*Article* **Defect Detection of Subway Tunnels Using Advanced U-Net Network**

**An Wang 1,***∗***, Ren Togo 2, Takahiro Ogawa <sup>2</sup> and Miki Haseyama <sup>2</sup>**


**Abstract:** In this paper, we present a novel defect detection model based on an improved U-Net architecture. As a semantic segmentation task, the defect detection task has the problems of background– foreground imbalance, multi-scale targets, and feature similarity between the background and defects in the real-world data. Conventionally, general convolutional neural network (CNN)-based networks mainly focus on natural image tasks, which are insensitive to the problems in our task. The proposed method has a network design for multi-scale segmentation based on the U-Net architecture including an atrous spatial pyramid pooling (ASPP) module and an inception module, and can detect various types of defects compared to conventional simple CNN-based methods. Through the experiments using a real-world subway tunnel image dataset, the proposed method showed a better performance than that of general semantic segmentation including state-of-the-art methods. Additionally, we showed that our method can achieve excellent detection balance among multi-scale defects.

**Keywords:** deep learning; semantic segmentation; U-Net; defect detection; subway tunnel
