underbuilding

Etymology

From underbuild (“to build beneath, lay a foundation for”), equivalent to underbuild + -ing.

noun

1. A substructure; foundation; infrastructure.
   It appears from the evidence that although the Dean of Guild authorities considered the defective underbuilding to have been sufficient to warrant the demolition of the wall, it must have otherwise been in a weak and decayed condition. 1900, Scotland. Courts, The Scottish law reporter
   The rubble concrete underbuilding has a uniform thickness of 5 feet, where the depth is less than 10 feet, but for greater depths the underbuilding is 6 feet thick at the top. 1904, Brysson Cunningham, A treatise on the principles and practice of dock engineering