## 3 On-Site Measurements
### Tunnel Section
#### 3.1.1 Admittance Measurements  
During a nighttime maintenance window, the absorber plates at the three tunnel cross-sections were removed over a length of about 2 m to place a vibration generator (weight: 20 t) and an impact tester as close as possible to the tunnel invert. By using both vibration sources, the differences in the measured transfer admittances per one-third octave band could be studied for different types of excitation.  
During excitation, the vibration velocities were measured both on the invert (at distances of 2, 4 and 8 m from the excitation point) and on the tunnel wall using a fixture, and at the surface above the tunnel to calculate the point transfer admittances at various positions.  
The vibrotruck's excitation force was transmitted into the ground through its baseplate, approx. 2 m wide and 1 m long, placed on the ground using hydraulics. Because of the baseplate's width, which exceeded the track gauge, the space between the invert slab and the tunnel's structural lining had to be filled using a temporary construction. This consisted of several timber beams (30 \(\times\) 25 \(\times\) 150 cm) each weighing approx. 80 kg. Assuming a loading area of 1 m\({}^{2}\), an E-modulus of 1000 N/m\({}^{2}\) (perpendicular to the fibers), and a truck weight of 20 tons (partially decoupled), a natural frequency for this setup of 100 Hz was estimated. As prior experiments were conducted to determine the resonance effect of the wooden construction, its influence could be eliminated numerically.  
For the excitation using the impact tester (falling mass 50 kg, drop height 1.25 m), multiple impacts were induced into the ground at the same cross-sections where the vibrotruck was positioned. The impacts took place on the tunnel's concrete invert without using the wooden construction, and the measured forces were used to calculate the point transfer admittance to all geophones at the same cross-section. The final transfer admittance was obtained by averaging over up to 30 impacts.  
For the subsequent analysis, one-third octave spectra were used for both force and vibration velocity, with the time weighting Slow (\(\tau\) = 1 s). Figure 7 shows the time history and spectrogram of the raw force signals measured during the excitation inside the tunnel with the truck (subplot (a)) and the impact tester (subplot (b)). Figure 8 shows the measured vibration velocity signals resulting from the impact tester (subplots (b) and (d) and the vibrotruck subplots (a) and (c)) at radial distances of 2 m and 97 m from the centerline of the track, recorded on the tunnel invert and at the surface above the tunnel, respectively. As can be seen in both, the raw time signal and the spectrogram, the impacts of the falling mass are clearly discernible even at considerable distances from the tunnel.  
Figure 6: Measurement layout of surface sensors (red dots on the map) along the tunnel track (solid orange line). Photo inserts on the left show the surface sensors, and the schematic insert on the right shows the tunnel sensors with their respective distances from the center of the track/rail head.  
Figure 7: Force signals emitted during the artificial excitation at Q2, time histories at the top and spectrograms at the bottom, (**a**) using the vibrotruck’s sweep (in this example, 32–92 Hz), (**b**) using the impulse generator.  
Figure 9 compares the point transfer admittances computed when using impulse excitation and when using sweep excitation at various receiver points--both underground and at the surface--around cross-section Q2. Excellent agreement between the two vibration sources can be observed across the entire investigated frequency range. The data from the impact tester were analyzed down to the 4 Hz band, while the data from the sweep excitation were available at bands of 8 Hz and above.