PROGNOSIS OF SURFACE DEFORMATION BY THE INFLUENCE FUNCTION METHOD IN TERMS OF PAROSENI MINE ( ROMANIA )

Purpose. The paper analyzes a problem of surface deformation as a result of low-inclined seam underground mining in Paroseni mine, Romania. Solving this problem makes it possible to predict the effects of the underground mining on the surface while offering the possibility of taking appropriate measures to protect the objects already available in the area or those to be built. Methods. In order to analyze the behavior of the surface affected by underground mining of the coal seams in Paroseni mine, the influence function method was applied. Findings. In the paper, the authors analyze how the underground mining acts on the surface in the case of the coal extraction in Paroseni mine, Jiu Valley, Romania. The possibility of important object placing in the area affected by the underground mining has also been analyzed. Originality. The paper is an original case study; the results, presented by the authors, have been obtained while using the influence function method, a well-known and widely applied method for investigating the subsidence phenomenon. Practical implications. The obtained results represent the prognosis of the surface deformation due to underground coal mining in Paroseni mine, Romania. These results were considered precise enough to determine the amount of surface deformation and the possibility of important object placing in the area affected by the extraction.


INTRODUCTION
The need to study the phenomenon of surface deformation as a result of underground mining of useful mineral substances has emerged with the development of mining and, in particular, with the transition from extraction predominantly to the surface (open-pit mining) to underground mining (Ortelecan, 1997;Marian, 2011;Vereș, Marian, Fissgus, & Ștefan, 2017).
The subsidence phenomenon is still being studied and will be studied in the future as it is a topical issue through the need to protect the surface and the constructions located in the area influenced by the underground mining (Onica & Marian, 2012;Zhu et al., 2016;Taherynia, Fatemi Aghda, Ghazifard, & Moradi, 2017).
The Paroseni mining perimeter (Fig. 1) is between the Lupeni perimeter to the west and the Vulcan perimeter to the east, in the region called Jiu Valley, Romania.

Figure 1. The mining perimeters from Jiu Valley, Romania
Underground mining of the coal seams from the Paroseni mining field has major implications regarding the stability of the surface and buildings.
Taking into account only the last 25 years (during this period coal seams No.3 and 5 were extracted), the volume of the voids left after the extraction of coal at Paroseni mine is approx.1700000 m 3 .The need to know the amount of surface displacement in the case of Paroseni mine is also important due to the fact that it is projected to build an important object which was initially designed to cross this mining field.
In the case described in this paper, for the surface subsidence prognosis due to underground mining, we applied the influence function developed by Knothe, also known as Knothe-Budryk method (Luo, Peng, & Zhu, 2008): where: S max = m•a -the maximum possible subsidence; m -the coal seam thickness; a -subsidence factor; R -the radius of principal influence; x -the coordinate of a point from the surface; y -the coordinate of the infinitesimal excavated element.This method is based on Gaussian distribution of the probabilities.

SURFACE SUBSIDENCE PROGNOSIS AT PAROȘENI MINE
In order to evaluate the deformations of the surface in the Paroseni mining field, Knothe's influence function method was applied to the panels extracted on the coal seams No.3 and 5 after 1990, which are considered to have some influence on the surface (it should be noted that in this mining field 4 panels are extracted from the Vulcan mine).Thus, the area of interest was delimited (Fig. 3), and by applying the aforementioned influence function and joining the subsidence trough associated with each panel (using the Excel and AutoCAD software), we obtained a common subsidence trough, 3D image of which is shown in Fig. 4. At the end of 2009, a monitoring station was situated in the area, but since it was done after the start of the underground mining, a large part of the surface movement was not recorded.It is also considered that the predicted subsidence trough is the final one.
In Figure 5 is the curve of predicted subsidence and the measured subsidence on 05.12.2015 in the monitoring station.
Figure 4 represents the location of the proposed object.

Figure 5. Predicted and measured subsidence curves
As can be seen (both in Figure 4 and in the graph presented in Figure 6) this object was not projected on a stable route and may suffer from subsidence of about 3 m.

Figure 6. Predicted subsidence curves on the route of the projected object
An important parameter related to the stability of the surface and of the objects located in the area is the horizontal displacement (after the two directions X and Y), which is graphically represented in Figures 7 -10.

Figure 7. Predicted horizontal displacement after X axis
As can be seen from the horizontal displacement graphs, the surface undergoes significant displacements in both directions due to the numerous panels under study.By evaluating the predicted horizontal displacements for the originally designed object, we can see that it can suffer transversal horizontal displacements of about 1 m (+300… -600 mm) and longitudinal horizontal displacements of approx. 2 m (+1500…-1500 mm).For these reasons, the originally designed route cannot be safe from the point of view of stability and it is recommended to deviate it to a more stable area.

CONCLUSIONS
The influence function method belongs to the empirical methods group applied for the prognosis of the surface deformation due to underground mining.Unlike the profile function method, this method can be applied and operated in the case of several extraction areas, or in the case of the areas with difficult extraction configurations.
By applying the influence function method to the case of the present research, we obtained an overview of the surface deformation as a result of underground mining at Paroseni mine (Jiu Valley, Romania).Following this analysis, it has been observed that the surface suffers major deformations caused by the underground mining both vertically (in the form of subsidence) and horizontally (as horizontal displacement).
By comparing the results obtained by applying the influence function to those measured at the surface, on the monitoring station, it can be observed (Fig. 5) that the displacements are higher than predicted.This is due to the fact that the monitoring station was placed long after the start of the extraction in the area (thus recording only a part of the movement) and the predicted displacements are considered to be the final ones.
Regarding the stability of the projected object (considered in the paper), its route cannot be realized without being affected in the future (more or less) by the underground mining, so it is recommended to move it to a stable area.
If, despite all the inconveniences, it is decided to place the object in this area, it is advisable to take additional safety measures to prevent both the surface deformations and the stresses generated by these deformations.

Figure 3 .Figure 4 .
Figure 3. Prognosis of surface subsidence in the case ofParoseni mine

Figure 8 .
Figure 8. Predicted horizontal displacement after X axis: (a) on the route of the projected object; (b) on the route of the monitoring station

Figure 9 .
Figure 9. Predicted horizontal displacement after Y axis