Section 7: Drainage Facility Placement
Anchor: #i1086981Overview
This section contains information on the following topics:
Anchor: #i1087000Introduction
In designing drainage systems, the primary objective is to properly accommodate surface run-off along and across highway right-of-way through the application of sound hydraulic principles. Consideration must also be given to incorporating safety into the design of drainage appurtenances. The best design would efficiently accommodate drainage and be traversable by an out-of-control vehicle without rollover or abrupt change in speed.
To meet safety needs, the designer may use one of the following treatments:
- Design or treat drainage appurtenances so that they will be traversable by a vehicle without rollover or abrupt change in speed.
- Locate appurtenances a sufficient distance, consistent with traffic volume, from the travel lanes so as to reduce the likelihood of accidental collision.
- Protect the driver through installation of traffic barrier shielding appurtenances.
The following guidelines are intended to improve roadside safety with respect to facilities accommodating drainage parallel to and crossing under highways. The guidelines apply to all rural, high-speed facilities and other facilities with posted speed limits of 50 mph [80 km/h] or more and with rural type (uncurbed) cross sections. Where reference is made to horizontal clearance requirements in these guidelines, see Table 2-11: Horizontal Clearances and the discussions regarding Slopes and Ditches, Roadside Design, and Horizontal Clearances to Obstructions. Desirable values for horizontal clearance width should generally be used and minimum horizontal clearance widths applied where unusual conditions are encountered. Site visits may be appropriate to ascertain terrain conditions and debris potential before arriving at design decisions for cross drainage culverts.
Designers should address and resolve culvert end treatment issues with involved parties early in project development. If there are doubts about the proper application of criteria on a given project or group of projects, then arrangements should be made for a project concept conference with the appropriate entities prior to in-depth development of P.S.&E.
Anchor: #BGBBEDDGDesign Treatment of Cross Drainage Culvert Ends
Cross drainage culverts are defined as those handling drainage across and beneath the highway. Selection of an appropriate end treatment is primarily related to culvert size, culvert end location, side slope rate, terrain characteristics, drift conditions, right-of-way availability, and other considerations that may influence treatment selection at individual sites. Recommended treatment options are in the following priority:
- Safety treat culvert ends.
- Meet horizontal clearance requirements.
- Shield with appropriate barrier or attenuator.
Roadside safety performance is related to horizontal clearance width and side slope rate. (For a discussion of safety performance and design guidelines related to side slopes, see Slopes and Ditches) In developing a design cross section, it is usual practice to vary embankment side slope rate with height of embankment using the values shown in Table 2-10: Earth Fill Slope Rates. . Where right-of-way availability and economic conditions permit, flatter slopes may be used.
Design values for horizontal clearances are shown in Table 2-11: Horizontal Clearances for new location and major reconstruction projects. Within the horizontal clearance, sideslopes should preferably be 1V:6H or flatter with 1V:4H as a maximum steepness in most cases.
Small Pipe Culverts. A small pipe culvert is defined as a single round pipe with 36 inches [900 mm] or less diameter, or multiple round pipes each with 30 inches [750 mm] or less diameter, each oriented on normal skew. (Note: For arch pipes, use span dimension instead of diameter.)
When skews are involved, the definition of a small pipe culvert is modified as shown in Table 2-12:
|
Skew (degree) |
Single Pipe |
Multiple Pipe |
|---|---|---|
|
15 |
30 inches [750 mm] |
30 inches [750 mm] |
|
30 |
24 inches [600 mm] |
24 inches [600 mm] |
|
45 |
24 inches [600 mm] |
18 inches [450 mm] |
Small pipe culverts with sloping, open ends have been crash tested and proven to be safely traversable by vehicles for a range of speeds. Pipe ends should be sloped at a rate of 1V:3H or flatter and should match side slope rate thereby providing a flush, traversable safety treatment.. Single box culverts on normal skew with spans of 36 inches [900 mm] or less may be effectively safety treated just as small pipes (open, match 1V:3H or flatter slope).
When vulnerable to run-off-the-road vehicles (i.e., unshielded by barrier), sloped ends should be provided on small pipe culverts regardless of culvert end location with respect to horizontal clearance dimensions.
For existing culverts, this often entails removing existing headwalls and may involve removing barrier treatment if no longer needed to protect an obstacle other than a culvert end. The resultant culvert with sloped end is both safe and inexpensive.
For new culverts or existing culverts that may need adjusting, culvert pipe length should be controlled by the intercept of the small pipe and the side slope planes. Side slopes should not be warped or flattened near culvert locations. Headwalls should not be used.
In summary, whether a small pipe culvert is new or existing, sloped open ends should normally be provided. Terrain in the vicinity of the culvert ends should be smooth and free of fixed objects.
Intermediate Size Single Box Culverts and (Single and Multiple) Pipe Culverts. An intermediate size pipe culvert is defined as a single round pipe with more than 36 inches [900 mm] diameter or multiple round pipes each with more than 30 inches [750 mm] diameter but having maximum diameter of 60 inches [1,500 mm]. (Note: For arch pipes, use span dimension instead of diameter.)
Single box culverts are defined as those having only one barrel with maximum height of 60 inches [1,500 mm]. Cross sectional area of the single box or individual pipe normally should not exceed 25 ft2 [2.3 m2].
The openings of intermediate size single barrel box and pipe culverts are too large to be safely traversable by a vehicle. Recommended safety treatment options are in the following priority:
- Slope ends with safety pipe runners.
- Provide flat side slopes and locating the ends outside the horizontal clearance.
- Use barrier to shield culvert ends.
Sloping end treatments with safety pipe runners are preferred from a safety standpoint and are generally cost effective for both new and existing culverts of this size. These end treatments should be sloped at a rate of 1V:3H or flatter and should match side slope rate thereby providing a flush, traversable safety treatment. Terrain in the vicinity of the culvert end should be smoothly shaped and traversable, and headwalls should not be used.
For new single barrel box and pipe culverts that are intermediate size, the preferred design treatment is to provide a sloped end treatment with safety pipe runners regardless of end location with respect to horizontal clearance criteria. Length of new culverts should be governed by the locations of the side slope plane/culvert intercepts rather than by horizontal clearance criteria.
For existing single barrel box and pipe culverts that are intermediate size, no treatment is warranted for certain culvert end offsets and traffic volumes as shown in Table 2-11: Horizontal Clearances. . Where an improved design is warranted using Table 2-11, the removal of headwalls and installation of sloped ends with safety pipe runners is the preferred safety treatment.
In certain situations (e.g., culvert skew exceeds 15 degrees, severe debris problems, etc.) treatment with safety pipe runners will be impractical. For these conditions, locating intermediate size culvert ends to meet desirable horizontal clearance values (see Table 2-11: Horizontal Clearances) is preferred over shielding with barrier. Designs having flared wing walls with safety pipe runners oriented parallel to flow and spaced at 30 inches [750 mm] (maximum) center to center thereby can minimize debris problems.
Multiple Box Culverts and Large Single Pipes or Boxes. Multiple box culverts are defined as those with more than one barrel and a total opening (i.e., distance) of 20 ft [6.1 m] or less between extreme inside faces as measured along the highway centerline. Large single pipes or single boxes are defined as those with diameter or height exceeding 5 ft [1,500 mm] or cross sectional area exceeding 25 ft2 [2.3 m2].
From a safety standpoint alone, treatment is in the following priority for both new and existing installations:
- Provide safety pipe runners
- Meet or exceed desirable horizontal clearance value
- Shield with barrier.
Designers should carefully consider several factors before opting to use safety pipe runners. First, multiple box culverts accommodate significantly greater flow quantities than single box or pipe culverts and often a defined channel crosses the highway right-of-way. Where a defined channel is present, it may be impossible or impractical to shape the terrain near the culvert end to provide for vehicular traversability. In other instances, debris may adversely affect hydraulic capacity and, considering the higher flow quantities, subject the highway or private property to inundation if safety pipe runners are used. Such circumstances as these would dictate that a more suitable, but lower priority, culvert end treatment be selected.
Meeting horizontal clearance criteria does not eliminate the obstacle of the culvert end, rather the obstacle is placed at a location where it is less likely to be struck. Although not as desirable as providing a traversable culvert end, it is preferred over barrier treatment where there is sufficient right-of-way and where the cost of providing the necessary culvert length is reasonable. Where the cost of added length for new culverts or of extension of existing culverts is three or more times the cost of shielding with barrier, treatment with barrier becomes an attractive alternative.
For low-volume (less than 750 current ADT) conditions, however, the treatment option that has the lowest initial (construction) cost is generally the most cost effective design if an improved design is warranted.
Bridge Class Drainage Culverts. Bridge class culverts are defined as those having an opening (i.e., distance) of more than 20 ft [6.1 m] between the extreme inside faces as measured along the highway centerline.
If the culvert end is not beyond the recommended horizontal clearance, these large culverts should be shielded with guardrail on the approach and across the culvert. Table 2-13 provides guidelines for installing guardrails.
|
Depth of Cover |
Post Spacing |
Foundation |
|---|---|---|
|
Less than 44 inches [1120 mm] |
6 ft – 3 inches [1,905 mm] |
Steel post welded to steel base plate and bolted to culvert ceiling |
|
44 inches [1120 mm] or more |
6 ft – 3 inches [1,905 mm] |
Standard embedment |
Where guardrail is carried across bridge class culverts, steep side slopes should be positioned to provide for lateral support of the guardrail, as shown on Figure 2-15.
Figure 2-17. Use of Guardrail on Bridge Class Culverts.
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Anchor: #BGBHEJGJParallel Drainage Culverts
The inlet and outlet points of culverts handling drainage parallel to the travel lanes, such as at driveways, side roads, and median crossovers, are concerns in providing a safe roadside environment. Flow quantities for these parallel drainage situations are generally low with drainage typically accommodated by a single pipe. The following guidelines apply to driveway, side road, and median crossover drainage facilities:
- Within the horizontal clearance, there should be no culvert headwalls or vertical ends. Outside the horizontal clearance, single pipe ends preferably should be sloped although not required.
- Where used, sloped pipe ends should be at a rate of 1V:6H or flatter. The sloping end may be terminated and a vertical section introduced at the top and bottom of the partial pipe section as shown in Figure 2-16.
- Median crossover, side road, and driveway embankment slope should be 1V:6H maximum steepness, with 1V:8H preferred, within the horizontal clearance dimensions.
- Where large (greater than 30 inches [750 mm] in diameter) pipe ends are located within the horizontal clearance, safety pipe runners should be provided with a maximum slope steepness of 1V:6H with 1V:8H preferred. Typical details for a driveway, side road, or median crossover grate are shown in Figure 2-17. Grates are not required on single, small (30 inches [750 mm] or less diameter) pipes regardless of end location with respect to horizontal clearance requirements; however, the ends of small pipes should be sloped as described above and appropriate measures taken to control erosion and stabilize the pipe end.
- The use of paved dips, instead of pipes, is encouraged particularly at infrequently used driveways such as those serving unimproved private property.
- For unusual situations, such as driveways on high fills or where multiple pipes or box culverts are necessary to accommodate side or median ditch drainage, the designer should consider the alternatives available and select an appropriate design.
Figure 2-18. Use of Small Sloping Pipe Ends Without Grates.
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Figure 2-19. Use of Large Sloping Pipe Ends With Grates.
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Anchor: #i1087298Side Ditches
For side ditches, attention to cross section design can reduce the likelihood of serious injuries during vehicular encroachments. Ditches with the cross sectional characteristics defined in Table 2-14 are preferred and should especially be sought when ditch location is within the horizontal clearance requirements. Where conditions dictate, such as insufficient existing right-of-way to accommodate the preferred ditch cross section or where ditches are located outside the horizontal clearance requirements, other ditch configurations may be used. Typically, guardrail is not necessary where the preferred ditch cross sections are provided.
|
- |
Preferred Maximum Back Slope (Vertical:Horizontal) |
|
|---|---|---|
|
Given Front Slope (Vertical:Horizontal) |
V-Shaped |
Trapezoidal-Shaped |
|
1V:8H |
1V:3.5H |
1V:2.5H |
|
1V:6H |
1V:4H |
1V:3H |
|
1V:4H |
1V:6H |
1V:4H |
|
1V:3H |
Level |
1V:8H |
Ditches that include retards to control erosion should be avoided inside the horizontal clearance requirements and should be located as far from the travel lanes as practical. Non-traversable catch or stilling basins should also be located outside the horizontal clearance requirements.