Objectif
- TO STUDY THE COMPOSITION OF THE VARIOUS (INERTS AND SUBSTITUTIVE MATERIALS) CEMENTED BACKFILLS FOR STOPINGS AND THEIR MECHANICAL CHARACTERISTICS. LABORATORY TESTS.
- TO DEVELOP OPTIMIZATION OF GEOMETRICAL SCHEME FOR EXPLOITATION OF A MINING ZONE.
At both Campo Pisano and San Benedetto Mines massive, irregular shaped orebodies are mined and the ore and the wallrock are characterized by low mechanical strength so that it is not possible to open up large stopes. Mining has to be done under a stable roof, which can only be produced artificially and any subsidence on the surface has to be avoided. Therefore, only a stoping method with complete filling of the cavities can be applied. The only stoping method which complies with the requirements appears to be an underhand cut and fill method with cemented backfill. Provided the slices are accessible by a ramp system, the method can be highly mechanized.
A backfill mix has been designed composed of low cost aggregates, portland cement, replaced partly by fly ash, and water. For cemented backfill, strength and workability are the most important parameters and they were both achieved as a result of and exhaustive preliminary testing programme. With the composition adopted, a compressive strength of 10 MPa after 28 days was steadily attained. The workability, together with an accurately designed geometry of the delivery system, made the full utilization of the mixing plant capacity (ie 200 cubic metres per 8-hour shift) possible. Particularly attractive features of this method are its high flexibility, the relatively simple infrastructure required underground and the low labour requirements.
In situ measurements so far have shown unexpectedly small stress values in the backfill. Even during undermining the stresses show no excessive values, at least not for a timespan of several months. In addition, no failures of the artificial roof of the stopes have been observed even with free spans of up to 10 m.
Results from numerical calculations, in particular with the finite element method, in comparison to the in situ measurements, have shown that this method is useful for estimating the stresses in the backfill during the mining process. The finite element method can therefore be used to optimize the stoping and backfilling sequence. It seems also possible to optimize the geometry of the stopes with this method. However, the method could only be applied under the assumption of continuity and homogeneity of the emplaced backfill. With application of cemented backfill, subsidence can be totally avoided. The application of numerical calculations revealed that an arching phenomenon takes place, which distributes the load from the overlying strata to the country rock thus considerably releaving the immediate stoping are from most of the load exerted by the overlying material.
The primary aim of the project was the development and optimization of cemented backfill stoping methods in some mines which comply with the following requirements: sufficient strength of the backfill to provide an absolutely safe support to the stoppe and to prevent and subsidence of the country rock with consequent environmental damage; formation of the cemented backfill mixes having resort to low price aggregates and binders; and competitivity with current stoping methods. The research conducted has been developed according to 3 main lines: identification of the aggregates and binders most suitable for achieving the proposed objectives, determination of the loads acting on the backfill for the various stoping geometries and optimization of the stoping methods. The successful development of a proper mix design and the adequate adjustment of the concrete delivery pipelines geometry has resulted in considerable improvements in stoping operations and in the achievement of a constant quality of cemented backfill. The regularity in operations, thus achieved, has made the instalation of a monitoring system possible. The observed lack of variations in the stress distribution within the cemented backfill might be an indication that the backfilling of each slice acts as a self supporting structure, at least for the moment.
MAINLY STEPS ARE:
- ELABORATION OF ANALYTICAL MODELS,
- TESTS ON THE CEMENTED BACKFILL AND MEASUREMENTS IN "SITU",
- DEVELOPMENT OF A MODEL, MONITORING, CONTROL AND USE FOR THE OPTIMIZATION OF EXPLOITATION PATTERN.
THE POTENTIAL ECONOMICAL BENEFITS WILL RESULT FROM THE ECONOMIC OPTIMIZATION OF THE STOPINGS. VIA:
1.- THE USE OF MATERIAL SUBSTITUTIVE OF THOSE COMMONLY AND TRADITIONALLY USED;
2.- TO CARRY OUT BACKFILLINGS WITH DIFFERENT MECHANICAL PROPERTIES ACCORDING TO AREAS OF THE MINING ZONE;
3.- STUDY OF VARIANTS TO THE TRADITIONAL SCHEMES OF STOPING WITH BACKFILL.
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CORDIS classe les projets avec EuroSciVoc, une taxonomie multilingue des domaines scientifiques, grâce à un processus semi-automatique basé sur des techniques TLN. Voir: Le vocabulaire scientifique européen.
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09016 Iglesias Cagliari
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