# Reinforcement Effect of a Concrete Mat to Prevent Ground Collapses Due to Buried Pipe Damage
## 3 Experimental Investigation
### Results  
Figure 5 shows the stress occurrence per depth according to the installation depth of concrete mat. Figure 5a shows the result obtained when the concrete mat was not applied. As the depth increased, the stress occurrence decreased nonlinearly under all load conditions. Moreover, as the stress level increased from 4 to 12 kPa, the reduction in stress occurrence increased with the depth. Figure 5b,c illustrate the experimental results according to the reinforced depth of the concrete mat at 0.1 and 0.2 m from the surface, respectively. When compared to the non-reinforced case, the stress occurrence was significantly reduced from the reinforced depth or below. Specifically, the stress-reducing effect occurring below the buried depth of the concrete mat appears to be due to the stiffness of the concrete mat.  
Figure 4: Experimental procedure: (**a**) earth pressure gauge installation in soil; (**b**) concrete mat installation; (**c**) loading plate installation; (**d**) reaction beam installation; (**e**) linear variable differential transformer (LVDT) installation.  
Using the experimental results, the stress reduction ratio was calculated per depth, according to the reinforcement of the concrete mat, as shown in Equation (1). Table 2 lists the calculation results.  
\[\text{Stress reduction ratio(\%)}=(\text{P}_{(non)}-\text{P}_{(rc)})/(\text{P}_{(non)}) \tag{1}\]  
where P\({}_{(non)}\) represents the stress occurrence per depth when not reinforced with the concrete mat and P\({}_{(rc)}\) represents the stress occurrence per depth when reinforced with the concrete mat.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Depth (m)**} & \multicolumn{3}{c}{**Reinforced at G.L. \(-\)0.1 m**} & \multicolumn{3}{c}{**Reinforced at G.L. \(-\)0.2 m**} \\ \cline{2-7}  & **4 kPa** & **8 kPa** & **12 kPa** & **4 kPa** & **8 kPa** & **12 kPa** \\ \hline
0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 \\
0.1 & 0.7 & 4.3 & 1.8 & 1.8 & 4.8 & 5.0 \\
0.2 & 28.5 & 13.4 & 6.2 & 6.6 & 2.1 & 1.4 \\
0.3 & 23.8 & 16.9 & 9.7 & 30.9 & 11.5 & 13.4 \\
0.4 & 24.5 & 16.5 & 14.5 & 18.9 & 3.2 & 7.3 \\
0.5 & 18.7 & 23.2 & 11.4 & 20.0 & 4.1 & 2.6 \\
0.6 & 13.7 & 13.7 & 11.6 & 22.4 & 1.2 & 1.8 \\
0.9 & 3.1 & 3.1 & 1.8 & 2.4 & 7.5 & 4.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of the reinforced ground and non-reinforced ground in terms of the stress reduction ratio.  
Figure 5: The relationship between the earth pressure and depth, according to the load step: (**a**) non-reinforced; (**b**) reinforced at G.L. \(-\)0.1 m; (**c**) reinforced at G.L. \(-\)0.2 m.  
When the concrete mat was installed at G.L. \(-\)0.1 m, the stress reduction continued below a 0.2 m depth, while the stress reduction ratio decreased. Moreover, the stress reduction ratio decreased as the applied load increased. When the concrete mat was installed at G.L. \(-\)0.2 m, the stress occurrence also decreased below the burial depth of the concrete mat. Despite the inconsistency, the stress reduction ratio also decreased as the applied load increased. Based on these results, a concrete mat reduces the stress by up to 28.5-30.9%, depending on the burial depth and the condition of the pressure applied to the ground surface.