Dr. Ed McCoy
Ohio State University
There are several reasons why a clay tile or perforated pipe drainage system may fail. These reasons include 1) sedimentation within the pipe itself, 2) filter-caking of a geotextile envelope, 3) sealing of drains with ochre, and 4) crushing or flattening of the drain. This article offers an explanation for each of these causes along with reasons why they should not occur in a PCD system.
The ability of a drainage pipe to convey flow may be compromised by fines entering into and settling within the drain pipe. While some soils have a greater tendency toward sedimentation than others, this problem often occurs immediately after construction when the backfill material is still loose and any pre-existing soil structure has been destroyed. Consequently, the force of water flowing to the pipe can dislodge individual particles and carry them into the pipe. The force of water flow depends on its velocity so that particle displacement occurs where velocities are greatest. Such is the case adjacent to deeply buried and widely spaced pipes that create large hydraulic gradients and individually collect large volumes of water.
The protocol for installing a PCD system does not involve soil excavation. Rather, these drainage elements are pulled into the ground resulting in minimal soil disruption. Further, these elements are placed relatively shallow and close together so that hydraulic gradients and flow velocities are modest, and individual elements are only required to collect small water volumes for adequate drainage. Thus, because of the installation method and the modest flow velocities, displacement of fines that may clog PCD is minimized.
Filter-caking of Geotextile Envelopes
To avoid sedimentation within drainage pipe – particularly when the drains are placed in sand – the pipe may be encased in a geotextile fabric. As with the sedimentation problem, when flow to the pipe is sufficiently large and/or the soil has been disturbed during installation; fines can be carried to and entrapped in the fabric envelope. If a thick layer of these fines builds up, then flow to the pipe becomes limited.
The reasons why filter-caking is not anticipated for a PCD system is the same as stated earlier. By not releasing fines during installation, and by designing for modest flow velocities, fines should not be conveyed to PCD elements in the first place.
Sealing of Drains with Ochre
Reduced iron in the soil and groundwater can oxidizes upon entering a drain pipe resulting in an iron hydroxide (ochre) sludge that gradually clogs the pipe. Ochre can also plug pipe perforations or seal geotextile wraps. The key to ochre formation is reducing (i.e. anaerobic) condition surrounding the pipe in conjunction with oxidizing (i.e. aerobic) conditions inside the pipe. And these conditions form when the soil adjacent to the pipe remains saturated for extended periods after flow has ceased. But how can there be saturated soil adjacent to a drainage pipe after flow has ceased? Well, the drastic size difference between soil pores and the pipe openings create a capillary break. And this capillary break results in a layer of saturated soil around the pipe; just as a capillary break results in perched water within a USGA putting green. So creation of a capillary break by placing a pipe in the soil leads to anaerobic and aerobic conditions in close proximity and the chance that ochre will form. Of course, iron is also needed for ochre to form; otherwise this problem would be much more prevalent.
This problem should not happen in a PCD system because extended periods of soil saturation directly adjacent to this drain should not occur. The pores within the capillary portion of a PCD element are similarly sized in comparison to many root zones so a capillary break is not present. Also, the hanging-water-column suction of a PCD system serves to drain pores that would otherwise be water filled. Thus the soil water conditions leading to ochre formation in pipes is absent with a PCD system.
Crushing or Flattening of the Drain
Clay tiles when exposed to excessive loads can crush due to brittle failure. Plastic pipe when exposed to excessive loads will bend and become flattened. The result in either case is a failure of the drainage system. Consequently there are standards that specify the minimum depth of pipe placement and the degree of cover firmness to avoid crushing or flattening of drainage pipes. Generally speaking, the standard for placement depth specifies that the thickness of the cover is proportional to the internal diameter of the pipe. Thus, deeper depths are required for larger diameter pipes and shallower depth may serve adequately for small diameter pipes. Considering the small, approximately 3/8-inch internal diameter of a PCD element, the typical placement depth of 9 to 10 inches should be sufficient to prevent the element from being flattened.
The firmness of the cover is also a consideration for protecting pipes from crushing or flattening. If the cover is loose and weak, then traffic can easily compact the cover, reducing its thickness and exposing the pipe to failure loads. Thus, in a conventional trenching and backfilling operation, it is recommended that the backfill material be compacted to gain strength. In a typical PCD installation there is no trenching so that firmness of the cover would not be an issue.