CosmicLog (read through Larry Kellogg’s “Lunar Update” mailing list) has an interview about innovative lunar structures with Robert Bigelow of “Inflatable Space Station” fame.
Bigelow does mention of an idea on how to bury the structure (but only with a couple of feet of soil, not the 12 or more required).
In fact the thought of spending more than a couple of days virtually unprotected on the Lunar surface should not enthuse anybody. It has been computed (*) that on average a maximum 20% of time should be spent by humans outside the protection of a minimum 4 meters of regolith.
(*) R Silberberg et al, ‘Radiation Transport of Cosmic Ray Nuclei in Lunar Material and Radiation Doses’, in W W Mendell, ed, ‘Lunar Bases and Space Activities of the 21st Century‘, Lunar and Planetary Institute, 1985, p668
Bigelow is right and wrong at the same time. If we seriously consider going back to the Moon, resources should be spent investigating how easy it will be to bury those Habitats (inflatable or otherwise).
But excavated regolith is only one option and not the most practical one given the amounts of soil that will have to be moved to make comfortable living out of a stay on the Moon.
Other ideas involve lava tubes, of which there should be aplenty, and artificial giant caves. Especially the caves should be easy to create with explosives, if there is no water in the lunar rocks.