## 3 Results and Discussion
### Field Test  
In order to verify the applicability of the SASW method, on-site inspection experiments were carried out on the basis of simulation calculations. A conveyance tunnel which diverts water from the Yellow River in Western China will not be put into operation until the lining has passed the quality inspection. The tunnel has a total length of 4.3 km with an arched-gate cross section, of which the width is 4.84 m, the height of its vertical wall is 3.5 m, and the crown radius is 2.42 m. The lining of the tunnel is constructed with C25 concrete with a doubly reinforced section and the steel protective cover is 35 mm thick. The lining thickness of the surrounding rock of Class III, IV, and V is 300 mm, 350 mm, and 400 mm, respectively. Due to the dense interior and exterior reinforcement of the concrete lining section, the steel bars strongly interfere with the GPR signal and the contact condition at the lining-rock interface cannot be read from the radar image with confidence. The SASW method is then introduced for further investigation. The schematic layout of SASW test stations at each test section is shown in Figure 15. As per the owner's request, a total of 10 test sections were arranged along the tunnel, of which the locations were chosen based on the GPR results, bedrock types, and apparent concrete quality. Figure 14: Phase spectrum and dispersion curve of the layered model 3 @ impact duration 100 us. Figure 13: Phase spectrum and dispersion curve of the layered model 3 @ impact duration 500 us. The nearest offset (i.e., distance between the impact point and the nearest transducer) is 40 mm, and the receiver spacing is 80 cm. The sampling interval is 4 us, the sampling points are 1024, and at each station, the impact test is performed four times. The typical phase spectrum and dispersion curve obtained from a mid-crown station at section 11+250 are shown in Figure 16. There are five regular and complete phase cycles in the phase spectrum, among which the first cycle has some strong interference. In the dispersion curve, when the R-wave length is below 40 cm, the R-wave velocity is around 2000 m/s, which reflects the overall quality of the concrete lining, but then a sharp drop to roughly 1500 m/s occurs at the R-wave length of 40 cm. These two irregularities are in good agreement with the results of finite element model analysis, indicating a quite probable void at the concrete-rock interface of the lining crown. Due to the gravity effect of the flowable concrete mixture during placement at the crown part of the lining and over-excavation, it is very easy for voids to form between the lining and bedrock above, and therefore most of the results of crown test stations, especially the mid-crown ones, have the same pattern as that in Figure 16. The voids could be easily detected by dry-drilling (since it is hard to core at the mid-crown). For the same reason, it is quite logical to imagine that the concrete-bedrock contact condition should be much better at both sidewall parts. The typical phase spectrum and dispersion curve obtained from the left sidewall station at section 12+250 are shown in Figure 17. The phase cycles exhibit a smooth and regular pattern and no obvious disturbance occurs, which leads to an essentially flat dispersion curve with the R-wave velocity fluctuating near 2200 m/s, indicating good concrete quality and concrete-bedrock connection. This was verified by c  
Figure 16: Phase spectrum and dispersion curve at the mid-crown test station at section 11+250. Figure 15: Schematic layout of SASW test stations at each test section. However, there are some sidewall stations where the phase cycles of the phase spectra exhibit some disturbances. As shown by the results of the right sidewall station at section 13+400 in Figure 19, a small but noticeable discontinuity occurs at the first phase cycle, causing scattering on the dispersion curve near the wave length of 40 cm, which is also the lining thickness at that location.