402 - Storm Sewers
GENERAL
The inspector’s preliminary investigation should determine if the storm sewer is correctly staked and the storm sewer is the type, size and class called for in the plans, or as allowed by specification. The storm sewer must be accompanied by the manufacturer’s certification, tested stock reports, or other requirements as specified in the Materials Quality Assurance Manual.
All sewer pipe must be clearly marked with the following information.
- Manufacturer’s name or trademark.
- Plant location.
- Date of manufacture.
- Class of pipe.
Other sewer pipe requirements that are type-specific follow.
Circular concrete pipe with elliptical reinforcement shall have lift hole cast in the pipe which will indicate the top of the pipe as installed. These lift holes are to be sealed with concrete plugs and waterproofed after installation.
Joints for smooth lined corrugated plastic pipe (CPE) shall be provided with homing marks to indicate correct joining of the pipe and the joint material during field installation.
MATERIALS
Some concrete storm sewer pipe does not contain reinforcement and therefore requires careful handling. Any pipe damaged in transit or by handling should be rejected by the Engineer or inspector. Storm sewer pipe will be subjected to rejection if any defects, noted in the previous section for culvert pipe, are found. In addition, non-reinforced concrete sewer pipe should be inspected for the following.
Non-reinforced Concrete Sewer
- Fractures and cracks passing through the shell or socket, except that a single crack not exceeding 2 inches (50 mm) in length at either end of a pipe, or a single fracture in the socket not exceeding 3 inches (75 mm) in width around the circumference of the pipe nor 2 inches (50 mm) in length into the joint, shall not be considered cause for rejection, unless these defects exist in more than 5 percent of the entire shipment or delivery.
- Blisters where the surface is broken or which project more than 1/8 inch (3 mm) above the surface.
- Defects which indicate imperfect mixing or molding.
- Crack sufficient to impair the strength, durability or serviceability of the pipe.
- Variations of more than 1/8 inch (3 mm) per lineal foot (300 mm) in alignment of pipe intended to be straight.
- Failure to give a clear ringing sound when placed on end and dry-tapped with a light hammer.
- Insecure attachment of branches and spurs.
- The complete absence of distinct web-like markings on the external surface of the pipe made by any process in which the forms are removed immediately after the concrete has been placed. This is indicative of a deficiency of water in the concrete mix.
Storm Sewer Pipe Alternates
When only the size and class of the storm sewer pipe is specified, the Contractor may furnish any of the alternates permitted in Table 402-1 of the Standard Specifications for Construction. Additional materials are now allowed for storm sewer pipe for use on state trunkline projects based on design life and proposed pipe use. Design life is 25 years for driveway culverts, 50 years for culverts and 70 years for sewers.
When a particular type of sewer pipe material is required in lieu of the classes designated in Table 402-1 of the Standard Specifications for Construction, the type and size of the sewer will be specified in the proposal or plans. A higher strength or greater thickness of sewer pipe may be substituted for the minimum strength or minimum thickness of sewer specified.
Flow Velocities for Sewer
The velocity of flow should be sufficient to prevent sedimentation at approximately 2 feet (0.61 m) per second. The minimum grades that will provide the self-cleaning velocities are as follows.
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CONSTRUCTION
Installation
Construction methods shall be according to the health and safety requirements in subsection 104.10 of the Standard Specifications for Construction.
The trench excavation should begin at the downstream end of the sewer and proceed toward the upstream end. The trench should be excavated to the lines and grades detailed on the plans, or as established by the Engineer. The trench width shall be at least the minimum width indicated in the Standard Plan R-83 Series, Standard Plan for Utility Trenches, or a width sufficient to provide free working space and to provide for the proper compaction of the backfill around the sewer pipe. The bottom of the trench is to be shaped to uniformly support the pipe.
Where rock or hardpan is encountered, the trench should be excavated a minimum of 6 inches (150 mm) below the proposed bottom of the pipe and backfilled and compacted with Granular Material Class II. If unstable soil conditions or obstructions other than rock are encountered, and the excavation of the sewer trench below the elevation shown on the plans is necessary, the excavation, backfill and compaction of the trench shall be directed by the Engineer. Backfill material for undercutting due to unstable soil conditions shall be Coarse Aggregate 6A.
House connections, water mains, gas mains, conduits and drains, when encountered in the sewer trench, must be supported and protected across the excavation.
Watertight Joints
All storm sewers are to be laid true to the lines and grades given, bells or grooves upgrade, ends fully and closely jointed. Each section shall have full, firm bearing throughout its length. All pipe sections/joint assemblies for use in sewers shall be selected from the Qualified Products List for Watertight Sewer and Culvert Joint Systems. A field assembly diagram shall be provided to the Engineer. All sewer pipe with diameters greater than 24 inches (600 mm) shall have the pipe joints wrapped with a non-woven geotextile fabric. The fabric shall have a minimum width of 36 inches (900 mm) and shall be centered on the joint.
If circular concrete pipe with elliptical reinforcement is installed, it should be installed with lift holes on top of the pipe. The manufacturer’s marks designating the top and bottom of the pipe shall not be more than 5 degrees from the vertical plane through the longitudinal axis of the pipe. After installation, the lift holes shall be sealed with suitable concrete plugs and waterproofed.
Reinforced concrete elliptical pipe shall be installed with the longer axis placed horizontally, unless otherwise specified.
Utility Service Interruptions
Existing live sewers and service leads shall be maintained and carefully protected during the construction of the sewers. Damaged sewer or service leads shall be repaired immediately or replaced, as directed by the Engineer. Service interruptions shall be kept to a minimum and may require coordination with the local municipality or utility company. Water, gas, telephone and electrical wire conduits can be moved above or below the sewer being constructed. Close liaison with the utility companies that might be in the area is particularly desirable. It is best to have the sewer trench excavated well ahead of the laying operation to allow lead time in making any necessary modifications. Abrupt changes in grade require manholes, as do changes in alignment, change in size of sewer and junction of sewer lines.
If it is found necessary to flatten the sewer grade to miss unforeseen obstructions, the capacity may be affected to the extent that a larger size pipe is required. Such matters should be discussed with the Engineer, who may also confer with the designer prior to making the change, especially as it may result in extensive redesign.
On occasion, a utility company prefers to encase its line in a conduit and run it through a manhole rather than adjust its grade. Be sure the manhole internal area is large enough so as not to constrict the flow. This is generally not the most desirable solution to this type of problem, as it may introduce contamination into water lines or a gas leak into the sewer system. In these situations, field decisions must be made promptly, yet with every consideration for the public’s well-being. If the facility being constructed is to eventually become the property of another agency, their concurrence should be obtained before making the changes.
Backfilling Sewer Pipe
Backfilling sewer pipe will be as specified in subsection 401.03 of the Standard Specifications for Construction and as described in this manual under culverts.
Sewer Pipe Jacked-in-Place
Follow the requirements of subsection 401.03 of the Standard Specifications for Construction and this manual under Section 401, Pipe Culverts Jacked-In-Place, except that joints for pipe 36 inches (900 mm) in diameter and smaller shall be sealed with flexible watertight rubber gaskets.
Sewer Taps
Connections to storm sewers owned by counties, municipalities or drain commissions shall be according to the regulations of the owner and according to the plans and specifications. If there is a conflict between the owner’s regulations and the Standard Specifications for Construction, the owner’s requirements will take precedence. Connections to existing storm sewers having a plug or bulkhead shall be made with a watertight joint. When tapping an existing pipe or structure, a minimum opening of the entering pipe’s outside diameter plus 6 inches (150 mm) shall be cut into the receiving structure. A minimum 3 inch (75 mm) mortar layer shall be packed completely around the entering pipe and struck smooth with the inner wall of the pipe or structure. Pipe passing through the pipe or structure wall is to be cut flush to conform to the inner wall. The entering pipe on the outside portion of the connection shall be encased to provide sufficient bearing under the pipe. Direct tapping sewers is restricted to entering pipes with outside diameters less than half the inside diameter of the trunk sewer. All other taps will require construction of a manhole structure according to Section 403 of the Standard Specifications for Construction and Section 403, Drainage Structures, of this manual.
Illicit Connections
If unusual or suspicious connections to MDOT’s storm sewer system are found during a construction project (e.g. the presence of a sudsy discharge or oily discharge is seen in the system), the Engineer should contact Region utilities/permits personnel to determine if the connection is permissible by permit. If no permit exists, guidance from the Region should be sought prior to reconnecting.
MEASUREMENT AND PAYMENT
Inspection After Installation
All of the general inspection points in the previous section on culverts apply to sewer pipes.
Mandrel Testing
If smooth lined corrugated plastic pipe (CPE) is installed on a project, at least 50 percent of the installed length of each size pipe shall be tested for deformation by the Contractor, using a nine point mandrel. The arms of the mandrel extend to a diameter equal to 95 percent of the nominal pipe diameter. The Contractor is to supply both the mandrel and a proving ring. The proving ring should be used by the inspector to verify that the diameter of the mandrel is correct before it is used.
The Contractor is to perform the mandrel testing after the required backfilling of the trench is achieved, but between 5 and 10 days prior to pavement surfacing or completion of final grade, except as otherwise approved by the Engineer. For enclosed pipe, the Contractor accesses the pipe from one drainage structure and pulls the mandrel through to the next drainage structure. If the mandrel pulls through the pipe without snagging, the pipe has retained its roundness and has an acceptable diameter (the diameter has been reduced less than 5 percent).
If the mandrel gets hung up on any section of the pipe as the Contractor pulls it through, it indicates that the pipe has not retained its roundness after installation and does not have an acceptable diameter (the diameter has been reduced by 5 percent or more). CPE pipe with a diameter reduced by 5 percent or more must be removed and either reinstalled, if the pipe is not damaged, or replaced at no cost to MDOT. The Contractor is responsible for all expenses and delays due to the replacement of deformed or damaged pipe. No re-rounding of CPE pipe with a diameter reduced by 5 percent or more is allowed.
Video Inspection of Installed Storm Sewers
As with culverts installed on the project, inspection by videotaping of all types of installed storm sewer is required for storm sewers with diameters from 12 to 36 inches (300 to 900 mm) with the following exceptions.
- Driveway culverts.
- Culvert extensions of less than 50 feet (15 m).
- Extension of existing catch basin leads of less than 20 feet (6 m).
- New culverts of less than 50 feet (15 m).
The purpose of the video inspection is to determine any damage or installation defects to the sewer, including pipe deformation, cracking, joint separation, corrosion, perforation, excessive siltation, excessive ponding of water, or any discernable feature observed in the video. For corrugated plastic pipe, videotaping of all pipe 12 to 36 inches (300 to 900 mm) in diameter, unless an exception as listed above, is required after installation, regardless of whether the same pipe is mandrel tested by specification. The sole purpose of the mandrel testing is to determine deformation.
The videotaping is to be performed by the Contractor. For sewers installed under the pavement, the inspection shall be conducted after the required backfill compaction of the trench has been achieved, and between 5 and 10 working days prior to pavement surfacing or completion of final grade, except as otherwise approved by the Engineer. For sewers not under pavement, the video inspection shall be conducted as close to project completion as possible, while still allowing sufficient time for corrective action determined to be necessary from the video inspection and as directed by the Engineer. All video inspection shall be conducted as described in subsection 402.03 of the Standard Specifications for Construction.