1st North American Rock Mechanics Symposium

ABSTRACT: In situ stope leach mining is pan of a clean mining research program designed to increase the domestic supply of metals for the United States. The U. S. Bureau of Mines (USBM) is developing this innovative mineral recovery technology to help preserve the ecosystem. The USBM is applying a systems approach to develop mineral recovery methods that minimize environmental impact by selectively extracting and recovering targeted minerals. The motivation for this approach derives from the conviction that we can no longer afford the environmental or economic cost of moving large volumes of rock to recover small amounts of metal. To accomplish a clean mining system, one must maximize the solutions contact with ore minerals and also maintain control of the leach solutions. This can be achieved by performing in situ fragmentation to obtain relatively uniform rock particle size to increase rock mass permeability, and also minimize blast damage to the perimeter of the excavation. The USBM is investigating the feasibility of in situ stope leach mining, where leaching solutions are passed through rubblized ore. Rubblizing blocks of ore by blasting for subsequent underground in-stope leaching requires special applications of blasting techniques.
This paper presents the results of in situ fragmentation of a simulated cylindrical stope which is 1.8 m in diameter and 6.1 m high. The blast design combined conventional and smoothwall blasting techniques. Three drop-raise and smoothwall blasts were used to fragment the simulated test stope.
The average powder factor for the stope fragmentation was 2.6 kg per metric ton, compared to 1.10 kg per metric ton for routine drift development work at the mine. All the material from the first blast, 14.2 metric tons, was sieved. The resulting distribution was compared to the distribution predicted by empirical equations. The best fit was found with a USBM developed equation based on over 50 sieved, reduced-scale (1-to 2-m) high wall blasts. Comparing the broken rock distribution data for round I with the photo image processing data for rounds 1, 2, and 3 shows that using photo image processing is cost effective and reasonable when compared to screening and weighing.

ABSTRACT: The edge function method for anisotropic elasticity is presented. Analytical solutions model field behavior in critical parts of the domain of interest. Boundary conditions are matched using a variational principle. Accurate results are obtained for a range of rock mechanics problems involving gravity forces, cavities, cracks and singular loadings.

ABSTRACT: This paper presents new capabilities of the discontinuous deformation analysis (DDA) when combined with "the artificial joint concept". These capabilities refine the determination of the refined stress distribution and permit simulating fracture propagation in intact blocks.

ABSTRACT: The design of rock slopes requires the quantification of many field parameters which are characterized by variability and uncertainty. In the past, average or expected values of those parameters were utilized for obtaining evaluations of slope stability in terms of safety factors. Today, engineering design requires to assess risk and to incorporate cost constraints in order to balance safety with economy before appropriate decisions are taken. Therefore, probabilistic analyses of slope stability must be based on a reliable framework involving all relevant factors, thus justifying a sound approach to formulate their variability and uncertainty. A description of these concepts and examples of their implementation are presented.

ABSTRACT: Creep of the rocks surrounding a horizontal tunnel is considered. It is shown where the rock becomes dilatant, where compressible, if and where failure will occur and how much rock will be involved, where and when creep failure is to be expected, and how creep failure spreads into the mass of rock. The influence of an internal pressure on the above phenomena is also studied.

ABSTRACT: The geometric evolution of a fracture set in a controlled laboratory experiment is accurately predicted using a physically-based network simulator (PBNS). The experimental fracture sets were produced by straining a brittle coating applied to an acrylic substrate. The physically-based fracture sets are generated using iterative solutions to the boundary value problem for multiple cracks in an otherwise homogeneous, isotropic, and linear elastic solid subject to uniform remote stress boundary conditions. A preliminary methodology for conditioning the PBNS networks to data from seismic surveys and well bore flow tests is also presented. The advantage of this methodology is that information concerning the elastic and fracture characteristics of the rocks and the mechanics of the rock fracture process can be combined with other well bore data to better estimate the geometry of a particular fractured formation.

ABSTRACT: As a part of the Canadian Rockburst Research. Programme (aimed at investigating the performance of rock support under rockbursting conditions) in-situ dynamic monitoring of various types of rockbolts loaded by blasts was performed. The aim of the monitoring was to investigate and explain the interaction of the dynamic load with the rock-rockbolt system. The measured strain waves on the bolt and the dynamic movement of the rock surface were successfully simulated numerically using a one dimensional finite difference representation of the mechanical system involving the wave source, the rockmass, the mechanical rockbolt, and the anchor and head connections. The model was calibrated using field data from the blasts, and parametric studies of the influence of various system parameters on the dynamic behaviour of the rockbolt were carried out. It was found that the wave source, rockmass modulus, tensile strength, connections at the rockbolt ends, and rockbolt pre-tension all play important roles in determining the dynamic stress levels in rockbolts.

ABSTRACT: Scaly clay is an argillaceous sediment, with a pervasive fabric of lenticular, highly polished, curved, slickensided surfaces and is commonly associated with mélange formations. In regions subject to tropical weathering regimes outcrops of this material erode rapidly giving rise to extensive badlands topography. Islands such as Barbados and Taiwan lose considerable areas of agricultural land to badlands generation each year and associated landslides disrupt settlements and communications.
A programme of laboratory studies has been completed to characterise the physical, mineralogical and mechanical properties of undisturbed samples of these materials. The samples, collected from different depths within weathering profiles, between ground surface and fresh materials with no sign of weathering at depth were each subject to similar experiments. These data illustrate important changes during weathering, including destruction of the original sedimentary structure, a large increase in pore volume and variations in strength.

ABSTRACT: The objective of this U.S. Bureau of Mines investigation was to characterize the change in the overburden due to mining of a 270 m wide longwall panel. The overburden thickness ranged from 95 to 105 m. Five-anchor, multipoint borehole extensometers (MPBX) were installed in each of five coreholes to measure subsurface displacement. Survey data of an array coincident with the boreholes show that surface movement began when the longwall face was about 45 m in front of any point. Over 90% of the total subsidence was achieved when the face was one overburden thickness past a point. Overburden failure was detected by the MPBX units when a borehole was undermined. Overburden failure occurred first in the center of the panel and then migrated outward toward the panel margins. The overburden appeared to collapse vertically as a unit and was laterally restricted to the mined-out area of the panel.

INTRODUCTION

Understanding and modeling seismic wave propagation from nuclear explosions require information on the behavior of rocks in the source region under appropriate loading conditions. Within the source region the rocks are loaded in uniaxial strain; during unloading damage occurs, permanently altering the rocks. Rock properties measured at the laboratory scale constrain the models. Static and dynamic elastic moduli and attenuation as a function of mean stress and strain amplitude are incorporated into forward models to determine particle velocity and displacements and time histories in the nearfield.
An underground nuclear explosion in granite was detonated near Semipalatinsk as part of a Joint Verification Experiment with the USSR. In order to model the range of possible responses and further our understanding of the relationship between static and dynamic moduli, a suite of experiments was performed on five granites representative of the source region.