Experiences from realized sealing-projects of abandoned shafts derivation of recommendations

Transcription

1 Experiences from realized sealing-projects of abandoned shafts derivation of recommendations Bodenstein, Jörg ; Sitz, Peter Summary Considering the protective aims for mine abandonment, the presentation will show with examples, how the basic concept of shaft sealing can be modified in relation to the given geotechnical and geological situation. The authors will explain recommendations deduced from experience. Introduction Because of the abandonment of mines in East Germany during the last 0 years, it was necessary to close and seal a large number of shafts. Furthermore, a lot of old shafts of potash and rock-salt mining have been abandoned and closed many decades before. From the current point of view, the actual closing construction of these shafts in most cases must be seen only as a temporary solution. In summary, between 990 and 000 the authors were involved in investigation as well as in planning and carrying out the sealing for about 40 shafts of abandoned potash, fluorspar, copper and coal mines and of almost 0 old potash shafts. Based on general protective aims, which must consider also the environmental legislation, a basic concept for shaft sealing can be defined. However, the experience shows, that every individual shaft requires careful investigations and partly comprehensive modifications of the sealing concept. The widespread experience in East Germany during the last ten years has contributed to develop the state-of-art in shaft sealing. Protective aims Dependent from the state of danger risk, shafts in time must be led to a finally sealing. However, a complex risk analysis, which considers the connected mine field, will be required. Based on the mining and environmental legislation, the general protective aims for the filling and sealing concept of shafts are as follows: - avoiding of the deep fall of persons into the shaft, - avoiding of sinkholes and surface subsidence, - protecting of the ground-water, - protecting of the mineral reserves. These general protective aims must be adapted on the site-given conditions as well as on the function of the abandoned mine. If shafts are completely situated in solid rock formations, the occurrence of sinkholes and subsidence normally must not be expected. For the abandonment of potash and salt mines the inflow of ground-water in every case has to be avoided, because of the risk of sinkholes and dangerous subsidence over the hole mine field. Furthermore a groundwater lowering as well as the salinity of ground-water can be induced. The long-term sealing of mines, which are utilized for underground waste storage, is of special importance, to protect the ground-water from intake of hazardous contaminants. In case of flooded shafts the sealing concept should consider the geological and hydrogeological conditions in the surroundings. If the mine is situated in ground-water bearing horizons or if connections between the ground-water and the mine can not be excluded, e.g. through tectonic faults, a special sealing will be useless.

2 3 Methodology of investigations and conceptual works In every case it will be necessary to start with a complex risk analysis for mine and surface damages, considering the whole connected mine field as well as mining areas in the neighbourhood. If an access into the shaft is possible, an estimation of the state of the shaft lining and the geological conditions has to be realized. For longstanding shafts without accessibility it is recommended to carry out additional investigations, such like sounding, camera-watching, survey, brine or water sample taking and analysis. Based on the results of the above mentioned investigations and estimations, it is needful to work out a shaft closing and sealing concept. The conceptual study should be discussed with the mining legislation body to declare the frame conditions for the approval proceedings. In the second stage, the basic design for the shaft sealing construction can be carried out. Before the approval by the mine legislation body, the detailed planning should include a verification by an accepted expert. 4 Basic concept and modified variants of shaft sealing Considering the general protective aims, the shaft abandonment must be carried out as a combination of - closing, - filling, - sealing. Following appropriate guidelines and normatives, the abandonment of shafts should prefer a total filling. Where this variant can not be realized, because of the site-given conditions, a stable partly filling should be carried out as well as possible. Especially for old shafts, depending on their accessibility, a partly filling in some cases can be the only solution, to avoid surface damages. In different cases, where the overburden is built by solid rock formations, the closing has been carried out only by a specially constructed shaft cover. To integrate a sealing element into the shaft filling, the accessibility of the built-in horizon will be necessary. Sealing elements should be stabilized by using materials with low settlement behavior or cohesive plugs. The long term functionality of sealing constructions must be proven by detailed exploration of the near field zones in the sealing horizons []. As an example, the abandonment of fluorspar shafts in a mountain region has included a filling of hard-rock gravel with low settlement behaviour and a steel-concrete shaft cover construction with an opening for subsequent filling []. Figure shows the abandonment solution for an old shaft of coal mining with a depth of up to 50 m. This shaft and the connected mine field are situated in an unconsolidated ground-water bearing rock formation. There was no accessibility for any work inside the shaft. In some cases these shafts are partly closed by caving materials or masses from the surface. A stable abandonment can be realized by filling with cohesive material, which often contains a suspension of coal filter ashes. The filling is built in from the surface by using of drillholes [3]. For the abandoned potash mines it was necessary to avoid the inflow of ground-water through the shaft. Because ot the accessibility of every shaft there were good conditions for the estimation of the shaft lining as well as for the built-in of the filling and sealing materials. The depth of the shafts rises up to.00 metres, what resulted in long filling columns and partly high hydraulic pressure. Figure shows the settlement behaviour of shaft filling columns, containing screened coarse gravel and gravel-sand [4]. Sudden settlements of about 90 metres were observed. A subsequent estimation came to the result that such settlements were induced by fast inflows of water into the filling column after closing the shaft.

3 Figure Abandonment of an old shaft in an unconsolidated ground without accessibility,0 % 0,0 % wet conditions, (water inflow after filling) Settlement in % 8,0 % 6,0 % 4,0 %,0 % 0 % 0 dry conditions months after filling Figure Settlement behaviour of shaft filling columns

4 In the past, in some cases, the abandonment of potash shafts was carried out by partly filling or by integration of only one sealing element. Water inflows into the mine has been observed after a short period. The experience shows, that a total filling, with a stabilisation of the sump and the loading points, and the integration of at least two sealing elements into the filling column is necessary. An additional sealing in the overburden is leading to a lower start-up hydraulic pressure and a time-delayed access of water to the main sealing. So, the compaction of the sealing material could increase. For the shafts of underground waste storages or mines with waste-backfill operations a third sealing element will be required (figure 3, [5]). NN brickwork lining Tubbing support 4,54 m +63,09 m NN 9,5 9,5 0, Filling Overburden sealing Higher salt formation sealing Obere Dichtung Salinar 388,0 m OK Salinar Lower salt formation sealing Stabilisation of the loading points depth up to 00 m Geotechnik und Bergbau mbh Figure 3 Sealing of a potash shaft Flooded shafts, which often are standing up to 00 years, required special solutions for their abandonment, because of the restricted accessibility. The fluid sample taking and analysis showed, that a closed hydraulic system with the connected mine field can be expected, as well as inflows of ground-water through the shaft lining. The sumping of flooded shafts was not practicable at all. These shafts often are filled with sand or unknown material in the lower part since many years. In some cases the sounding showed a caving of brickwork lining and rock masses in progress [6]. The fittings were removed as well as possible in the

5 accessible part of the shaft. A total filling with hard-rock gravel has been attempted. If a sealing element was necessary, and if the built-in horizon was accessible to remove the lining and the damaged near-field zone, it could be stabilized by a cohesive plug. Partly the apply of underwater concrete was needful (figure 4, [7]). Figure 4 Abandonment of an old flooded potash shaft 5 Recommendations For the abandonment of shafts the authors give the following recommendations: The basic concept for shaft sealing must be modified in relation to the site-given conditions. A complex risk analysis, additional investigations and a stepwise planning phase for the sealing project must be required. The abandonment of shafts should prefer a total filling. For the shaft filling, the use of broken hard-rock material with low settlement behavior should be preferred. As a requirement, the authors see the removal of fittings before filling. Only if the accessibility can not be guaranteed, e.g. in case of flooding, the fittings should be left (not removed). In such cases the use of cohesive materials could be necessary. For keeping the mine air-filled the integration of at least two sealings should be preferred as well as possible. To integrate a sealing element into the shaft filling, the accessibility of the built-in horizon will be necessary. The brickwork lining and the damaged near-field zone in the sealing horizon must be removed. The sumping of old flooded shafts in most cases will not be practicable, as the experience shows.

6 Literature [] SITZ, P.; WEYER, J.: State-of-the-art and development in the field of the closure of shafts with special consideration to salt mining.- Vortrag, Ostrava, 999 [] ERCOSPLAN Ingenieurgesellschaft Geotechnik und Bergbau mbh: Ausführungsplanung Abdeckplatte Hauptschacht Schönbrunn.- Unveröffentlichter Bericht, Hartsteinwerke Vogtland GmbH, 998 [3] ERCOSPLAN Ingenieurgesellschaft Geotechnik und Bergbau mbh: Ausführungsplanung Versatz untertägiger Hohlräume zur Sicherung der B 9.- Unveröffentlichter Bericht, LMBV mbh, 000 [4] MARTIN, J.; BODENSTEIN, J.: Untersuchungen zum Setzungsverhalten von Verfüllsäulen in verwahrten Schächten.- Vortrag, 5. Fachtagung Schachtbau, Sondershausen, 999 [5] ERCOSPLAN GmbH: Ausführungsplanung zum Vorhaben Verwahrung Schacht Kleinbodungen II.- Unveröffentlichter Bericht, GVV mbh, 993 [6] ERCOSPLAN Ingenieurgesellschaft Geotechnik und Bergbau mbh: Geotechnischer Bericht zur Auswertung der Kamerabefahrung im Altkalischacht Felsenfest.- Unveröffentlichter Bericht, Thüringer Oberbergamt, 999 [7] LEUSCHNER, J.: Verwahrung des Schachtes Conow.- Vortrag, 4. Fachtagung Schachtbau, Bad Salzungen, 997