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Content archived on 2024-04-16



To improve the profitability of the underground mining of soft rock deposits by analyzing the stability of mine workings using numerical models and on site inspections and by technically and economically optimizing the dimensioning and reinforcement of pillars.
For a soft rock mining method where the principle is that of creating stopes that are 35 to 50 m long, 6 m wide and 32 m high, separated by rib pillars of 25-30 m width, the constant preoccupation of the mine manager is to improve the profitability of the operation without endangering the safety of the mining staff and equipment. To increase the recovery ratio, the possibility of leaving higher and wider stopes, without backfill, for a period of up to several months, combined with a reinforcement of the rib pillars by cable bolting, was examined. To this end, it was necessary to carry out the following works in an experimental stope: a complete geostructural and geomechanical survey of the rock mass and laboratory tests (talc and serpentinite walls); monitoring of stress change in the pillars, using in particular hydraulic tests on pre-existing fractures (the HTPF method), before and during the mining works; measurement of the variation and distribution of forces acting upon the cable bolts that reinforce the stope walls, with special extensiometric gauges.

In situ stress measurements using the HTPF method made it possible to determine the initial stress rate and to monitor stress changes in a rib pillar during mining of the adjacent stope. The natural stress tensor is characterized by a principal horizontal component direction parallel to the talc deposit axis, which corresponds to a major shear zone. The mode of emplacement and structure of the deposit govern the stress field at the scale of the mine. The initial vertical stress (approximately 28 MPa) was found to be greater than the weight of overburden (at a depth of 600 m, approximately 16 MPa). Interpretation of the tests made in a rib pillar has not only shown the influence of mine working (rotation of the horizontal principal stress direction as well as significant variation of its modulus), but also the existence of a stress discontinuity that delimits two rock compartments with different mechanical behaviour.

From the extensiometric gauge measurements, a principal component analysis was done to correlate strain with blasting and to see if a relationship existed among the gauges themselves. The gauge strain was used to define a tentative block size within the zone of influence of the cable bolts, taking into account the results of the structural study of the fractured rock mass (joint sets).

To obtain a better understanding of the mechanical behaviour of the soft and fractured rock mass, as well as of the interaction between such rock and its reinforcement, the investigations and measurements carried out in the experimental stope were considered in 2-dimensional and 3-dimensional analyses. Numerical modelling, using respectively a distinct element method and a finite element method, was developed for the large scale simulation of the effects on the natural stress state and its variation, of topography, geometrical conditions of the orebody, and the mechanical properties of the talc rock mass and its serpentinite host rock. In this way, the impact of the Young modulus contrast between talc and serpentinite on the calculated stress modulus values could be investigated. The results indicate a good correlation between numerical models and measured stress values, when the in situ deformation modulus of the serpentinite is about 8 times higher than that of talc. Compared with the results of deformability tests carried out in the mine and the laboratory, this contrast value seems to be acceptable.

The experiments and measurements carried out in the underground Lanzada talc mine served to obtain a better understanding of the mechanical behaviour of a soft fractured rock mass at great depth, and have led to the technical and economic optimization of design and reinforcement by cable bolting of the mine workings, which will guarantee a further improvement in stability and safety within the mine as well as greater profitability of the exploitation as a whole.
The project consists of the following 6 stages.

Stage 1 is the structural and geochemical exploration of the deposit. Politecnico di Torino (PdT) has been given the task of studying the structure of the rock mass by means of seismic tomography and surveys of the fracturing of side walls and oriented drill cores to be made by UNITALC. BRGM will determine the state of the natural stresses in an unworked zone by the technique of fracturation and/or hydraulic stimulation of natural or provoked fractures. Several tests will be carried out on the drill cores to determine the geochemical character of the mineralized structure and its surrounding rock.

Stage 2 is the experimentation in a pilot stope. In close consultation with the other participants, UNITALC will set up a pilot stope together with the additional infrastructural works this will require. In this experimental working, all the observations and measurements necessary for numerical modelling will be carried out in actual mining conditions.

Stage 3 is the installation of measuring and monitoring devices. UNITALC will supply and help to instal anchor cables and various devices for measuring the vertical and horizontal components of movements of the rock mass during excavation (convergence indicators, strain gauges) and for measuring variations in total pressure by means of load cells. Variations in the stress state in a pillar in the underground pilot working will be determined by BRGM using the hydraulic stimulation technique, studying throughout the experiments the same fractures preselected from the results of the first hydraulic tests carried out during the geochemical characterization of the deposit.

Stage 4 is the numerical modelling of the mechanical behaviour of the reinforced rock mass. A behaviour model of the rock mass, reinforced as the excavation work proceeds, will be set up using all data collected.

Stage 5 is the study of achievements as regards profitability and mining safety. The results of these various models should make possible a deeper and wider understanding of the mechanism governing the behaviour of fractured soft rock masses and the role of reinforcement by long roof bolting. The experiments will also lead to the development of new methods for working deposits of soft rock by a better design of systematic reinforcement by long roof bolting using passive anchor cables. These new methods should make it possible to optimize the rate of recovery from the deposit while at the same time ensuring the safety of the operation.

Stage 6 is drawing up the final report.


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Bureau de Recherches Géologiques et Minières
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Avenue de Concyr
45060 Orléans - La Source

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