I'm having some difficulties understanding the construction 崩れかけた in
博士たちは崩れかけた門をくぐり、中へ入っていった。
I couldn't find it as a single word, so I take it is constructed from 崩れる and かける. My guess is that it's the stem form of 崩れる joined with かける (equivalent to the て form to join to verbs), but I'm not sure.
崩れる means something like collapsed so I was thinking it referred to a collapsed gate. I don't know what かける adds to this though...
Can someone shine some light on this?