In block caves, the flow characteristic of the caved rock strongly influences ore recovery and dilution content. Consequently, caved rock flow characteristics plays a fundamental role in determining, at an engineering level, optimal production level layout and draw control practices. Since the first applications of block caving, a significant amount of research has been focused towards an understanding of the flow characteristics of the caved rock.
Laubscher's gravity flow principles for block caving mine design, widely used by the industry, were initially derived in the 1980's using a combination of sand physical models and observation in the field of the flow of finely fragmented caved rock. Those principles have involved the definition of the geometry of the Isolated Movement Zone (IMZ) and the interaction limit for defining the production level layout. Afterwards, other researchers have continued with sand modelling work confirming and improving on Laubscher's gravity flow principles. In today' s mining practice, block caving methods are being applied to a range of competence of rock masses and block geometries which have resulted in a wide range of caved rock sizes from fine to very coarse. There is a general concern in the industry about the applicability of the principles of gravity flow derived from sand models to the full range of modem block cave conditions.
This has motivated in the last years studies on the mechanisms of flow in coarse caved rock through large physical models and numerical modelling. Physical modelling work in large three dimensional models using gravel as the model media has been very helpful in understanding the location of the extracted material when drawing from a single drawpoint. However, there is still debate on the controlling parameters and mechanisms under isolated draw. Additionally, to date large three dimensional physical modelling has not been conducted to study the mechanisms of draw under multiple drawpoints using coarse fragmented materials. Numerical modelling using both particle flow codes and continuum models has been used to help describe the flow of coarse cohesionless granular materials. Mechanisms of isolated draw and interaction of IMZs as observed in sand models have also been observed in numerical models but those require to be validated through experiments.
In this research, the mechanisms of the flow of coarse caved rock were studied by conducting controlled experiments using the largest ever three-dimensional physical model built to date.
This research aimed mainly to investigate the mechanics of flow in both isolated and multiple drawpoint draw and to incorporate those observations into a cellular automata numerical approach to simulate flow. Previous to the experimental work, the physical modelling conditions were defined by means of an analysis of similitude based and on a benchmark study of block caving practices. Experimental studies of the mechanisms of isolated draw showed that the main controlling parameters that define the geometry of the IMZ are the mass drawn and the column height. The influence of other parameters such as particle size and drawpoint dimensions were varied but showed not to have a statistical major role in determining flow behaviour under the experimental conditions. Detailed modelling of the experimental data along with stress measurements during draw were used to describe the mechanisms controlling the evolution of the geometry of the isolated movement zone.
Experiments were conducted to study the mechanisms of mass flow and the limit of interaction under multiple drawpoints using different drawpoint configurations. The results indicated that when the distance between drawpoints was such that IMZs overlap mass flow was initiated under concurrent (ideal) draw. When the distance between drawpoints was such that IMZ did not overlap, vertical stresses in the unmoved zone increased. However, movement zones between adjacent drawpoints only interacted when the distance between drawpoints was smaller than that of the IMZ's width.
Stress data in the large physical model also showed that at model conditions there was significant stress arching expected to occur in a confined granular material. Analysis of vertical stresses and material characteristics in previous sand models were conducted which indicated an absence of stress arching and the use of weak under shear material conditions that could have induced failure of unmoved areas between IMZs.
In the current mining design criterion, interaction is the main mechanisms to justify the spacing of drawpoints at up to 1.5 wIMZ. The current research and those from sand models suggest that interaction is a function of stresses and material shear strength characteristics (friction angle and cohesion). These conditions characteristics under full scale are not well understood. This suggest that it essential to investigate in-situ and induced stresses during flow and caved rock shear strength characteristics if interaction is to be used as part of the design criterion in block caving mines. Advanced numerical models using results of this research may be an avenue of investigating the observed behaviour in the physical model but under full scale conditions.