SECTION 5: SANITARY SEWER SYSTEM DESIGN

Transcription

1 SECTION 5: SANITARY SEWER SYSTEM DESIGN 5.01 GENERAL Sanitary sewer improvements shall be designed to serve the ultimate level of City development as defined in the General Plan and the Wastewater Facilities Master Plan. All improvements shall conform to the requirements of the Yolo County Health Department, the Uniform Plumbing Code and the Standard Specifications of the City PLAN REQUIREMENTS Street Plan and Profile sheets shall show sanitary sewer (geometric) design improvements. Required information shall include main and lateral sizes and slopes, SDR or wall thickness value, utility crossings, manholes, cleanouts, invert elevations and calculations used in design DESIGN FLOW The design sanitary sewer flow shall be computed using the following formula: Q d = Q p + I Q d is the design flow in gal. per day Q p is the peak flow in gal. per day I is the infiltration and inflow in gallons per day The peak flow (Qp) in all areas for 12 diameter pipes or greater is defined as 2.5 times the average flow, for 10 diameter pipes it is 3 times the average flow and for 8 diameter pipes or smaller it is 3.5 times the average flow, with the average flow being computed from the following basic assumptions: DESCRIPTION WASTEWATER FLOW RATE UNIT Residential 225 gpd/unit Central commercial 8,000 gpd/gross acre Other commercial 1,500 gpd/gross acre Industry 1,800 gpd/gross acre DESCRIPTION WASTEWATER FLOW RATE UNIT Business park 1,000 gpd/gross acre Public facilities 2,000 gpd/gross acre School 25 gpd/student City of Woodland 1-65 Standard Specifications and Details 2007

2 The infiltration and inflow (I+I) shall be 600 gallons per acre per day for all areas PIPE DESIGN CAPACITY Sewer mains shall be sized so that the design flow (Q d ) results in a depth of flow (d) that does not exceed the following ratios. Pipe Diameter (D) Maximum Allowed Less than or equal to 12 inch d/d 0.50 Larger than 12 inch d/d 0.70 Manning's Formula: Where: Q = 1.49 A R 2 / 3 S ½ n Q = flow rate, (ft 3 /sec); A = cross-sectional area of flow, (ft 2 ); n = R = S = coefficient of roughness; hydraulic radius, (ft); and slope of pipe, (ft/ft). Should be used to compute the required size of sewer pipe flowing full. The "n" value shall be for vitrified clay pipe (VCP) and ductile iron pipe (DIP) and shall be.011 for polyvinyl chloride pipe (PVC) pipe or the pipe manufacturer s recommendation; whichever is greater. Minimum sewer velocity, at maximum allowed depth, shall be two feet (2') per second PIPE SIZES AND SLOPES PIPE DIAMETER MINIMUM SLOPE LATERALS 4".020 6".010 MAINS 8" TYPE OF PIPE 10" ".0020 Pipe used for sanitary sewer improvements shall be extra strength VCP or PVC pipe. Special designs such as less than minimum cover requirements will require City of Woodland 1-66 Standard Specifications and Details 2007

3 the use of special pipe materials in accordance with the Materials and Construction Methods Section Improvements using PVC pipe shall require a statement by a Geotechnical Engineer stating that the native soil on the project site within the pipe zone area will have a minimum soils reaction modulus (E ) of 150 psi. A Geotechnical Investigation Report that includes test results, evaluation of conditions and recommendations for groundwater handling shall be required for all plans installing public sewer facilities or private sewer systems constructed in high groundwater areas. Recommendations acceptable to the City will be incorporated into the proposed work. PVC pipe shall conform to the provisions of the Materials and Construction Methods Section 9, Sanitary Sewers. Ductile iron or other high-strength pipe, approved by the City Engineer, shall be used whenever cover is greater than 20 feet (20 ) or extra support strength is required (such as to resist traffic loading). Ductile iron pipe or other high strength pipe shall be used whenever cover is less than four feet (4 ). If insufficient clearance exists between the sewer pipe and rigid or load transmitting structures, DI or Class 200 (SDR-14) PVC pipe conforming to the requirements of AWWA C900 may be used PIPE COVER AND CLEARANCE Sewer mains shall have six feet (6') minimum cover from top of pipe to finished grade. Whenever deviation from this requirement is proposed, the pipes designed with less than minimum cover shall be clearly identified as such on the plans. All sanitary sewers shall be placed within City right of way dedicated for public streets unless the use of easements is specifically approved by the City Engineer. In some streets, dual collectors may be required. Permanent sewer easements shall be a minimum of 15 feet (15 ) wide for sewers pipe up to 18 inches (18 ) and 20 feet (20 ) minimum for pipe larger than 18 inches (18 ). A trench wall slope of 1.5:1 (Hor:Vert) shall be the basis on which the easement width is determined. The slope may be adjusted as required be existing soil conditions for adequate stability. Temporary construction easements of adequate dimensions shall be provided to allow the construction within the permanent easement to be completed in a safe and reasonable manner. If it is necessary to install a sewer main within a private road, the sewer easement shall be the width of the paving plus one foot each side. In areas of high groundwater, design techniques shall be incorporated to minimize the depth of laterals and mains. Sewer laterals shall have five feet (5') minimum and six feet (6') maximum cover from top of pipe to finished grade at the property line. City of Woodland 1-67 Standard Specifications and Details 2007

4 Sewer mains shall be located at least ten feet (10') horizontally (outside clearance) from and one foot (1') minimum (outside clearance) below water mains. Vertical clearance between sewer mains and all other (non-potable water) utilities shall be a minimum of one foot (1 ) HORIZONTAL AND VERTICAL ALIGNMENT Mains and trunk sewers shall be located parallel with and six feet (6') off the centerline of streets, and on the opposite side of the centerline as the water main. Curved alignments shall be designed based upon ASTM C425 maximum allowable deflection and NCPI Engineering Manual radius of curvature for VCP and the latest version of the Handbook of PVC Pipe Design and Construction for PVC pipe. Vertical alignment shall provide a constant slope between manholes. If a change in grade is necessary, construction of a manhole shall be required unless if otherwise approved by the City Engineer. In such case, elevations shall be shown at ten-foot intervals throughout the length of the vertical curve. Joint deflections in excess of 66% of the pipe manufacture s recommended maximum will not be allowed. Only factory joints will be allowed SERVICE LATERAL A separate service lateral shall be provided for each individual lot or parcel. Service laterals shall be located six (6 ) inches below and eighteen (18 ) inches away from the water lateral for residential lots. A cleanout shall be provided for all service laterals. Residential service laterals shall be four (4 ) inches minimum. Commercial and industrial service laterals shall be six (6 ) inches minimum. Six-inch (6") laterals may be required to tie into a manhole, depending on the proposed commercial or industrial land use. See Standard Detail 0505 for additional requirements. In areas where the service lateral conflicts with the storm drain, double long sweep 45 degree bends are allowed for use as approved by the City Engineer. See Standard Detail 0501 for additional requirements MANHOLES The spacing of manholes shall be: PIPE SIZE, INCHES MAXIMUM MANHOLE SPACING, FEET & Larger Anything greater than 500 must be approved by the City Engineer. City of Woodland 1-68 Standard Specifications and Details 2007

5 Manholes shall be located at the beginning and end of curved sewers, the end of cul-de-sacs, at any change in pipe size or slope, at the connection point of any service lateral where adjacent manhole spacing is greater than 500 feet, and at points of angular change of more than 20 degrees. Manholes with different pipe sizes shall be connected so that the crown of the smaller main is no lower than the crown of the larger main and channeled to allow for a smooth transition of flow. Provide 0.1 foot drop through the manhole if main flow changes directions by more than 30 degrees or side sewer mains are connected to the manhole. Maximum drop shall be 2 feet (2 ) without drop structure construction. Drop manholes shall not be used unless approved by City Engineer ONSITE CONNECTIONS There shall be no design connections that introduce storm runoff into the sanitary sewer SPECIAL DESIGN Special designs of sanitary sewer facilities or other unusual structures (clean-outs or manholes upstream of sewer lines) shall require a study and approval of City Engineer PUMP STATIONS To the extent practicable, regional wastewater pump stations shall be avoided. In unavoidable cases, a regional pump station may be considered for ownership by the City after submission of a detailed engineering report documenting reasons for proposing a pump station. This report shall establish that there is no reasonable alternative other than a pump station to serve the proposed tributary area. This report will be reviewed by the Public Works Department and a decision will be made by the Public Works Director or authorized representative to accept City ownership of such a pump station. All publicly owned wastewater pump stations shall comply with the City of Woodland Wastewater Pump Station Design Standards. Single parcel pump stations for required detention facilities shall be owned and maintained privately by the parcel owner TRENCH LOADING CONDITIONS Flexible Conduit Loading: On flexible conduits, Marston's formula for flexible conduits as shown in the ASCE Manual and Report of Engineering Practice No. 60 and in other similar handbooks shall be used to determine the load placed on the pipe by the backfill. The maximum load allowable shall be determined by pipe deflections computed by the Iowa Deflection Formula (or Spangler's Formula). The soils reaction modulus (E') shall be estimated using a method acceptable to the City Engineer, and shall consider the modulus values of both the native and the bedding materials (ATV method). The bedding soils reaction modulus (E') used in the deflection calculation shall be 1,000 psi for bedding utilizing imported material to twelve inches above the top of the pipe. Deflection lag factor shall be 1.5. If specific soil data are unavailable as determined by a Soil Engineer, City of Woodland 1-69 Standard Specifications and Details 2007

6 a soil weight of 120 p.c.f., a bedding constant (k) of shall be used. Placement of flexible conduit within soils equivalent to Class IV and Class V of table 1, ASTM D2321 material will not be permitted unless approved by the City Engineer. Allowable Deflection: On flexible conduits, the maximum allowable deflection shall be 5% of the nominal inside diameter. City of Woodland 1-70 Standard Specifications and Details 2007