Greg wrote:
You can keep the THHN/THWN single stranded, it is only the NM (AKA Romex) that
has to go.

BTW it is not necessarily a leak that causes underground raceways to collect
water. It is usually condensation. They breathe as temperatures rise and fall.
Moisture condenses over time.

Conduits can be kept dry by charging them with low pressure dry nitrogen
or air but for most purposes it is not worth the effort. You will find
that nearly all Thermoplastic insulated, High High temperature rated
[90C], Nylon sheathed, (THHN) wire is also listed as THWN with the W
meaning WET locations. Just run the THWN or other wet location rated
wire in the conduit. The advantages of conduit are its greater
resistance to physical damage, it's shallower depth of bury, and the
ability to replace or add conductors without excavating.

As to your junction boxes you can replace them with ground boxes that
are made of high density plastic. Such boxes are available at
electrical supply houses and the code permits them to be covered by
readily removable stuff such as aggregate, gravel, and so forth.

[314.29 Boxes and Conduit Bodies to Be Accessible.
Boxes and conduit bodies shall be installed so that the wiring contained
in them can be rendered accessible without removing any part of the
building or, in underground circuits, without excavating sidewalks,
paving, earth, or other substance that is to be used to establish the
finished grade.
Exception: Listed boxes shall be permitted where covered by gravel,
light aggregate, or noncohesive granulated soil if their location is
effectively identified and accessible for excavation.] Copyright 2002
the National Fire Protection Association.

Note that the box has to be listed for burial.
--
Tom H