Page 24 - Buried Pipe Design
P. 24
2 Chapter One
Engineers and planners realize that the subsurface infrastructure is
an absolute necessity to the modern community. It is true we must
“build down” before we can “build up.” The underground water systems
serve as arteries to the cities, and the sewer systems serve as veins to
carry off the waste. The water system is the lifeblood of the city, pro-
viding culinary, irrigation, and fire protection needs. The average man
or woman on the street takes these systems for granted, being some-
what unaware of their existence unless they fail. In the United States
today, people demand water of high quality to be available, instanta-
neously, on demand. To ensure adequate quality, the distribution sys-
tems must be designed and constructed so as not to introduce
contaminants.
Sewage is collected at its source and carried via buried conduits to a
treatment facility. Treatment standards and controls are becoming con-
tinually more stringent, and treatment costs are high. Because of these
higher standards, the infiltration of groundwater or surface water into
sewer systems has become a major issue. In the past, sewer pipe join-
ing systems were not tight and permitted infiltration. Today, however,
tight rubber ring joints or cemented joints have become mandatory.
Even though septic tanks and cesspools are still widely used today,
they are no longer accepted in urban or suburban regions. Only in the
truly rural (farm) areas are they sanctioned by health departments.
Today, more sewer systems are being installed. This produces a
demand for quality piping systems. Thus, the need for water systems
that deliver quality water and for tight sanitary sewers has produced
a demand for high-quality piping materials and precisely designed sys-
tems that are properly installed.
Old and deteriorating conduits frequently fail. These failures can
cause substantial property damage that results in tremendous cost,
inconvenience, and loss of public goodwill. Utilities have programs to
replace or rejuvenate deteriorating pipes to minimize failures and
associated costs. In urban areas, trenching to remove the old and
install the new can be very difficult and extremely expensive. Relining
and microtunneling are viable options in certain situations where it is
difficult and extremely disruptive to construct using an open trench.
Soil Mechanics
Various parameters must be considered in the design of a buried pip-
ing system. However, no design should overlook pipe material proper-
ties or the characteristics of the soil envelope surrounding the pipe.
The word soil means different things to different people. To engineers,
soil is any earthen material excluding bedrock. The solid particles of
which soil is composed are products of both physical and chemical
action, sometimes called weathering of rock.