《资治通鉴·汉纪·汉纪五十六》
五月，曹操毁去土山、地道，开凿壕沟，包围邺城，围圈达四十里。最初让挖得很浅，看去好像可以越过。
五月，操毁土山、地道，凿堑围城，周回四十里，初令浅，示若可越。
In May, Cao Cao destroyed the earth ramparts and underground tunnels, and dug a moat to surround Ye City. The moat was forty li in circumference. At first, he ordered it to be dug shallow, so that it looked as though it could be crossed.