The original Horizon plans called for used Saturn stage boosters to be buried under the regolith. After '69 the discovered density of the regolith and no topsoil means it required far more than a few men digging to bury or cover such a "instant shielding" moon-base.

Tom's probably right that lava tubes would act as an easy pre-made receptacle for pre-fab base structures. Craters of sufficient depth would also act to shield pre-fab moonbase structures. I'm disappointed at the reduction of vision evident today. Hiroshi Kumagai designed a lunar village in 1958 for both scientists and visitors that was far-seeing and I have his blueprints in my collection. That sort of vision has been dwindled to a plethora of one-cheeked efforts and plans.