There was a space competition a while ago that had teams design the best habitat for either Mars or the Moon, and the winning design was a cone-like building using a large 3D printer. I saw it and wondered why they couldn't just use a robot to dig a small hole, widen the hole out around 6 ft deep, and use the excavated material to make a plug for the hole? It seems like it would be cheaper and easier than using a large 3D printer that you have to make sure didn’t leave any air leaks anywhere on the structure. Is that true?
It all depends on the initial conditions. If you need to create a large volume for the accumulation of resources in the shortest time, then this is ideal. For example, as accumulators of oxygen produced from regolith. For residential premises, it makes sense to deliver from Earth ready-made modules to the surface and bury them in trenches. It will be safer and more reliable.
The best way would be an explosion, which creates a spherical chamber with glass-sealed walls. But this technology is not available to private companies.
Soviet scientists successfully created underground spherical tanks and storage facilities that had walls 10 cm thick from molten rock. Those. in regolith conditions it will be something like glass. This was the most productive part of the Soviet program of underground nuclear explosions for peaceful purposes. The Soviet technique makes it possible to create stable cavities at depths of up to 1000 meters and a volume of more than 60 thousand cubic meters.
With an explosion power of 1 kiloton at a depth of 1 kilometer, the cavity radius is 10 meters. If the power of the explosive charge is increased to 8 kilotons, then the radius of the cavity will be 20 meters. Etc. An essential fact is that if such explosions are carried out in underground rock salt masses, then containers are formed that are particularly durable. In other geological conditions, they do not have great strength and quickly collapse.
For lunar conditions, it is necessary to conduct research - different chemical composition and gravity.
Sources in Russian about the history of the creation of large underground storage tanks:
Адамский В. Б., Клишин В. И., Смирнов Ю. Н. Советская программа подземных ядерных взрывов в мирных целях: надежды и результаты Adamskiy V.B., Klishin V.I., Smirnov Yu.N. Soviet program of underground nuclear explosions for peaceful purposes: hopes and results http://elib.biblioatom.ru/text/istoriya-sovetskogo-atomnogo-proekta_v2_2002/go,190/
НЕКОТОРЫЕ ЭКОЛОГИЧЕСКИЕ АСПЕКТЫ ПРИМЕНЕНИЯ ПОДЗЕМНЫХ ЯДЕРНЫХ ВЗРЫВОВ В МИРНЫХ ЦЕЛЯХ © 2010 г. А. К. Калиновский, В. А. Краснов, Э. М. Пазухин SOME ENVIRONMENTAL ASPECTS OF UNDERGROUND APPLICATION NUCLEAR EXPLOSIONS FOR PEACEFUL PURPOSES A. K. Kalinovskii, V. A. Krasnov, and E. M. Pazukhin http://mntc.smn.com.ua/downloads/2010_14/c113.pdf