402 - Storm Sewers

402 Storm Sewers 2012 STANDARD SPECIFICATIONS FOR CONSTRUCTION - SECTION 402

GENERAL DESCRIPTION

Definitions of Terms

Catch Basin: A catch basin is a curbside opening that collects surface runoff.

Curb and Gutter: Curb and gutter is the edge of the paved roadway that collects water and is designed to drain to a catch basins, downspouts, and spillways.

Detention Pond: A detention pond is a temporary storage area that is engineered to control storm water. Excess storm water is collected within a detention pond during heavy rain events to help mitigate flooding and erosion downstream. A detention pond typically dries out between rain events.

Outfall: An outfall is the outlet where a storm sewer discharges water to a body of water or basin.

Retention Pond: A retention pond is a permanent storage area for excess storm water runoff. Excess storm water is collected in the retention pond aiding in the control of erosion. Often, there is a weir or another filtration system within the retention pond, which is used for filtering the storm water as it travels through the pond. Retention ponds can serve as artificial wetlands for the surrounding area increasing biodiversity.

Runoff: Runoff is drainage that leaves an area as surface flow and enters a storm sewer system.

Sedimentation: Sedimentation is the process of depositing soils that were eroded by the flow of water.

Storm Sewer System: A storm sewer system is a network of underground pipes designed to collect and discharge water.

Sump: A sump is the lower portion of a catch basin below the outlet pipe that is used to settle out debris and suspended material prior to storm water exiting the catch basin.

Watershed: A watershed is an area of land that drains to a specific point.

General Overview

Storm sewers are an underground infrastructure system designed to manage excess surface runoff and groundwater from roadways. These systems collect water from various sources, such as curb and gutter, drainage ditches, and underdrain pipes, by utilizing the natural slope of the land to facilitate drainage through gravity. Storm sewer systems discharge untreated water into ditches, basins, streams, or rivers.

The key components of storm sewer systems include:

1. Inlet: Inlets are typically catch basins with grates to permit water to enter into the system. The grates serve to prevent large objects from entering while facilitating the entry of water and sediment. Catch basins, which have a diameter measuring in inches, allow water to descend into the storm sewer for downstream conveyance. Some catch basins are designed with a sump below the pipe elevation, aiding in sediment settlement to prevent its entry into a body of water.
2. Pipe: Pipe is the conduit responsible for transporting water to the outlet. Project documents specify the size and class of the pipes.
3. Outlet: Outlets serve as the endpoint of the storm sewer system pipe. The outlet discharges water to a body of water or basin.

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MATERIALS

All materials must meet acceptance requirements in the and be listed on the Contractor-provided Material Source Lists in the project files.

Description of Materials

The pipe material and class are noted on the plans. MDOT defines the pipe by class, which is based on the amount of cover over the pipe. Refer to for acceptable pipe in each class.

Common storm sewer pipe materials include:

• Plastic:
  • Smooth-lined corrugated polypropylene pipe (PPP). Grey in color.
  • Smooth-lined corrugated polyethylene pipe (CPE) Type S. Black in color.
  • Corrugated polyethylene pipe (CPE) Type C. Black in color.
  • Corrugated polyvinylchloride pipe (CPV). White in color.
• Concrete:
  • Reinforced concrete pipe (RCP).
  • Non-reinforced concrete pipe.
• Metal:
  • Corrugated metal pipe (CMP).
  • Corrugated and spiral ribbed pipe (CSP).

Each pipe section, regardless of type, must be clearly marked with the following:

• Pipe class and specification designation (ASTM or AASHTO).
• Date of manufacture.
• Name or trademark of the manufacturer.
• Plant identification.

The marking is either painted on the inside or scribed on outside of concrete pipes, and is painted on the outside of metal and plastic pipe.

To aid in the joining of pipe segments and to seal the pipe so the joints do not leak, lubricant and gaskets are used.

Any of these materials found with defects (cracks, fractures, exposed rebar, deformities, etc.) prior to or during installation are to be rejected and removed from the project site. Any damage caused by the Contractor’s operation to existing storm sewers that are intended to be salvaged must be removed and replaced with similar material without damage or defects at no cost to the contract.

Plastic Pipe (PPP, CPE, and CPV)

Plastic pipe is more susceptible to shipping and storage deformation than concrete or metal pipe. Improper lifting and storage can lead to misshapen sections that are difficult to lay at a proper grade if the curve is placed vertically, and difficult to lay at a proper alignment if the curve is laid horizontally. Plastic pipe should not be deformed (out-of-round) or have blemishes in the extrusion. It should not have gouges or tears on the exterior, the interior, or the bell ends. Plastic pipes should have protective gaskets installed on the spigot ends. Gaskets should be clean and not dried out during installation.

Concrete Pipe

Concrete pipe is precast by a manufacturer and is made of cementitious materials, aggregates, and in most cases steel reinforcement.

Concrete pipe and end sections must not be cracked or chipped, especially on the bells and spigots that are to make durable watertight joints. The concrete used must be free of segregation and honeycombing. If RCP is used, the internal reinforcement steel must not be visible on the exterior.

Metal Pipe (CMP and CSP)

Corrugated metal pipe should not be out-of-round, exhibit deviation from a straight alignment, or have dents or bends in the metal that can create loose, poorly formed joints. Bruised or broken zinc, aluminum, or polymer coating must be repaired by the Contractor in accordance with the manufacturer’s specifications at no cost to the contract.

Bedding and Backfill

Bedding material is comprised of granular material Class IIA, or Michigan Series 34R if undercuts are required. The material used is determined by the existing subgrade material at the bottom of the trench.

Backfill material varies depending on what is above the storm sewer being installed and the type of pipe that is used. Typical backfill consists of granular Class II, IIAA, III, or IIIA material, suitable excavated material, or a concrete encasement. Refer to the for details.

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EQUIPMENT

The following equipment can be used for storm sewer work:

• Excavator
• Backhoe
• Front end loader
• Trench box
• Occasionally, a bulldozer and a vibratory roller for aggregate compaction
• Plate compactor
• Ho-Pac attachment
• Gas powered rammer tamping machine
• Air powered pogo thumper hammer
• Sand/stone box
• Portable concrete mixer
• Cement/mortar mixing tub
• Portable generator
• Electric submersible pump
• Trash pump
• Pipe laser with target
• Transit or laser level
• Leveling/grade pole
• Electric impact wrench
• Pipe/spud bar
• Small hand tools (shovel, level, pipe “cutoff” saw, etc.)
• Direct tap machine for sewer tap

Description of Equipment

Typically, an excavator is used to remove existing storm sewers and dig the trench for new storm sewer. A laser or other acceptable alignment device is used to install the storm sewer pipe to proper grade as it is laid in the trench by the excavator. A front-end loader or excavator is used to place backfill around the pipe to be compacted in layers by a Ho-Pac or other compaction equipment. In some cases, with a large trench, a truck will dump the backfill material directly into the trench to be graded by a bulldozer before compaction.

Pictures of Equipment

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PRECONSTRUCTION

Prior to the start of construction, the Inspector should perform the following:

1. Inspect the pipe upon arrival to the site.
   • Verify the pipe meets the requirements contained in the plans, specifications, and any addenda.
     • The pipe must be accompanied by the manufacturer’s certification, tested stock reports, or other requirements as specified in the Materials Quality Assurance Procedures Manual.
     • Review the cover over the top of the pipe as called for in the plans and verify against the pipe chart (Table 402-1 of the Standard Specifications for Construction) for acceptable pipe installation depth and class.
   • Visually inspect the pipe for defects originating from the manufacturing process or occurring during delivery and unloading at the site (such as cracks, fractures, exposed rebar, deformities etc.). Any damaged pipe should be promptly rejected by the Engineer.
     • Inspect the bell and spigot end of the pipe to ensure there are no chips or cracks that might impede a watertight joint connection.
     • Inspect plastic storm sewer pipe to ensure it does not exhibit deformities that would hinder water flow.
     • Using survey paint, mark any rejected pipe with an “R” to indicate rejection of the pipe. It is good practice to mark both the inside and outside of the pipe.
2. Verify the survey stakes are placed according to the plans and the appropriate invert elevations, rim elevations, and pipe and structure IDs are labeled on the stakes.
   • Ensure proper offset stakes are placed as stakes placed in the location of the proposed sewer will be removed when excavation begins.
3. Verify existing underground utility markings.
   • Take note of any potential utility crossings and reference the plans and profiles to ensure minimum clearances are met.
4. Review the plans and the Special Provision for Maintenance of Traffic. There may be restrictions or staging requirements for installation of the storm sewer. Installation restrictions are typically associated with staging requirements on a project and access to side streets and driveways.
5. Hold an onsite meeting with the Contractor to discuss:
   • The construction method that will be used to complete the work.
   • Required traffic control measures, proximity of traffic to the work area, and maintaining driveways during construction.
   • Communication required if impacting residents or businesses during installation.
     • Residents and businesses should be contacted if there will be disruptions to services. Coordination by the Contractor with impacted users may be required if disruption is necessary.
   • Review of local ordinances.
   • Disposal of excavated material and material stockpile locations. The Contractor should provide authorization letters to the Engineer noting material stockpile and disposal locations.
   • Soil erosion and sedimentation control measures that will be utilized to minimize soil erosion and subsequent sedimentation.
   • Methods for securing the site during work operations and at the end of each workday.
     • Including maintaining appropriate cover over the top of the pipe to minimize damage to the pipe from construction traffic.

A review of the proposed sewer depth on the profile sheets should be compared to all known existing utilities that will be crossed, and any other proposed utilities and structures to be built, with attention paid to elevation conflicts. The size of the storm sewer must be considered. The plan invert elevation might not be in conflict, but the height of the pipe above the invert might be. Likewise, an elevation shot on “top” of an existing utility needs to consider the size of the utility below the top.

Example: The proposed invert of a 12” concrete pipe is 100.00 at the outlet and 100.50 at the inlet. The storm sewer is 100’ long. A 6” gas main exists at the middle of the run (50’ out) with a top-of-pipe elevation of 101.50. 101.5 - 0.5’ = 101.00 at the bottom of the gas pipe. Calculate the pipe grade using rise/run. 0.5’ rise/100’ run = 0.005’ rise per foot. 0.005 x 50’ = 0.25 rise to the point where the gas main crosses. Invert 100.00 + 0.25 = 100.25 at 50’ out + 12” (pipe size), + 0.25 pipe thickness = 101.50 at the top of the sewer 50’ out. There is an elevation conflict between the proposed 12” storm sewer and the existing 6” gas main.

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CONSTRUCTION

Trench Excavation

Construction of a storm sewer begins with trench excavation. Excavation may include the removal of existing storm sewer and drainage structures. These removal quantities should be recorded as called for in the specifications.

There are many factors that determine the safe and proper width of a pipe trench. Refer to , for all applicable sloping, shoring, and soil classification safety standards for excavation. Existing soil material, groundwater elevation, utilities, trees (roots), and proximity of paved surfaces (roads, sidewalks, and curbs and gutters) and buildings may all be considered by the Contractor when deciding how the trench will be excavated.

In accordance with Subpart P, soil can be classified as 3 different types:

1. Type A: A cohesive soil with unconfined compressive strength of 1.5 ton per square foot (tsf) or greater. Examples include clay, silty clay, sandy clay, clay loam, and in some cases silty clay loan and sandy clay loam. Cemented soils like caliche and hardpan are also considered Type A soils. However, the soil cannot be classified as Type A if one or more of the following conditions are met:
   • The soil is fissured.
   • The soil is subject to vibration from heavy traffic, pile driving, or similar effects.
   • The soil has been previously disturbed.
   • The soil is part of a sloped, layered system where the layers dip into the excavation on a slope of 4H:1V or greater.
   • The material is subject to factors that would require it to be classified as a less stable material.
2. Type B: A cohesive soil with unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf. Granular cohesionless soils include angular gravel (crushed rock), silt, silt loam, sandy loam, and in some cases silty clay loam and sandy clay loam. Other characteristics include:
   • Previously disturbed soils except those which would otherwise be classified as Type C.
   • Soil that meets the compressive strength or cementation requirements for Type A but is fissured or subject to vibration.
   • Dry rock that is not stable.
   • Material that is part of a sloped, layered system where the layers dip into the excavation on a slope less steep than 4H:1V, but only if the material would be otherwise classified as Type B.
3. Type C: A cohesive soil with an unconfined compressive strength less than 0.5 tsf. Examples include gravel, sand, and loamy sand. Other characteristics include:
   • Submerged soil or soil from which water is freely seeping.
   • Submerged rock that is not stable.
   • Material in a sloped, layered system where the layers dip into the excavation on a slope of 4H:1V or steeper.

Following are various sloping requirements in accordance with OSHA regulations.

The width of the trench should be sufficient to enable a worker to place and compact material under the pipe haunches. The provides direction for minimum trench width as a function of the pipe size.

Trench boxes and/or sheeting can be used to “steepen” the banks of a trench. By limiting the width of the trench, the Contractor can minimize excavation and backfill quantities and avoid conflicts with existing utilities, tree roots, and removal of more pavements than otherwise necessary. Some sheeting systems or cofferdams require “engineering” due to depth of material retained. Refer to , which describes the design for sheeting and cofferdams for retaining soil in a trench greater than 6 feet deep.

The trench must be excavated and graded to the proper alignment and grade, leaving room for at least 4 inches of granular material as a bedding layer below the pipe. The bedding material must be graded uncompacted uniformly so the entire length of the pipe is firmly supported. If the pipe has bells that are larger in diameter than the barrel of the pipe, the surface of the bedding at the location of the bell can be lowered, so the bell is not propped up creating a void beneath the barrel or a misalignment of the pipe. The Contractor should not adjust the grade of the as-placed pipe in the trench with an excavator or other equipment.

Where rock or unyielding hardpan is encountered in the storm sewer trench, it should be excavated to a minimum of 6 inches below the proposed bottom of the pipe, backfilled with granular material Class IIIA, and compacted. The provides details for bedding and filling around storm sewers in various applications and should be strictly followed. Backfill should be thoroughly compacted under the haunches of the pipe to provide uniform bearing throughout.

Dewatering

Dewatering of the trench may be required if either the groundwater elevation or water in the existing ditches is inundating the trench bottom. If pumps are employed to control the water, proper soil erosion and sedimentation control should be employed as well. Running the discharge of a pump through a filter bag is a common requirement. In some cases, multiple filter bags may be required. Direct discharge of pumped water to a watercourse is prohibited.

Undercutting

Undercutting the proposed trench bottom may be required if the material in the bottom is unstable and will not sufficiently support the pipe. Trench undercutting has a separate pay item (Trench Undercut and Backfill) that must be calculated by the cubic yard. Refer to for construction details and method of payment, respectively. The additional depth below the intended trench bottom must be backfilled with Michigan Series 6A, 17A, or 46G aggregate. Granular material (Class I, II, or III) and other “suitable material” are not acceptable and not included in the pay item.

Bedding

Storm sewer should be placed on suitable bedding material. Soft or spongy materials do not provide a firm bed for storm sewers. Excavation and bedding for storm sewers must be in accordance with . Storm sewer bedding must be constructed uncompacted using a minimum 4-inch layer of granular Class III or IIIA material in accordance with the details in the .

Pipe Placement

Pipe laying typically begins at the outlet end and proceeds uphill with the bells on the higher end. Sometimes storm sewers must be laid backwards from the inlet end downhill. In this case, the pipe bells should still be laid to the higher end of the individual pipe sections.

The first piece of pipe laid should be set to the plan invert elevation and reasonably at the plan grade. Typically, a pipe laser is used to control the alignment and the grade by directing the beam at a target placed in the pipe. The plan grade is entered into the pipe laser and the laser is directed in the plan alignment of the storm sewer by sighting the centerline of the opposite end where the target is placed, which is measured from the offset stakes. By checking the invert elevation of the first pipe with the “cut/fill” of the offset stakes and checking the invert elevation (rise/run) periodically throughout the pipe laying operation, the Inspector will know if the plan elevations are being maintained.

The Contractor should not use a backhoe bucket or other excavation equipment to adjust the grade of the pipe. If the grade needs to be increased or decreased, the end of the pipe should be lifted to remove excess material or add additional material. Using a backhoe bucket or other excavation equipment to adjust the grade will damage the pipe.

The Inspector should verify the pipe sections are pushed home completely during installation. Ensure the Contractor does not forcefully slam the pipe connections together.

Prior to backfilling, all pipe joints must be wrapped with a non-woven geotextile blanket. The fabric must have a minimum width of 36 inches, be centered on the joint, and overlap on the ends by at least 12 inches.

The joints of CMP are different than the description above because there are no bells and spigots. The ends of the pipe sections are joined using couplers made of an open steel band that is passed around the joint and then secured with a bolted connection. To make the joint watertight, a gasket must be placed around the pipe between the coupler and the pipe. After the bolt(s) have been tightened on the coupler, the joint is wrapped, as described above, with geotextile blanket.

Elliptical concrete pipe should be installed with the lift holes on the top of the pipe. The manufacturer’s marks designating the top and bottom of the pipe should not be more than 5 degrees from the vertical plane through the longitudinal axis of the pipe. After installation, the lift holes should be sealed with suitable concrete plugs and waterproofed. Flow velocities within a pipe must be sufficient to prevent sedimentation at an approximate flow of 2 feet per second (fps). The minimum pipe slopes to provide a self-cleaning velocity are detailed in the following table:

Diameter (inches)	Storm (fps)	Sanitary (fps)
12	0.33	0.17
15	0.25	0.14
18	0.22	0.12
21	0.21	0.12
24	0.17	0.10
30	0.15	0.08
36-42	0.12	0.07
48	0.10	0.06
54-60	0.09	0.05
66-78	0.08	-
84	0.07	-

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