The following is from Michael Swan's Practical English Usage:
Relative clauses can also be combined with if-clauses in sentences like the following.
I am enclosing an application form, which I should be grateful if you would sign and return.
Then, can relative clauses also be combined with other kinds of adverbial clauses, for example since-clauses in sentences like the following?
It seems something is wrong with this PC, which barely a month has passed since I bought.