Numerous drainage products are available in steel with protective coatings and in aluminum. These include corrugated pipe, spiral-rib pipe, structural-plate pipe, box cul­verts, and, where a tunnel is required, tunnel liner plates. Figure 5.24 shows the variety of profiles available for the wall cross-section of steel drainage products. For additional profiles, see “Corrugated Steel Pipe Design Manual,” National Corrugated Steel Pipe Association, 2008. The arc-and-tangent profiles shown with depths of through 1 in (6.5 through 25 mm) are wall profiles for pipe factory-corrugated to the full pipe

cross-section. The 2-in-deep (51-mm) profile, which is used for structural-plate pipe and box culverts, is corrugated and curved into arc segments that can be bolted together in the field. The 552-in-deep (140-mm) profile is a similar product used for longer-span structures. The 54- and 1-in-deep (19- and 25-mm) rectangular profiles are for factory-corrugated spiral-rib pipe. Figure 5.25 illustrates the shapes of the products, the range of sizes available in steel, and common uses. Some corrugation profiles and size ranges vary for aluminum products. The larger sizes of structural-plate products and box culverts in steel or aluminum are often used as replacements for short-span bridges. Factory-made box culverts are available in spans up to 26 ft (7.8 m) and even longer with special designs.

Corrugated Steel. Most of the metal pipes used are corrugated from coils of coated sheet steel. Coatings, which are applied by the continuous hot-dip process in the production of the steel coil, include zinc (galvanizing) and aluminum. In addition, coils are available precoated with a polymer (on one or both sides) to provide extra protection against corrosion and/or abrasion. Most corrugated pipes have a continuous helical lockseam, but some manufacturers use a continuous helical welded seam, or a longitudinal riveted or spot-welded seam. Wall profiles from 1/2 X ^4 in (38 X 6.5 mm) to 5 X 1 in (125 X 25 mm) are factory-corrugated to the full pipe cross-section. The pipe is furnished in lengths (typically 20 ft or 6 m) and joined in the field by coupling bands. Diameters through 120 in (3000 mm) are available, depending on the wall profile. Pipe-arch shapes for installations with low cover are formed to shape from lengths of round pipe.

Corrugated Aluminum. Corrugated aluminum pipe is usually furnished with one of the following wall profiles: 152 X 34 in (38 X 6.5 mm), 253 X 52 in (68 X 13 mm), or 3 X 1 in (75 X 25 mm). The pipe may have a helical lockseam or a riveted seam. It is furnished in lengths similar to steel pipe and joined in the field by coupling bands. Diameters through 120 in (3000 mm) are available, depending on the wall profile, and pipe-arch shapes are formed to shape from lengths of round pipe.

Spiral-Rib Pipe. This is a newer type of steel pipe that is helically corrugated to the rectangular profiles shown in Fig. 5.24. The cross-section profile has been developed so

that flow characteristics are similar to that of a smooth-walled pipe. It is available in either coated steel or aluminum, as either round pipe through 108-in (2700-mm) diameter, or as pipe-arch.

Structural-Plate Pipe. This product type is available in either zinc-coated steel or aluminum.

Steel. The 6- X 2-in (152- X 51-mm) profile used for structural-plate pipe and box culverts is corrugated and curved into arc segments. The segments provide an arc length of up to about 86 in (2184 mm), in lengths of 10 or 12 ft (3.0 to 3.7 m). The segments are joined together with high-strength bolts in a sequential manner during construction. All of the shapes illustrated in Fig. 5.25 can be constructed with this product. The 15- X 5L-in (381- X 140-mm) profile can be used for the larger struc­tures. With spans up to about 50 ft (15 m), structural-plate structures can provide an economical alternative for replacing short-span bridges. Field coatings can be applied to enhance durability.

Aluminum. The 9-in-wide (230-mm) by 2i2-in-deep (64-mm) profile is used for the aluminum structural-plate pipe and box culvert structures. Product characteristics are generally similar to those of the steel product.

Long-Span Structures. Long-span structural-plate structures are defined as having either special shapes that involve a relatively large radius in the crown or side plates, or a span that exceeds certain structural design criteria as specified in AASHTO Standard Specifications for Highway Bridges. These structures generally have spans in the range of 20 to 50 ft (6 to 15 m). They are advantageous where headroom is restricted and can often provide the required waterway area at a lower cost than building a short-span bridge. Long-span structures are made up of a structural-plate barrel of coated steel or aluminum and integral special features that enable the structure to reach long spans. Special features include either (1) continuous longitudinal stiffeners of metal and/or reinforced concrete attached to the plates at the sides of the top arc, or (2) circumferential reinforcing ribs curved from structural shapes and attached to the plates to provide additional stiffness. Typical sections of each are illustrated in Fig. 5.26. They may be constructed to most of the shapes shown in Fig. 5.25 except box culverts.

Box Culverts. This product type is available in either zinc-coated steel or aluminum.

Steel. Box culverts are available in three types, including (1) 6- X 2-in (152- X 51-mm) corrugated plate shell with 6- X 3-in (152- X 76-mm) corrugated rib stiffeners (inside, outside, or both), (2) 6- X 2-in (152- X 51-mm) corrugated plate shell with 3- X 5-in (76- X 127-mm) hot rolled angle rib stiffeners, and (3) 15- X 5.5-in (381- X 140-mm) corrugated plate shell without stiffeners. Sizes range as shown in Fig. 5.25. The structures usually have an open bottom and are supported on a base channel or corrugated footing pads, on either a concrete footing or compacted soil, depending on size and other factors. They are also available with full invert plates.

Aluminum. Box culverts have a 9- X 2i2-in (230- X 64-mm) corrugated shell plate with extruded bulb angle rib stiffeners. Size ranges are similar to those for steel box culverts. Figure 5.27 shows a typical section and rib cross-sections. Stronger ribs, including a box-section rib, are available.

Tunnel Liners. Tunnel liners are press-formed from steel in an arc segment 16 or 18 in (400 or 450 mm) long. A corrugated profile is pressed in to make the wall cross­section, and flanges are formed on the sides. Two styles are available: (1) two-flange plates that are bolted through the flanges on the two longitudinal sides and lap-bolted

Concrete Thrust Beam

Thrust Beam to be made integral with Headwall

Span

Structural member attached to structural

plate corrugation

Staggered joints except at radius change

Span

FIGURE 5.26 Typical sections of long-span structural-plate structures. (a) Longitudinally stiffened with concrete thrust beam. (b) Transversely stiffened with structural members. (From Highway Design Manual, California Department of Transportation, with permission)

on the other two sides, and (2) four-flange plates that are bolted together through flanges on all four sides. Installation and assembly can be done entirely from the inside as the tunnel is constructed. The assembled liner plates may then act as a tem­porary structure that is lined by concrete, or may act alone as a permanent conduit. In addition to tunneling, the liner plates can be used in rehabilitation work, such as for lining a deteriorated culvert.

5.6.2 Plastic Pipe

Both high-density polyethylene (HDPE) and polyvinyl chloride (PVC) are used for drainage pipe. HDPE pipe may be single-wall corrugated, smooth-wall (double-wall), or ribbed. Common diameters are 4 to 24 in (100 to 600 mm) for single wall, 4 to 60 in (100 to 1500 mm) for double wall, and 18 to 96 in (450 to 2400 mm) for ribbed pipe. Single-wall pipe has a deep corrugation, whereas a smooth internal liner is added for double­wall pipe. Wall profile details vary with the manufacturer. PVC pipe may be either smooth-wall or ribbed, with diameters ranging up to 54 in (1350 mm). Plastic pipe is fur­nished in lengths (typically about 20 ft (6 m) for HDPE and 13 ft (4 m) for PVC) and joined in the field by coupling bands. It is available only as round pipe.

The main types of pipe used in highway construction are concrete pipe, metal pipe (steel or aluminum), and plastic pipe (high-density polyethylene and polyvinyl chloride). They are available in a wide array of sizes, shapes, and properties. Table 5.10 gives the ASTM and AASHTO standards for the most common highway drainage pipes. Some of the characteristics of these pipes are reviewed below.

5.6.1 Concrete Pipe

Concrete pipe is manufactured as nonreinforced, reinforced, or cast-in-place pipe; as box culverts and special shapes; and as field-constructed pipe. Shapes, as shown in Fig. 5.21, include round, horizontal and vertical ellipse, and arch configuration.

Factory-Made Pipe. Nonreinforced pipe is used for smaller diameters, whereas pipe with steel reinforcement is used for larger diameters and greater loads. Both are manu­factured in a plant, cured, and shipped to the job site. They are furnished in relatively short lengths and coupled with a bell-and-spigot or tongue-and-groove type joint.

Nonreinforced concrete pipe is available in diameters from 4 to 36 in (100 to 900 mm) and three strength classes. Nonreinforced concrete pipe is available as round pipe only. Reinforced concrete pipe is available in diameters from 12 to 144 in (300 to 3600 mm). The strength of reinforced concrete pipe can be specified according to five standard pipe classes (ASTM C 76), with Class I pipe being the most economical and Class V offering the greatest structural strength; according to required D-load strength (ASTM C 655); or according to a direct wall design (ASTM C 1417). Wall thickness of reinforced concrete pipe can be varied to meet in-field conditions. The standard “class” specifications for pipe give wall thickness according to three distinct types, which vary from Wall A, being the thinnest, to Wall C, being the thickest.

Steel reinforcing for reinforced concrete pipe can be arranged in many combina­tions to meet the given structural requirements. Figure 5.22 shows some of the steel reinforcement layouts used in manufacturing reinforced concrete pipe.

Cast-in-Place Pipe. This type of nonreinforced pipe is formed in a trench using a continuous process. First a trench is excavated so that it has a semicircular bottom and vertical or near vertical sidewalls, which serve as the outer form for the bottom and sides. The upper portion of the pipe is cast against an inner arch form as illustrated in Fig. 5.23. The form is pulled along the trench while concrete is poured into a hopper

located above. Powered spading mechanisms and variable-speed vibrators aid the flow of the concrete.

Box Culverts. Box culverts are rectangular shapes with flat sides, top, and bottom. These shapes are constructed with steel reinforcement. Factory-made boxes are shipped in sections 4 to 8 ft (1200 to 2400 mm) long and joined in the field to make a structure of the required length.

Precast Three-Sided Culverts. Three-sided culverts, sometimes called “three-sided box culverts,” are rectangular in shape. These shapes are constructed with steel reinforcement

that may or may not be pretensioned. They are available in spans between 12 and 34 ft (3.7 and 10.4 m), and rises up to 10 ft (3 m). These structures usually have an open bottom and are constructed on concrete footings.

Special Shapes. Other shapes are also manufactured. One example is a reinforced concrete section made up of an arch top and vertical sidewalls. Another example is a reinforced concrete arch that can be fabricated in either one or two piece segments. Both examples are joined in the field to make up the required length. With spans of 12 to 84 ft (3.7 to 25 m) or larger they are suitable as replacement structures for short span bridges. Additionally, segmental tunnel liners can be furnished as precast concrete segments.

Field-Constructed Pipe. Large reinforced structures may be constructed at the job site using appropriate formwork. Large arches and box culverts are often constructed in this manner.

FIGURE 5.22 Concrete pipe culvert reinforcement notation. (From PIPECAR: User and Programmer Reference Manual, FHWA, 1989, with permission)

Because of its hydraulic characteristics, the outlet velocity of a culvert is usually higher than the velocity in the discharge channel. The outlet velocity may be calculated either using Manning’s equation, Eq. (5.11), if the culvert is under inlet control, or by divid­ing the discharge by the cross-sectional area of the flow if under outlet control. Under outlet control, if the tailwater is above the crown of the pipe, or if the discharge is high enough to result in a critical depth equal to the depth of the culvert barrel, then the flow area may be taken as the area of the barrel. If the tailwater depth is low, the area of flow, and thus the velocity, may be determined using the chart in Fig. 5.19 or 5.20. To use these charts, first calculate the normal depth or tailwater TW (ft) in the channel; the ratio TW/D, where D is the structure height (ft); and the flow parameter Q/BD3/2, where B (ft) is the width of the barrel and Q (ft3/s) is the discharge. Enter the chart with TW/D and find YJD at the intersection of the appropriate curve. Multiply by D to determine the depth of flow at the outlet end of the culvert, Yo. The flow area is then calculated for Yo and the velocity for the flow Q from the continuity equation, Eq. (5.10). o

Recommended maximum channel velocities were presented in Table 5.5. The velocity at the outlet should be kept at or below these values, or, if this is not possible, the channel should be protected from erosion. The controlling parameters for the cul­vert velocity are its slope and roughness. If the recommended velocity is exceeded, consider decreasing the slope or using a culvert with a greater roughness coefficient.

If the velocity at the outlet cannot be reduced by these means, channel protection or energy dissipaters should be used to protect against erosion. Channel protection may consist of treatments such as concrete aprons or cutoff walls. In some cases, concrete or rock riprap may be required. These types of protection do not necessarily dissipate the energy, but protect against erosion. Energy-dissipating devices may be necessary either separately or in conjunction with channel protection where flow velocities are high. Dissipation devices, if used, are generally located at the outlet end or in the inte­rior near the end of the culvert. If such devices are used, consideration must be given to the effects on possible debris collection. (See “Hydraulic Design of Energy Dissipators for Culverts and Channels,” HEC 14, FHWA.)