In most respects, installing corrugated high-density polyethylene (HDPE) pipe is like working with any other material: The pipe system's strength is a combination of the pipe itself and the backfill envelope. Proper construction maximizes the drainage capabilities designed into the pipe by maintaining alignment and load-carrying ability.
In a properly constructed backfill envelope, loads are distributed across the pipe's crown to the backfill material along the sides of the pipe and then to the pipe bedding and foundation. With flexible material like corrugated HDPE, however, this load arching effect tends to reduce the total soil column load applied to the crown.
The load that a flexible pipe will carry is related to the backfill envelope construction. The load-carrying capacity of a pipe/backfill system will be determined by a combination of the backfill material, the level of compaction, and the placement of the backfill material. The application may also influence what type of backfill is required.
Material selection is the first step to creating a structurally sound backfill envelope. In general, it should be of an aggregate nature and have the ability to be compacted, if necessary, into a structurally sound arrangement. A variety of materials, including some native soils, meet these requirements.
Backfill offers passive resistance to the pressure on the pipe by the soil surrounding it, termed the “modulus of soil reaction.” The modulus of soil reaction is determined by a combination of the material and the amount of compaction. The type of material (sand, gravel, clay, etc.) and compaction level (standard proctor density, which should be used at the discretion of the engineer) determine overall strength of the backfill. Some research indicates that other factors, such as trench walls that are at least as strong as the backfill material, may add to the conservancy of the backfill strength, although those relationships are often neglected.
The combination of material type and compaction level determines soil strength. When a variety of options will work in a particular installation, the final decision can depend on what's most available locally to keep cost to a minimum. Using native soil, for example, eliminates having to import material and the effort spent grading or hauling the excavated material offsite.
— Joe Babcanec (firstname.lastname@example.org) is application engineering manager for Advanced Drainage Systems Inc. of Hilliard, Ohio. For more information from the ADS Drainage Handbook, visit www.ads-pipe.com.
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How to avert settling and shifting
The best construction practices place corrugated HDPE pipe on a firm foundation for maximum performance and structural integrity throughout the system's design life.
It may be necessary to perform subsurface evaluations of the soil conditions where muck, rock, or other unsuitable conditions are suspected. Zones of soft material, such as muck, allow the pipe to settle, potentially affecting the system's structural integrity and hydraulic characteristics. Rock and rock protrusions apply point loads where they contact the pipe, which also can compromise hydraulics and structural integrity.
It's a good idea to excavate unsuitable foundation material before installation proceeds. If you find soft soil or other unsuitable material, ask the design engineer or a geotechnical engineer to determine the extent to which the undesirable material is to be excavated.
As this trench detail shows, once you've got a suitable foundation:
- Bedding. Place and compact at least 4 to 6 inches of stable and uniform bedding on the foundation and around protrusions like joints and fittings to equalize load distributions along the pipe invert.
- Under the pipe invert. The middle of the bedding should be equal to 1/3 of the pipe outside diameter (O.D.). Place it loosely and compact the rest to minimum 90% standard proctor density.
- The haunching. The most important backfill layer because it supports the pipe against the soil and traffic loadings. Place in lifts and compact in accordance with ASTM D2321.
Initial backfill. Extend from the spring line to 6 inches above the crown to anchor the pipe and ensure loads are distributed as evenly as possible into the haunching.
If your backfill material requires compaction, don't use mechanical compaction equipment directly on the pipe itself. ASTM D2321 explains how to place and compact the material in lifts until the initial backfill zone is completed.
Final backfill. Extending from the initial backfill layer to the ground surface, this layer doesn't directly support the pipe. Depending on the intended use at the surface, you may be able to use excavated material.
Selection, placement, and compaction shall be as directed by the design engineer.
When placing, consider compaction equipment and construction loads operating over the top of the pipe.
Although proper compaction isn't nearly as critical for the pipe as the other layers, you'll need a relatively high degree to prevent pavement from settling if traffic will be crossing above the pipe. PW