"He felt as if the ground were slipping beneath his feet." is correct sentence.

Adverb clause: as if the ground were slipping beneath his feet.

Adverbial clause of Manner: As the name states, this kind of adverb clause refers to how the action of the verb takes place. Some of the conjunctions used by this type of clause are though, although, as though, as if, etc. Here the manner in which the action takes place is described by introducing the adverbial clause starting with "as if".