Page 29 - Buried Pipe Design

Introduction and Overview 7

Wastewater Systems
A sewage system is made up of a collection system and a treatment
system. We are concerned only with the collection part. For the most
part, sanitary sewers and storm (street) sewers are separate. However,
there are a few older cities in North America which use combined
sewers. The ills of these combined sewers have been recognized by
modern engineers, and such systems are no longer designed. Most
state and regional engineering and public works officials and agencies
no longer permit installation of these dual-purpose lines.
Unfortunately, many combined sewers are used throughout the world,
and some still exist in the United States.
Some sanitary sewers are pressurized lines (sewer force mains), but
most are gravity flow lines. The sanitary sewer is usually buried quite
deep to allow for the pickup of water flow from basements. Due to this
added depth, higher soil pressures, which act on the pipe, are probable.
To resist these pressures, pipe strength and/or pipe stiffness become
important parameters in the design. Soil backfill and its placement and
compaction also become important to the design engineer. The installa-
tion may take place below the water table so construction procedures
may include dewatering and wide trenching. For such a system, the pipe
should be easy to join with a tight joint that will prevent infiltration.
The soil-pipe system should be designed and constructed to support the
soil load. The pipe material should be chemically inert with respect to
soil and sewage, including possible hydrogen sulfides. The inside wall
should be relatively smooth so as not to impede the fluid flow.
Storm sewer design conditions are not as rigorous as they are for
sanitary sewers. Storm sewers are normally not as deep. The require-
ments for the joining system are often very lax and usually allow exfil-
tration and infiltration. Because of the above, loose joining systems are
often acceptable for storm sewers. The design life for any sewer system
should also be 100 years minimum.

Design for Value
The piping system must be strong enough to withstand induced
stresses, have relatively smooth walls, have a tight joining system, and
be somewhat chemically inert with respect to soil and water. The piping
systems must be designed to perform for an extended period. The nor-
mal design life for such systems should be 50 years minimum. However,
50 years is not long enough. Government and private agencies cannot
afford to replace all the buried pipe infrastructures on a 50-year basis.
A 100-year design life should be considered minimum. Pipe manufac-
turers warrant their products to be free from manufacturing defects,
but cannot guarantee the pipe will perform for a given length of time.