# Spatiotemporal Monitoring and Evaluation Method for Sand-Filling of Immersed Tube Tunnel Foundation
## 2 Elastic Wave Monitoring for Sand-Filling of Immersed Tube Tunnel
### Elastic Wave Monitoring  
#### 2.3.1 Elastic Wave Monitoring System  
The elastic wave monitoring system consists of a detector and coupling device, connection line, power cord, excitation device, excitation hammer, computer, power supply device, etc. A 24-channel Geode digital seismograph was used as the data acquisition equipment, connected to 24-channel single-component detectors. The frequency of the detector was 100 Hz. A 300-g hammer was used as the elastic wave exciter. There were 24 detectors, and the spacing between detectors was 50 cm. The detector used could measure vertical vibration velocity. The data collection method of one excitation and one reception was adopted. The specific parameters of the monitoring system are listed in Table 2.  
To obtain the elastic wave at receiving point Ri, a hammer was used to excite at the impact point I\({}_{i}\). At the same time, the trigger activated the waveform signal record system, and the response waveform of the sand-filling model was recorded and saved. Afterwards, the knocking point of the hammer was moved to the next receiving point to record the response waveform. The data record of a line containing 24 detectors was completed by  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sand-Filling Hole** & **Sand-Filling Time (h)** & **Pipe Section Lifting State** \\ \hline W8 & 13 & Not uplifted \\ W7 & 7 & Not uplifted \\ W6 & 11 & Not uplifted \\ W5 & 8 & Not uplifted \\ W4 & 6 & The pipe was lifted by 4 cm at \\ W3 & 11 & Not uplifted \\ W2 & 12 & Not uplifted \\ W1 & 12 & Not uplifted \\ E9 & 13 & Not uplifted \\ E8 & 8 & Not uplifted \\ E7 & 11 & Not uplifted \\ E6 & 7 & Not uplifted \\ E5 & 12 & Not uplifted \\ E4 & 13 & Not uplifted \\ E3 & 12 & The pipe was lifted by 4 cm at \\  & & the 10 th hour \\ \hline \hline \end{tabular}
\end{table}
Table 1: Data record of the P6 pipe section.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Part** & **Parameters** \\ \hline \hline \multirow{4}{*}{Geode seismograph} & Recording channel: 24 channels \\  & Analogue-to-digital conversion: 24 bit \\  & Minimum sampling interval: 0.02 ms \\  & Low cutoff frequency: 10 Hz \\  & Manufacturer: Geode, USA \\ \hline \multirow{4}{*}{Detector and coupling device} & Moving coil-type velocity detector \\  & Natural frequency: 100 Hz \\  & Manufacturer: China Chongqing Geological \\  & Instrument Factory \\ \hline \multirow{2}{*}{Computer} & Installed with self-developed data acquisition \\  & software \\ \hline Excitation hammer & Weight: 300 g \\ \hline Power supply device & DC power supply: 24 V \\ \hline Connection lines & Long enough \\ \hline \hline \end{tabular}
\end{table}
Table 2: Device parameters.  
analogy. In this way, the obtained waveform was saved, and the data acquisition was accomplished.  
The process of data acquisition is shown in Figure 4. The sand-filling process usually takes about 10 h empirically based on the designed sand-filling volume and sand-filling speed. Therefore, the monitoring frequency was taken as 1 h. The length of sampling time was determined according to the thickness of propagation medium and wave velocity. The collected wave data had decayed at the time of 0.1 s. Therefore, the sampling time was set to 0.1 s. The uplift state of the pipe section was recorded along with the measurement results of the level gauge, including the cushion filling state, in detail.