750.7 Non-Hydraulic Considerations
|Metal and Concrete Pipe|
|High Density Polyethylene (HDPE) Pipe Culverts|
|See also: Innovation Library|
- 1 750.7.1 General
- 2 750.7.2 Types
- 2.1 7184.108.40.206 Roadways with ADT > 3500
- 2.2 7220.127.116.11 Roadways with ADT ≤ 3500
- 2.3 718.104.22.168 Roadways and Roadside Applications with ADT < 1700
- 2.4 722.214.171.124 Other Roadway Drainage Structures
- 2.5 7126.96.36.199 Installations for Special Situations
- 2.6 7188.8.131.52 Storm Sewers
- 2.7 7184.108.40.206 Multiple Opening Installations
- 2.8 7220.127.116.11 Temporary Installations
- 3 750.7.3 Box Culverts
- 4 750.7.4 Pipe Headwalls
- 5 750.7.5 End Treatment
- 6 750.7.6 Reinforced Concrete Pipes
- 7 750.7.7 Fill Settlements
- 8 750.7.8 Camber in Culverts
- 9 750.7.9 Culvert Extensions
- 10 750.7.10 Overfill Heights
- 11 750.7.11 Culvert Grades
- 12 750.7.12 Culvert Lengths
- 13 750.7.13 Excavation for Structure
- 14 750.7.14 Connections
- 15 750.7.15 Culvert Cleanout
- 16 750.7.16 Slotted Drains
Drainage structures are located and designed to adequately handle runoff across improvements and to handle runoff from the improvement. Non-Hydraulic Considerations presents criteria pertaining to the selection of culvert and storm sewer material and appurtenances. The hydraulic design of culverts and other drainage facilities is discussed in other articles under Hydraulic Analysis.
Permissible culvert pipe types are separated into the following groups.
|Group A (ADT>3500)||Group B (ADT≤3500)||Group C (Other Applications)|
|Reinforced Concrete||Group A Pipe||Group A Pipe|
|Vitrified Clay||Polymer Coated Corrugated Metal||Group B Pipe|
|Corrugated Aluminum Alloy||Corrugated Zinc-Coated Steel|
|Corrugated Polyethylene||Corrugated Bituminous-Coated Steel|
|Corrugated PVC (Polyvinyl Chloride)|
|Corrugated Aluminum-Coated Steel|
In general, there are two methods of specifying the permissible culvert types dependent upon the design ADT. For most applications, a group of culvert types is specified as described in succeeding sections. For special situations, a qualified pipe group may be specified, as described in Installations for Special Situations. The final selection of the structure type is based on requirements in the standard specifications, on good engineering judgment, and economy with consideration of service and maintenance costs.
718.104.22.168 Roadways with ADT > 3500
For roadways with ADT > 3500, reinforced concrete pipe will be specified for crossroad structures, except for the following conditions:
- Reinforced concrete box culverts are specified when it is more economical to build the reinforced box culvert than it is to provide an equivalent pipe culvert.
- Vitrified clay pipe (extra strength) is specified when the purpose of the culvert is a sanitary sewer.
- Group C pipe should be specified for the portion of median outlet pipes outside the edge of pavement where such pipes are located on high fills requiring a break in flowline grade. Details for such installations are illustrated in Pipe Grades For Median Drop Inlets.
- Group B pipe is specified to drain drop inlets into crossroad drainage structures when such installation necessitates a steep flowline grade and when the pipe will not extend under the pavement or in other non-traffic areas, such as behind guardrail at median piers.
Pipes of 12 in. and 15 in. are not used except as outlets from drop inlets and in storm sewer systems.
The requirements for using reinforced concrete pipe or vitrified clay pipe for structures may be waived if conditions warrant, such as poor structure foundation conditions, high fills, simplification of handling traffic, etc.
722.214.171.124 Roadways with ADT ≤ 3500
For roadways with ADT ≤ 3500, Group B pipe should be specified for crossroad structures. The hydraulic design computations for Group B pipe should be performed for both corrugated and smooth wall pipe. The pay item for the corrugated pipe size should be used. At each pipe location on the plans, both the corrugated and equivalent smooth wall diameters should be shown as in the following examples:
- 36" Group B Pipe (30")
- 36" Group B Pipe (36")
The equivalent smooth wall diameter must be shown whether the size is equal to or less than the corrugated size. The standard specifications for pipe and end sections describe the nomenclature for this procedure. Some exceptions to specifying Group B on the plans are the following:
- Reinforced concrete box culverts should be considered for pipes larger than 60 in. diameter.
- Reinforced concrete pipe, corrugated polyethylene pipe or polyvinyl chloride pipe should be specified for locations where high acidity or alkalinity of soils or waters or other abrasive or corrosive elements are present.
- Corrugated metallic-coated steel pipe-arch structures in sizes B-5 and larger may be specified where necessary because of limited allowable structure height. A battery of round pipes or a single elliptical reinforced concrete pipe may be considered in lieu of B-1 through B-4 corrugated metallic-coated steel pipe-arch structures.
- Elliptical reinforced concrete pipe may be specified in special cases, usually for storm sewers, where necessary because of limited allowable structure height.
Pipes of 12 in. and 15 in. are not used for crossroad culverts, except where the use of an 18 in. pipe will create an unsightly or impracticable drainage condition.
Corrugated polyethylene pipe (Type S) and polyvinyl chloride pipe are double walled, full circular cross section pipes, with an outer corrugated wall and a smooth inner liner. Only 12 to 60 in. diameter sizes of corrugated polyethylene, or 12 to 48 in. diameter sizes of PVC pipe, are approved for use on highway projects. Corrugations may be either annular or helical. Headwall protection is provided by means of a beveled pipe end treatment, safety slope end section, or metal or concrete flared end sections. Section 728 of the Standard Specifications require all PVC pipes to have an end section of one of the other Group B pipe materials to protect from ultraviolet degradation where the end of the pipe is exposed.
7126.96.36.199 Roadways and Roadside Applications with ADT < 1700
For roadways, side roads, and entrances with ADT < 1700 Group C pipe should be specified using the same method for corrugated and smooth wall Group B pipe as explained in Roadways with ADT ≤ 3500. For information regarding replacement of failed driveway drainage pipes see EPG 941.29 Maintenance of Residential, Commercial and Public Road Entrances.
7188.8.131.52 Other Roadway Drainage Structures
Outer roadway drainage structures shall be selected by ADT as described above. Continuous drainage structures extending under outer roadways are designed to the same standard as required for the portion of the structure under the main roadway. Since a continuous drainage structure usually increases the standard for the portion under the outer roadway, it is usually more economical to use independent structures. Where continuous structures are used, the runoff between the outer roadway and the main roadway is usually carried into the crossroad structure by drop inlets and pipe. Where the crossroad structure is a relatively small pipe, the drop inlet is constructed in the crossroad structure.
For drainage applications other than crossroad pipes, such as entrances, side roads and median drainage, Group C pipe may be specified.
Flared end sections or beveled pipe end treatment are specified at both ends of pipe structures 66 in. or less in diameter. In special cases where low clearance exists and the structure is essentially at right angles on roads with less than 400 ADT, pipe arches with flared end sections may be specified.
7184.108.40.206 Installations for Special Situations
For installations on a project which normally would require a pipe group option, special conditions may exist which would justify the specifying of a qualified pipe group type. Justification for the selection of a qualified group pipe type include, but are not limited to, unstable foundation, high embankments, high erosive forces, highly abrasive or corrosive conditions, high fire hazard or other pertinent reasons. When any one or a combination of these factors exist, the culvert pipe type(s) best suited to resist such destructive forces is selected and specified by excluding pipe types from a specified group with a note on the plans or Job Special Provision. When a qualified pipe group is specified, the reasons for such selection are included in the letter of transmittal of the plans.
7220.127.116.11 Storm Sewers
The permissible storm sewer type under the paved portion and any planned widening of roadways with ADT greater than 3500 is Group A pipe. All other applications of storm sewer are Group B pipe. The standard specifications require that corrugated metal culvert pipes used for storm sewer be smooth interior pipe types, so that consistent hydraulic characteristics may be assumed during design of the entire interconnected system.
718.104.22.168 Multiple Opening Installations
Multiple opening structures, either boxes or pipes, are used only as required where the allowable structure height is restricted. Where multiple pipes are constructed, the pipes are separated by a distance of 1/2 their outside diameter, or a minimum of 1 ft., whichever is greater. Multiple box structures require that the plans be done by Bridge. Where such plans are required, Bridge is furnished with the culvert section, grade across the structure, typical section, and any other necessary information. For drainage areas of 1000 acres and under which require a structure designed by Bridge (multiple cell box, etc.), the district shall also make the necessary analysis and provide to Bridge the cell sizes, drainage area, the magnitude of the discharge, frequency, and design highwater for placement on the plans for each structure. A summary of responsibilities for multiple opening installations is contained in the table below.
|Drainage Area (Acres)||Number of Cells||Design Fill in Standard Plans||Responsible Entity||Bridge Number||Pay Items, Job Specials, Estimate|
|<1000||2 or 3||N/A||District||Bridge Division||Yes||Bridge|
|>1000||1, 2 or 3||N/A||Bridge Division||Bridge Division||Yes||Bridge|
|*Bridge Division will produce a typical section for the District to use in roadway plans (including quantities per foot)|
722.214.171.124 Temporary Installations
For bypasses, crossovers or other temporary installations, regardless of design ADT, Group C pipe should be specified using the same method as for corrugated and smooth wall Group B pipe as explained in Roadways with ADT ≤ 3500.
750.7.3 Box Culverts
7126.96.36.199 Standard Plans
See Standard plans 703.10 through 703.87. The HL-93 design loading is used for all culverts, including those located in commercial zones, at major river crossings and on NHS routes.
Hydraulic factors will control the required shape of the box culvert. The shape of the box culvert will affect fill slopes and the culvert length. In special cases, a more economical box culvert may be attained by increasing the culvert opening. Box culvert sizes are indicated on the plans as "(span) x (height)".
7188.8.131.52 Environmental Requirement for Multi-Cell Box Culverts
As part of this condition, for multi-cell box culverts with three or more cells on class P or C streams, the opening of the center cell must be slightly lower than the adjacent cells to concentrate low flows for the passage of aquatic organisms.
To determine whether a culvert is affected by this regional condition, the designer should carefully review the stream list referenced in the regional condition to determine if the structure crosses an affected class P or C stream segment (DNR Stream Classifications). If so, contact the appropriate wetland specialist to determine the appropriate course of action (special conditions may vary depending on whether the project is authorized by Nationwide Permit or Individual Permit).
At some locations, it may be desirable to effectively lower an outer cell in lieu of the center cell.
750.7.4 Pipe Headwalls
Type S pipe headwalls may be used in lieu of drop inlets for median pipes for medians 60 ft. wide or wider. Details for Type S pipe headwalls are shown on Standard Plan 604.05.
750.7.5 End Treatment
End treatments on culverts provide safety benefits for errant vehicles, improved structural support for pipe ends, and improved hydraulic flow for pipe entrances. All pipe ends in the clear zone require some type of end treatment. Outside of the clear zone, considerations of hydraulics, aesthetics or soil erosion may warrant the use of an end section.
The following table contains minimum useage of end treatments as discussed in the Roadside Design Guide. This table and the following discussion should be used with site specific factors, including maintenance concerns, to determine the safe and cost-effective end treatment for each culvert. The Roadside Design Guide (Chapter 3) has additional discussion of cross-drainage and parallel drainage structures.
|Within Clear Zone||Outside Clear Zone|
|FES Flared End Section
BET Beveled Pipe End Treatment
SSES Safety Slope End Section
7184.108.40.206 Flared End Sections
Flared end sections may be used on concrete, metal, or plastic pipes as shown on Standard Plan 732.00. Where flared end sections are used on skewed pipe, the section is placed on the same line as the pipe and the fill slope is warped to fit.
7220.127.116.11 Beveled Pipe End Treatment
Beveled pipe end treatment may be used on either corrugated metal or plastic pipes as shown on Standard Plan 732.05. All beveled pipes use encasement. Corrugated steel pipe-arches are not beveled. In general, the bevel should not be flatter than 1V:6H nor should the skew exceed 15 degrees. If these controls are exceeded, special consideration is given to the use of headwalls, riprap, or slope pavement to stiffen the structure against uneven loading from the embankment and the dynamic forces of the water. Proposed designs for these conditions are submitted to the Support Center for approval.
718.104.22.168 Safety Slope End Sections
Safety slope end sections are used to reduce the potential for vehicle snagging. End sections with two types of safety treatment for 1V:4H, 1V:6H and 1V:10H slopes are shown in Standard Plan 732.10. The 2B (Summary of Quantities) sheet should indicate the slope of the end section and whether the end section is for a crossroad or parallel drainage structure.
Floodgates are specified for the outlet ends of pipes where required to prevent floodwater from backing through the pipe. Type 1 floodgates are specified for concrete pipes. Floodgates for concrete box structures will require a special item number and special provision. Type 2 floodgates are specified on corrugated metal pipes. The hydraulic head should be specified on the plans. If the hydraulic head is not specified on the plans, the height of fill above the pipe will be considered the hydraulic head. The number of floodgates is listed on the plans in accordance with pipe sizes.
For information on maintenance of floodgates, see Drainage Fittings.
750.7.6 Reinforced Concrete Pipes
722.214.171.124 Classes of Strength
Reinforced concrete pipe is available and is specified on the plans as any one of five classes designated as Class I, II, III, IV or V. Class V pipe is the strongest design.
7126.96.36.199 Use of Various Classes
Class I and Class II reinforced concrete pipe is used only for sewers in trenches outside roadbed and street limits. Class I pipe is provided in 60 to 108 in. diameters, inclusive, and is used in trenches 14 ft. or less in depth. Class II pipe is provided in sizes from 12 to 108 in. diameters, inclusive, and is used in trenches 18 ft. or less in depth. Deeper trenches require Class III, IV or V pipe. Class III, IV or V pipe is used for all other drainage structures and are provided in sizes from 12 to 108 in. diameters, inclusive.
7188.8.131.52 Selection of Pipe Class and Installation Type
In order to select the most economical class of reinforced concrete pipe, the existing soil conditions and fill height must be known. As shown on Standard Plan 726.30, a range of installation types can be used for each soil category; however, the proper installation type will correspond to the most economical, lowest class of pipe for a given fill height.
7184.108.40.206 Specifying Pipe Class and Installation Type on Plans
When the culvert pipe is shown on the plans as Group B or Group C, neither the class of reinforced concrete pipe nor the installation type is specified. When reinforced concrete pipe is specified directly on the plans, the class of reinforced concrete pipe and installation type is specified. For fill heights greater than 50 ft, reinforced concrete pipe is specified on the plans and the contractor is responsible for acquiring a special design. The Job Special Provision (JSP) "Special Design Reinforced Concrete Pipes and Flared End Sections" should be used. Contact the Bridge Liaison Engineer to obtain the D-load for insertion into the JSP. Approval of this special design is required from the Construction and Materials and the Bridge Divisions.
7220.127.116.11 Payment for Bedding
There is no direct payment or bid item for any bedding category; however, when an unsuitable bedding material exists, such as rock, payment for a specific bedding material needed for the installation type specified will be included in the Class 3 excavation.
750.7.7 Fill Settlements
Fill settlements can seriously affect concrete box structures by opening joints and cracks sufficiently to allow the fill around the culvert to infiltrate into the culvert, thereby creating voids which can cause the roadbed to fail. In areas subject to large settlements, other structure types are considered or the box culvert is designed to withstand the settlement. This requires special box culvert designs and where box culverts are to be so designed, Bridge is furnished with full information, including culvert sections, grades, and anticipated settlement. Box culverts with special collars around joints have been successfully designed and used in areas subject to large fill settlements. Since such structures are expensive, it is sometimes more economical to use other structure types, such as flexible pipe.
750.7.8 Camber in Culverts
Camber, as used in culvert design, is defined as the distance the central portion of crossroad structures is constructed above final flowline grade to compensate for anticipated settlement. Typical details for cambering culverts are shown on Standard Plan 726.30. A structure designed with proper camber will settle to near flowline grade and elevation when it reaches final settlement. All culverts, except those on non-yielding foundations, are cambered at a minimum rate of 0.01 ft/ft of overfill. Cambers of 0.1 ft. or less are not shown on plans. Where the fill settlement is known, culverts are designed with a camber equal to the anticipated settlement. The camber is shown on the culvert section at the roadbed shoulders by amount and flowline elevation.
750.7.9 Culvert Extensions
All culvert extensions, both boxes and pipes, are extended with structures meeting current design requirements and standards, regardless of the type of standard of the existing structure. Pipe collars, as detailed on Standard Plan 604.40, are used to connect different types of pipe, and concrete pipe to concrete pipe. Box culverts are extended in accordance with details shown on Standard Plan 703.38 and Culvert Extensions. Additional fills on existing box culverts may require a structural analysis of the existing structure by the Bridge Division. If so, Bridge is furnished a print of the completed culvert section and the standard to which the existing structure was designed, if known, for their use in making the analysis.
The unit cost for pipe or box extensions is typically greater than the unit cost for new construction of pipe or box culverts.
For floodplain permit requirements, see National Flood Insurance Program (NFIP) - Culvert Extensions for structures designed by Bridge or Floodplain Management and the Regulatory Floodway for roadway culverts.
750.7.10 Overfill Heights
718.104.22.168 Minimum Fill Heights
The minimum allowable fill or cover for all structures is 1 ft. at the shoulder line, with the following exceptions:
- The minimum fill for structural-plate pipe structures is tabulated in Allowable Overfills for Structural Plate Pipe.
- The minimum clearance from the top of structures to the bottom of bases is 6 in. Exceptions are special box culverts designed to carry traffic on the top slab.
- For roadways with an ADT ≤ 1700, the minimum fill at the shoulder on the inside of superelevated curves is 18 in.
- Minimum fill heights for vitrified clay pipe (extra strength) are 4 ft. for the 8 to 21 in. diameters and 3 ft. for the 24 to 36 in. diameters.
Overfill heights which are less than those indicated as allowable for any one pipe type are not considered as justification for the elimination of specifying pipe types by "Group" provided other criteria are satisfactory.
722.214.171.124 Design Fill Heights
If any question develops regarding the fill heights to be used, and where the fill height is between values tabulated for design, the design fill height is taken to the next increment requiring the higher design. Pipe culverts are designed throughout their length for the maximum design condition except in the case of structural plate pipe.
A single fill height, representing the maximum height of fill over the culvert, can be used for most box culverts. Additional fill heights may be used for longer culverts, as shown in Culvert Extensions. Generally, no more than two or three fill heights should be required. When multiple fill heights are used, consideration should be given to the potential for future widening of the roadbed.
Design fill heights for all pipe culverts specified by "Group" are shown on the "B" sheets. The allowable overfill heights for corrugated metal pipe-arches and structural plate pipes are tabulated in Allowable Overfills for Corrugated Metal Pipe-Arches and Allowable Overfills for Structural Plate Pipe. These overfill heights indicate both a minimum and a maximum, neither of which should be exceeded. The column headed "Standard" under gage for pipe-arches in Figure 9-10.6 refers to the gage required for the particular structure by the standard specifications. If overfill heights exceed the range shown, a different gage may be necessary and a special design is requested from the Support Center. A special design is also requested for pipe-arches of a size not listed in Allowable Overfills for Corrugated Metal Pipe-Arches. If a different gage is necessary, the plans specify the gage required. Where overfill heights are greater than shown in the figure, consideration should be given to round pipe. The gage for structural plate pipe is specified on the plans and may be changed throughout the length of the structure, where economically feasible, dependent on the fill heights in accordance with Allowable Overfills for Structural Plate Pipe.
7126.96.36.199 Maximum Fill Heights
7188.8.131.52.1 Box Culverts
For fill heights exceeding the values tabulated on the standard plans, a special design request is made to Bridge. A plat showing the culvert location and a cross section at the culvert location should be provided with the request. A special sheet with structural details will be returned to the district. Input on pay item quantities will also be provided.
Design overfill heights which are in excess of those indicated as allowable for any one pipe type are not considered as justification for the elimination of specifying pipe types by "Group" provided other criteria are satisfactory.
750.7.11 Culvert Grades
Crossroad drainage structures are usually placed on a grade equal to the natural ditch grade or the ditch grade in which the culvert is being placed. Controlling grades for storm and sanitary sewers are given in preceding sections. Erosion may be a problem at the outlet end of culverts on steep grades, which sometimes can be reduced by breaking the grade through the culvert. Grade breaks can be used to reduce structure excavation. Drop structures can be used at the inlet end of culverts to reduce the grade through the culvert. Drop structures are used with discretion because of the ponding upstream, and because of the unstable condition that may be created by the ponding. The grade for pipes for median drop inlets is broken in accordance with the requirements and details illustrated on Pipe Grades For Median Drop Inlets.
750.7.12 Culvert Lengths
Culvert lengths are determined graphically by scaling from the culvert sections. The lengths are obtained by intersection of the structure with slope lines as shown on the culvert standard plans, and as described in the following sections. Precise lengths are not computed. In questionable cases a longer length is used. Skewed slopes used for culvert sections are shown on Slopes for Skewed Culvert Sections. Intermediate values are interpolated.
7184.108.40.206 Box Culverts
The length of box culverts is the barrel length which is the distance between headwalls measured along the centerline of culvert, and is scaled to the next higher 1.0 ft. Barrel lengths over 80 ft. are built in sections and therefore transverse joints shall be shown on the plan culvert sections as shown on Culvert Extensions. The minimum barrel length is 2 ft. greater than the roadway width measured normal to the centerline of the roadway.
The length of pipe culverts with headwalls is 2 ft. longer than the distance between headwalls. Pipe headwalls are designed on a flat grade, regardless of the grade of the pipe. The length of pipe culverts not beveled and without headwalls is the distance between the slope lines at the flowline. Corrugated metallic-coated steel pipe lengths are scaled to the next higher even foot. Other pipe lengths are scaled to the next higher 1.0 ft. The length of corrugated metallic-coated steel pipes with beveled ends is 2 ft. longer than the distance between the intersection of the slope lines and the centerline of the pipe scaled to the next higher even foot. Pipe bends and special connections are not listed as a pay item on the plans. The plans should include notes to the effect that such items are required and that their costs are included in other items. The plans include, usually on the culvert sections, sufficient dimensions and detail to fabricate pipe with bends or special connections.
750.7.13 Excavation for Structure
The plans provide for connecting new structures to existing structures, and connecting different types of new structures. The plans do not include an item for the connection of pipes to existing manholes, box culverts, drop inlets or sewer pipes. The plans do include the pipe collar item for connecting different types of pipe or different sizes of pipes. Details for pipe collars are shown on Standard Plan 604.40.
750.7.15 Culvert Cleanout
Information needed to properly estimate culvert cleanout items are:
- Location of culvert by station
- Type of culvert, i.e. corrugated metal or concrete
- Length of culvert
- Size of culvert
This information should be listed for each culvert to be cleaned out on the 2B sheets under the heading "Culvert Cleanout, Estimated." This information should be obtained from existing plans; no field measurement is required.
750.7.16 Slotted Drains
Slotted drains may be used to assist in drainage across entrances or very short sections of the roadway edge. Consistent hydraulic characteristics are not available, so slotted drains should not be expected to completely drain a pavement area. Chapter 4 of the FHWA Hydraulic Engineering Circular 22, "HEC-22 Urban Drainage Design" provides guidance for the design of slotted drains. The plans provide for the contractor to select among different styles of slotted drains (See Standard Plan 604.70). The diameter of pipe and length of slotted drain are specified in the plans.