement width from the center of the
pipe shall generally be 1.0 times the sewer depth rounded up to the nearest 5
feet.
D. The minimum easement width is 15 feet.
8.13 LIFT STATIONS & FORCE MAINS:
A. It is the City's intention and preference to have all extensions or additions
to the sanitary sewer collection system be gravity sewer systems. If gravity
sewer is not feasible, lift stations shall be designed to have regional service
areas.
B. Telemetered alarm systems are required.
C. Provisions for the connection of standby power or auxiliary pumping are
required.
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Revised July 2004
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