Pipe Stiffness and Flattening Tests in Coilable HDPE Conduit; and Its Relationship to Burial Depth in Conduit Applications
|Publication Date:||1 May 2011|
Continuous length, coilable HDPE conduit has enjoyed tremendous growth not only as a protection for electrical cable, but in large part due to the huge expansion of the telecommunications industry in the US and abroad to protect fiber optic cable. Its availability in long lengths combined with HDPE's ductility, strength and durability make it ideally suitable for the "long haul" and trenchless installation technologies such as HDD (horizontal directional drilling) installations in the power and telecom industries. Buried cables installed in conduit provide long term and improved protection from damage caused by storms and vandalism when compared to aerial cables. Conduit also extends the life and reliability of underground cables by providing added protection from ground movement or poor soil conditions. These permanent raceways allow for easier and less costly future cable replacements.
HDPE conduit has been used for decades with great success due to its low coefficient of friction, resistance to corrosion, excellent chemical resistance and ability to remain ductile (flexible) even at low temperatures. Because the properties of HDPE result in a wide range of flexibility, HDPE conduit can be provided on coils or on steel reels in long lengths specifically for trenchless or plowing technologies (i.e. a reel of 1 ¼" diameter HDPE conduit could hold as much as an 8,500 ft continuous length). Larger diameter HDPE conduit such as 8" or greater is available in sticks up to 50-ft. An alternative plastic used in the conduit industry is PVC where typical lengths are limited to 20-ft.
When HDPE conduit is considered for underground installations, there have been cases where PS (pipe stiffness) and the term "Crush Strength" are used to determine strength relative to other materials regardless of their property differences. (In reality, "Crush Strength" is a term misapplied to thermoplastic, flexible pipes (i.e. HDPE and PVC). Crush Strength implies a brittle failure, where flexible pipes can deflect in excess of 20% with no signs of wall buckling, cracking or splitting. Flattening is the correct and more appropriate engineering term to measure how much deflection a flexible pipe can take without damage.) When HDPE is compared to PVC at similar diameters and wall thicknesses, PVC will inherently have a higher PS attributed to the modulus being more than twice that of standard HDPE conduit materials1. However, this in no way indicates how HDPE will ultimately perform compared to PVC conduit.
PS has applicability in calculations of burial depth but it should not be used to compare dissimilar materials. Also, too often, engineers consider "Crush Strength" or Flattening to determine allowable burial depth. Flattening does not relate to burial depth and should be considered a quality control test. Finally, the national specifications calculate PS inconsistently. This technical note provides the reader with detailed information showing that HDPE conduit's PS is more than adequate at typical burial depths. Furthermore, because of the inconsistencies in the national specifications, PS and Flattening should be considered only as quality control tests.