# Liquidation of Shallow-Lying Post-Mining Excavations
## 3 Liquidation of Shallow Voids in the Rock Mass
The problem results from the inaccessibility of the voids and, therefore, limited possibilities of their location and, thus, inaccurate knowledge of their dimensions and shapes. The voids are located on the basis of old mining maps with an estimated consideration of their development due to occurring collapses and on the basis of modern methods of examining the rock mass described in Section 2. It should be noted that the geophysical location of voids with currently available equipment does not give fully satisfactory results due to large disruptions of the geophysical underground reading and surface reinforcement of the area. Reconnaissance by drilling is the most expensive, but it gives good results, and, in addition, most of the holes made are used as backfill holes. The backfill injection technique requires the installation of a head pumped by pipelines on the casing pipe of the borehole. Ash is transported by cement trucks (Figure 3). The ash tank is connected to the compressor and the backfilling pipeline. The working pressure of the fly ash pumping ranges from 30  
Figure 1: Installation for preparing ash–water suspension in the Borynia hard coal mine in Poland. to 45 kN/m\({}^{2}\). The pumping capacity is approximately 0.6 to 1.0 m\({}^{3}\)/min. Air consumption for transporting 1 m3 of ash is 0.9 to 1.5 m\({}^{3}\). The backfilling pipeline outlet is placed directly under the ceiling of the void or, in the case of large voids, it is moved during the backfilling process, starting from a certain distance above the floor upwards at specific intervals (e.g., every 1.5 m). After filling the void and dismantling the pressure pipelines, the quality of the backfill is checked. The boreholes are drilled to the original depth, and water is pumped into the hole, maintaining a pressure of 100 N/cm\({}^{2}\) for 15 min. If the pressure does not drop during this time, the backfill quality is considered good; otherwise, fly ash is pumped in until positive results of the backfill control are obtained. A separate issue is determining the necessary number of backfill holes for proper filling of the void and support of its ceiling. The ash backfill flowing out of the nozzle of the backfill pipeline suspended under the ceiling of the void takes the form of a truncated cone, the bases of which are located on the  
Figure 3: Fly ash backfilling of shallow voids: 1—tanks with fly ash; 2—compressor; 3—valve; 4—hose; 5—head; 6—pipeline; 7—filling drillholes; 8—filling void; 9–10—filling cones. Figure 2: Diagram of the installation for placing ash–water suspensions with carbon dioxide in the post-mining space in an underground hard coal mine. floor and roof of the excavation (Figure 3). From the point of view of designing the density of backfill holes, it is important to know the angle of the cone slope and the radius of its upper base. As a result of industrial tests [26], the slope angle (\(\alpha\)) is assumed to be 9\({}^{\circ}\), and the radius (r) of the upper sub-base is 7 m. Therefore, assuming that the backfilled void has the shape of a chamber or a gallery with the width s \(\leq\) 2 r and height h (for larger widths, backfill holes should be located in two or more lines), the following cases are considered (Figure 4):  
Case I: When complete support of the roof is required. The distance between the backfill holes located in the axis of the excavation should be  
\[L=\sqrt{4r^{2}-s^{2}} \tag{1}\]  
Case II: When the holes are located on the axis of the excavation at a distance of L = 2r. The relationship between the percentage of roof support (X) and the width of the excavation is  
\[X=\frac{0.873\;s^{2}(180-2\gamma)+25\;s^{2}tg\gamma}{rs},(\mathrm{m}) \tag{2}\]  
where  
\(\gamma\) can be determined from the dependence as follows:  
\[cos\gamma=\frac{s}{2r}, \tag{3}\]  
Case III: When the holes are located in the axis of the excavation at a distance of \(L>2r\). The volume of the unbacked part of the void will be limited by the planes of the roof and the sides and the surfaces of two interpenetrating cones.