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 They do not interfere with aboveground traffic.
T h e y are less unsightly.
T h e piping is protected from freezing when buried below the frost
line.
3 . 2 S o i l C h a r a c t e r i s t i c s . T h e following soil characteristics determine the
type of conduit system used.
T h i s characteristic is influenced by the elevation
Permeability.
of the water table in the ground. See NAVFAC DM-3, Design Manual:
Mechanical Engineering; and NAVFAC DM-7.2, Design Manual, Foundations
a n d Earth Structures for field permeability tests.
S o i l C o r r o s i v i t y . When metal casings are used as underground conduits, the
S o i l properties such
c o r r o s i v i t y of the soil is an important consideration.
a s r e s i s t i v i t y , p H , a e r a t i o n , a n d the presence of certain soil bacteria can
i n f l u e n c e t h e c o r r o s i v i t y o f t h e s o i l . Other factors such as the presence of
s t r a y electrical currents in the ground can affect the corrosion of buried
structures.
The requirement for cathodic protection should be based upon past
experience in the specific location being considered if possible. If
underground corrosion
h a s been a problem, then either relocate the
s y s t e m to a less corrosive site, use an above ground system, or use cathodic
p r o t e c t i o n to control corrosion.
I f there has been insufficient experience in
t h e specific location being considered, then soil corrosivity as determined
through a corrosion survey of the site can be used to make a preliminary
a s s e s s m e n t of soil corrosivity. S o i l resistivity can be used as a general
g u i d e for determining the need for cathodic protection. Where soil
r e s i s t i v i t y is less than 10,000 ohms per cubic centimeter, cathodic protection
w i l l probably be necessary to prevent premature system failure. D e t e c t i o n of
s t r a y electrical currents in the ground should be determined through a
c o r r o s i o n survey of the site. P r o c e d u r e s for performing a corrosion survey of
the site are given in NAVFAC DM-4, Design Manual, Electrical Engineering.
S o i l S t a b i l i t y . NAVFAC DM-7.2, Design Manual, Foundations and
Earth Structures, provides criteria for determining soil stability.
3 . 3 F a c t o r s A f f e c t i n g I n s t a l l a t i o n . S e v e r a l factors may affect an
u n d e r g r o u n d installation, i n c l u d i n g t h e f o l l o w i n g s i t u a t i o n s :
P o s s i b i l i t y of surface runoff seeping into a backfilled trench and
p e r c o l a t i n g down at a greater rate than the draining capability of
the ground below the system
Areas where pending may occur
Permeability of the ground below the system
3-6
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