Chapter 3 - Design Requirements for Sanitary Sewers - The City of ...

City of WentzvilleEngineering Design Criteria3.03.03 BeddingThe project Plans and Project Specifications shall indicate the specific type ortypes of bedding, cradling, or encasement required in the various parts of thesanitary sewer construction if different than the current City of WentzvilleStandard Specifications and Construction Details.Special provisions shall be made for pipes laid under or over fills orembankments in shallow or partial trenches either by specifying extrastrength pipe for the additional loads due to differential settlement, or byspecial construction methods, including ninety (90) percent modified proctorcompaction of fill, to prevent or to minimize such additional loads.Compacted granular backfill shall be required in all trench excavation withinpublic (or private) rights-of-way or areas where street rights-of-way areanticipated to be dedicated for public use. Under areas to be paved, thecompacted granular backfill shall be placed to the sub-grade of thepavement. Under unpaved areas, the compacted granular backfill shall beplaced to within two (2) feet of the finished surface.Pipes having a cover of less than three (3) feet shall be encased in concrete,unless otherwise directed by the City.If the storm and sanitary sewers are parallel and in the same trench, theupper pipe shall be placed on a shelf and the lower pipe shall be bedded incompacted granular fill to the flow line of the upper pipe.3.03.04 Pipe or Conduit Under Streets and Pavements3.03.05 JointsAny pipe or conduit material beneath a highway, road, street, or pavement, orwith reasonable probability of being so located, shall have ample strength forall vertical loads, including the live load required by the highway authorityhaving jurisdiction, and in no case shall provide for less than an AASHTOHS-20 loading. For other locations, the minimum live load shall be the HS-10loading. Special considerations may be required for adverse conditions.Granular backfill, consisting of 1” clean stone, shall be utilized in utilitytrenches for fill, up to the base of the pavement.The joint type required for the type of pipe used and the application shallconform to the latest standards set forth in the City of Wentzville StandardSpecifications and Construction Details or as approved by the City Engineer.3.03.06 Monolithic StructuresMonolithic reinforced concrete structures shall be designed structurally ascontinuous rigid units.3 -4 Approved 6/10/09

City of WentzvilleEngineering Design Criteria3.03.07 Alignment3.03.08 LocationSanitary sewer alignments are normally limited by the available easementswhich in turn should reflect proper alignment requirements.Sanitary sewers shall be aligned:1. To be in straight line between structures for all pipe sewers thirty (30)inches in diameter and smaller.2. To be parallel with or perpendicular to the centerline of straight streetsunless otherwise unavoidable. Deviations may be made only withapproval of the City.3. To avoid meandering, off-setting and unnecessary angular changes.4. To make angular changes in alignment for sewers thirty (30) inches indiameter or smaller in a manhole located at an angle point and forsewers thirty-three (33) inches in diameter or larger, by a uniformcurve between two tangents. Curves shall have a minimum radius often times the pipe diameter.5. To avoid angular changes in direction greater than necessary and anyexceeding ninety (90) degrees.Sanitary sewer locations are determined primarily by the requirements ofservice and purpose. It is also necessary to consider accessibility forconstruction and maintenance, site availability and competing uses, andeffects of easements on private property.Sanitary Sewers shall be located:1. To serve all property conveniently and to best advantage.2. In public streets, roads, alleys, rights-of-way, or in sewer easementsdedicated to the City.3. In easements on private property only when unavoidable.4. On private property along property lines or immediately adjacent topublic streets, avoiding crossing through the property.5. At a sufficient distance from existing and/or proposed buildings(including footings) and underground utilities or other sewers to avoidencroachment and reduce construction hazards.6. To avoid interference between house connections to foul water orsanitary sewers and stormwater sewers.3 -5 Approved 6/10/09

City of WentzvilleEngineering Design Criteria3.03.09 Flowline3.03.10 Manholes7. In unpaved or unimproved areas whenever possible.8. To avoid, whenever possible, any locations known to be or probablyto be beneath curbs, paving or other improvements particularly whenlaid parallel to centerlines.9. To avoid sinkholes and creeks.The flowline of sanitary sewers shall meet the following requirements:1. The flowline shall be straight or without gradient change between theinner walls of connected structures.2. Gradient changes in successive reaches normally shall be consistentand regular, with small or insignificant differences in successivereaches. Gradient designations of less than the nearest 0.001 foot perfoot, except under special circumstances and for larger sewers, shallbe avoided.3. Sewer depths shall be determined primarily by the requirements ofpipe or conduit size, utility obstructions, required connection, futureextensions, and adequate cover.4. For sanitary, the hydraulic grade line shall not rise above the intradosof the pipe.5. When the grade of a sewer is twenty (20) percent or greater, aconcrete cradle or collars are required. For grades exceeding fifty (50)percent a special design and Project Specifications are required.6. A 0.2’ drop shall be provided from the incoming to outgoing flowline ofmanholes.Manholes provide access to sewers for purposes of inspection, maintenanceand repair. They also serve as junction structures for connecting lines.Requirements of sewer maintenance determine the main characteristics ofmanholes.1. Manholes shall be located at changes in direction, changes of pipesize, flowline gradient, and at junction points with connecting sewers.For sewers thirty-three (33) inches in diameter and larger, manholesshall be located on special structures, at junction points with othersewers, and at changes of size or gradient.3 -6 Approved 6/10/09

City of WentzvilleEngineering Design Criteria2. Spacing of manholes shall not exceed four hundred (400) feet for pipesewers thirty-six (36) inches in diameter and smaller and five hundred(500) feet for pipe sewers forty-two (42) inches in diameter and larger,except under special approved conditions. Spacing shall beapproximately equal, whenever possible.3. At stream and channel crossings, manholes shall be located on bothsides of the crossing at changes in pipe material. The manhole shallbe located a minimum of twenty (20) feet from the top of the bank onboth sides of the crossing.4. All manholes on sanitary sewers that are built within the 100-yearflood limits, in the Stormwater overflow path, or in areas determined tobe subject to flooding shall be provided with lock-type watertightmanhole covers.5. Manholes located on 8” sanitary sewers shall be 42” diameter,minimum size. Manholes located on 10” sanitary sewers and largershall be 48” diameter, minimum size. Manholes having one (1) insidedrop shall be 48” diameter, minimum size. Manholes having two (2)inside drops shall be 60” diameter, minimum size. Terminal manholesthat could facilitate a future extension of the sanitary system shall be48” diameter manholes, minimum.3.03.11 Sewage Treatment Facilities3.04 Design Requirements3.04.01 General3.04.02 GradientsNew treatment plants will not be allowed.All Sanitary Sewers shall be so designed and constructed as to conform tothese design requirements. Hydraulic Calculations shall be submitted whenrequested by the City. Calculations must include existing and ultimateupstream development conditions.The following minimum slopes of sanitary pipe sewers are those giving atleast 3 feet per second velocities flowing full, based on Manning's formulausing an "n" value of 0.013 unless otherwise directed by the City. Slopesgreater than these minimums shall be used wherever possible.Pipe SizeMinimum Slope in Ft.per 100 Ft. (% Grade)8 1.010 0.63 -7 Approved 6/10/09

City of WentzvilleEngineering Design Criteria12 0.615 0.418 0.321 0.324 0.227 0.230 0.236 0.1Pipes larger than thirty (36) inches in diameter shall maintain a cleansingvelocity of three (3) feet per second.Sanitary Sewer laterals, which shall be a minimum size of 6”, shall have aminimum slope of 2%. (2 feet per 100 feet)3.04.03 Depth and Minimum Cover3.04.04 Flow DesignThe minimum depth of collecting and lateral sewers should be nine (9) feetbelow the finish street grade to flowline except where the topographyindicates this depth is not necessary. Where the nine (9) feet depth is notrequired, the minimum vertical distance from the low point of a basement orlow floor to the flowline of a sanitary sewer at the corresponding houseconnection shall be not less than the diameter of the sewer plus a verticaldistance of two and one-half (2 l/2) feet. This minimum vertical distance shallbe increased, if the length of the lateral requires, obtaining a minimum slopeof two (2) percent for six (6) inch house laterals. These minimum depths willnot apply to interceptor sewers in low areas whose only function is to carry offthe accumulated flow to existing or proposed trunk sewers.All collecting and outfall, sub-trunk and trunk line sewers in low areas in closeproximity to natural watercourses shall be constructed at depths low enoughto permit all proposed and existing connecting lateral sewers to make thechannel crossing so as to have at least two (2) feet of cover over the top ofthe lateral pipe. All sanitary sewers shall have a minimum of two (2) feet ofcover where crossing streams or channels. All sanitary sewers shall be laiddeep enough to allow for future extension of the system to fully serve thewatershed. The foregoing depth requirements may be modified in specialcases when such modifications are feasible and will not jeopardize the designwhen considered as an integral part of an overall system. The City shallapprove all modifications.All sewer mains twenty (20) feet in depth or greater shall be C900 PVC, saidpipe extending from Manhole to Manhole. All sewer laterals connected toC900 PVC main, shall also be of C900 PVC, extending up to the building lineof the property being servedAll lateral and sub-main or collecting sewers shall be designed on the basis ofan average per capita use In the case of industrial flow, when the rate and3 -8 Approved 6/10/09

City of WentzvilleEngineering Design Criteriavolume can be predetermined with a reasonable degree of accuracy, nodilutions or diminishing factor shall be applied against this flow in the outfall,sub-trunk or trunk sewers.3.04.05 Sanitary Flow TableFlow (Gallons perday per capita)Persons perunitSingle Family Homes 100 3.7Apartment or Condominium(1 Bedroom) 100 2.0(2 Bedroom) 100 3.0(3 Bedroom) 100 3.7The Population Equivalent (PE) shall be equal to 100 gallons per day perperson. When flows are calculated from commercial or non-residentialdevelopments, the PE shall be calculated, along with the basis for same.Calculations shall be provided to the Engineering Department for review forall other types of establishments and uses. Flow quantities must bereferenced from an acceptable industry standard or studies. All sewers, liftstations and force mains shall be designed for the peak flow rate using thepeaking factor as calculated from the following formula:Peaking Factor = 18 +PEPE4 +1000 10003.04.06 InfiltrationAn additional amount of flow due to infiltration shall be evaluated. All sanitarysewers shall be limited to a maximum of 200 gallons per inch of diameter perday per mile of line, as required by MDNR Specifications, when tested byactual infiltration conditions. In addition, there shall be no visible leaks. Allmanholes on sanitary sewers that are built within the 100 year flood limits orin other areas determined to be subject to flooding shall be provided with locktype watertight manhole covers.3.04.07 Special Situations and Design Requirements1. Connections to Manholesa. Foulwater drops are required only for sewers containingsanitary flow and then only when it is necessary that sanitaryflow enter a manhole at a height of two (2) feet or greaterabove its flowline. If an inside drop is to be used, a forty-eight(48) inch diameter manhole is required. If two inside drops areto be used, a sixty (60) inch diameter manhole is required.Sewer-lines shall not enter the manhole in the transitionconical section, or through a joint. The slope on incomingpipes should be limited to a maximum one percent for inside3 -9 Approved 6/10/09

City of WentzvilleEngineering Design Criteriadrops. Manhole inverts should be shaped to assure properflow through drop structures. All drop sewer lines are to beductile iron for the first 20 feet upstream from the manhole.b. If it is necessary to enter a manhole with a force main thisshould be done within twelve (12) inches of the flowline of themanhole, and the manhole invert should be shaped to insureproper flow through the structure.Consideration shall be given to the detention time of thesewage in the force main, and the potential detrimental effectsof the release of hydrogen sulfide from the force main on theconcrete structure of the manhole. The receiving manhole andthe next 2 downstream concrete manholes, at a minimummust be protected by the application of an epoxy coating.c. The number of lines coming into one manhole should be keptto a minimum. A special detail may be required to assure theproper constructability and maintenance of the structure.d. Pipes entering and exiting manholes at the flowline shouldproject through the center of the structure and the manholeinvert should be shaped to assure proper flow through thestructure.e. Private house lateral connections should be made to the mainsewer, not to the manholes.f. All connections to sanitary manholes are subject to City ofWentzville review and approval and will be made at the City’sdiscretion.g. Connections to existing structures may require rehabilitation orreconstruction of the structure being utilized. This work will beconsidered part of the project being proposed.2. Adjusting Manholes to GradeWhen a project requires a manhole to be adjusted to grade, amaximum of twelve (12) inches of rise is allowed if not previouslyadjusted. Adjustments greater than twelve (12) inches, or lowering,will generally require rebuilding all or part of the manhole.3. Swimming PoolsSwimming pool backwash connections to the sanitary sewer must notexceed fifty gallons per minute (50 gpm).3 -10 Approved 6/10/09

City of WentzvilleEngineering Design Criteria4. Channel CrossingsIf a sanitary sewer must cross under a channel with less than two (2)feet of cover, ductile iron pipe with restrained joints must be usedunless otherwise directed by the City. A manhole shall be providedon both sides of the crossing at changes of pipe materials at aminimum distance of twenty (20) feet from top of bank of both sides ofthe crossing.5. Private Force Main ConnectionsWhen site topography does not allow for gravity lateral service to thesanitary sewer, a private force main connection may be made. TheCity requires that a grinder pump or sewage ejector be used in thesecases. As indicated above, in Section 3.04.07 1.b., the connectionshall be made into a manhole, in accordance with the requirementsset forth therein.6. Storm Sewers Crossing Over Sanitary SewersWhen a storm pipe crosses over a sanitary sewer and the verticalclearance is less than two (2) feet, the sanitary sewer must beencased in concrete through the crossing and for ten (10) lineal feeteach side of the crossing.7. Location in Conjunction with Water ServiceSanitary sewers shall be at least ten (10) feet horizontally from anyexisting or proposed water main. On crossings, a minimum verticalclearance of eighteen (18) inches shall be provided between theoutside of the water main and outside of the sanitary sewer. If theseminimum clearances cannot be met, then the requirements set forth inthe Standard Specifications shall be followed.8. Abandonment of Sanitary Sewer ServicesSanitary sewer laterals, from buildings to be demolished, shall beplugged with concrete for their entire length, or removed. Portions of alateral may be left in service if those portions of the lateral are to becontinued in service to replacement buildings, and existing buildingswhich remain on the lateral. All laterals permanently abandoned shallbe cut off and capped at the sewer main as directed by the City.Sewer mains that are to be abandoned shall either be plugged withconcrete for their entire length or removed. Manholes that are to beabandoned shall be plugged with either concrete or the bottomsbroken out, with the inlet and outlet piping being plugged. The topsection of the manhole, consisting of the cone section lid and coverare to be removed, and the void filled with 1” clean stone.3 -11 Approved 6/10/09

City of WentzvilleEngineering Design Criteria9. Oil/Gas Separators and Sand Filters3.05 Sanitary Detention RequirementsIf required by the City, grease, oil and sand interceptors or traps shallbe provided when such devices are necessary for the proper handlingof liquid wastes containing grease or oil in excessive amounts or anyflammable wastes, sand, or other harmful materials which can betrapped. Such interceptors or traps shall not be required for privatedwelling units. Prior to the installation of any interceptor or trap,drawings and specifications shall be submitted to the City forapproval. All interceptors and traps shall be located so as to be readilyaccessible for cleaning and inspection.Grease and oil interceptors or traps shall be constructed of imperviousmaterials capable of withstanding sudden and extreme changes intemperature. All such devices shall be of substantial construction,water-tight, and equipped with easily removable covers which, whenbolted in place, shall be gas tight and watertight, unless otherwiseapproved by the City.All grease, oil and sand interceptors or traps shall be maintained ineffective operation at all times by and at the expense of the user.3.05.01 Surcharged Sanitary SewersWhen it has been determined that the outfall sewer or the downstreamsystem serving a proposed development is overcharged, the City may requirethe developer to provide special facilities that the City deems necessary. Asan example, the downstream system may need to be upgraded in order toprovide additional capacity, by the replacement of an inadequately sized pipe,by a pipe sized for the additional flows, or the construction of a pipe, locatedparallel to the pipe that is undersized.In the design of such facilities, consideration should be given for theprotection of structures and equipment against corrosive and/or explosivegasses that may result from the detention of sewage.3 -12 Approved 6/10/09