## 3 Validation of the Proposed Method
### In Field Measuring Process  
As shown in Figure 9, two transducers were grouped in cement mortar using the developed steel mold to produce a sensing column before the test. During the process, a release agent was first brushed onto the mold surface. Two transducers were placed in the mold with different orientations and the connection devices were fixed in the side hole of the mold. The cap of the mold then was closed tightly with bolts and the cement mortar was injected into the mold. The sensing column was demolded  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test Location** & **Rock Type** & \begin{tabular}{c} **Elastic** \\ **Modulus/GPa** \\ \end{tabular} & \
\begin{tabular}{c} **Poisson’s** \\ **Ratio** \\ \end{tabular} &
\begin{tabular}{c} **Uniaxial Compressive** \\ **Strength/MPa** \\ \end{tabular} \\ \hline PDS No. 1 coal mine & Sandy mudstone & 8.325 & 0.221 & 22.56 \\ PDS No. 11 coal mine & Mudstone & 5.764 & 0.263 & 17.85 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Average value of rock parameters obtained from uniaxial compression tests.  
Figure 8: Roadways distribution of test location in PDS No. 11 coal mine.  
Figure 7: Roadways distribution of test location in PDS No. 1 coal mine.  
after 24 h and maintained for 28 days in wet steam. Since the mechanical properties of cement mortar should be consistent with those of the rock mass, the grade of cement mortar was chosen as M20 or M10, based on the mechanical parameters of the rock shown in Table 1. The detail ratios of cement mortar and the parameters of the grouted specimens obtained from the uniaxial compression test are presented in Table 2.  
The test hole, with a diameter of 130 mm and dip angle of 2\({}^{\circ}\), were drilled in side walls of different roadways in PDS No. 1 and No. 11 coal mines. The spans of the test roadways in the No. 1 and No. 11 coal mines were 4.5 m and 4.8 m, respectively. As a result, the depths of drilled holes were 23 m and 25 m, which are five times of the spans in each roadway. After drilling, the sensing column was pushed to the bottom of the test hole using the installing equipment. The actual buried depths of the sensing columns were 22.5 m and 24 m as debris accumulates at the bottom of the drilled holes. After the grouting pipe was pushed near to the sensing column, the test hole was sealed and grouted with the cement mortar and the grouting pressure was 0.3 MPa. The detailed testing process is presented in Figure 10.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Grade** & \begin{tabular}{c} **Ratio/Cement:** \\ **Mortar: Water** \\ \end{tabular} & \begin{tabular}{c} **Elastic** \\ **Modulus/GPa** \\ \end{tabular} & \
\begin{tabular}{c} **Poisson’s** \\ **Ratio** \\ \end{tabular} &
\begin{tabular}{c} **Uniaxial Compressive** \\ **Strength/MPa** \\ \end{tabular} \\ \hline M25 & 4:4.53:1.03 & 9.52 & 0.30 & 24.38 \\ M20 & 1:4.03:0.92 & 6.48 & 0.29 & 18.23 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Average value of cement mortar obtained from uniaxial compression tests.  
Figure 9: Production process for the column transducers: (**a**) Steel mold; (**b**) placing the 3D comprehensive pressure transducers; (**c**) column transducer.