Grammar: relative clause

It's so heavy that I can't carry it.

No, it is not a relative clause. It is an adverbial clause

"so ... that" is a kind of adverbial clause to indicate the result of something.

2011-11-05 21:16:08 補充：
Refer to *** Kinds of adverbial clauses ***

It isn't a relative clause but a 'so that' structured sentence instead.

"it" as the impersonal subject in the position It is so....
Rearranging gives:-Because it's so heavy,so much so that I can't carry it.
(grammar)each of two or more parts of a sentence,often joined by "that" that make separate statements that each have an equal importance.It might also be known as subordinate clause to a main clause. Carry or take with you sb from place to place instead.