China has undergone rapid development in urban rail transit; in 1965, the first subway was built in Beijing and, by the end of 2019, the nation’s urban rail transit lines reached 6736.2 km, including 5180.6 km of metro lines (76.9%). In 2019, the length of newly added operation lines was 974.8 km, with a total daily passenger traffic volume of 66.371 million persons; the average daily passenger traffic volume of Beijing and Shanghai numbered more than 10 million persons. However, according to incomplete statistics, 1416 delay events of 5 min or more occurred in 2019 [
1]. Because of the influences of geology, stress, and groundwater changes, internal deformation will inevitably occur in tunnels at different degrees. Therefore, to ensure normal subway operations and ensure the safety of people’s lives and property, it is especially important to monitor the deformation of the lining ring in a tunnel. Tunnel clearance detection is a part of tunnel deformation monitoring. To ensure that the pipeline and other accessories in a tunnel do not intrude the vehicle gauge, the normal operation of vehicles can be judged for being disturbed or damaged by the positional relationship between the gauge and objects, such as pipeline equipment in the tunnel.
Over many years, several ideas have been proposed for tunnel deformation detection. To detect tunnel lining cracks and other state information, the contact detection method was adopted. In this method, an extensible mechanical arm is installed on the detection vehicle, and the tunnel lining state is measured by mechanical vibration. If there is a boundary invasion, the equipment will sound an alarm. Because of the limitations of the equipment itself, this method is prone to equipment damage or providing an insufficient measurement range [
5]. In the noncontact detection method, the total station, measuring robot [
6,
7], laser profiler, three-dimensional laser scanner, and other equipment are often used. In the application of a total station in tunnel clearance detection, through observing the fixed positions at the top and bottom of tunnel section, the transverse, longitudinal, and settlement deformations of the tunnel section are calculated to predict the rock mass change. Because of the low density of collected cross-section points, it is difficult to detect an entire section, and the operation efficiency is low [
8,
9]. The application of a laser profiler in the quality inspection of tunnel lining can aid in completely obtaining information for an entire section, but the laser profiler must always be perpendicular to the track center line, which is difficult to achieve in practice [
10]. The station 3D laser scanner can obtain high-precision, point cloud information in a tunnel with a high density, which can accurately reflect the real state of the tunnel. However, in data processing, the algorithm is relatively complex, requiring multi-station splicing and cross-section extraction. In the actual operation process, station-to-station observations are also required, which leads to a low operation efficiency and heavy postprocessing workload [
11,
12,
13,
14,
15,
16,
17,
18,
19]. From the aspect of multi-sensor fusion, Liu et al. [
20] designed a tunnel monitoring car integrating two laser ranging sensors, four displacement sensors, an odometer, and a point laser, and used Peripheral Component Interconnect (PCI) and computer real-time communication to display the measured information of a tunnel section. Zhou et al. [
21] integrated a laser scanner and a high-precision navigation and positioning system on a tunnel car. Through the track extraction of a three-dimensional point cloud, the track center was fitted and the tunnel section clearance was calculated. Du et al. [
22] placed a laser scanner, odometer, and displacement sensor on a track car to detect the deformation of a tunnel section. Boavida et al. and Puente et al. [
23,
24] developed a set of systems for clearance detection of highway tunnels. A lidar scanner, GNSS antenna, inertial measurement unit, and odometer were assembled on a vehicle for a clearance detection of a highway tunnel. However, owing to the vibration of the vehicle body, the accuracy of the clearance detection was low. The Amberg company of Switzerland developed GRP5000 track detection car [
25] that included a PROFILER5002/5003 3D laser scanner, displacement sensor, inclinometer, and odometer. Their system demonstrated an acceptable performance in gauge detection, completion acceptance, holographic image acquisition, and so on. Developed by Leica company, the SiTrack:One track movement detection system [
26] integrates a P40 scanner, noncontact laser odometer, high-precision inertial navigation measurement unit, and GNSS antenna; this system is suitable for existing outdoor railways as well as metro tunnels without a GNSS signal. Through using postprocessing software, a relative point cloud accuracy of 3–4 mm can be obtained and the track can be extracted automatically, mileage can be calculated, and collision detection and detection reports can be generated automatically. Hao et al. [
27] proposed a mobile laser scanning system for shield tunnels, extracted tunnel appendages through wavelet filtering, fitted ellipses, calculated tunnel convergence diameter, and generated a gray image to detect tunnel circumferential cracks. Du et al. [
28,
29,
30,
31] analyzed the cross-section deformation of a shield tunnel from the aspects of tunnel section extraction, convergence diameter calculation, and generated a gray image of a tunnel point cloud using a mobile laser scanning system that integrated scanner and odometer. Researchers have obtained tunnel information via laser point clouds and have attained certain achievements in the development of digital imaging technology [
32,
33,
34]. However, owing to the complexity of digital image acquisition, the accuracy of tunnel structure data cannot be guaranteed and the lack of light in tunnels will also have a certain impact on data acquisition.
In the above research on tunnel clearance detection methods, owing to the low detection efficiency of total stations and other equipment, the postprocessing workflows of station scanner data are excessively complex. However, the working mode, software, and hardware cost of the tunnel detection system developed by Amberg company and Leica company, as well as its applicability to the Chinese market, must be improved. There is a lack of relevant research on tunnel deformation detection using the mining method. Therefore, an efficient, high-precision, cost-effective tunnel detection system is increasingly necessary. This study introduces a tunnel detection system that focuses on the detection of tunnel section deformation using the mining method. The system integrates a laser scanner, inertial measurement unit, and GNSS antenna, and is mounted on a rail car that can travel at a uniform speed. Outdoors, the GNSS time received by GNSS antenna is synchronized with the time system of the scanner and IMU through a Pulse Per Second (PPS) signal. Combined with setting control points, the system attitude angle and car speed are calibrated. Through the control points and system calibration parameters to calculate the track center, the point cloud data and inertial navigation data are combined to calculate the tunnel clearance to determine whether there is deformation inside the tunnel.