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(1) Sewer Mains and Laterals. See also Sewage Lift Stations, Article IV of this chapter, for additional requirements on lift station force mains.

Sewer mains to be installed shall be of material noted below. Pipe material shall be consistent and unchanged between manholes, with the material type determined by the deepest cover requirement.




Max. Slope

Gravity Sewer and Laterals

PVC – ASTM D3034 SDR 35

5' – 18'

18% for all unless otherwise approved.

Pipe anchors may be required if over 18%.

Ductile Iron, Class 52

≥ 18' and/or 3' – 5'

C900 – PVC AWWA DR 18

≥ 18' and/or 3' – 5'*

Force Mains

Ductile Iron or HDPE*

4' – 10'

*Locate tape required for shallow C900 or HDPE.

PVC pipe shall be a minimum Class S.D.R. 35 and be manufactured in accordance with ASTM D3034. The pipe and fittings shall be furnished with bells and spigots, which are integral with the pipe wall. Pipe and fittings shall be of the same material. Pipe joints shall use flexible elastomeric gaskets conforming to ASTM D3212. Nominal laying lengths shall be 20 feet and 13 feet. PVC C900 pipe shall conform to AWWA C900 DR 18, and shall be installed with locate tape in shallow applications.

Each length shall be plainly marked with the manufacturer’s identification and class of pipe. The pipe shall be furnished with mechanical joint or push-on type joint, except where plans call for flanged ends. Joints shall conform to AWWA C111.

All pipe shall be jointed by the manufacturer’s standard coupling, be all of one manufacturer, be carefully installed in complete compliance with the manufacturer’s recommendations. Welded HDPE pipe connections and joints shall only be made by certified personnel. The interior of the welded joints shall be made flush with the surrounding pipe surface prior to final acceptance.

Ductile iron pipe shall conform to AWWA C151 and shall be Class 52. Pipe and fittings shall be of the same material. The pipe shall be epoxy lined to a nominal thickness of 40 mils with a minimum lining thickness of 30 mils. Products meeting this standard are “Protecto 401” or district approved equal. The exterior shall be coated with an asphaltic coating. If needed, all ductile iron fittings shall be short-bodied complying with AWWA C110 or C153 for 350 psi pressure rated mechanical joint fittings and 250 psi pressure rated flanged fittings. All fittings shall be epoxy lined.

Restrained joint ductile iron or C900 pipe is required for installation in areas of fill or previously disturbed soil for both main line and district owned lateral pipe. Fittings in areas requiring restrained joints shall be mechanical joint fittings with a mechanical joint restraint device. The mechanical joint restraint device shall have a working pressure of at least 250 psi with a minimum safety factor of 2:1 and shall be EBAA Iron, Inc., MEGALUG, or approved equal for ductile iron pipe. All ductile iron couplings shall be mechanical joint sleeves.

Restrained joint ductile iron pipe shall be push-on joint pipe with Field Lok, TR Flex or district approved equal gaskets. Restrained joint C900 pipe connections shall be approved by the district.

The sewer pipe shall be laid upgrade from point of connection on the existing sewer or from a designated starting point. The sewer pipe shall be installed with the bell end forward or upgrade. When pipe laying is not in progress, the forward end of the pipe shall be kept tightly closed with an approved temporary plug.

Wherever movable shoring is used, pipe shall be bedded, backfilled and anchored to the satisfaction of the district before each move. Any indication that joints are not being held shall be sufficient reason for the district to require restraints, whether or not movable shoring is being used.

All gravity pipe shall be laid in straight lines and at uniform rate of grade between manholes. Variance from established line and grade shall not be greater than one-half inch unless the variation results in a level or reverse sloping invert. Any corrections required in line and grade shall be reviewed with the district and shall be made at the expense of the developer.

All extensions, additions and revisions to the sewer system, unless otherwise indicated, shall be made with sewer pipe jointed by means of a flexible gasket, which shall be fabricated and installed in accordance with the manufacturer’s specifications.

All joints shall be made up in strict compliance with the manufacturer’s recommendations and all sewer pipe manufacture and handling shall meet or exceed the ASTM recommended specifications.

Pipe handling after the gasket has been affixed shall be carefully controlled to avoid disturbing the gasket and knocking it out of position, or loading it with dirt or other foreign material. Any gaskets so disturbed shall be removed, cleaned, relubricated if required, and replaced before the rejoining is attempted.

Care shall be taken to properly align the pipe before joints are entirely forced home. During insertion of the tongue or spigot, the pipe shall be partially supported by hand, sling or crane to minimize unequal lateral pressure on the gasket and to maintain concentricity until the gasket is properly positioned. Since most flexible gasketed joints tend to creep apart when the end pipe is deflected and straightened, such movement shall be held to a minimum once the joint is home.

Sufficient pressure shall be applied in making the joint to assure that it is home, as described in the installation instructions provided by the pipe manufacturer. Sufficient restraint shall be applied to the line to assure that joints once home are held so, until fill material under and alongside the pipe has been sufficiently compacted. At the end of the workday, the last pipe laid shall be blocked in an effective way to prevent creep.

For the joining of dissimilar pipes, suitable adapter couplings shall be used which have been approved by the district.

(2) Bedding and Backfill. All gravity sewer pipe shall be bedded with pea gravel. The pipe shall be bedded from a depth of four inches below the pipe to 12 inches above the pipe. The bedding material shall extend across the full width of the trench and shall be compacted under the haunches of the pipe.

Clay or bentonite dams shall be installed across the trench and to the full depth of the granular material in all areas of steep slopes, high groundwater, stream crossings and wetlands to prevent migration of water along the pipeline. Drainage facilities shall be installed to accommodate water buildup behind the dams in accordance with district standard details.

Backfill in roadway sections or structural areas shall be placed and compacted to a minimum of 95 percent of the modified Proctor dry maximum density per ASTM D1557 in accordance with district standard details, as well as Snohomish County, city and/or state requirements.

Backfill in unpaved easements or nonstructural areas shall be placed and compacted to a minimum of 90 percent of the modified Proctor dry maximum density per ASTM D1557. Recycled concrete is not allowed for use in district trench sections, no exceptions.

Backfill compaction shall occur in sufficiently thin lifts to achieve the density requirements specified. Copies of the compaction results shall be provided to the district prior to final acceptance.

When metallic pipes are backfilled or in contact with corrosive or cementitious material (i.e., CDF), the metallic pipe shall be wrapped using American V-Bio polyethylene encasement or equal and the pipes bedded and covered with 12 inches of clean, imported material.

(3) Manholes. Manholes shall be of the offset type and shall be precast concrete sections with either a cast in place base, or a precast base made from a 3,000 psi structural concrete. Joints between precast wall sections shall be grouted on the exterior and confined O-ring or as otherwise specified.

All sewer mains shall end with a manhole unless otherwise approved by the district. Lamp holes or cleanouts are not allowed on the end of sewer main lines.

The minimum diameter for manholes shall be 48 inches to a depth of 18 feet, and 54 inches for depths of 18 feet and greater. The district may require the diameter to be increased beyond the minimum based on future needs or size of pipe connections in accordance with the district standard details. Shallow manholes shall be 54 inches in diameter minimum.

For connections to existing systems, a concrete coring machine suitable for this type of work shall be utilized in making the connection. The connection shall be made with a watertight flexible rubber boot, Kor-N-Seal or district approved equal. Grouted sand collars shall only be used with district approval. The new pipe connection shall be sealed with a watertight plug until the new pipe system has been installed and approved.

Smaller diameter extension pipes shall match the crown elevation of larger existing main line sewer pipes in the manhole. The existing manhole shall be rechanneled as required, and coring through the existing bench may be considered and authorized by the district as appropriate for the conditions.

The developer shall be responsible for any existing defects in the existing manhole unless these defects are witnessed by a representative of the district prior to any work being performed to make the connection. The contractor shall be required to remove any and all deleterious material in the existing manhole and downstream reaches as a result of this work.

Manholes located in easements or outside of paved areas shall have reinforced concrete collars with a minimum size of 48 inches diameter by 12 inches thick.

(a) Manhole Sections. Manhole sections shall be placed and aligned so as to provide vertical sides and vertical alignment of the ladder steps. The completed manhole shall be rigid, true to dimension, and be watertight. Rough, uneven surfaces will not be permitted.

(b) Manhole Steps and Ladders. Manhole steps shall be polypropylene, Lane International Corp. No. P13938 or equal and installed in accordance with district standard details. Ladders shall be polypropylene, Lane International Corp. or equal, and shall be compatible with steps.

(c) Grade Adjustment. The manhole shall be set to provide not less than eight inches or more than 18 inches of adjustment between the top of the cone or slab and the top of the manhole frame.

Manhole adjusting rings shall conform to the ASTM C2, Grade MA. The outside and inside of manhole adjusting rings and the joints of precast concrete sections shall be plastered and troweled smooth with one-half inch (minimum) of mortar to attain a watertight surface. No wood shall be used for adjustment.

(d) Channels. Channels shall be made to conform accurately to the sewer grade and shall be brought together smoothly with well-rounded junctions, satisfactory to the district. The channels shall be field poured with concrete, no other fillers are permitted, after the inlet and outlet pipes have been laid and firmly grouted into place at the proper elevation. Allowances shall be made for a one-tenth-foot drop in elevation across the manhole in the direction of flow. Channel sides shall be carried up vertically from the invert to three-quarters of the diameter of the various pipes. The concrete shelf shall be warped evenly and sloped three-eighths inch per foot to drain. Rough, uneven surfaces will not be permitted. Channels shall be constructed to allow the installation and use of a mechanical plug or flow meter of the appropriate size. Terminating manholes, where sewer extension may occur, shall be channeled accordingly.

(e) Drop Manholes. Drop manholes on new construction require district approval and shall be constructed with an inside drop structure in a 54-inch manhole. Outside drop connections are not allowed. Approval will be limited to future extensions from deep sewers. Where extension from deep sewers is concurrent with deep sewer construction, drop manholes will not be allowed. Drop manholes shall, in all respects, be constructed as a standard manhole with the exception of the drop connection.

(f) Lift Holes and Steel Loops. All lift holes shall be completely filled with expanding mortar, smoothed both inside and outside, to ensure watertightness. All steel loops shall be removed, flush with the manhole wall. The stubs shall be covered with mortar and smoothed. Rough, uneven surfaces will not be permitted.

(g) Frames and Covers. Frames shall be cast iron and covers shall be ductile iron. Castings shall be free of porosity, shrink cavities, cold shuts or cracks, or any surface defects, which would impair serviceability. Repair of defects by welding, or by the use of “smooth-on” or similar material, will not be permitted. Frames and covers shall be machine finished or ground on seating surfaces so as to assure non-rocking fit in any position and interchangeability of covers. Frames and covers shall be provided with three-bolt locking lids. Rings and covers shall be positioned so one of the three locking bolts is located over the manhole steps and shall be adjusted to conform to the final finished surface grade of the street or easement to the satisfaction of the district. Manhole frames and covers shall be in accordance with district standard details. Must meet the (AASHTO) M306 specification and have replaceable lock down nuts.

(h) Manhole Marker Posts. A fiberglass manhole marker post shall be located adjacent to all manholes located in easement areas. The marker post shall be green in color, three and three-quarters inches wide (flat), 60 inches long and furnished with a three-inch-by-three-inch high intensity white reflector (250 candle power) and a flexible anchor barb. Each post shall include the following decal: “Caution Sewer Manhole. Before digging, Call 811, Utility Underground Location Center.” Manhole markers shall be Carsonite Utility Marker CUM 375.

The marker posts shall be set to leave 36 inches of the post exposed above grade.

Distance from the marker to the manhole shall be stenciled on the marker with two-inch letters.

(i) Manhole Casting Adjustment. Manhole castings shall be adjusted to final grade after asphalt paving is completed on new development construction. Existing manhole castings within the impacted development work area shall be replaced and upgraded to current standards in accordance with the district standard details. [Res. 835 § 2 (Exh. 2), 2022; Res. 735 § 1, 2017.]