Proper underground piping installation is one of the most misunderstood piping applications. Underground piping is expected to support not only the earth load, but the live (traffic) load above it as well, all while limiting deflection that can cause joint leaks.
Proper installation of plastic pipe increases the installed cost of the product dramatically. Defined as flexible systems, plastic piping is subject to deflection if not properly installed. The point of failure for plastic pipe is deflection in excess of just 5% of the inner diameter of the pipe, according to the ASTM D 2321 standard. This 5% limit means only a quarter inch in deflection in a four-inch pipe is defined as a failure.
Cast iron soil piping, defined as a rigid material, can handle these loads with no deflection.
With plastic installations, the sidefill stiffness of the trench is critical to support the pipe. For thermoplastic pipe, the trench is required to be the width of the pipe O.D. plus 16 inches or pipe O.D. times 1.25 plus 12 inches.
For example, a six-inch pipe would require a 20-inch wide trench – that’s a lot of extra digging on the front end and extra compaction on the backfill. Compaction in six-inch maximum layers – done by hand – is required by ASTM D2321. Depending on the soil type, minimum compaction density can range from 85% to 95%.
Because cast iron is up to 10 times stronger than thermoplastic materials, it does not need the compaction of the sidefill to support the pipe wall, so the trench can be as narrow as the installer wants.
Also, the trench for plastic pipe involves special bedding requirements – a minimum of four inches of material depending on the soil type to support the pipe. Cast iron requires no such bedding, only that the trench bottom be flat so that the pipe is uniformly supported. Again – a lot of extra work and materials when it comes to plastics and a lot of extra cost.