Correct passive form of the sentence is: We had dug the canal.

• Past perfect tense যুক্ত active voice কে passive voice এ রুপান্তর করার নিয়ম-
- Object এর subject হবে + had been + verb এর past participle form + by + subject এর object form বসবে।
Example:
Active: We had dug the canal.
Passive: The canal had been dug by us.