Real distribution systems are usually operated in open loop, but designed in closed loop to be capable of load transfer under contingencies. This letter proposes an approach for distribution network expansion planning problem under N-1 criterion, which presents both optimal closed-loop investment and open-loop operation plans. To alleviate the computational burden of the model, constraints for N-1 criterion are first relaxed and thereafter the problem is solved with iteratively added binding contingency constraints, which are identified by a designed method. The proposed methodology is validated with numerical experiments on a 138-node test system.