R.J. Pine

Rock mass properties for engineering design

The determination of rock mass properties for engineering design is considered from twoperspectives. These are in-situ measurement, including classification-based methods, and the limitations of the classification approach. Several measurement methods are available which will give useful results, if used appropriately. The choice of different methods will depend on the nature of therock mass. The required accuracy should be considered realistically and, in many cases, high levels of accuracy are not, in fact, necessary. Examples are included, which show how the mass strengths of mine pillars were determined with acceptable accuracy using well known rock mass classification schemes, modified as necessary to accommodate local conditions. Rock mass classification is a widely used, economical and extremely useful basis for determining properties, but there are dangers in uncritical application. The classification methodology is critically examined, and the use of multivariate statistics applied in a multi-dimensional space is considered to optimize the usefulness of the measured data.

In-Situ Hydraulic Parameters for the Canirnenellis Granite Hot Dry Rock Geothermal Energy Research Reservoir

A research hot dry rock (HDR) geothermal energy reservoir is currently under development in the Carnmenellis granite in Cornwall, U.K., at a depth of about 6,600 ft (2000 m). This paper details the performance, analysis, and interpretation of hydraulic tests using low to medium pressures and flow rates. These tests allowed an evaluation of the in-situ hydraulic parameters relevant to initial water storage and long-term leakage. Tests were analyzed using both diffusion theory and an explicit, coupled hydraulic/mechanical computer code. At low fluid overpressures, the permeabilities, in terms of uniform diffusion, were in the range of 1 to 10 ..mu..d, rising to about 60 ..mu..d at overpressures of about 725 psi (5 MPa). These figures imply that long-term leakage from an HDR reservoir is minor. The computer model was able to simulate several cycles of injections and shut-ins at progressively higher pressures and flow rates with a single set of variables.

A probabilistic method for predicting the formation of key blocks

Abstract Probabilistic distributions have been fitted to joint mapping data obtained from South Crofty tin mine. This allowed a simple deterministic model of rock jointing to be developed. Using this model, key block analysis of the 380 Fm level footwall drive, of the No. 8 lode was carried out. The inherent limitations present in this simplified analysis are noted and the need for a probabilistic approach to key block formation is shown. The limitations of existing probabilistic methods of analysis are also examined. A different form of probabilistic analysis for key block determination has been developed using the computer code B3LHS. This method does not suffer from the inadequacies associated with other methods of probabilistic analysis. A comparison was made between the results from B3LHS and deterministic key block analysis. It was found that this new model predicted the formation of more key block types than could be generated using the deterministic model. In addition, the probability of the formation of key blocks can now be determined. Threshold values have been suggested that link the probability of key block formation with the use of specialist or non specialist staff in the implementation of rock bolt support.

Vipiteno mine, Italy – underground mining for marble

Abstract There is increasing emphasis on the underground mining of industrial minerals in the UK and Europe. A recent example is Vipiteno mine in Northern Italy, whose products are crushed and powdered translucent white marble. The marble is in seams with minimal dark mineral impurities, about 20–30 m thick, dipping at about 45–50° extending about 1·5 km along strike and several hundred metres down dip. The mine is in mountainous terrain subject to heavy winter snowfalls. Production strategies have been developed using high and low elevation underground mines. A combination of large cross-section development and bulk mining methods has been adopted, notably open stoping with drawpoint extraction. Stopes excavated to date are up to 50 m high x 20 m true thickness x 80 m along strike. Higher stopes are planned. The rock mass is of good quality, with a GSI rating of up to 70 and a Q rating of up to 15. It is dominated by very continuous bedding, which controls hanging-wall stability. In some locations, faulting also exerts control on hanging-wall stability. The mass strength is about 24 MPa and the in situ stresses are estimated at 5–10 MPa. Pillars have been designed conservatively and the major design challenge is the hanging-wall stability. Extensometer instrumentation has been installed to monitor hanging-wall stability in some early stopes and these indicate stable behaviour to date. It is planned to modify current mining methods to minimise development costs by simplifying and eventually eliminating drawpoints.

Analytical and Numerical Modeling of High Pressure Fluid-Rock Mechanical Interaction in HDR Geothermal Energy Reservoirs

Publisher Summary This chapter discusses analytical and numerical modeling of high-pressure fluid-rock mechanical interaction in hot dry rock (HDR) geothermal energy reservoirs. In the case of HDR exploitation, the contribution of microseismic location and interpretation is of fundamental importance for a satisfactory modeling interpretation of processes, dimensions, magnitudes, stress orientations, and locations. Models are evolved gradually in response to field observations, commencing with relatively simple analytical models and progressing to more complex numerical models. Pseudo-3D and true 3D modeling are now possible. Further modeling sophistication is possible and can certainly develop. However, there can always be a limit to the quantity and quality of field data available because of the remoteness of the environment. The following are the main variables considered in the modeling: (1) joint orientation, spacing, aperture, shear and normal compliance, shear strength, dilation, and connectivity; (2) rock mass deformability; (3) fluid flow regime in joint networks; and (4) far field stress and hydraulic boundary conditions.