TEXTILES.ORG
Identification, history, testing, and usage.
By George R. Koerner
Perhaps the original geosynthetic material is buried plastic pipe, which is called “geopipe” in this article. Geopipe is being used in many aspects of geotechnical, transportation and environmental engineering. It sees uses in water and gas distribution, sewer and wastewater, oil and gas production, industrial and mining uses, power and communications, duct and irrigation systems.
Geopipe is strongly represented in all of these applications by the Plastics Pipe Institute Inc. (PPI). Founded in 1950, it is the major trade association representing the plastics piping industry. PPI currently categorizes plastic pipe in the following 7 material (resin) types.
- Polyvinyl Chloride (PVC)
- Chlorinated Polyvinyl Chloride (CPVC)
- Polyethylene (PE)
- Crosslinked Polyethylene (PEX)
- Polyacetal (Polyoxymethylene – POM)
- Polyvinylidene Fluoride (PVDF)
- Polyamide (PA)
These 7 resin types have 13 designation codes with 6 different grades associated with them. This makes a huge matrix of plastic pipe to choose from. In the geosynthetics area, we deal almost exclusively with PVC and HDPE pipe—both smooth-wall transmission pipe and corrugated drainage pipe.
Polyvinyl chloride was accidentally discovered on at least two different occasions in the 19th century, first in 1835 by Henri Victor Regnault and in 1872 by Eugen Baumann. On both occasions, the polymer appeared as a white solid on the bottom of flasks of vinyl chloride that had been left exposed to sunlight. In the early 20th century, the Russian chemist Ivan Ostromislensky and Fritz Klatte of the German chemical company Griesheim-Elektron both attempted to use PVC in commercial products, but difficulties in processing the rigid, sometimes brittle polymer hindered their efforts. In 1926, Waldo Semon of B. F. Goodrich developed a method to plasticize PVC by blending it with various additives. The result was a more flexible and more easily processed material that soon achieved widespread commercial use.
Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while heating diazonmethane. Eric Fawcett and Reginald Gibson of ICI Chemicals in the United Kingdom discovered the first practical polyethylene synthetic in 1933. Upon applying extremely high pressure to a mixture of ethylene and benzaldehyde, they also produced a white waxy material. Since the reaction had been initiated by trace oxygen contamination in their apparatus, the experiment was at first difficult to produce.
It was not until 1935 that another ICI chemist, Michael Perrin, developed this accidental process into a reproducible high-pressure synthesis for polyethylene. Subsequent landmarks in polyethylene synthesis have centered on the development of several types of catalysts that promote ethylene polymerization at more mild temperatures and pressures. The first of these was a chromium trioxide based catalyst discovered in 1951 by Robert Banks and John Hogan at Phillips Petroleum. In 1953, the German chemist Karl Ziegler developed a catalytic system based on titanium halides and organoaluminum compounds that worked at even milder conditions than the Phillips catalyst. For this he received the Nobel Prize.
To state that the sales of geopipe are brisk is a gross understatement. PPI data indicates that North American sales of HDPE pipe exceed $1B in 2005. PVC sales were near this mark if one combines both pressure and drainage applications.
PVC pipe specifications are covered under ASTM F794, ASTM D3034, ASTM F794, ASTM F679 and AASHTO M304 depending on the application. HDPE pipe is covered under AASHTO M252, AASHTO M294, ASTM F405 and ASTM F667. It is interesting that ASTM leads PVC specification development while AASHTO leads HDPE. These specification are application oriented and generally are formed around a master table. It identifies the property, method, units, value, and frequency. Key pipe properties are listed below.
- Dimensions
- Mass
- Density
- Carbon Black Content
- Melt Flow Index (MFI)
- Tensile
- Compression
- Burst
- Impact
- Stress Crack Resistance (called ESCR)
- Abrasion resistance
- Creep
- OIT by DSC
Design of the pipe is the responsibility of the engineering design community. Topics of main consideration include the following:
- Hydraulic Issues:
- Hazen-Williams formula with Manning Roughness coefficients
- Collection and removal equations; McEnroe or Giroud methods
- Distribution within pipes; CTI model
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- Deflection analyses:
- Burial
- Installation
- Service
- Buckling considerations
- Wall crush considerations
- Thermal effects with respect to time
- Connection/Perforation/Joining/Manhole and Sump details
An issue that cannot be overemphasized when using geopipe is its placement and backfilling. Generally trenches are excavated and sides will be stable under all working conditions. Trench walls are sloped or supported for the walls in conformance with all local and national standards for safety, e.g., OSHA Regulations.
Pipe should not be laid or embedded in standing or running water. Surface water should be presented from entering the trench and the trench should be dewatered when necessary to maintain stability. Trench bottoms containing rock, soft areas of muck or other material needs to be supported. Also, one must provide a width sufficient, but no greater than necessary, to ensure working room to properly and safety place and compact haunching and other embedment materials. If mechanical compaction of the fill material is required, the space between the pipe and the trench wall must be wider than the compaction equipment used in the pipe zone.
Supports may be used to maintain the trench sidewalls throughout the installation; the supports should be tight enough to prevent washing out of the trench walls from behind the supports. Unless specified by the engineer, supports should be left in place as long as support is necessary in that vicinity. When removing trench supports the pipe and the embedment material should not be disturbed, this also holds true for movable trench supports.
A layer of granular bedding soil will create a consistent surface free of unstable or unsuitable materials. The trench should be excavated to allow for a minimum of four inches of bedding layer, unless otherwise specified. It may be necessary to excavate additional trench to provide more bedding in situations where unstable or deformable trench bottoms are encountered. The bedding shall be done with Class I uncompacted materials or Class II materials compacted to 85 Proctor Density. Materials and description appropriate for bedding may be found in the ASTM D3774 and are either sands, gravel, or their mixtures.
A layer of granular bedding soil will create a consistent surface free of unstable or unsuitable materials. The trench should be excavated to allow for a minimum of four inches of bedding layer, unless otherwise specified. It may be necessary to excavate additional trench to provide more bedding in situations where unstable or deformable trench bottoms are encountered. The bedding shall be done with Class I uncompacted materials or Class II materials compacted to 85 Proctor Density. Materials and description appropriate for bedding may be found in the ASTM D3774 and are either sands, gravel, or their mixtures.
Backfilling the haunches (haunching) of the pipe is the most important part of the installation process since this soil provides the primary support for the pipe under all loading conditions. Haunching should be placed in maximum six-inch layers uniformly on both sides of the pipe. The soil must be tamped or rammed to achieve the specified compaction. Construction of each layer should be repeated up to the springline. The haunching may be done with Class I uncompacted materials or Class II materials compacted to 80% proctor density.
Backfilling the haunches (haunching) of the pipe is the most important part of the installation process since this soil provides the primary support for the pipe under all loading conditions. Haunching should be placed in maximum six-inch layers uniformly on both sides of the pipe. The soil must be tamped or rammed to achieve the specified compaction. Construction of each layer should be repeated up to the springline. The haunching may be done with Class I uncompacted materials or Class II materials compacted to 80% proctor density.
The final backfill extends from the initial backfill to the ground surface. It does not support the pipe, but plays an important role in allowing the load to be distributed over the pipe. Compaction of this area is only critical if live loads such as a roadway or equipment movement will be over the pipe trench so as to prevent pavement settlement. The final backfill can be done with Class I material and Class II material compacted to the design engineer’s specifications. The native material excavated as well as Classes III or IV backfill materials will need to be reviewed by the design engineer before they may be used as a final backfill material.
The minimum coverage in trafficked areas is one foot above the crown. Pavement layers may sometimes be included as part of minimum cover. Under flexible pavements the paving equipment and the amount of cover over the pipe must be considered in determining if the pipe can support the resultant load. Minimum cover calculations for flexible pavement are measured from the top of the pipe to the bottom of the pavement section. Minimum cover calculated for rigid pavements is measured from the top of the pipe to the top of the pavement section.
Summary
As industrial, domestic, and agricultural development increases, plastic pipe is used routinely in systems with the goal of reliable and sustainable development.
PVC and HDPE plastic pipe (both transmission and drainage) provides excellent materials used to design engineering solutions for a variety of applications. These products can provide high flow capability to convey surge water release in applications with little or no grade. Plastic pipe can improve water flow rates making downsizing possible in certain situations. The anti-adhesive, non-polar, slick nature of plastic, minimizes solid waste build-up, reduces abrasion, and can reduce cleaning and maintenance costs.
Going forward, the entire geosynthetics community will all be increasingly involved with designing, testing, and specifying large amounts of plastic pipe. It has a very bright future ahead.