## 5 Experiments and Results  
After the numerical simulation, the proposed ultrasonic CT based on CS algorithm was employed to inspect the concrete-filled tube of the pipe-roofing STS structure of Olympic Metro Station on Shenyang Metro line 9. Olympic Metro Station, an interchange station between line 9 and line 2 that was completed in 2016, is an application of the STS curtain method in Shenyang, China (Figure 7a). The station is oriented in an east-west direction across Qingnian Avenue. To connect the new station (line 9) to the built one (line 2), a triple-deck tunnel of rectangular shape was constructed with the cut-and-cover method, and the middle section under the prosperous avenue was tunnelled to three layers with the STS curtain method. The cross-section size is approximately 22.9 m in span and 21.24 m in height, as shown in Figure 7b. There are 40 tubes in total in the STS curtain system, including 28 tubes in the tubular roof. Considering the complex environments, including an overburden soil depth of more than 4 m, the heavy traffic flows on the Qingnian Avenue, and the high-rise buildings in this modern urban area, each concrete-filled steel tube needs to be inspected using ultrasonic CT before the service stage to guarantee the load-carrying capacity of the STS structure. Thus, all 40 concrete-filled steel tubes are required to be inspected by ultrasonic CT. Both the proposed ultrasonic CT technique and the conventional one are employed to complete the inspection project. Generally, the implication of sparsity \(K\) is the number of microstructures containing construction defects in the concrete-filled tube. However, for the non-destructive ultrasonic inspection, it is impossible to grab the number of anomalous elements before the detection process. Thus, there is an assumption that the number of damaged microstructures is less than 10% of the total number, which is based on the previous damage inspection results of the STS structure. Therefore, the number of selected paths is chosen as 80 to satisfy the requirement of the CS theory in this research. The accuracy of tomography is dependent on the sensor spacing and microstructure size. In a real situation, the essential goal of both the proposed method and conventional computerized tomography is to identify the significant void part and crack in the concrete, which might cause a major catastrophe instead of minor anomalous events. Although the proposed computerized tomography does not have as dense a net of ray paths as the conventional method, it is capable of capturing the dramatic change of travel time caused by the desired large-scale defect. Considering the size of the aggregate (maximum size aggregate in the concrete is 25 mm) in the concrete-filled tube and the Chinese standard for quality control of concrete, the threshold of the difference of the slowness is determined as 0.004, which means that defects below the size of 15 mm are discarded as 'noise'. Figure 7: \(Cont.\)Figure 7: _Cont._  
An ultrasonic NDT instrument with two transducers, NU62, is employed as the detection equipment in the experiments. The sampling frequency of the NU62 is 25 MHz, and the launched ultrasonic wave is a series of square waves centered at 50 kHz with the amplitude of 250 V and a 0.08 m wavelength. One transducer emits ultrasonic waves and the other is used as sensor to capture the waves propagating in the structure.