Let's don't get carried away with economics. Remember that there are sometimes legal requirements that require burial, as per this example covering installations in water and plastic pipe on land and many other jurisdictions and landowners along the ROW may also require burial as a condition of granting the Right-of-Way.
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CFR Title 49 Part 192
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§ 192.319 Installation of pipe in a
(c) All offshore pipe in water at least
12 feet (3.7 meters) deep but not more
than 200 feet (61 meters) deep, as measured
from the mean low tide, except
pipe in the Gulf of Mexico and its inlets
under 15 feet (4.6 meters) of water,
must be installed so that the top of the
pipe is below the natural bottom unless
the pipe is supported by stanchions,
held in place by anchors or heavy concrete
coating, or protected by an equivalent
means. Pipe in the Gulf of Mexico
and its inlets under 15 feet (4.6 meters)
of water must be installed so that the
top of the pipe is 36 inches (914 millimeters)
below the seabed for normal
excavation or 18 inches (457 millimeters)
for rock excavation.
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§ 192.321 Installation of plastic pipe.
(a) Plastic pipe must be installed
below ground level except as provided
by paragraphs (g) and (h) of this section.
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192.325
(e) Except as provided in paragraph
(c) of this section, all pipe installed in
a navigable river, stream, or harbor
must be installed with a minimum
cover of 48 inches (1,219 millimeters) in
soil or 24 inches (610 millimeters) in
consolidated rock between the top of
the pipe and the underwater natural
bottom (as determined by recognized
and generally accepted practices).
(f) All pipe installed offshore, except
in the Gulf of Mexico and its inlets,
under water not more than 200 feet (60
meters) deep, as measured from the
mean low tide, must be installed as follows:
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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain