# Liquidation of Shallow-Lying Post-Mining Excavations
## 5 Discussion  
The proposed technology for filling shallow excavations using a mixture of cement, fly ash, water, and various contents of liquefiers is characterized by a setting time ranging from 2.5 h to 6 h. The filling time of the excavations does not play a very important role in the case of inactive excavations because there are no additional stresses from the moving exploitation front. However, it is significant in the event of a dynamic outflow of groundwater, which can directly contribute to a decrease in the efficiency of the backfilling process. [PERSON] et al. [38] conducted laboratory tests in which they determined the relationship between paste backfill material with the addition of fly ash and water, indicating that water \begin{table}
\begin{tabular}{l l l l l l} \hline \hline \multicolumn{1}{c}{**Parameter**} & \multicolumn{1}{c}{**Recipe 1**} & \multicolumn{1}{c}{**Recipe 2**} & \multicolumn{1}{c}{**Recipe 3**} & \multicolumn{1}{c}{**Recipe 4**} \\ \hline Bending strength after 7 days & MPa & 1.134 & 1.054 & 0.865 & 0.810 \\ Compressive strength after 7 days & MPa & 4.511 & 2.876 & 2.412 & 2.004 \\ Bending strength after 14 days & MPa & 1.288 & 1.134 & 1.014 & 1.087 \\ Compressive strength after 14 days & MPa & 8.036 & 4.252 & 4.158 & 4.098 \\ Bending strength after 28 days & MPa & 2.271 & 3.078 & 2.658 & 2.474 \\ Compressive strength after 28 days & MPa & 9.157 & 9.045 & 9.267 & 6.978 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameters for hardened sealing slurry recipes. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & **Contents** & & **Recipe 1** & **Recipe 2** & **Recipe 3** & **Recipe 4** \\ \hline \multirow{3}{*}{Mixing (M)} & Cement & \% & 10 & 10 & 10 & 10 \\  & Ash & \% & 90 & 90 & 90 & 90 \\ Liquefier & BWOM & 0 & 0.1 & 0.3 & 0.5 \\ Water (W) & W/M & 0.75 & 0.75 & 0.75 & 0.75 \\ \hline \hline \multicolumn{5}{l}{BWOM—by weight of dry binding ingredients.} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of the proposed sealing slurries. \begin{table}
\begin{tabular}{l l c c c c} \hline \hline \multicolumn{1}{c}{**Parameter**} & \multicolumn{1}{c}{**Recipe 1**} & \multicolumn{1}{c}{**Recipe 2**} & \multicolumn{1}{c}{**Recipe 3**} & \multicolumn{1}{c}{**Recipe 4**} \\ \hline Density & g/cm\({}^{3}\) & 1.54 & 1.54 & 1.54 & 1.58 \\ Flowability & mm & 190 & 195 & 210 & 230 \\ Resting & \% & 4 & 4 & 4 & 4 \\ Start of setting time & h:min & 17:40 & 31:00 & 33:00 & 35:00 \\ End of setting time & h:min & 22:50 & 33:30 & 36:30 & 38:50 \\ Binding time & h:min & 6:00 & 2:30 & 3:30 & 3:50 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters for fresh sealing slurry recipes. plays a significant role in the hydration process. The natural inflow of water to the cement backfill material can contribute to a significant weakening of its strength parameters [39]. The basic parameter of the cement-based backfill mixture is its flowability. In the proposed recipe, a relationship was found in which the flowability also increases with the increase in the fluidity content. In the studies conducted by [PERSON] et al. [40], it was found that the plasticity limit of the backfill mixture could be increased for an increased content of solids while reducing flowability. Particularly dangerous for shallow-lying excavations are rock mass tremors, which contribute to changing the structure of rock layers by weakening them and whose effects are very often visible on the ground surface [41]. In the case of cracked rock mass, the possibility of penetrating the cracks on one side is a beneficial phenomenon because it additionally seals the rock layers, preventing the migration of gases. However, taking into account the amount of material that would have to be supplied to fill the void with additional cracks, it may turn out that the costs increase significantly considering the 10% cement content.