Several pipe materials are acceptable to WSDOT, depending on the pipe classification (see the Standard Specifications). WSDOT’s policy is to allow and encourage all schedule pipe alternatives that will function properly at a reasonable cost.
If one or more of the schedule pipe alternatives at any location are not satisfactory, or if the project has been designed for a specific pipe material, the schedule alternate or alternates shall be so stated on the plans, usually on the structure note sheet. Pipe materials shall conform to the Hydraulics Manual, the Standard Specifications, and the Standard Plans.
Justification for not providing a pipe material, as limited by the allowable fill heights, corrosion zones, soil resistivity, and limitations of pH for steel and aluminum pipe shall be justified in the hydraulic report (Chapter 1) and within the PS&E. Cost will not normally be a sufficient reason except in large structures such as box culverts or structural plate pipes. Frequently, structural requirements may have more control over acceptable material than hydraulic requirements.
When drain, culvert, or sewer pipe is being constructed for the benefit of cities or counties as part of the reconstruction of their facilities and they request a certain type of pipe, the PEO may specify a particular type without alternatives; however, the city or county must submit a letter stating its justification. Existing culverts should be extended with the same pipe material and no alternatives are required.
8-3.1 Concrete Pipe
This section presents design criteria for concrete pipe, including drain pipe; underdrain pipe; and culvert, storm, and sanitary sewer pipe.
8-3.1.1 Concrete Drain Pipe
Concrete drain pipe is non-reinforced. The strength requirements for concrete drain pipe are less than the strength requirements for other types of concrete pipe. Also, concrete drain pipe can be installed without the use of O-ring gaskets or mortar, which tends to permit water movement into and out of joints.
8-3.1.2 Concrete Underdrain Pipe
Concrete underdrain pipe is no longer used. Additional guidance will be provided in future revisions to the Hydraulics Manual.
8-3.1.3 Concrete Culvert, Storm, and Sanitary Sewer Pipe
Concrete culvert, storm, and sanitary sewer pipe can be either plain or reinforced. Plain concrete pipe does not include steel reinforcing. Reinforced concrete pipe is available in Classes I through V. The amount of reinforcement in the pipe increases as the class designation increases. Correspondingly, the structural capacity of the pipe also increases. Because of its lack of strength, Class I reinforced concrete pipe is rarely used and is not listed in the fill height tables of Section 8-12.
The reinforcement placed in concrete pipe can be either circular or elliptical. Elliptically designed reinforcing steel is positioned for tensile loading near the inside of the barrel at the crown and invert, and at the outside of the barrel at the springline. As shown in Figure 8-15, a vertical line drawn through the crown and invert is referred to as the minor axis of reinforcement. The minor axis of reinforcement will be clearly marked by the manufacturer; the pipe must be handled and installed with the axis placed in the vertical position.
Concrete joints use rubber O-ring gaskets, allowing the pipe to meet the pressure-testing requirements for storm sewer applications. The joints, however, do not have any tensile strength and in some cases can pull apart, as discussed in Section 8-7. For this reason, concrete pipe shall not be used on grades over 10 percent without the use of pipe anchors, as discussed in Section 8-8.
Concrete pipe is permitted anywhere in the state, regardless of corrosion zone, pH, or resistivity. It has a smooth interior surface, which gives it a relatively low Manning’s roughness coefficient (Table 4-1). The maximum fill height for concrete pipe is limited to about 30 feet or less. However, concrete pipe is structurally superior for carrying wheel loads with shallow cover. For installations with less than 2 feet of cover, concrete pipe is an acceptable alternative. Table 8-3 lists the class of pipe that should be specified under these conditions.
Concrete is classified as a rigid pipe, which means that applied loads are resisted primarily by the strength of the pipe material, with some additional support given by the strength of the surrounding bedding and backfill. Additional information regarding the structural behavior of rigid pipes is provided in Section 8-10.3. During the installation process, pipe should be uniformly supported to prevent point load concentrations from occurring along the barrel or at the joints.
Potential difficulties during installation include the weight of concrete pipe and, for sanitary sewer applications, hydrogen sulfide buildup. The PEO shall follow the recommendations of the local sewer district or municipality when deciding if concrete pipe is an acceptable alternate at a given location.
Metal Pipe: General
Metal pipe is available in galvanized steel, aluminized steel, or aluminum alloy. All three types of material can be produced with helical corrugations, annular corrugations, or as spiral rib pipe.
Metal pipe is classified as a flexible pipe, which means that applied loads are resisted primarily by the strength of the bedding and backfill surrounding the pipe, with some additional support given by the pipe material itself. Because of the dependence upon bedding strength and backfill material, it is critical that metal pipe be installed in accordance with the requirements of Section 8-10.4 to ensure proper performance.
Metal pipe is available in a wide range of sizes and shapes and, depending on the type of material corrugation configuration, can be used with fill heights up to 100 feet or more. Metal pipe is susceptible to both corrosion and abrasion; methods for limiting these issues are covered in Sections 8-5.3 and 8-6.
8-3.2.1 Helical Corrugations
Most metal pipe produced today is helically wound, where the corrugations are spiraled along the flow line. The seam for this type of pipe is continuous, and also runs helically along the pipe. The seam can be either an ungasketed lock seam (not pressure testable) or it could be gasketed lock seams (pressure-testable seams). If ungasketed lock seam pipe is used in storm sewer applications, it is generally necessary to coat the pipe with Treatment 1 (Section 8-5.3.1) for the pipe to pass the pressure testing requirements.
Helically wound corrugations are available in several standard sizes, including 2⅔-inch pitch by ½-inch depth, 3-inch by 1-inch, and 5-inch by 1-inch. Corrugation sizes are available in several gage thicknesses, depending on the pipe diameter and fill height. Larger corrugation sizes are used as the pipe diameter exceeds about 60 inches. A typical corrugation section is shown in Figure 8-3.
As a result of the helical manufacturing process, the Manning’s roughness coefficient for smaller-diameter—24 inches or less—metal pipe approaches the Manning’s roughness coefficient for smooth wall pipe materials, such as concrete and thermoplastic pipe. This similarity will generally allow metal pipe to be specified as an alternative to smooth wall pipe without increasing the diameter. However, in situations where small changes in the headwater or head loss through a system are critical, or where the pipe diameter is greater than 24 inches, the PEO shall use the Manning’s roughness coefficient specified in Table 4-1 to determine if a larger-diameter metal pipe alternative is required.
8-3.2.2 Annular Corrugations
Metal pipe can be produced with annular corrugations, where the corrugations are perpendicular to the flow line of the pipe. The seams for this type of pipe are both circumferential and longitudinal and are joined by rivets. The Manning’s roughness coefficient for all annularly corrugated metal pipes is specified in Table 4-1. The fill heights shown in Section 8-12 apply to both helical and annular corrugated metal pipe.
The typical corrugation section shown in Figure 8-3 is the same for annular corrugations, except that annular corrugations are available only in 2⅔-inch by ½-inch and 3-inch by 1-inch sizes.
8-3.2.3 Spiral Rib
Spiral rib pipe uses the same manufacturing process as helically wound pipe but, instead of using a standard corrugation pitch and depth, spiral rib pipe comprises rectangular ribs between flat wall areas. A typical spiral rib section is shown in Figure 8-4. Two profile configurations are available: ¾-inch width by ¾-inch depth by 7½-inch pitch or 1-inch by 1-inch by 11-inch. The seams for spiral rib pipe are either ungasketed-lock seams for non-pressure-testable applications or gasketed-lock seam for pressure-testable applications. If ungasketed lock seam pipe is used in storm sewer applications, it is generally necessary to coat the pipe with protective Treatment 1 (Section 8-5.3.1) for the pipe to pass the pressure-testing requirements.
The primary advantage of spiral rib pipe is that the rectangular rib configuration provides a hydraulically smooth pipe surface for all diameters, with a Manning’s roughness coefficient specified in Table 4-1.
8-3.2.4 Galvanized Steel
Galvanized steel consists of corrugated or spiral rib steel pipe with 1 oz/ft2 of galvanized coating on each surface of the pipe. Plain galvanized steel pipe is the least durable pipe from a corrosion standpoint and is not permitted when the pH is less than 5.0 or greater than 8.5 or if the soil resistivity is less than 1,000 ohm-cm. Galvanized steel pipe will, however, meet the required 50-year life expectancy for culvert and storm sewers installed in Corrosion Zone I, as described in Section 8-4. In more corrosive environments, such as Corrosion Zone II or III described in Section 8-4, galvanized-steel pipe must be treated with a protective coating for the pipe to attain the required 50-year service life.
8-3.2.5 Aluminized Steel
Aluminized steel consists of corrugated or spiral rib steel pipe with an aluminum protective coating applied both inside and out. The aluminized coating is more resistant to corrosion than galvanized-steel pipe and is considered to meet the 50-year life expectancy in both Corrosion Zones I and II without the use of protective coatings. Aluminized steel is not permitted when the pH is less than 5.0 or greater than 8.5 or if the soil resistivity is less than 1,000 ohm-cm.
8-3.2.6 Aluminum Alloy
Aluminum alloy (aluminum) consists of corrugated or spiral rib pipe and has been shown to be more resistant to corrosion than either galvanized or aluminized steel. When aluminum is exposed to water and air, an oxide layer forms on the metal surface, creating a barrier between the corrosive environment and the pipe surface. As long as
this barrier is allowed to form, and is not disturbed once it forms, aluminum pipe will function well.
Aluminum meets the 50-year life expectancy for both Corrosion Zones I and II. It can also be used in Corrosion Zone III, provided that the pH is between 4 and 9; the resistivity is 500 ohm-cm or greater; and the pipe is backfilled with clean, well-draining, granular material. The backfill specified in Section 8-10.4 will meet this requirement.
Aluminum shall not be used when backfill material has a high clay content, because the backfill material can prevent oxygen from getting to the pipe surface and consequently, the protective oxide layer will not form. For the same reason, aluminum pipe generally shall not be coated with the protective treatments discussed in Section 8-5.3.1.
8-3.2.7 Ductile-Iron Pipe
Ductile-iron pipe is an extremely strong, durable pipe designed primarily for use in high-pressure water distribution and sanitary sewer systems. Ductile-iron pipe is acceptable for culvert and storm sewers use; it is more expensive but is useful for shallow cover and deep installations. Ductile-iron pipe is acceptable with as little as 0.5 foot of cover in most installations. Deep fill heights are available from manufacturers and concurrence with the State Hydraulics Office. Joint systems for ductile-iron pipe include push-on, mechanical, or flanged. Depending on the type of joint, the pipe may be plain end, grooved, or flanged.
8-3.3 Thermoplastic Pipe: General
Thermoplastic is a term used to describe several types of pipes including corrugated polyethylene, solid-wall high-density polyethylene (HDPE), polypropylene (PP), and polyvinyl chloride (PVC). These pipes are allowed for use in drain, underdrain, culvert, storm sewer, and sanitary sewer applications, although not all types of thermoplastic pipe are allowed for use in all applications. The PEO must reference the appropriate section of the Standard Specifications to determine the allowable thermoplastic pipe for a given application.
Thermoplastic pipe is classified as a flexible pipe, which means that applied loads are resisted primarily by the strength of the bedding and backfill surrounding the pipe, with some additional support given by the pipe material itself. Because of the dependence upon the strength of the bedding and backfill material, it is critical that thermoplastic pipe be installed in accordance with the requirements of Section 8-10.4 to ensure proper performance.
The physical properties of thermoplastic pipe are such that the pipe is resistant to both pH and resistivity. As a result, thermoplastic pipe is an acceptable alternative in all three corrosion zones statewide, and no protective treatment is required. Laboratory testing indicates that the resistance of thermoplastic pipe to abrasive bed loads is equal to or greater than that of other types of pipe material. However, because thermoplastic pipe cannot be structurally reinforced, it shall not be used for severely abrasive conditions as described in Table 8-1.
Thermoplastic pipe is lightweight when compared to other pipe alternatives. This can simplify pipe handling because large equipment may not be necessary during installation. However, the light pipe weight can lead to soil or water flotation problems in the trench, requiring additional effort to secure the line and grade of the pipe. The allowable fill height and diameter range for thermoplastic pipe are somewhat limited. This may preclude thermoplastic pipe being specified for use in some situations.
Any exposed end of thermoplastic pipe used for culvert or storm sewer applications shall be mitered to match the surrounding embankment or ditch slope. The ends shall be mitered no flatter than 4H:1V, as a loss of structural integrity tends to occur after that point. It also becomes difficult to adequately secure the end of the pipe to the ground.
The minimum length of a section of mitered pipe shall be at least 6 times the diameter of the pipe, measured from the toe of the miter to the first joint under the fill slope. This distance into the fill slope will provide enough cover over the top of the pipe to counteract typical hydraulic uplift forces that may occur. For thermoplastic pipe 30 inches in diameter and larger, a Standard Plan B-75.20-03 headwall shall be used in conjunction with a mitered end.
8-3.3.1 Corrugated Polyethylene for Drains and Underdrains
Corrugated polyethylene used for drains and underdrains is a single-wall pipe, corrugated inside and outside. It is available in diameters up to 10 inches. This type of pipe is extremely flexible and can be manipulated easily on the job site should it become necessary to bypass obstructions during installation (see Chapter 3 for treating the exposed end for flotation.)
8-3.3.2 PVC Drain and Underdrain Pipe
PVC drain and underdrain pipe is a solid-wall pipe with a smooth interior and exterior. It is available in diameters up to 8 inches. This type of pipe is delivered to the job site in 20-foot lengths and has a significant amount of longitudinal beam strength. This characteristic is useful when placing the pipe at a continuous grade but can also make it more difficult to bypass obstructions during installation (see Chapter 3 for treating the exposed end for flotation).
8-3.3.3 Corrugated Polyethylene Culvert and Storm Sewer Pipe
Corrugated polyethylene used for culverts and storm sewers is double-walled, with a corrugated outer wall and a smooth interior. This type of pipe can be used under all state highways, subject to the fill height and diameter limits described in Section 8-12 and the Standard Specifications.
The primary difference between polyethylene used for culvert applications and polyethylene used for storm sewer applications is the type of joint specified. In culvert applications, the joint is not completely watertight and may allow an insignificant amount of infiltration. The culvert joint will prevent soils from migrating out of the pipe zone and is intended to be similar in performance to the coupling band and gasket required for metal pipe. If a culvert is to be installed where a combination of a high water table and fine-grained soils near the trench are expected, the joint used for storm sewer applications shall be specified. The storm sewer joint will eliminate the possibility of soil migration out of the pipe zone and will provide an improved connection between sections of pipe.
In storm sewer applications, all joints must be capable of passing WSDOT’s pressure test requirements. Because of this requirement, the allowable pipe diameter for storm sewer applications may possibly be less than the allowable diameter for culvert applications. The PEO shall consult WSDOT’s Qualified Products List for the current maximum allowable pipe diameter for both applications. Corrugated polyethylene is a petroleum-based product and may ignite under certain conditions. If maintenance practices such as ditch or field burning are anticipated near the inlet or outlet of a pipe, polyethylene shall not be allowed as a pipe alternative.
8-3.3.4 Solid-Wall PVC Culvert, Storm, and Sanitary Sewer Pipe
Solid-wall PVC culvert, storm, and sanitary sewer pipe is a solid-wall pipe with a smooth interior and exterior. This type of pipe can be used under all state highways, subject to the fill height and diameter limits described in Section 8-12 and the Standard Specifications. This type of pipe is used primarily in water line and sanitary sewer applications but may occasionally be used for culverts or storm sewers. The only joint available for this type of PVC pipe is a watertight joint conforming to the requirements of the Standard Specifications.
8-3.3.5 Profile-Wall PVC Culvert and Storm Sewer Pipe
Profile-wall PVC culvert and storm sewer pipe consists of pipe with an essentially smooth waterway wall braced circumferentially or spirally with projections or ribs, as shown in Figure 8-5. The pipe may have an open profile, where the ribs are exposed, or the pipe may have a closed profile, where the ribs are enclosed in an outer wall. This pipe can be used under all state highways, subject to the fill height and diameter limits described in Section 8-12 and the Standard Specifications. The only joint available for profile-wall PVC culvert and storm sewer pipe is a watertight joint conforming to the requirements of the Standard Specifications.
