Page 30 - Buried Pipe Design

8 Chapter One

This is because the life of the pipe, after it is installed, is not just a
function of the pipe material, but is largely a function of the loading
conditions and the environment to which it is subjected. It is the
design engineer’s responsibility to assess all factors and formulate a
design with a predicted design life. The cost of the system should be
based on life considerations, not just initial cost.
Most piping system contracts are awarded to the lowest bidder.
Contractors will usually bid materials and construction methods
which allow for the lower initial cost with little thought to future
maintenance or life of the system. Even for the owner, the lowest ini-
tial cost is often the overriding factor. However, the owner and the
engineer should insist on a design based on value. For engineers, eco-
nomics is always an important consideration; any economic evaluation
must include more than just initial cost. Annual maintenance and life
of the system must also be considered.
Initial cost may include such things as piping materials, trenching,
select backfill, compaction, site improvements and restoration, and
engineering and inspection. Pipe cost is related to pipe material and to
pipe diameter. Diameter is controlled by the design flow rate and pipe
roughness. That is, a smaller diameter may be possible if a pipe with
a smooth interior wall is selected. Annual maintenance cost includes
cleaning, repair, and replacement due to erosion, corrosion, and so
forth. Life is directly related to durability and is affected by such
things as severe loading conditions, corrosion, erosion, and other types
of environmental degradation. It is important to design the installa-
tion to minimize detrimental effects.
The question is not whether the pipe will last, but how long it will per-
form its designed function. Generally, metals corrode in wet clayey soils
and corrode at an accelerated rate in the presence of hydrogen sulfide
sewer gas. Concrete-type structures are also attacked by hydrogen sul-
fide and the resulting sulfuric acid. Care should be taken when select-
ing a pipe product for any service application and installation conditions
to ensure that environmental effects upon the life of the system have
been taken into consideration. The system should be designed for value.

References
1. American Society for Testing and Materials. Standard Practice for Underground
Installation of Flexible Thermoplastic Sewer Pipe, D 2321. Pennsylvania,
Philadelphia: ASTM.
2. The Asphalt Institute. 1997. Soils Manual MS-10 (reprinted). 5d ed. Lexington,
Kentucky: Asphalt Institute.