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20 BULLETIN 155, U. S. DEPARTMENT OF AGRICULTUHE. 
vent decay, some engineers advocate 50 feet as the minimum so far 
as it is possible to secure such location, while others place it at 25 
feet. With reference to the relation of pressure to durability of the 
wood, much may depend on other conditions of the location, par- 
Penlariy as to whether or not the pipe is placed 1 in contact with the 
soil. If the pipe is placed in the ground or in contact with the soil, 
a pressure head of 50 feet or more is preferable to anything less, 
but if it is kept free from contact with the soil, 15 feet below the 
hydraulic gradient is as good as 50. By locating the pipe close to 
the hydraulic gradient fewer bands are required, but nothing is 
saved in keeping the pressure lower than 20 feet of head. 
Evidence based on the experience of the past 20 years appears to 
be sufficient to show that, in general, continuous stave pipe lines 
should be located above ground and free from all contact with it, 
though opinions diametrically opposite with reference to this point 
have prevailed and still prevail. By those who favor locating pipes in 
the ground, it is argued that they are thus better protected pani injury 
from fire, freezing, falling rocks, falling trees, landslides, etc., and 
that the life of the wood will be prolonged. In answer to which it may 
be claimed that a pipe line properly patrolled and maintained is seldom 
in serious danger from fire; the velocities as a rule are a sufficient 
safeguard against freezing in most places where such pipes are used, 
though wood pipes, even if frozen, may be easily repaired; in a region 
so rough that danger from landslides or falling rocks is a matter for 
consideration, the cost of excavating a trench is usually very great 
and material suitable for backfilling difficult or impossible to obtain, 
so that other means of protecting the pipe from such injury may be 
much more economical; and while under ideal conditions as to char- 
acter of soil, depth of covering, pressure, etc., the life of a pipe in 
the ground might be longer than that of one fully exposed, ex- 
perience shows conclusively that in practice there is great uncertainty 
as to conditions; that they are seldom ideal in all respects, and that 
burying has shortened the life of many pipes, both by decay of wood 
and by corrosion of bands. The conditions of a pipe above ground 
may be easily determined at any time, and if repairs are required 
they can be made with much less difficulty and expense than would 
otherwise be possible. If, however, reasons appear sufficient to 
justify placing a pipe in the ground, as they may in some instances, 
it is best to insure a deep covering of a nature that will most nearly 
exclude air from the pipe, particularly if the water pressure is light. 
Gravel, shell rock, or other porous material is not satisfactory for 
backfilling. 
Summits and depressions in the line should be avoided as far as 
consistent with economical location. Where water courses are to be 
