We don't have synonyms for underfoot yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are tenderfoot, undercoat and underwood.
"The workers were all big, burly, hard-hearted men, tromping through the marsh in their heavy boots without sparing so much as a single thought for the masses of tiny frogs they crushed underfoot."
"The workers were all big, burly, hard-hearted men, tromping through the marsh in their heavy boots without sparing so much as a single thought for the masses of tiny frogs they crushed underfoot."