|High-density polyethylene (HDPE) is a versatile material and has some ideal characteristics for use in underground structures. HDPE pipe is relatively lightweight allowing for easier and less costly transportation and installation costs. It is not brittle and therefore not susceptible to cracking during pipe handling and installation activities. Once formed into a pipe, HDPE has a smooth surface, which is resistant to abrasion, corrosion and chemical scouring. The smooth surface provides excellent pipeline flow characteristics. HDPE pipe is structurally strong and has the ability to support large loads, and has the ability to relax under stress. This characteristic provides advantages for underground structures and also helps define limitations of use. As HDPE pipe is loaded, the pipe relaxes immediately, and over time, allows the load to be transferred to the adjacent soil. This characteristic allows the pipe to off-load points of local stress. Stress relaxation may result in slight pipe reformation over time to accommodate in place loading conditions. Such re-formations are believed to cause long-term structural
Corrugated HDPE is an excellent choice for gravity flow or low-head pipeline situations. Although HDPE pipe products are versatile, the primary use of corrugated HDPE is for gravity flow water management. Examples of these water management systems include storm drainage, subsurface drainage, sanitary sewers, leachate collection and detention/retention stormwater management systems. The structural stability of corrugated HDPE pipe is produced by three pipe designs. According to AASHTO M294, they are defined as:
� Type C - This pipe shall have a full circular cross section, with an annular corrugated surface both inside and outside.
� Type S - This pipe is a full circular dual-wall cross section, with an outer corrugated pipe wall and a smooth inner liner.
� Type D - This pipe is a circular cross section consisting of an essentially smooth inner wall joined to an essentially smooth outer wall with annular or spiral connecting elements.
Typically, Type C pipe with interior and exterior corrugated walls is available in 3-inch through 24-inch diameters. Interior and exterior corrugated pipe in smaller diameters are connected with separate snap-on connections with no gasket. In larger diameters, the connections are made with corrugated bands secured with plastic ties.
The specific design of the pipe varies by manufacturer. Each section is associated with specific structural properties and performance characteristics. Those characteristics are available from the manufacturer for use in load calculations. Pipe manufacturers provide various pipe joining methods depending on the pipe style and project requirements. Coupling bands, with or without a gasket, wrap around the pipe and are secured with plastic ties. Gasket bell and spigot joints are also widely used. Non-rated and non-pressure tested watertight joints are suitable for the majority of non-pressure (gravity flow) drainage applications and typically do not experience significant leakage. For environmental and other reasons, most manufacturers also have a pressure-rated watertight joint suited for non-pressure applications. Joints are rated at either 10.8 psi or 5.0 psi when tested in accordance with ASTM D 3212.
HDPE pipes find application in:
Storm Drainage: Corrugated HPDE is a durable and cost-effective pipe material for these on-site drainage facilities.
Subsurface Drainage: Subdrainage systems are used to collect leachate under landfill sites. Subdrainage systems also are used to control and direct underground water transport and to encourage proper surface water percolation in golf courses, athletic fields, hillside development projects and in agricultural fields. Corrugated HDPE pipe can also be produced with perforations. The perforations allow subsurface water to be collected and transplanted to favorable locations for discharge. Often, subdrainage systems are used to lower the groundwater table. For athletic field development, subdrainage systems have been connected to air vacuum systems to encourage the downward movement of surface and subsurface water. Perforated corrugated HDPE pipe is often used to control water levels in agricultural land. Perforated pipe is installed to collect and transport subsurface drainage and/or groundwater or to control the depth to groundwater.
Sanitary Sewers: HDPE pipe is an ideal septic system leach pipeline material. One type of HDPE pipe has been specifically designed with special perforations to allow percolation.
Leachate Collection: The mining industry has a special application of subdrainage that is ideal for corrugated HDPE perforated pipe. A technique called heap leaching is used to recover low-grade deposits of copper, gold and silver. A cyanide solution sprayed over soil containing gold or silver converts the minerals to a chemical compound. The solution is collected in a perforated pipe subdrainage system and transported to ponds. The gold or silver is recovered from the ponds using carbon absorption or precipitation. HDPE is well-suited to this process because it is highly resistant to chemical attack. Tests have shown little or no degradation of HDPE with long-term exposure to a pH range from 1.5 to 14.0.
Detention/Retention Storm Water Management Systems: Current regulations in most areas limit the rate of storm water runoff as well as the level of pollutants allowed in discharged storm water. Urbanization of land can dramatically alter the natural movement of water. When runoff is transported away from critical areas, it can cause problems where recharge of aquifiers is necessary to maintain a steady groundwater supply. To counter these problems, storm water retention systems hold runoff until the surrounding soil can accept it via percolation, allowing aquifiers to be recharged. In other cases, the existing storm drainage trunk system is not designed to accept increased peak flow and the runoff must be retained until the peak flow has subsided. Storm water retention and detention systems can be either above-ground ponds or subsurface piping. Ponds are the least prone to early siltation and clogging, but could present child safety and long-term aesthetic problems such as insect breeding, weed growth, odor and refuse control. Subsurface retention/detention systems use available land efficiently at a low maintenance cost, while posing little or no public safety or aesthetic problems. Underground storage facilities developed by placing several pipes in series are a common use of corrugated HDPE pipe.
Corrugated polyethylene pipe is used in a wide variety of other applications, several of which are described below.
Roof Leader and Landscape Area Drainage: Residential, industrial and commercial buildings all have demand for roof leader and landscape area drainage facilities. Small diameter corrugated interior and exterior HDPE is the most commonly used product available for these types of uses. The combination of flexibility, durability and strength is not offered by other materials.
Perforated corrugated HDPE pipe also has become the product of choice for ventilation systems. Pipe placed in the bottom of grain storage bins introduce air via blowers to evaporate moisture from the grain piles. Another application utilizes perforated HDPE pipe to collect air from the discharge of an air scrubber for disbursement under a filter media to remove contaminant particles.
Earth Cooling Tubes: Are a viable method for space cooling, and are being used as an alternative to conventional air conditioning. In these systems, warm air is moved through the cool earth via tubes. The air is then used to achieve a cooling effect. Corrugated polyethylene tubing is particularly suited for this application because the corrugations provide a greater surface area for the heat transfer process to take place.
Floating Systems: HDPE is resistant to corrosion and chemical attack. Those properties, along with its relative light weight, have allowed it to be used as a holding vessel for floats. Polystyrene-filled corrugated HDPE pipe is used as floats in various applications. A common use of these floats is as pontoons for floating boat docks. Various dock materials are easily attached to the corrugated HDPE pontoons of any length to form the appropriately shaped floating dock. Similar floats also have been used to provide the support for polyurethane covers of liquid waste and chemical storage ponds.
Subsurface Irrigation and Drainage: Relining of Failed Pipes with Corrugated HDPE- Corrugated HDPE pipe can be used as a structural insert inside failing culverts, storm drains or sewers made of corrugated metal or concrete. The HDPE pipe becomes the load-bearing structure after the annulus is filled with grout. Corrugated HDPE pipe with a smooth interior must be inserted from a pit, or at the end of the existing culvert. The inserted HDPE pipe will reduce the original inlet area. If the reduction is too drastic, a short, specially designed HDPE taper may be attached to the inlet end to increase the inlet area.
Aeration in Sewer Sludge Composting: Perforated corrugated polyethylene pipe is an integral component when composting sewer sludge. The perforations allow controlled aeration of the sludge. Many communities have found that they can compost sewer sludge and market the finished compost.
(Source Courtsey: The Plastics Pipe Institute Inc. (PPI))