Clay Pipe Engineering Manual

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CLAY PIPE

ENGINEERING MANUAL

NationalHeadquartersLakeGeneva,WisconsinWesternRegionOfficeCorona,California

Price $25.00 Copyright 2006 by National Clay Pipe Institute Printed in U.S.A.


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CLAY PIPE

ENGINEERING MANUAL

Foreword

The ClayPipeEngineering Manual has been prepared by the National ClayPipe Institute and is offered by the member manufacturers of the Institute as

an aid to those requiring engineering reference data applicable to the design

and construction of sewer systems and other projects in which Vitrified Clay

Pipe should be used.

Design and construction techniques encountered throughout the country aremany and varied. Those described here are considered sound, although it is

recognized that there may be other equally satisfactory methods. Technical

data presented are considered reliable, but no guarantee is made or liability

assumed.

The recommendations in this Manual should not be substituted for the judgment

of a professional engineer as to the best way of achieving specific design

requirements.

The Engineering Staff of the Institute and of its member companies are

available to assist the reader.

An electronic version of this document is available for download at NCPI.org.

AcknowledgementsThe National Clay Pipe Institute wishes to acknowledge the following members

of the NCPI Technical Services Committee who both individually and collectively

have contributed to this edition.

Richard Brandt Dan Cross Edward SikoraRudy Brandt David Gill Pat V. Symons

Jeff Boshert, P. E. Steve King, P. E. Larry G. Tolby

Mark Bruce Edwin C. Lamb, P. E. Michael VanDine, P.E.

John Butler Todd McClave Bryan Vansell

David Rausch

Equally appreciated are the efforts of the many others who generously gave

their time and talent throughout the development of this manual.

NATIONALCLAYPIPEINSTITUTE20062


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TABLEOFCONTENTSPageNo.

Chapter1. VITRIFIEDCLAYPIPEAnEngineeringMaterial 5 The Manufacture of Vitrified Clay Pipe100-Year Sewers Research and Development

Chapter2. GRAVITYSEWERDESIGNPartI. PlanningandLayout 10

Sewer PlanningDesign PeriodDrainage AreaDesign Flows Population EstimatesConvert Population Data to AverageFlowPeak FactorsExtraneous FlowsInflowInfiltrationAdvantages of Flexible Compression JointsSummations of Flow

Flow Monitoring

Land Use Coefficients

Sample Land Use Map

Flow EstimatingPeak Factor Table

PartII. HydraulicDesign 18 Basic Premises for Calculating Flow in SewersFlowCharacteristics DiagramThe Hydraulic ProfileDesignRequirementsDetermination of Pipe SizesSelecting the Sizesfor the New Sewer LineTypical Plan and ProfileQuantity andVelocity EquationsThe Manning Equations Discussion of Valuesfor nDesign Capacity GraphsComputer DesignConveyanceFactors and Tables

Chapter3. CORROSIONINSANITARYSEWERS 29Vitrified Clay Pipe is Chemically InertHydrogen SulfideAcid Resistance Aggressive Soils and Other Hostile Environments

Chapter4. RIGIDCONDUITS,UNDERGROUNDPartI. StructuralAnalysis 32

Importance of Predetermining Loads AccuratelyComputerDesignLoads Can be Accurately DeterminedTrench LoadEquationsFrictional Forces in the BackfillThe Effect of Trench

WidthMarston EquationLoad Computation DiagramBackfillSoils Classification ChartEmbankment LoadsSuperimposedLoadsConcentrated LoadsTable of Load CoefficientsWheelLoadsDistributed LoadsImpact LoadsTrench Width, Depth ofFill and Soil Characteristics Using Trench Load Tables TypicalLoad Computation for Highway Work Summary

PartII. StructuralDesign 44 Design Load Versus Actual LoadThe Effect of Trench Width The Effect of Moving the Trench Box or Removing the Sheeting The Effect of Sloping Trench WallsSupporting Strength of

Vitrified Clay PipePhysical Properties of Vitrified Clay Pipe Bearing Strength Tests Foundation Proper Bedding toDevelop Design Supporting Strength

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TABLEOFCONTENTS(Continued) PageNo. Bell or Coupling HolesBedding MaterialsSuitable BeddingMaterial Native BeddingBedding Sieve Analysis GeneralGuidelines

Load Factors

Field Supporting Strength

Supporting

Strength in Trench Conditions Design Saftey Factor SampleProblems Solutions Bedding DiagramsUse of Concrete toDevelop Added Support Principles of Concrete Design

Chapter5. TRENCHLOADTABLES 59 Based on Marstons Equations Showing Loads on Pipe byBackfilling With Various MaterialsTRENCH LOAD, AComputer Design Trench Load Program

Chapter6. CONSTRUCTION 90 ExcavationTrench WallsUse of Shoring, Sheeting andTrench BoxFoundation and Trench Bottom Preparation PipeLayingPipe Joining Industrial Application Pipe BeddingInitialBackfill Final BackfillCompactionMechanical CompactionWater CompactionSpecialized EncasementPiling and SpecialFoundations DewateringGeotextilesService Connections

Chapter7. TRENCHLESSTECHNOLOGY 101A Technology for Clay PipeMicrotunnelingPipe Bursting Pilot Tube Method

Chapter8. CONSTRUCTIONOFSPECIALSTRUCTURES 103 ManholesManhole Frames, Covers and StepsDropManholesTerminal Cleanout StructuresAbovegroundSewersMeasuring and Sampling Flow in Sewers

Chapter9. INSPECTIONANDTESTING 107 QualificationsOn the JobDutiesAcceptance Testing Test MethodsInfiltration TestingCalculation ofInfiltration RateAir TestingSummary of Method

ProceduresSafety

Chapter10. RESIDENTIALBUILDINGSEWERS 112 Lateral SewersTrench ExcavationInstallation Backfilling

Chapter11. APPENDIX 114 Applicable StandardsVelocity HeadsHydraulic Propertiesof Clay Pipe at Design DepthHydraulic Properties of CircularSewersMetric Units and SymbolsConversion Table

Conversion FactorsRadius of Curvature and Angle ofDeflection for Curvilinear SewersPerforated Clay PipeIndex

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Vitrified clay pipe is truly unique in itscorrosion and abrasion resistant quali-ties. It is manufactured from clays andshales, the earthy mineral aggregates

which are the end products producedby the weathering forces of nature.Through centuries of time the solubleand reactive minerals have beenleached from rock and soil, leaving aninert material. This chemically inertmaterial is transformed into a dense,hard, almost homogeneous, clay bodythrough firing in kilns at temperaturesabout 2000oF (1100oC). At this pointvitrification occurs, as the clay miner-

al particles become fused into an inert,chemically stable compound, integrallybonded by its very nature, independentof any outside or artificial agent.

Only specialized raw clay materials,found in hydrous alumina silicates, aresuitable for the manufacture of vitri-fied clay pipe. The requisite character-istics are:

1. Plasticity essential for extrusion,

2. Suitable vitrification properties and

3. Stability at high temperatures.

Clay pipe manufacturers blend the fire

clays and shales to develop the inherent

strength and load bearing capacities ofthe pipe. As noted in the manufacturing

chart on pages 6 and 7, the principal

steps in the manufacture of clay are

mining, blending, grinding, pugging,

forming, finishing, drying, firing and

testing the pipe and joint.

The high quality of vitrified clay pipe

manufacture and performance is main-

tained in accordance with Standards

issued by the American Society for

Testing and Materials (ASTM). These

specifications are prepared by a com-mittee consisting of engineers from theconsulting, governmental, laboratorytesting and educational fields. The

engineering team continually reviewsand upgrades clay pipe standards forquality and performance as the latest

manufacturing methods and automated

processes are introduced.

When clay pipe manufacturers apply

modern day automated procedures to

natures perfect material conforming to

standards established by recognized

technical societies, the finest engineer-ing material available today for gravity

sewers is produced. It should be noted

that Vitrified Clay is the only piping

material exclusively designed to convey

the full range of materials that a com-

munity or an industry may discharge

into it. It will not rust, shrink, elongate,

bend, deflect, erode, oxidize or deterio-

rate. It is structurally sound, with a

permanently fused body independent ofchemically reactive bonding agents.

Clay Pipe Engineering Manual

CHAPTER 1

VITRIFIEDCLAYPIPEAnEngineeringMaterial

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For these reasons, vitrified clay pipe

can satisfy all of the following factors

which must be considered in the selec-

tion of materials for sewer construction.

1. Life expectancy: over 100 years of

proven performance.

2. Chemically inert: resistant to inter-

nal and external attack from solvents,

acids, alkalies, gases, etc.

3. Flow characteristics: low friction

coefficient.

4. Structural integrity: inherent load

bearing capacity.

5. Joint tightness: factory applied flex-

ible compression joints.

6. Abrasion resistance: exceptional re-

sistance to abrasion and scour.

7. Availability: available in a full range

of sizes, fittings and adapters.

8. Handling: easy to handle and install.

9. Economics: best total value consider-

ing cost of material, installation, main-

tenance and useful life.

Industrial users regularly specify clay pipe

to carry aggressive effluent.

VITRIFIEDCLAYPIPE...is one of mans most enduring materials.

The manufacture of Clay Pipe involves

many important steps.

This manufacturing process produces a

homogeneous, heat-bonded, chemically

inert material which provides a perma-

nent and durable product for all sewer

systems. Vitrified Clay Pipe installed

over one-hundred-fifty years ago are

still operational.

1.MININGTHE CLAY Vitrified Clay Pipe is made of selected

clays and shales. Laboratory tests

determine the correct properties of allraw materials for maximum strength

and other physical characteristics.

2. BLENDINGTHECLAYSMany clays are aged to various degrees

and then blended in the proper combi-

nations.

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3. GRINDINGTHE CLAYSClay are ground in heavy, perforatedmetal pans by large crushing wheels.

The clay is ground fine enough to fallthrough the perforations in the metal

pan.

5. FORMINGTHEPIPEThe moistened clay is extruded underextreme pressure to form the pipe.

7. DRYINGTHEPIPEThe pipe is transferred to large, heateddrying rooms to remove moisture.

9.FIRING THEPIPEThe temperature in the kiln is gradu-

ally increased to the intense heat

required for vitrification of the pipe,

which is approximately 2000O

F (1100O

C).

4. PUGGINGTHECLAYSGround raw materials are mixed with

water in a pug mill. This material isforced through a vacuum, de-airing

chamber to produce a smooth, densemixture.

6.FINISHINGTHE EXTRUDED PIPE Automatic machines trim and finish

the moist pipe.

8. SETTINGTHE KILNThe pipe is then set on tunnel kiln cars,

as illustrated, or in the familiar beehivekiln.

10.TESTINGTHE PIPEANDJOINTRepresentative samples are tested for

performance.

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1. Washington, D.C. 1815

2. Philadelphia, PA 1829

3. Boston, MA 1829

4. Sydney, N.S. Wales 1832

5. Manchester, England 1845

6. Liverpool, England 1846

7. London, England 1848

8. Clinton, IA 1850

9. Edinburgh, Scotland 1850

10. Rigby, England 1851

11. Croydon, England 1851

12. Darlington, England 1852

13. Chicago, IL 1856

14. Cleveland, OH 1861

15. New York, NY 1866

16. Erie, PA 1868

17. Grand Rapids, MI 1869

18. St. Louis, MO 1869

19. Hartford, CT 1870

20. Indianapolis, IN 1872

21. Los Angeles, CA 1873

22. New Haven, CT 1873

23. St. Paul, MN 1873

24. Portland, OR 1873

25. Raleigh, NC 1873

26. Lawrence, KS 1874

27. Baltimore, MD 1875

28. Portland, ME 1875

29. San Francisco, CA 1876

30. Jacksonville, FL 1876

31. Albany, GA 1876

32. St. Joseph, MO 1876

33. Davenport, IA 1877

34. Kansas City, MO 1877

35. New Bedford, MA 1877

36. Bucyrus, OH 1877

37. Omaha, NE 1878

38. Camden, NJ 1879

39. Memphis, TN 1879

40. Parkersburg, WV 1879

41. Providence, RI 1879

42. Nashville, TN 1879

43. Rome, GA 1880

44. Rockford, IL 1880

45. Terre Haute, IN 1880

46. Sioux City, IA 1880

47. Red Wing, MN 1880

48. Reno, NV 1880

49. Fargo, ND 1880

50. Dallas, TX 1880

51. Denver, CO 1880

52. Napa, CA 1880

53. Sacramento, CA 1880

54. Woodland, CA 1880

55. Kalamazoo, MI 1881

56. Le Mars, IA 1884

57. Salt Lake City, UT 1888

58. San Jose, CA 1890

59 Phoenix, AZ 1892

60. Massilon, OH 1892

61. Santa Cruz, CA 1895

62. Atlanta, GA 1895

63. Highlands, NJ 1895

8

100 - YEARSEWERSThese are only a few of the many municipalities where vitrified clay pipe

sewers have served for one hundred years or more.

Date First

Clay PipeCity Was InstalledDate First

Clay PipeCity Was Installed


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The National Clay Pipe Institute is an

organization established for the purpose

of providing technical information con-

cerning vitrified clay pipe to the con-sulting engineering profession and to

the various federal, state and local

agencies involved in the design and

construction of sanitary sewer systems.

NCPI maintains offices in Corona,

California and in Lake Geneva, Wiscon-

sin. These offices provide engineering

information pertaining to the design

and construction of sewerage systems.

Sponsored research is routinely con-

ducted at major universities and by

NCPI member manufacturers to fur-ther improve the product and to develop

advanced techniques related to the

design and installation of sewer lines.

Typical projects include:

Installation and performance ofclay

pipe in varying embedment conditions.

Finite element analysis of trenchloads and bedding factors.

Corrosive and abrasive conditionsencountered by sewer systems and the

resistance of various types of sewer

piping materials to these conditions.

Computer analysis of factors

relating to the selection, performance

and economic justification of sewer pipe

materials.

Innovative and alternative design

and installation technologies i.e. deepburial, flat slopes, trenchless technology,

rehabilitation and replacement criteria.

The design of compression type

joints, including the chemical resist-

ance of jointing materials.

The physical properties of vitrified

clay pipe as related to the mineralog-

ical characteristics of types of clays

required in clay pipe manufacture.

NCPI Field test to evaluate Controlled Low Strength Material as a pipe bedding.

9

NationalClayPipeInstitute(NCPI)


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SewerPlanningPlanning for the economical devel-

opment of a sewer system requires

information on current flows and fore-casts of future flows. The projection of

flow increases should provide sufficient

lead time to formulate economic propos-

als, secure approvals, arrange financ-

ing, design, construct and place in oper-

ation the necessary sewers to carry

domestic, commercial and industrial

wastewater from a community to a

point of treatment.

DesignPeriod A design period must be chosen and

sewer capacity planned that will be

adequate. Professional planners are

reluctant to predict land use or

population changes for more than 20

years into the future. However, when

planning, designing, financing and con-

struction are considered together with

the relatively minor additional cost of

providing extra capacity, a 50 year

hydraulic design period should be

considered. Planners should design for

ultimate development where special

conditions exist such as remote areas

near the boundary of a drainage area.

Also to be considered are areas where

special construction, such as tunnels

and siphons, may be required. The cost

of additional capacity is minimal com-

pared to the cost of relief lines installed

at a later date.

Mainline sewers should be designed for

the population density expected in the

areas served, since the quantity of

domestic sewage is a function of the

population and of water consumption.

Trunk and interceptor sewers should be

designed for the tributary areas, land

use and the projected population. For

these larger sewers, past and futuretrends in population, water use and

sewage flows must be considered. The

life expectancy of the pipe is critical.

Clay pipe has a demonstrated life

expectancy in excess of 100 years.

DrainageArea A drainage area is the territory being

considered within which it is possible to

find a continuously downhill surface

route from any point to the established

outlet. Drainage areas should also

include areas that are tributary by

gravity that will be served by future

sewer construction and areas that are

not tributary by gravity which could be

served by pumping or other means. It

should be noted that natural drainage

boundaries do not necessarily coincide

with political boundaries.


CHAPTER 2

GRAVITYSEWERDESIGNPartI - PlanningandLayout

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DesignFlowsA sanitary sewer has two main functions:

(1) to carry the peak discharge forwhich it is designed and (2) to transportsuspended solids so that deposits inthe sewer are kept to a minimum. It isessential, therefore, that the sewerhave adequate capacity for the peakflow and that it function properly atminimum flows.

The peak flow determines the hydrauliccapacity of sewers, pump stations andtreatment plants. Minimum flows mustbe considered in design of sewers and

siphons to insure reasonable cleansingvelocities.

Population EstimatesThe best tool to use for estimatingfuture sewage flow is population data.Forecasts of commercial and industrialflows are also helpful. A long rangepopulation forecast is needed and, if

possible, an estimate of future commer-cial and industrial development. Alarger value for gallons/capita/day (gcd)should be used when these estimatesare not available.

Population data should be collected forthe total drainage area. Populationprojections for large areas are generallymore accurate than for smaller areasbecause historic records are morereadily available and local changeshave less influence.

ConvertPopulationDatatoAverageFlowConvert population data to quantity ofsewage using an average gallons/capita/day (gcd). This per capita flow varies

from 50 to 140 and in some areas ashigh as 160 where industrial flows are

included. The minimum and maximumaverage daily quantities for the initialand final years of the design period arenecessary for designing siphons andtreatment plants.

A value of 100 gcd has been found to be

a reasonable average flow. This does

not include commercial and industrial

flows. An over-all figure of about 125

gcd may be used to convert population

to average flow including commercial

and industrial flow. The Land Use

Coefficients (page 13) can be used to

predict flow from future land use. These

coefficients should be adjusted in accor-

dance with flow studies in the local area.These rates make no allowances for flow

from foundation drains, roofs or yard

drains, none of which should be con-

nected to a sanitary sewer. Plot a pro-

jection of average flow for the drainage

area. A factor is applied to account for

the variation between average flow and

peak flow. This variation is primarily

the result of the time of concentration

since peak flows do not reach a point ina sewer at the same time. The use of

a higher factor for small area flows

(mainline sewers) as compared to large

area flows (trunk sewers) is justified

because small flows are particularly

sensitive to changes, where a slight

increase in rate of flow represents a

large percentage increase. Larger areas

and larger flows have a greater time of

concentration that reduces the resultingvariation.

PeakFactorsThe Peak Factor Table (page 17) maybe used to raise average flow to peakflow. Peak Factors are the relation-ship between average daily dry weather

flow and the highest dry weather peakof the year and varies from 1.3 to 3.5.

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This method yields a reasonable esti-

mate of the peak factors. As flows

increase, the peak factor decreases. If

possible, the peak factors should be

adjusted to flow studies in the local

area.

Extraneous Flows

Sanitary sewer design quantities

should include consideration of the var-

ious non-sewage components which

inevitably become a part of the total

flow. The cost of transporting, pump-

ing and treating sewage obviously

increases as the quantity of flow deliv-

ered to the pumps or treatment facilityincreases. Thus, extraneous flow

should be kept within economically jus-

tifiable limits by proper design and

construction practices and adequately

enforced connection regulations.

Inflow

A very few illicit roof drains connected

to the sanitary sewer can result in

a surcharge of smaller sewers. Forexample, a rainfall of 1 in. per hour on

1,200 sq. ft. of roof area, would contrib-

ute more than 12 gpm.

Connection of roof, yard and founda-

tion drains to sanitary sewers should

be legally prohibited and steps taken to

eliminate them. Water from these

sources and surface run off should be

directed to a storm drainage system.

Tests indicate that leakage through

manhole covers may be from 20 to 70

gpm with a depth of 1 in. of water over

the cover. Such leakage may contribute

amounts of storm water exceeding the

average sanitary flow.

Infiltration

Prominent sources of excessive infil-tration can be poorly constructed man-

holes and or connections and improperly

laid house laterals. Laterals frequently

have a total length greater than the

collecting system and may contribute

as much as 90% of infiltration. House

connections should receive the same

specifications, construction and inspec-tion as public sewers.

In the past, sewer designers allowed

higher amounts of infiltration to aid in

transporting solids. Infiltration must

now be kept to a minimum.

Advantages of FlexibleCompression Joints

Flexible compression joints conforming

to ASTM C 425 Compression Joints forVitrified Clay Pipe and Fittings havereplaced all other forms of joining

vitrified clay pipe. Obsolete field joining

systems can be major contributors to

infiltration. The advantages of limiting

infiltration, exfiltration and roots while

providing flexibility and durability have

been widely demonstrated. A tight andflexible joint is clearly desirable whether

the sewer is above or below ground

water.

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Summations of Flow

Starting at the upper end of the sewer

under review, add projected average

flows for 50 or more years in the future.

As the projected average flows from

each drainage area are totaled, multi-ply by the appropriate peak factor

(page 17) to determine the peak flow for

each reach of the line. These values are

the design capacities for the proposed

sewer (page 15).

Flow Monitoring

A sewer flow monitoring program is

necessary to determine when existing

sewers will reach hydraulic designcapacity. Monitoring methods vary

from high water markers that record

maximum depths to hand held mechan-

ical tools or electronic devices. With a

history of flow data, projections can

forecast the year the peak flow will

reach the design capacity of the sewer.

Check adjacent population, gaugings,

water consumption, rainfall and any

other available data to determine if the

measured quantity of flow is reason-

able. If adjacent measurements or the

estimate is greatly different from the

gauged amount, the cause should beidentified and corrected before proceed-

ing with a relief sewer. With a long

range projection of peak flow based on

population and a short range projection

based on past gaugings, a reasonable

estimate utilizing both can be made.

As new or more reliable information

becomes available, the projection

should be updated. Planning for relief

sewers must begin with sufficient leadtime before the projection reaches the

design capacity of the sewer.

Sewer line modeling computer pro-

grams are available to analyze existing

systems and establish quantities for

the design of relief sewers.

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LAND USE COEFFICIENTS


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1. The coefficients of discharge used in

this example are as follows:

(cfs per acre from page 13)

Low Density LD .0031

Medium Density MD .0116High Density HD .0217

Commercial Comm .006

2. Peak Factors (Pf) are shown in the

Peak Factor Table (page 17).

3. Qav flows are accumulated as they

become tributary to the line. See

Sample Land Use Map (page 14) and

Sample of Flow Estimating Calcula-tions (page 15). Dr. Area 1 average

flow is totaled and converted to Qpk in

Manhole (MH) A, Dr. Area 2 is added at

MH B, Dr. Area 3 is added at MH C,

Dr. Area 4 is added at MH D. Dr. Area

5 is served by a number of house con-

nection sewers directly tributary to the

study sewer all along the Drainage

Area. To simplify calculations the flow

from this area has been lumped togeth-er and added at one point. The point

arbitrary selected was MH D so Dr.

Areas 4 and 5 are both added at that

point. As each Qav is added to the

sum of upstream Qavs the subtotal is

converted to Qpk with the Pf. The Qpk

or Qd downstream from MH D in this

example is 4.5 cfs.

4. If a larger sewer was being studied,

this entire area could be considered oneDrainage Area with the same proce-

dures followed to accumulate Qav and

then convert to Qpk using the Peak

Factors.

5. If a relief sewer was proposed that

would intercept a portion of this Study

Area the average flow from Drainage

Areas or parts of Drainage Areas tribu-

tary to the new line would be added tothe relief line and subtracted from the

existing line. The average flows would

be totaled and converted to Peak using

Peak Factors.

6. The estimated Qav and Qpks are

shown on the Sample Land Use Map.

The Qavs are shown because they can

be easily added and subtracted and are

useful when studying alternate routes,etc. The Qpks are the quantities to be

used to determine the adequacy of an

existing sewer or to design a new one.

These Qpks can also be called Qd.

16

Many industries specify vitrified clay pipe to carry wastes. The chemical

composition of the discharged effluent can vary greatly.


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PartII - HydraulicDesignBasic Premises for CalculatingFlow in Sewers

This section on hydraulics of sewersdeals only with uniform flow. Standardhydraulic handbooks should be con-sulted for special conditions.

Since the flow characteristics of sewageand water are similar, the surface ofthe sewage will seek to level itselfwhen introduced into a channel with a

sloping invert. This physical phenome-non induces movement known as grav-ity flow. Most sewers are of this type.

The flow in a pipe with a free watersurface is defined as open channelflow. Steady flow means a constantquantity of flow and uniform flowmeans a steady flow in the samesize conduit with the same depth and

velocity. Although these conditions sel-

dom occur in practice, it is necessary toassume uniform flow conditions inorder to simplify the hydraulic design.

There are times when sewers becomesurcharged, encounter obstacles re-quiring an inverted siphon or requirepumping. Under these conditions thesewer line will flow full and be underhead or internal pressure.

The Flow Characteristics Diagram de-monstrates the theory and terminologyapplied to flow in open channels. To

simplify the diagram, all slopes aresubcritical and it is assumed that atpoint D a constant supply of water orsewage is being supplied. Between Dand E the slope of the conduit isgreater than is required to carry thewater at its initial velocity, and isgreater than the retarding effect offriction, which causes acceleration tooccur. At any point between E and F,

the potential energy of the water equals

18

GRAVITYSEWERDESIGN


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the loss of head due to friction and the

velocity remains constant. This is uni-

form flow. Between F and G the effect

of downstream conditions are causing a

decrease in the velocity.

The Hydraulic Profile

Three distinct slope lines are common-

ly referred to in hydraulic design of

sewers as shown on page 18.

1. The Slopeof theInvertof theSewer.This is fixed in location and elevation

by construction. Except in rare cases,

the invert slopes downstream in the

direction of flow.

2. The Slopeof theHydraulicGradient(H.G.). This is sometimes referred toas the water surface. In open channel

flow, this is the top surface of the liquid

flowing in the sewer. Except for a few

cases, the hydraulic gradient slopes

downstream in the direction of flow.

3. The EnergyGradient (E.G.). This islocated above the hydraulic gradient, adistance equal to the velocity head

which is the velocity squared divided

by two times the acceleration due to

gravity (v2/2g). This slope is always

downstream in the direction of flow.

For uniform flow, the slope of the ener-

gy gradient, the slope of the hydraulic

surface and the slope of the invert are

parallel to one another but at different

elevations.

Design Requirements

In sewer system design the following

hydraulic requirements must be met:

1. The velocity must be sufficiently

high to prevent the deposition of solids in

the pipe but not high enough to induce

excessive turbulence. The minimum

scouring velocity is 2 feet per second.

Clay pipe is being used successfully

where velocities exceed 20 feet per

second.

2. Where changes are made in the

horizontal direction of the sewer line,

in the pipe diameter, or in the quantity

of flow, invert elevations must be

adjusted in such a manner that the

change in the energy gradient eleva-

tion allows for the head loss.

3. Sanitary sewers through 15-inch

diameter should be designed to runhalf-full at peak flow and larger sewers

designed to run three-quarters full at

peak flow. This also provides neces-

sary air space to transfer sewer gases.

After flow estimates have been pre-

pared, (page 15) including all allowances

for future increases and the layout of

the system has been determined, the

next step is to establish the slope foreach line. Using the land use map

(page 14) working profile sheets are

prepared. The profile sheets show the

surface elevations, subsurface struc-

tures and any other control points,

such as house connections and other

sewer connections. A typical profile for

sewer design is shown on page 21.

Using the profile sheet, a tentativeslope of the sewer is determined begin-

ning at the lower end and working

upstream between street intersections

or control points. The slope is located

as shallow as possible to serve the

adjacent area and tributary areas with

consideration to street grade and any

control points or obstructions.

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Determination of Pipe Sizes

Knowing the peak flow and the tentative

slope, a tentative pipe size can beselected for each reach. Diagrams based

on Mannings Equations showing quan-tity, slope, pipe size and velocity can

be used to find pipe sizes. The dia-grams show quantities for one-halfdepth for small pipe up through 15-inch diameter and three-quartersdepth for 18-inch and larger sizes. Then values range from .009 to .013(pages 24-26). Enter the diagram withQ and slope and read the larger pipesize. Except for cases where there are

large head losses, the tentative pipesize will be the final pipe size.

Selecting the Sizes for theNew Sewer Line

Using the flows (Qd) from the SampleLand Use Map (page 14), the pipe sizesmay be selected after determining theslope of the line and the n value to beused.

The slope is obtained by drawing a pre-

liminary profile showing control points,

such as, sewers to be intercepted, major

substructures, ground lines, outlet

sewer, etc. The n value is selected by

the user or specifying agency.

If the available slope is .004 along thisreach and n equal to .011 was selected

for design, use the n = .011 DesignCapacity Graph shown on page 25.Locate the intersection of the .004 slopeand Qd and read the larger pipe size.In the reach downstream from MH Athe Qd is .96 cfs. This Qd intersectsthe .004 slope between a 10-inch and a12-inch pipe. The larger pipe is usually

selected. In the reach downstream fromMH B, the Qd is 1.53 cfs, indicating

that a 15-inch pipe will be required.

Further downstream, the outflow fromMH F is 9.4 cfs, and a 21-inch pipeis necessary.

As a final check, plot the pipe lines onthe profile, set the outlet elevation and

work upstream through each conflu-ence, making sure there is adequateclearance for substructures, and thatthe line meets all other controls. Thepipe size will have to be recheckedif the slope has been changed for anyreason.

Knowing the quantity of flow and thepipe size, the velocity can be calculated

using the Manning Equation, the Velocity Variation Table (page 22) orthe Design Capacity Graphs (pages 24-26). The velocity head can be calculated

to give the energy gradient.

In many cases, especially with largediameter sewers, it is necessary tocarefully plot the energy gradient ofthe sewer to determine that thehydraulic design requirements are

met.

In these cases, start at the downstreamend of the profile and mark the energygradient at that point. Where the flowenters another sewer it will be theenergy gradient of that sewer.

A line to represent a tentative locationfor the energy gradient for the firstsection of sewer being designed is then

drawn upstream following the avail-able surface slope to the next controlpoint on the profile. As discussed earlier,

this could be a point where flow isadded, a street intersection, an abruptchange in surface slope or other control

points. Care must be taken to see thatthe final design of the sewer providesadequate cover and that the sewerclears all subsurface obstructions. The

profile can now be finalized.

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Quantity and VelocityEquations

The following equations are providedto show the basis for flow diagramsand to supply equations for moreaccurate hydraulic calculations. The

designer is reminded that precisecalculations of hydraulic data are notpossible except under controlledconditions.

The Manning Equations

The most commonly used velocity and

quantity equations are:

V=1.486

r2 / 3s1 / 2 (Velocity)n

Q=1.486

ar2 / 3s1 / 2 (Quantity)n

V is the velocity of flow (averagedover the cross-section of the flow)measured in feet per second. For

sewers flowing at design depth,Vshould exceed 2 feet per second to

prevent settlement of solids in the

pipe. Conversely, velocities exceed-

ing 20 feet per second should beavoided where possible. Clay Pipe

can handle high velocities without

damage, however, manholes, struc-

tures and angle points must bedesigned carefully to avoid prob-lems.

Q is the quantity of flow measured

in cubic feet per second.

n is a coefficient of roughness whichis used in Mannings Equation tocalculate flow in a pipe. (See the

following discussion of n values.)

a represents the cross-sectional areaof the flowing water in square feet.

r represents the hydraulic radiusof the wetted cross-section of thepipe measured in feet. It is ob-

tained by dividing a by thelength of the wetted perimeter.

s represents the slope of the energygradient. It is numerically equalto the slope of the invert and thehydraulic surface in uniform flow.

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DiscussionofValuesfor nThe value of n for smooth bore pipe isaffected by depth of flow, velocity offlow and quality of construction. Incontrolled experiments, using clean

water, values of n under 0.009 haveconsistently been obtained for vitrifiedclay pipe and some other sewer materi-als. Many design engineers recommendthat a more conservative value of n beused in design because of (1) the varia-tions in n due to variable flow condi-tions, (2) the deposition of debris, gritand other foreign materials which findtheir way into a sewer system, (3) the

build-up of slime and grease on all pipesurfaces, (4) the loss of hydrauliccapacity of flexible pipe due to ringdeflection and (5) misalignment due toconstruction or settlement. Basedupon current data, it appears that n

values of .009 - .013 can be applied toall types of smooth bore pipe. Afterpipe lines have been in place for severalyears, measurements may indicate nvalues which differ from the designvalue. These new values can be used forfuture flow calculations. Factors fordetermining Qs at different n valuesare shown on the Design CapacityGraphs (pages 24-26).

23

Flow is retarded by increased friction. A typical smooth bore clay pipe.

These photographs reproduced from Flow of Water Through Culverts, Bulletin 1, University

of Iowa Studies in Engineering, illustrate the effect of pipe smoothness on the flow.


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24


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25


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26


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Computer Design

The National Clay Pipe Institute hasdeveloped a hydraulic design program,HYFLOW, using the Manning equa-tions. This program can select pipesize, flow quantities or velocity in grav-

ity flow sanitary sewers. It is availablefrom the Institute or one of the membercompanies.

Conveyance Factors

Conveyance Factors equal Q/Qd ex-pressed as a percent. Q is the amountof flow at any depth and Qd is the

amount of flow when the depth is atdesign depth. Design depth for pipe15-inch and smaller, is one-half full(.5D) and for pipe 18-inch and larger,three-quarters full (.75D). Depths areexpressed in terms of d/D, where d isthe depth and D is the diameter. TheConveyance Factor Tables are shownon page 28.

Examples 1 and 2 demonstrate the useof the .5D Table for pipe 15-inch andless in diameter.

Example No. 1 Determination ofPercentage of Design Capacity of anExistingSewerThe depth of flow measured in a 10-inch sewer is 0.35 feet. The diameter ofa 10-inch pipe expressed in feet is 0.83feet. Use the .5D Table to calculate d/D(pg. 28). 0.35 divided by 0.83, equals0.42. Enter table with 0.42 (.4 verticaland .02 horizontal) and read 73%. Forthe size, slope and n, read Qd fromthe appropriate Design CapacityGraph. If 1.2 cfs is the Qd then multi-ply by 0.73 to find Q equal to 0.9 cfs.

Example No. 2 Determination of theDepthof Flow WhentheQ isKnownThe same 10-inch sewer has a designcapacity of 1.2 cfs. The estimated flowwill be 0.7 cfs. To find the depth, divide

0.7 by 1.2 which equals 58%. EnterTable with 58% and read d/D of 0.37.Multiply by the diameter 0.83 feet tofind depth of 0.31 feet.

Examples 3 and 4 demonstrate the useof the .75D Table for pipe 18-inch andlarger in diameter.

Example No. 3 Determination of theQuantity of FlowThe depth of flow in a 21-inch sewer is1.12 feet. d/D is 1.12 divided by 1.75 or0.64. Use the .75D Table and read 81%.If the Qd from the Design CapacityGraph for this line is 9.2 cfs, multiply81% times 9.2 for a Q of 7.5 cfs.

Example No. 4 Determination of theDepthof Flow WhentheQisKnownIf the Q is 8 cfs and Qd is 9.2 cfs, divide8 by 9.2 to find the Conveyance Factorof 87%. Enter the Table with 87% andread d/D of 0.67. The depth for a Q of 8cfs is 0.67 times the diameter 1.75,which is 1.17 feet.

Vitrified clay pipe is chemically inert.

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It is not vulnerable to damage due todomestic sewage , sulfide attack, mostindustrial wastes and solvents oraggressive soils.

Hydrogen Sulfide

The relationship between the chem-istry of sewage to the pipe materialsconveying it is of primary concern inthe design of sanitary sewer systems.A brief outline of the factors involved inthe ever-present generation of hydro-gen sulfide gas is provided to point outthe variety of conditions which can

exist and must therefore be anticipatedin sanitary sewers. The protection ofthe sewer system from the ravages ofsewer gas attack is of fundamentalimportance in designing and providingpermanent, trouble-free lines. Failureto fully and properly evaluate any ofthe contributing factors may lead to

subsequent failure of the sewer line.Factors contributing to sulfide gener-

ation and evolution are:

1. Temperature of sewage2. Strength of sewage3. Velocity of flow4. Age of sewage5. pH of sewage6. Sulfate concentration

Sulfides are generated in the slimelayer which forms between the sewerpipe and the flowing sewage. This

action takes place by the bacterialconversion of sulfates into sulfides.The sulfides form hydrogen sulfide gaswhich first diffuses into the sewage andthen, unless destroyed or neutralized,escapes into the sewer atmosphere.

Once the gas is created within the lineand released to the atmosphere abovethe sewage, it comes in contact with themoist surface in the upper part of thepipe and is oxidized very rapidly,by the action of bacteria, into dilutesulfuric acid. The sulfuric acid collectson the exposed arch of the pipe and

begins a chemical attack unless thepipe material is chemically inert andinvulnerable to corrosive acid action.

In solution, H2S is in equilibrium withits partly ionized form HS. The twocomprise what is called dissolvedsulfide. The proportion of dissolved

sulfide existing as H2S varies with pH. At pH = 6, the H2S concentration is

91% of the dissolved sulfide; at pH = 7,the proportion is 50%; and at pH = 8,the proportion is 9.1%. Actual fieldinvestigations of hydrogen sulfide andacid formation in sewers reveal thecrown moisture to have a pH = 2 eventhough the pH of the sewage was closeto neutral (pH = 7).

Under certain conditions, the sulfideswhich originally form in the slimelayer, and which diffuse into the

29


CHAPTER 3

CORROSIONINSANITARYSEWERSVitrifiedClayPipeisChemicallyInert


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sewage, are destroyed more rapidlythan they are formed. Under otherconditions, accumulation, or build-uptakes place.

From the chemical standpoint, it is rec-

ognized that for hydrogen sulfide gasgeneration to occur, there must be asupply of sulfate present. Sulfate isalways present in sewage. Even in acommunity where the water supplycontains no sulfate, the sewage willcontain sulfate in sufficient concentra-tion to produce severe sulfide condi-tions. It has been amply demonstratedthat sulfide is produced just as rapidly

where there is little or no sulfate in thewater supply where a large amount isavailable. Sulfide generation will con-tinue until all sulfate and other sulfurcompounds in the sewage have beenconverted to sulfide.

The factor which determines whethersulfide build-up occurs in a stream ofsewage is whether or not oxygen isabsorbed at the surface of the streamfast enough to oxidize the hydrogensulfide diffusing out of the slime. The

oxygen demand varies from one sewageto another. Oxygen absorption dependsprincipally upon flow velocity. A highflow velocity may reduce sulfide build-up depending upon the strength andtemperature of the sewage.

However, high velocity may also bedamaging if any hydrogen sulfide ispresent in a stream of sewage. Therate of sulfide release increases withincreased flow rate. Turbulence, due to junctions, changes of pipe size, drops,etc. will cause a relatively rapid releaseof hydrogen sulfide gas.

30

A. Bacteria in the slime under flowingsewage converts to sulfides.

C. Hydrogen sulfide gas in atmospheremakes contact with moisture in arch in pipe

which contains more bacteria. Bacterialaction converts gas to sulfuric acid.

B. Sulfides in the liquid make their way tothe surface and are released into the sewer

atmosphere as hydrogen sulfide gas.

D. If pipe is of corrodible material, sulfuricacid attacks it causing ultimate failure.

Vitrified clay pipe is chemically inert andnot vulnerable to acid attack.


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One of the major causes for the increas-

ing sulfide damage in modern sewer

systems is the dumping of vast quanti-

ties of organic matter from household

garbage grinders into such systems.

This condition increases deposits in

sewer lines, thus retarding the flowand providing a source of increased

sulfide generation. It also substantially

increases the B. O. D. which increases

the difficulty of meeting the oxygen

required to limit sulfide build-up.

Force mains are a cause of sulfide prob-

lems in sewers, particularly if the

sewage is retained for any appreciable

length of time. High sulfide concentra-tions will not damage the interior of the

filled pipe, but may cause odor nui-

sances and damage to downstream

structures.

Sewage temperature is a contributing

factor in the rate of development of

sewer gas. Household appliances such

as dishwashers, washing machines,

etc., have resulted in large quantities ofhot water being discharged into the

sewer system. When consideration is

given to the fact that for every increase

of 10 degrees in sewer temperature,

there is a 100% increase in the effective

B.O.D., it shows why it is difficult to

prevent hydrogen sulfide generation in

sewers.

When corrodible pipe materials are

attacked by sulfuric acid, disintegra-tion begins on the upper surface of the

pipe leaving a soft residue. Sometimes

the soft or pasty material is washed

away by high water exposing new

surfaces to corrosive attack. Even when

this does not occur, acid formed at the

exposed surface continues to diffuse

through this residue and attacks the

underlying pipe material. When the

arch is too weak to support the earth

load, it collapses and the sewer

becomes inoperable.

Acid Resistance

Test procedures to determine the acid

resistant qualities and other properties

of vitrified clay pipe are also outlined in

ASTM C 301.

Aggressive Soils and OtherHostile Environments

Some sanitary sewers are subject to

constant attack by a multitude ofwastes from industry, homes and

businesses. Ordinary domestic sewage

includes detergents, drain cleaners,

scouring powders, bleaches and other

household chemicals. From business

and industry come other and more

aggressive chemicals, solvents, acids

and alkalis.

Sanitary sewer pipe may also be

subject to corrosion from acidic oralkaline soils, electrolytic decompo-

sition attack and temperature induced

damage. Different pipe materials dis-

play various levels of resistance to

these factors. Cement bonded and

metallic pipe materials normally re-

quire special protection.

Temperature and solvent sensitive

plastic materials should be avoidedwhere the potential exists for these

factors to occur.

Preliminary soils and site investigation

should be required if conditions in the

area selected for installation are un-

known or suspected to cause damage

to candidate materials.

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CHAPTER 4

RIGIDCONDUITS,UNDERGROUNDPartI - StructuralAnalysis

This section deals with the examination

and evaluation of all those forces whichaffect or influence the structural sta-bility and useful life of vitrified claypipe.

Methods are outlined by which trench

loads may be considered and analyzedfor the purpose of accomplishingrequired structural support.

Importance of PredeterminingLoads Accurately

There is a tendency to think of sewerpipe from the hydraulic standpoint only

and to neglect the importance of pipeas a structural element. It must, above

all else, maintain structural stability.

Nearly all building codes impose legalstandards upon designers to insureagainst the failure of building struc-tures. Standard practice in highwaywork and railroad work also provides

for predetermined structural safety.

Computer Design

The National Clay Pipe Institute hasdeveloped TRENCH LOAD, a comput-er program which can be used to deter-mine backfill loads, safety factors andbedding classes. It is available fromthe Institute or one of the membercompanies.

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Loads Can be AccuratelyDetermined

Just as the safety of ordinary structural

members involves the application ofmechanics to cases of assumed liveloadings, the safety in underground

pipe work involves application of soilmechanics for determining the load onthe pipe. The amount of load to be sup-ported by the pipe can be computed andthe result will be safe and accurate inthe same sense that predeterminationof beam strength is safe and accurate.

To provide engineers with a convenient

method of predetermining loads andstrength requirements for clay pipe,complete reference tables are includedin Chapter 5. These tables show theestimated load according to pipe size,trench depth and width and type ofbackfill. Other references (pages 44-51)

provide data for determining the effectof the type of bedding or support givento the pipe.

Trench Load Equation

To determine a reliable equation forcomputing the relationship betweenvarious kinds of loads and the re-quired test strength of pipe, a series

of studies have been made at theEngineering Experiment Station ofIowa State College. The result is theMarston Equation named for its origi-nator, Anson Marston, who wasPresident of the American Society ofCivil Engineers and Dean Emeritus ofthe College. It is a widely recognized,conservative equation for computingtrench loads on pipe.

An understanding of the MarstonEquation, and the factors involved, ishelpful when using the trench load

tables.

Essentially, any structure installedbelow the surface of the earth supportsthe weight of all the materials above it,

depending upon certain characteristicsof the fill. These characteristics, (prin-cipally internal soil friction) tend toincrease or diminish the backfill loadon the pipe structure.

This is true for both trench andembankment loads. Considering astructure of circular cross-section suchas a sewer pipe, the backfill material

directly above the pipe is that materialwhich lies between vertical planestangent to the outside of the pipe barrel(page 34). The net load on the pipeexclusive of live load, is the actualweight of such backfill material plus orminus an amount which depends onwhether internal soil friction assists inthe support of the mass of backfill overthe pipe or not.

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Cross section of a typical trench

showing primary and secondary planes.

The drawing above demonstrates the

cross-section of a typical sewer trench

showing the location of planes tangent

to the sides of the pipe. These are

called the primary planes. When the

backfill in a trench is compacted uni-

formly, uniform settlement (further

compaction) can be expected with the

passage of time. The depth of the back-

fill between the primary tangent

planes will be reduced through such

settlement by a fairly definite amount,

depending upon the nature and com-

paction of the original backfill.

The backfill between the trench walls

and the primary planes on either sideof the pipe will also settle in time.

Frictional Forces in the Backfill

Since the depth of the backfill between

the pipe and the trench sidewalls is

greater than the depth of the backfill

directly over the pipe, it will settle or

compact more than the material directlyover the pipe. This movement will be

restricted by friction between the back-

fill particles on each side of the primary

tangent planes. The increased settle-

ment of the backfill on both sides of the

pipe tends to tranfer load to that por-

tion of the backfill located directly

above the pipe, thereby transmitting

additional load to the pipe.

Secondary vertical planes are assumedto be between the primary planes and

the walls of the trench as shown in the

drawing. As mentioned previously, the

backfill between the primary and sec-

ondary planes is prevented from set-

tling to a maximum amount by the

action of a frictional force along the

primary vertical planes. This increases

the load supported by the pipe in the

trench condition.

The remainder of the backfill which

lies between the secondary planes and

the trench walls is supported in part by

friction along the trench walls. This

reduces the load on the pipe.

The Effect of Trench Width

It will be seen that, as the secondary

plane is moved away from the pipe, the

differential settlement on opposite

sides of the plane will become less. It is

therefore possible to locate a definite

position where the differential settle-

ment on opposite sides of the secondary

plane is so small that no frictional

forces are transmitted across it. When

this location is within the cross section

of the trench, the weight of backfill

between the secondary plane and

trench wall can add nothing to the loadon the pipe. In other words, the trench

34


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width may be increased from this pointon without adding to the weight on thepipe.

The minimum distance which meetsthe above qualifications is called the

transition width of the trench. It isthe trench width at which furtherwidening will have no effect on theload on the pipe.

When the actual width is less than thetransition width, friction in the planeof the trench wall tends to supportpart of the load and to lessen the loadon the pipe. This phenomenon isgraphically illustrated by the curvemarked surface curve after settlementin the previously referenced drawing.Wherever this curve deflects down-ward from its origin directly over thecenter of the pipe, internal friction inthe backfill transmits weight to thepipe. Where the curve deflects upward(as alongside the trench wall) backfill

weight is transmitted to the side wallof the trench.

MarstonEquationThe Marston Equation applies the pre-ceding reasoning to the calculation ofloads on pipes. Actual tests have beenperformed on many types of soil todetermine the weight, frictional charac-teristics and the relative settlement ofeach type. These measurable quanti-ties have been combined into a single

expression to produce for each case acomputation of the total weight sup-ported by the pipe.

The factors taken into consideration inthe following Marston Equation are:

Depth of backfill cover over the topof the pipe.

Width of trench measured at the

level of the top of the pipe.

Unit weight of backfill.

Values for frictional characteristics

of the backfill material.

The Marston Equation for pipe in nar-

row trenches is:

Wc=CdwBd2

where Wc= The vertical external load ona closed conduit due to fill materials

(lb/ft of length),

Cd = Load calculation coefficient forconduits completely buried in ditches,

abstract number (see Computation

Diagram on page 36),

w = The unit weight of fill materials,(lb/ft3) and

Bd = Breadth of Ditch (trench widthmeasured at top of pipe barrel, ft.)

By substitution of available data in the

Marston equation, a direct result is

obtained for the load on the pipe interms of pounds per linear foot. The

computation of loads is simplified by the

use of this equation and the Computa-

tion Diagram on page 36, which repre-

sents the plotted solution of the Load

Calculation Coefficient equation:

-2K'{HBd}

Cd=1- e

2K'where e= 2.7182818 which equals base

of natural logarithms, an abstract

number,

K = Ratio of active horizontal pressureat any point in the fill to the vertical

pressure which caused the active hori-

zontal pressure, an abstract number,

'= The coefficient of sliding friction

between the fill material and sides of thetrench, an abstract number, and

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*Reference USBR Standards 5000 and 5005

37


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H=Vertical height from top of conduit tothe upper surface of fill in feet.

The Computation Diagram is based onvarious types of soil conditions, andmay be used to obtain the values of the

load calculation coefficient Cd.

The Trench Load Tables in Chapter 5have been compiled using the Marstonequation previously described. The soilweights are based upon an arbitraryvalue of 100 lbs./cu.ft. When the actu-al soil weight is known to vary from 100lbs./cu.ft., the tabulated loads may beadjusted up or down by direct ratio.

Embankment Loads

Although these Trench Load Tablesshow loads on pipe in trenches, theyare equally applicable for pipe installedunder embankment or wide trenchconditions. As the width of the trenchincreases, other factors remainingconstant, the load on the pipe increases

until it reaches a limiting value equalto the embankment load on the pipe.This limiting value is called the tran-sition width. The transition widthsshown in the Trench Load Tables havebeen calculated using the equation forpositive projecting conduits in widetrenches.

Superimposed Loads

Concentrated and distributed superim-posed loads should be considered in thestructural design of sewers, especiallywhere the depth of earth cover is lessthan 8 ft. Where these loads are antici-pated, they are added to the predeter-mined trench load. Superimposedloads are calculated by use of Hollsand Newmarks modifications toBoussinesqs equation (page 40).

ConcentratedLoadsHolls integration of Boussinesqs solu-tion leads to the following equation fordetermining loads due to superimposedconcentrated load, such as a truckwheel load (Diagram 1):

Wsc= Cs

P F

L--------

in which Wsc is the load on the conduitin lb/ft of length; P is the concentratedload in lbs; F is the impact factor; Cs isthe load coefficient, a function ofBc / (2H) and L/ (2H); H is the

height of fill from the top of conduit toground surface in ft; Bc is the widthof conduit in ft; and L is the effectivelength of conduit in ft. (For values ofFand Cssee pages 40 and 41).

Diagram 1. - Superimposed concentrated

load vertically centered over pipe.

An effective length, L, equal to 3 ft. forpipe greater than 3 ft. long, and theactual length for pipe shorter than 3 ft.is recommended.

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H-20 wheel loadings are standard forhighway and bridge design and areapplicable for estimating traffic loadson sewers. However, engineers andcontractors shall consider constructionloads imposed upon sewers subsequent

to their installation. Large earthmov-ing equipment traveling over sewersand construction activities from subse-quent installation of nearby structuresshould be reviewed for additional

imposed loads on installed pipes.Wheel loads from large constructionequipment may exceed 50,000 lbs.

H-20 refers to wheel loading resulting

from the passage of trucks having agross weight of 40,000 lbs., 80% ofwhich is on the rear axle, with axlespacing of 14 ft., center to center, and awheel gauge of 6 ft., each rear wheelcarrying one half this load or 8 tons(16,000 lbs.) without impact.

Example: Determine the load on a 15inch, 5 ft. length of pipe with 5 ft. ofcover caused by a concentrated H-20

wheel load. For pipe greater than 3 ft.long, use 3 ft. as the effective length, L(pg. 38).

P = 16,000 lbF = 1.5 (Highway)L = 3.0 ft.d= pipe diameter = 15 inch

t = wall thickness = 1.5 inches

Then Bc = 15 + 3 = 18 inches = 1.5 ft.H = 5.0 ft.Bc/(2H) = 1.5/10 = 0.15L/(2H) = 3/10 = 0.30Cs = 0.078

(by interpolation from the Cs table,page 40)

Substituting in the equation:

If the concentrated load is not centeredvertically over the pipe, but is displaced

laterally and longitudinally, the loadon the pipe can be computed by addingthe effect of the concentrated load.Dividing the tabular values of Cs by 4will give the result for this condition.

An alternative method of determiningconcentrated or superimposed loads ona buried conduit, is to use thePercentages of Wheel Loads shown inthe Table on page 40. These percent-ages have been determined directlyfrom data contained in Theory ofExternal Loads on Closed Conduits,Bulletin 96, published by the Engineer-

ing Experiment Station at Iowa StateCollege. Note that an allowance for

impact must be added to the percent-age figures shown in the table. Thetable does not apply to distributedsuperimposed loads.

Example: Referring to the previousexample problem.

P = 16,000 lb

F = 1.5 (Highway)Percentage of load for 15-inch pipe with5 ft. depth of cover = 2.6% (0.026)

(from the Percentage of Wheel LoadTable, page 40)

Substituting in the equation:

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40

* These figures make no allowance for impact. see table of impact factors on page 41.

* Influence coefficients for solution of Holls and Newmarks Integration of the Boussinesq equation for vertical stress.


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Distributed Loads

For determining loads on pipe due to

superimposed loads distributed over a

surface area (Diagram 2) the following

equation was developed:

in which Wsd is the load on the conduit

in lb/ft of length; p is the intensity of

distributed load in psf; F is the impact

factor; Bc is the width of the conduit in

ft; Cs is the load coefficient, a function

of D/(2H) and M/(2H); H is the height

from the top of the conduit to theground surface in ft; and D and M are

the width and length, respectively, of

the area over which the distributed

load acts, in ft. (For values of Cs and F,

see pages 40 and 41.)

Diagram 2 - Superimposed distributed load

vertically centered over pipe.

If the area of the distributed superim-

posed load is not centered vertically

over the pipe, but is displaced laterally

and longitudinally, the load on the pipe

can be computed by adding algebraical-

ly the effect of various rectangles ofloaded area. It is more convenient to

work in terms of load under one corner

of a rectangular loaded area rather

than at the center. Dividing the tabu-

lar values of Cs by 4 will give the effect

for this condition.

Impact Loads

Impact factors must be considered to

account for the influence of impact

loading due to traffic and construction

activities after sewer installation.

The following table shows suggested

values.

Impact Factors (F)Traffic ImpactFactorHighway 1.50

Railway 1.75

Runways/Airfield 1.00

Taxiways, aprons, hardstands 1.50

Extremely high impact loads can be

imparted to the pipe especially whenwheeled construction equipment travels

over the trench. The engineer and con-tractor need to consider construction

impact loads during the initial project

and any subsequent construction.

Trench Width, Depth of Fill andSoil Characteristics

41

To properly approach the analysis of

loads imposed on the pipe, it is neces-

sary to decide, for each size of pipe,

what the minimum practicable designtrench width at the top of the pipe is to

be and still permit good workmanship.

The design trench width, the depth of

fill over the pipe, and the soil charac-

teristics of the fill, will produce the

load which must be supported by the

pipe and its bedding. This load is readily

available from either the Trench Load

Tables or the NCPI trench load com-

puter program when the above factors

are known.


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Using Trench Load Tables

The correct use of the Trench Load

Tables, which are given in Chapter 5, is

demonstrated by the following hypo-

thetical case where a designer wants to

know the trench load imposed by the

following conditions:

A 12-inch sewer is to be installed in an

area of gravel K= 0.165 with an aver-

age weight of 120 lb./cu. ft. The top of

the pipe is 8 ft. below ground surface

and the trench width is 30 inches.

Pipe size - 12 in.

Backfill-Gravel K = 0.165Trench width - 30 in.

Cover depth - 8 ft.

Backfill weight - 120 lb./cu. ft.

Backfill load = 1240 lb./lin.ft x 120/100

= 1488 lb./lin.ft.

Typical Load Computation forHighway Work

Suppose that plans call for the installa-

tion of a 15-inch sewer line with 5 ft. of

cover in a 3 ft. wide trench of silt and

clay K=0.110 weighing 95 lb./cu. ft.

and that construction equipment wheel

loads of 16,000 lbs. each will pass over

the backfilled trench before the pave-

ment is placed. This is the maximum

loading condition. What is the totalload on the pipe? To determine the

trench load use the Trench Load Table

on page 69.

Pipe size - 15 in.

Backfill- Silt and Clay K = 0.110

Trench width - 36 in.

Cover depth - 5 ft.

Backfill weight = 95 lb./cu.ft.

Backfill load =(1170 lb./lin. ft. x 95/100)=1112 lb./lin. ft.

(The live load has been calculated.

See example on page 39.)

Live load = 624 lb./lin. ft.

Total trench load = 1736 lb./lin. ft.

Note that the concentrated and dis-

tributed live load equations shown on

pages 38 and 41 include an allowancefor impact and that values of F are list-

ed on page 41.

42

In computing the load to be supported by the pipe line illustrated above,live load must be added to the backfill load.


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SUMMARYPart I has dealt with the examination

and evaluation of those forces which

affect or influence the structural stabil-

ity of vitrified clay pipe underground.

Clay pipe can be installed in deep trenches

with appropriate design and proper instal-

lation.

Having completed the analysis of deter-

mining LOADS on the pipe, the method

of designing the pipeline to SUPPORT

these loads will be developed in Part II

Structural Design.

Vitrification produces a strong and inert

body composition which enables a properly

installed clay pipe system to permanentlysupport the trench loads.

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Structural support is achieved

by selecting and providing proper

trench and bedding conditions.

This section deals with the structuraldesign as outlined by the analysis of the

trench loads previously developed in

Part I, Structural Analysis. Structural

support is achieved by selecting andproviding proper trench and beddingconditions. The following text describes

the methods by which the trench loads

must be supported.

It is of fundamental importance to rec-ognize the variable supporting strengths

of pipe in the trench, including a designfactor of safety, under various beddingand field construction conditions.

The several factors influencing thestructural stability of the proposedinstallation must first be considered.These factors include:

1. Design Load versus Actual Load2. Trench Width3. Moving of Trench Box or Removal of

Sheeting4. Sloping Trench Walls

When these factors have been takeninto consideration, the supporting

strength of Vitrified Clay Pipe can thenbe calculated.

1. Design Load Versus ActualLoad

The design load is the actual load adjusted by a factor of safety. The factorof safety is determined by dividing thefield supporting strength of the pipe by

the total trench load.

It should be clearly recognized that allloads considered in Part I, have beenthe actual loads imposed upon a con-duit in a given installation. In struc-tural design, all actual loads must betranslated into design loads so that the

factor of safety is incorporated in thefinal design.

An engineer determines the factor ofsafety based upon his knowledge oflocal soil conditions, construction prac-tices, future development of the area

and any unusual variations of land use.Solutions to sample problems areshown on pages 51 and 52.

2. The Effect of Trench Width

The trench width at the top of the pipeis one of the most important factors in-volved not only in design but through-out construction.

As shown in the equation Wc = CdwBd2,

the load on the pipe increases in rela-tion to the square of the trench width.Therefore, even a relatively smallincrease in width results in a largeincrease in load.

44

RIGIDCONDUITS,UNDERGROUNDPartII- StructuralDesign


45/124

For example, an 8 inch pipe laid in a

sandy soil weighing 100 pounds per

cubic foot at a cover depth of 14 feet

and 24 inch trench width will have a

load imposed of 1170 lbs./lin.ft. If the

trench width is increased only 6 inches

to 30 inches, the load imposed willincrease to 1700 lbs./lin.ft., or almost

50 percent. (Load Table pg. 62)

The design trench width at the top of

pipe equals the sum of the outsidediameter of the pipe, the minimum

working space on each side of the pipe,

and the thickness of sheeting ifremoved or the trench box wall on each

side of the trench.

Controls during the course of construc-

tion which will preserve the designtrench width are vital to the structural

performance and useful life of the pipe.

3. The Effect of Moving theTrench Box or Removingthe Sheeting

When a trench box is moved or sheeting

is removed from a trench after beddinghas been placed, a space may be creat-

ed at the sides of the trench. Sufficient

bedding material shall be placed sothat the bedding meets the require-

ments of the specified class of beddingfollowing removal of any trench sheet-

ing or box.

Good engineering practice recommendsthat timber sheeting be cut off at thetop of the pipe. The upper portion may

be removed without harming the sup-

port conditions. Thin steel sheetingmay be carefully withdrawn.

4. The Effect of Sloping TrenchWalls

Since the load on the pipe increases

with the square of the width of thetrench at the top of the pipe, it follows

that trenches should be as narrow aspossible.

All available evidence shows that thewidth or shape of the trench above the

level of the top of the pipe does notincrease the load on the pipe. The

trench walls above that level may be

sloped outward without adding to the

load on the pipe.

45


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BellorCouplingHolesBell or coupling holes should be c a r e -fully excavated so that no part ofthe load is supported by the bellsor couplings. The pipe barrel is

designed to support the trench load.Consolidation of material around andunder the bell and couplings duringbedding and backfilling should beavoided because it may create a concen-trated load resulting in a decreasedfield supporting strength.

Provide uniform and continuous support of

pipe barrel between bell or coupling holes

for all classes of bedding.

The field supporting strength of thepipe is substantially reduced when the

pipe is improperly bedded. The engi-neer should insure that the class ofbedding specified is actually providedduring construction. The need forimplementation of proper installationprocedures is clearly demonstrated bysignificant losses in the field support-ing strength of the pipe as a result ofimproper bedding.

4. Bedding Materials

The National Clay Pipe Institute hasconducted extensive laboratory andfield research on bedding materials,load factors and trench load develop-ment. Subsequent field experience hasconfirmed that pipe movement is theleading cause of structural problems.Consequently, the objective of a quality

installation must be to develop a stablepipe bedding system which will mini-mize pipe movement in the long term.

It is known that not all bedding mate-rials provide the same longitudinal andcircumferential pipe support.

An ideal bedding material can bedefined as one that (a) provides uni-

form support over the greatest pipearea, (b) does not develop point load, (c)does not migrate under various trenchconditions, (d) is easily placed with lit-tle or no compaction and (e) is widelyavailable.

Suitable Bedding Material

Suitable material is well-graded 3/4 to1

/4 in. crushed stone, having a mini-mum of one fractured face; or otherangular, non-consolidating beddingmaterial not subject to migration(NOTE 1, pg. 50).

In practice, the precise gradation is notcritical but the bedding materialshould be well-graded.

Standard aggregate sizes for beddingmaterials are in accordance with Table1 - ASTM D 448 Standard Sizes ofProcessed Aggregate (Pg. 48).

Native Bedding

Many native materials taken from thetrench will provide suitable support forclay pipe and may be the most costefficient method of installation. Caremust be exercised to remove largestones which could cause point loading.

Local materials may be used when therequired load factor design can beachieved.

47

Nominal Pipe Size Aggregate Size

Less than 15 67,7 or 8

15 to 30 6 or 67

Greater than 30 57,6 or 67


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General Guidelines

These guidelines are not applicable to

every field condition encountered.

They are offered as a basis for judg-

ment and practical application.

Well-graded, angular bedding

materials are more stable, allow lesspipe movement and are more resistant

to migration when flooded than round-

ed bedding materials of equal grada-

tion.

Rounded materials and gravel are

less stable than angular material and

are not recommended to achieve bed-

ding factors higher than 1.5.

The stability of a bedding material

increases as its particle size increases.

However, gradations containing parti-

cles greater than 1 inch become in-

creasingly more difficult to shovel-

slice into the pipe haunch area and

may result in uneven support.

Fine materials, are subject to more

movement than those of a larger sieve

size. Compaction is required.

Sand is suitable as a bedding mate-rial in a total sand environment.

However, where high or rapidly chang-

ing water tables are present in the

pipe zone, consideration should be

given to the use of angular bedding

with a geo-fabric material. Sand is not

appropriate for bedding or haunching

in a trench cut by blasting or in

trenches through hard clay soil. The

maximum load factor for sand beddingis 1.5.

Controlled Low Strength Material

(50-300 psi) has been shown to be an

economic alternative to other bedding

materials and classes. It assists in

utilizing the inherent strength of the

pipe, completely fills the haunch area

and reduces the trench load on thepipe.

5. Load Factors

The load which a pipe can support

varies according to the class of

bedding.

Trench details shown on page 49

depict the recommended classes of

bedding. Load factors have been deter-

mined for each bedding class. The load

factor is the ratio of the supporting

strength of the pipe in the trench to

its three-edge bearing test strength.

It does not include a design factor

of safety. The three-edge bearing

strength has been established as a

base and is considered equivalent to a

load factor of 1.0.

48

From Table 1, ASTM D 448 Standard Sizes of Processed Aggregate


49/124


50/124

Field Supporting Strength

The load factor is used to compute thefield supporting strength of vitrifiedclay pipe with any designated beddingclass. The specified minimum three-

edge bearing strength of vitrified claypipe is multiplied by the appropriateload factor to obtain the field support-ing strength of the pipe. Therefore, it ispossible to provide the necessary fieldsupporting strength to exceed the calcu-lated trench loads. Field supportingstrengths of extra strength clay pipe(ASTM C 700) are shown in the tableon page 49. Also see the reference tofield supporting strength on pg. 47.

Supporting Strength inTrench Conditions

Class D Load Factor = 1.1 (Fig. 1, pg. 53)

The pipe shall be placed on a firm andunyielding trench bottom with bellholes provided. The bottom of the entirepipe barrel shall have a continuous anduniform bearing support. The initial

backfill shall be of selected material(NOTE 2).

Class C Load Factor = 1.5 (Fig. 2, pg. 53)

The pipe shall be bedded in suitablematerial (NOTE 1), gravel or otherlocally available non-cohesive materi-als on a firm and unyielding trench bot-tom. There shall be a minimum thick-ness beneath the pipe of one-eighth of

the outside pipe diameter but not lessthan 4 inches and sliced into thehaunches of the pipe with a shovel orother suitable tool to a height of one-sixth of the outside diameter of thepipe. The initial backfill shall be ofselected material (NOTE 2).

NOTE 1: Suitable material is well-graded

crushed stone, having a minimum of one frac-

tured face, or other angular, non-consolidating

bedding material not subject to migration.Material shall be shovel-sliced to fill and support

the haunch area and encase the pipe to the limits

shown in the trench diagrams (pgs. 53-56).

NOTE 2: Selected Material shall consist of finely

divided material free of debris, organic material

and large rock and stones.

Class B Load Factor = 1.9 (Fig. 3, pg. 54)

The pipe shall be bedded in crushedstone or other suitable material whichis non-consolidating and not subject tomigration. The bedding shall be placedon a firm and unyielding trench bottomwith a minimum thickness beneath thepipe of one-eighth the outside pipediameter, but not less than 4 inches.

The bedding shall be sliced into thehaunches of the pipe with a shovel orother suitable tool to a height of one-half the outside pipe diameter, or to thehorizontal centerline. Shovel-slicingthe bedding material into the haunch-es of the pipe is essential if the totalload factor is to be realized. The initialbackfill shall be of selected material(NOTE 2).

Crushed Stone EncasementLoad Factor = 2.2 (Fig. 4, pg. 54)

The pipe shall be bedded and encasedin crushed stone or other suitablematerial (NOTE 1) which is non-consol-idating and not subject to migration.The bedding shall be placed on a firmand unyielding trench bottom with aminimum thickness beneath the pipe ofone-eighth the outside pipe diameter,

but not less than 4 inches. The beddingshall be sliced into the haunches of thepipe with a shovel or other suitabletool. Shovel-slicing the bedding mate-rial into the haunches of the pipe isessential if the total load factor is to berealized. The encasement materialshall extend laterally to the specifiedtrench width and upward to a horizon-tal plane at the top of the pipe barrelfollowing removal of any trench sheet-

ing or boxes. The initial backfill shallbe of selected material (NOTE 2).

50


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Controlled Low Strength Material(CLSM) Load Factor = 2.8 (Fig. 5, pg. 55)

The pipe shall be bedded on crushed

stone or other suitable material (NOTE

1, pg. 50). The bedding shall be placed

on a firm and unyielding trench bottomwith a minimum thickness beneath the

pipe of one eighth of the outside pipe

diameter but not less than 4 inches.

CLSM shall be carefully discharged to

the top of the pipe and allowed to flow

approximately equal to both sides to

prevent misalignment. The fill can be

made in a single pour to the top of the

pipe or it can be made in two or more

lifts if desired. Clay Pipe has been

widely used in CLSM installations

where buoyancy was not a problem.However, given the variability of CLSM

mix designs, application methods

and discharge rates, contractors should

recognize that the potential exists for

flotation to occur. It is recommended

that the CLSM material be continuously

agitated prior to discharge due to rapid

segregation of materials.

The CLSM material shall have a 28

day compressive strength of 50-300 psi.

Local and State mix designs are avail-

able which have early setting times

allowing backfilling to proceed quickly.

The initial backfill shall be of selected

material (NOTE 2, pg. 50). Furtherevaluation may be necessary where

native soils are expansive.

Class A: Concrete Cradle, Arch or Full

Encasement (Figs. 6-8, pgs. 55-56)

Three types of Class A bedding are

available giving the designer a wide

selection of load factors. The angular

material shall be crushed stone or other

suitable material (NOTE 1, pg. 50). The

bedding shall be placed on a firm anunyielding trench bottom with a mini-

mum thickness beneath the pipe of one-

eighth of the outside pipe diameter but

not less than 4 inches. The concrete

shall be 3000 psi or greater strength.

Design Safety Factor

The design safety factor is a discre-tionary decision for the ProfessionalEngineer during design based on thenumber and magnitude of unknownvariables. The greater effect theseunknown variables may have, thegreater the need for a large safety fac-tor. Hence, the safety factor is intend-ed to insure a successful project, with-out adding unnecessary costs.

During trench design for Vitrified Clay

Pipe, a safety factor having a valuebetween 1.0 and 1.5 is typically speci-fied. This may be accomplished byusing the different bedding classes.

Sample Problems

Assume a 24-inch clay pipe line is to beinstalled in an area of CH (silts andclays) which has a weight of 107pounds per cubic foot and a K= 0.110

(see Soil Classification Chart on page37). The depth of cover over the top ofthe pipe is 18 feet and the trench widthat the top of the pipe is 48 inches.Determine a structurally sound andeconomic bedding design.

The trench load is determined by usingthe Trench Load Tables or the NCPItrench load design computer program.

Pipe size - 24 in.Depth of cover - 18 ft.Backfill -CH (K= 0.110) @ 107 lbs/cu. ft.Trench width - 48 in.

(From Trench Load Table pg. 75)4570 x 107/100

Total Trench Load = 4890 lbs/lin. ft.

With the trench load on the pipe deter-

mined, the next step is to calculate the

field supporting strength and the safetyfactor.

51

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