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Hydraulic Structures & Safety: Plastic Pipe Used in Embankment Dams

More hydropower projects are being designed and constructed using plastic pipe. But most dam designers have never been trained on the behavior of plastics and must make decisions on the use of plastic pipe by weighing the cost, operating requirements, dependability, and long-term performance.

By Chuck R. Cooper

Plastic pipe has been used for many decades in water and sewer applications. More recently, plastic pipe has been used in new embankment dam construction and in the renovation of existing conduits.

However, most of the available design information is targeted toward water distribution and sewer pipes and does not address the unique factors involved in using plastic pipe in embankment dams.

In general, information on plastic pipe is too dispersed for the best use of lessons learned from past performance. Due to the absence of any single recognized standard for plastic pipe used in embankment dams, there is significant inconsistency in the design and construction rationale.

Plastic pipe used in embankment dams usually has limited accessibility should something go wrong. Plastic pipe used in dams is often buried deeply, where access is nearly impossible due to the amount of overburden above it and a reservoir pool.

Failure of plastic pipe in water and sewer applications rarely results in loss of life. However, failure of plastic pipe in dams can have catastrophic consequences.

Removal and replacement can be difficult, time consuming, and costly. For these reasons, dam designers considering the use of plastic pipe must be cautious and select pipe that meets or exceeds conservative design criteria affecting watertightness, durability, structural performance, and design life.

Plastic pipe used in dams must be conservatively designed to provide for a long service life, strength to accommodate all loading conditions and foundation movements, and have adequate access for cleaning and inspection. For a discussion of the importance of good design and construction and the ramifications that can result if these are lacking, refer to the Federal Emergency Management Agency's (FEMA) Technical Manual: Conduits through Embankment Dams (2005) (FEMA-484).

Significant and high hazard potential dams require stringent and conservative design measures, because failure or misoperation could result in loss of human life or economic setbacks. Generally, this is not the case for low hazard potential dams.

While low hazard potential dams could certainly benefit from similar design measures, some of these may be considered overly conservative for this type of structure. Designers of low hazard potential dams need to carefully consider the requirements of their particular application.

While no standardized guidelines exist for the design of plastic pipe used for embankment conduits and drainpipes, numerous codes, standards, and recommended practices do exist that regulate and influence the plastic pipe industry.

These publications cover a wide range of product performance requirements, materials, manufacture, and test methods related to plastic pipe. ASTM International publishes standard specifications, practices, and test methods. Standard specifications define specific performance and product requirements, standard practices define how a particular activity is to be performed, and standard test methods define how a particular test is to be performed.

The American Water Works Association (AWWA) also publishes standards. ASTM International and AWWA are consensus standards and are voluntary. They only become mandatory when specified by some user or entity such as a government agency. As changes in plastic pipe are made and newer products, applications, or test methods are developed, the standards are revised accordingly.

Most dam designers have never had training on the behavior of plastics and must weigh decisions on the use of plastic pipe by considering the initial costs, operating requirements, maintenance costs, dependability, and long-term performance.

Dam safety officials have attempted to address the use of plastic pipe in their policies and regulations since the early 1990's. Their efforts have resulted in various design requirements, including reinforced concrete encasement, restrictions on the use of plastic pipe, and use restrictions based on dam hazard classification.

However, because of the many potential benefits, more projects are being designed and constructed using plastic pipe.

The manufacture of plastic pipe will continue to evolve, based on the requirements of the engineering community. Continued improvements in manufacturing processes will provide products with enhanced strength, durability, and efficiency.

FEMA, as the lead agency for the National Dam Safety Program, sponsored development of a technical manual in conjunction with the Association of State Dam Safety Officials, Bureau of Reclamation, Mine Safety and Health Administration, Natural Resources Conservation Service, U.S. Army Corps of Engineers, and the private sector. The Technical Manual: Plastic Pipe Used in Embankment Dams - Best Practices for Design, Construction, Problem Identification and Evaluation, Inspection, Maintenance, Renovation, and Repair (2007) (P-675) is now available. The information used in this article has been adapted from this manual.

Plastic pipe is lightweight, which facilitates installation.
Plastic pipe is lightweight, which facilitates installation. In addition, it is relatively inexpensive, readily available in many diameters, and can be quickly installed.

Using plastic pipe in embankment dams

Plastic pipe is lightweight, abrasion resistant, and inert to most forms of chemical attack.

This facilitates installation and benefits durability and service life.

Plastic pipe is often used in drain systems for collecting and measuring seepage and safely discharging it into a channel located downstream from the dam. Collecting and measuring seepage flows through and under embankment dams is an integral part of safe and reliable monitoring of performance.

Plastic pipe has been commonly used for embankment dam drainage systems (e.g., drainpipes) since the early 1970's. Drainage applications in dams are typically nonpressurized. Plastic pipe is used for drain construction, since it is relatively inexpensive, readily available in many diameters, can be manufactured with slots or perforations, and can be rapidly installed.

The use of plastic pipe for embankment conduit applications (e.g., outlet works and spillways) within traditional earthen dams is less common. Plastic pipe has been used in the construction and modification of embankment conduits since the early 1990's. Common uses have included sliplining of deteriorating outlet works conduits.

Plastic pipe is preferred for sliplining due to its ease of installation, ability to reestablish the watertightness of the conduit, and improved hydraulic performance.

These types of applications can either be pressurized or nonpressurized.

The mining industry has used plastic pipe in tailings dams and slurry impoundments since the mid-1980's for decant pipes, internal-drain collector pipes, and delivery pipes for slurry or tailings disposal.

What types of plastic pipe have been used?

Many types of plastic pipe are available, but not all types should be used in dams.

The formulations used for the production of plastic pipe can vary slightly from manufacturer to manufacturer. The designer must specify the type, grade, and class required for each plastic pipe application. Due to the numerous options available, selection of the proper plastic pipe can become a bewildering experience for the designer.

Fortunately, many standards, such as those from ASTM and AWWA, have been developed to ensure plastic pipe products have uniform characteristics, regardless of the manufacturer.

Plastic pipe used in dams are primarily two types: thermoplastic and thermoset plastic.

Thermoplastic

Thermoplastics and polyvinyl chloride (PVC) are plastics that can be repeatedly softened by heating and hardened by cooling without deterioration of their properties.

The extrusion process produces an inherently strong finished product. The extrusion process continuously forces molten polymer material through an angular die by a turning screw. The die shapes the molten material into a cylinder. The speed at which the molten material is drawn away from the extruder determines the wall thickness.

After a number of additional processes, such as cooling of the extruded pipe, the final product can be handled without distortion and can be cut into the specified pipe lengths. The process is typically used for solid wall pipe. Additional steps are required in the manufacturing process for adding corrugations or belling the ends of the pipe.

High-density polyethylene pipe

Two general classes of high-density polyethylene (HDPE) materials are commonly used to make pipe for dam applications. One material (ASTM F 714) is classified by ASTM D 3350 as having a hydrostatic design basis and is suitable for pressure applications. The other material (ASTM D 3035) is classified by ASTM D 3350, but is not pressure rated. This material is used to make corrugated pipe manufactured to American Association of State Highway and Transportation Officials (AASHTO) standards M252 and M294 respectively. Types of HDPE pipe used in dam construction include:

- Solid wall.-Solid wall pipe is made of a continuous wall of HDPE with uniform thickness. Solid wall pipe has smooth interior and exterior surfaces. Although solid wall pipe is pressure rated to meet the requirements as specified in ASTM F 714, dual-wall containment pipe should be used for pressurized embankment conduit applications in dams due to its added factor of safety. Solid wall pipe is available in diameters up to about 63 inches in typical lengths of 40 to 50 feet.

- Dual-wall containment.-Dual-wall containment pipe is made from two solid wall pipes. Dual-wall containment pipe consists of an inside pipe (carrier pipe) which is centered within an outer pipe (containment pipe). Dual-wall containment pipe should be used use in pressurized embankment conduits, since it affords the added protection of a second pipe. The annular space between the carrier and containment pipes allows for quick detection of leaks in the carrier pipe. The manufacturer can preassemble this type of pipe at the factory, or the pipe can be assembled at the job site using two solid wall pipes. The containment pipe and carrier pipe should be pressure rated to meet the requirements as specified in ASTM F 714. Dual-wall containment pipe is available in diameters up to about 54 inches for the carrier pipe and 63 inches for the containment pipe.

- Corrugated (single wall).-Single wall corrugated pipe has corrugated interior and exterior surfaces. Single wall corrugated pipe is distributed in coils. Single wall corrugated pipe is available in both perforated and nonperforated products. Perforations can be slots or circular holes. Single wall corrugated pipe is available in diameters up to about 24 inches. Due to deformations experienced, this pipe has limited use in dam construction.

- Corrugated (profile wall).-Profile wall corrugated pipe has a smooth interior surface and a corrugated exterior surface. The corrugations add ring stiffness to the pipe to assist in maintaining cross-sectional shape.

The smooth interior surface reduces friction and resistance to flow. Profile wall corrugated pipe economizes on the amount of material needed for fabrication; by altering the wall the same stiffness may be achieved with less material. Most designers prefer profile wall corrugated pipe over single wall pipe due to its higher wall strength and smoother interior. Profile wall corrugated pipe is available in both perforated and nonperforated products.

Perforations can be slots or holes.This pipe is supplied in standard 20-foot lengths. Profile wall corrugated pipe is available in diameters up to about 60 inches.

The most common method used to join solid wall pipe and dual wall containment pipe is by heat fusion (ASTM D 2657). Although a number of different fusion techniques exist, the butt fusion technique is the most widely used and industry-accepted method for joining sections of HDPE pipe.

Butt fusion is typically used to join pipes that have the same nominal outside diameter and wall thickness. Butt fusion is accomplished by heating two surfaces to a designated temperature, and fusing them together by application of sufficient force.

The application of force causes the melted materials to flow and mix together. As the joint cools, the molecules return to their crystalline form, the original joint interfaces are gone, and the two pipes have become one homogenous pipe.

If performed according to recommended procedures, the fused joint is watertight and as strong, or stronger, than the HDPE pipe itself in both tensile and compressive properties.

Butt fusion is performed at the site by an operator who has been trained by an experienced pipe distributor, fusion equipment manufacturer, or pipe manufacturer using a portable fusion machine.

Improper operation of the equipment can produce a poor fusion. Fusion machines are available for pipe sizes up to 63 inches in diameter. Manufacturers' recommended procedures should always be observed for butt fusion.

Other joining methods for HDPE pipe include:

- Joints made by extrusion welding.-Many prefabricated fittings can be joined to solid wall HDPE pipe with heat fusion (ASTM D 3261) in the field using an extrusion gun. Extrusion welding is a manual process utilizing a hand held extruder. Extrusion welding has also been successfully used for connecting HDPE grout and air vent pipes to plastic pipe slipliners. Extrusion welding cannot be used to repair damaged HDPE pipe and should not be used as a substitute for butt fusion.

- Mechanical joints.-Mechanical joints are used to join solid wall HDPE pipe and fittings to themselves or to other types of pipe materials. The most common mechanical joint is the flange adapter. Flanged connections are often used to connect HDPE pipe to steel pipe. Flanged connections allow for easy assembly and disassembly of the joint.

- Couplers.-Corrugated pipe is most often used in embankment dams for drainpipe applications, requiring nonrated and nonpressure joints. Manufacturers typically offer a variety of joints to meet specific project requirements. Corrugated pipe products are joined using the following methods: (1) single wall pipe using an external split or snap coupler and (2) profile wall pipe using an external split coupler, snap coupler, bell/bell gasketed coupler, or integral bell and spigot gasketed joint.

The corrugated metal pipe (CMP) in this outlet works conduit was deteriorating and was sliplined with high-density polyethylene (HDPE) pipe to restore watertightness. The small diameter pipes attached to the slipliner were used to pump grout into the annulus between the existing CMP and new HDPE pipe.

PVC Pipe

Pressure and nonpressure PVC pipe is available in solid wall, which has smooth interior and exterior surfaces. Solid wall PVC pipe is commonly available in 4- to 48-inch diameters in standard 20 foot lengths for pressure pipe. ASTM D 3034 nonpressure pipe is available in 14- or 20-foot lengths and ASTM F 679 nonpressure pipe is available in 14-foot lengths. Note that AWWA C900 and C905 are the only standards that specify a length.

The common joining system for PVC pipe is a bell and spigot flexible gasketed joint. Care must be taken to avoid over- or underinsertion of the spigot end into the bell end.

Common uses for PVC pipe have included drainpipes. Since embankment conduits in significant and high hazard potential dams require a high degree of conservatism, bell and spigot joints should not be used.

Bell and spigot joints are susceptible to separation as the embankment dam settles and are best suited only for low hazard applications.

Thermoset plastic

Thermosetting plastics refer to a variety of polymer materials that cure, through the addition of energy, to a stronger form. The energy may be in the form of heat or through a chemical reaction. The curing process transforms the resin into a plastic by cross-linking.

Thermoset plastic polymer molecules are cross-linked (chemically bonded) with another set of molecules to form a "net like" or "ladder-like" structure. Once cross-linking has occurred, a thermoset plastic does not soften, melt, or flow and will disintegrate when sufficient heat is added.

However, if the cross-linking occurs within a mold, the shape of the mold will be formed.

A thermoset material cannot be melted and remolded after it is cured. The thermoset class of materials includes unsaturated polyester, epoxy, and polyurethane.

Two sections of plastic pipe are joined
Two sections of plastic pipe are joined, using a process known as butt fusion.

The most commonly used thermoset plastic in dam applications has been cured-in-place pipe (CIPP).

CIPP liners have been used primarily for lining embankment conduits, as an alternative renovation method.

CIPP liners are constructed to be slightly smaller than the inner diameter of the existing pipe that is being renovated.

CIPP consists of a flexible polyester needle-felt or glass fiber/felt tube preimpregnated with resin. The preimpregnation process is usually done at the factory for quality control purposes.

Unsaturated polyester, vinyl ester, and epoxy resins are available, with unsaturated polyester being the most widely used.

These resins have a wide range of capability allowing CIPP to be designed for specific applications, unlike other types of plastic pipe, which have fixed properties.

The fabric tube carries and supports the resin until it is in the final position and cured. The fabric tube must withstand stresses from installation and stretch to expand against irregularities within the existing pipe.

On the inner surface of the CIPP liner is generally a coating or membrane of polyester, polyethylene, surlyn, or polyurethane, depending on the type of application. The membrane provides a low friction and hydraulically efficient inner surface to the CIPP liner.

A variety of installation methods are available, including using water or air pressure to invert the tube through the existing pipe or a winch to pull the tube through the existing pipe.

When pressure is applied for rounding out the tube, the saturated fabric stretches to conform to the inner surface of the existing pipe.

Although inversion is the preferred method of installation, winching may be pursued in situations where sufficient water pressure is unavailable or scaffold towers required for inversion are not practicable.

Combinations or variations of these methods are sometimes used.

Most often, hot water or steam is used to heat the resin and allow it to harden and cure after the liner has been formed within the existing pipe. When completed, the CIPP process forms at continuous tight-fitting, pipe-within-a-pipe containing no joints.

Many CIPP systems are available today.

The primary differences between these systems are in the composition and structure of the tube, method of resin impregnation, installation procedure, and curing process.

Common standards for specification and installation of CIPP are ASTM D 5813 and F 1216.

CIPP is applicable for lining existing conduits with diameters ranging from 4 to 132 inches.

At the larger diameters, the weight and cost of the materials become significant and the economics of the process may be adversely affected.

The design life of plastic pipe

Plastic pipe has many desirable characteristics.

Unlike metal and concrete pipe, which can deteriorate over time from galvanic or chemical corrosion, plastic pipe does not rust, rot, or corrode.

Aggressive soils do not affect plastic pipe, and it tolerates subzero temperatures well.

Much has been written regarding the projected design life for plastic pipe, but there is general agreement that 50 years is a conservative estimate.

The designer should consider all aspects of the project, installation conditions, end-use application, product specifications compliance, and established codes of practice when designing for a design life of more than 50 years.

If high quality materials are used in the manufacture of plastic pipe and installation is performed in compliance with established codes of practice, a design life exceeding 50 years may be possible.

Choosing the right embedment and encasement material

The embedment or encasement is the material immediately surrounding the pipe. The nature and placement of this material are critical to the structural performance.

For instance, a properly shaped reinforced cast-in-place concrete encasement is required in significant and high hazard potential dams to facilitate the compaction of earthfill against the conduit to minimize differential settlement and the potential development of internal erosion.

The type of embedment or encasement material dictates whether the plastic pipe is designed according to flexible or encased plastic pipe design procedures.

In a flexible plastic pipe design, the pipe deflects into the embedment material. As the pipe deflects, load is transferred to the material surrounding the pipe, which results in a shifting of load away from the pipe.

The embedment material should provide adequate strength, stiffness, uniformity of contact, and stability to minimize deformation of the pipe due to earth pressures.

An encased plastic pipe design is necessary if stress redistribution is limited by concrete or grout, which limits deflection.

Methods available for inspecting plastic pipe after installation

Periodic inspection of the condition of plastic pipe is essential in detecting problems and evaluating its long-term safety and reliability.

Periodic inspection may reveal trends that indicate more serious problems are developing.

However, plastic pipe used in embankment conduits and drainpipes is often not inspected as part of an overall inspection of the embankment dam and appurtenant features.

Generally, structural defects and deterioration develop progressively over time. A trained and experienced inspector can identify defects and potential problems before existing conditions in the dam and conduit become serious.

On occasion, situations can arise suddenly that cause serious damage in a short period of time.

The use of closed circuit television has rapidly become the most common method used for inspection of plastic pipe.

Closed circuit television is a method of inspection utilizing a television camera system and appropriate transport and lighting equipment to view the interior surface of the pipe.

New or renovated pipe installations should always be designed to accommodate CCTV inspection.


Technical Manual now Available

The Technical Manual: Plastic Pipe Used in Embankment Dams - Best Practices for Design, Construction, Problem Identification and Evaluation, Inspection, Maintenance, Renovation, and Repair (2007)(FEMA P-675) is now available.

Copies of the manual may be obtained free of charge by calling Federal Emergency Management Agency's (FEMA) Publications Warehouse at (1) (800) 480-2520.

The manual is available on DVD or can be downloaded from the FEMA website at www.fema.gov/plan/prevent/damfailure/publications.shtm. The manual contains over 100 illustrative figures, 24 case histories, and an in-depth glossary. The DVD has built-in Adobe Acrobat Reader Software, hyperlink, and search capabilities.

FEMA sponsored development of this technical manual in conjunction with the Association of State Dam Safety Officials, Bureau of Reclamation, Mine Safety and Health Administration, Natural Resources Conservation Service, U.S. Army Corps of Engineers, and the private sector.

The manual provides procedures and guidance for "best practices" associated with plastic pipe used in embankment dams.

Chuck Cooper, P.E., is a civil engineer for the Technical Service Center of the U.S. Bureau of Reclamation. He served as chairman of the national committee responsible for developing the technical manual, Plastic Pipe Used in Embankment Dams - Best Practices for Design, Construction, Problem Identification and Evaluation, Inspection, Maintenance, Renovation, and Repair.

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