ABSTRACT: Laboratory drilling experiments were conducted to determine the effects of different test variables on the development of fracture-like breakouts in high porosity Berea sandstone. Tests showed that breakout length is directly proportional to the horizontal principal far-field stress differential, suggesting a potential use of this correlation to estimate in situ stress magnitudes. Larger boreholes produced increasingly longer breakouts, from which we infer that breakouts could reach lengths of several meters in typical oil-field wellbores. Within the ranges tested, neither drill-bit penetration rate, nor drilling-fluid flowrate, had a discernable effect on breakout dimensions or shape. One consistent characteristic of fracture-like breakouts, regardless of testing conditions, was their near-constant width, which averaged 3.3±0.3 mm in all tests. Long breakouts, expected to develop in regions of high differential stresses in rock having similar characteristics to Berea sandstone, may be a potential source of sand production.
ABSTRACT: This paper defines the principal aspects of primary and secondary rock fracture by impact. It discusses mechanical rock fragmentation and cutting tool design. And it investigates those physical rock properties that can be used to predict excavation rate performance. Items that are referred to for specific discussion are:
ABSTRACT: Instrumented cable bolt support [SMART - Stretch Measurement for Assessment of Reinforcement Tension - cables] was developed in Canada [Canadian Patent #2,200,834] and has been in use for the past three years. These instruments have achieved wide acceptance in the Canadian mining sector and are now being exported internationally. A significant case study database has been developed using SMART cables, SMART MPBX's and SMART contractometers under a wide array of mining situations and ground conditions. The instrumentation is routinely used today to help achieve both operational and safety goals. The paper briefly summarizes findings from various case studies that illustrate the positive economic impact this technology brings to operations, both from an economic and safety perspective. A recent case study illustrating cable bolt performance [measured load steps] under dynamic rock burst load conditions is discussed in detail.
Chang, K. (Korea Electric Power Corporation) | Lee, E.-Y. (Korea Electric Power Corporation) | Kim, C.-L. (Korea Electric Power Corporation) | Park, J.-W. (Korea Electric Power Corporation) | Kwon, S. (Korea Atomic Energy Research Institute)
ABSTRACT : Reasonably good agreement was found between the distribution of conductivity estimated from simulated and actual packer tests. It was also found that the correlation between the block conductivity and conductivity estimates from the simulation of packer test was weak. The numerical experiment was carried out to investigate the scale-dependency of groundwater inflow into a tunnel by using discrete fracture network (DFN) and equivalent porous medium (EPM) approaches. For REV scale, the discrepancy of average groundwater influx between the two approaches was not significant regardless of the ratio of tunnel length to the length of model section. The groundwater influx estimated from DFN approach seemed not to be reasonable for below REV scale. And it was found that there was a large discrepancy of groundwater influx between DFN and EPM approaches for below REV scale.
ABSTRACT: At low mean stresses, porous geomaterials fail by shear localization, and at higher mean stresses, they undergo strain-hardening behavior. Cap plasticity models attempt to model this behavior using a pressure-dependent shear yield and/or shear limit-state envelope with a hardening or hardening/softening elliptical end cap to define pore collapse. While these traditional models describe compactive yield and ultimate shear failure, difficulties arise when the behavior involves a transition from compactive to dilatant deformation that occurs before the shear failure or limit-state shear stress is reached. In this work, a continuous surface cap plasticity model is used to predict compactive and dilatant pre-failure deformation. During loading the stress point can pass freely through the critical state point separating compactive from dilatant deformation. The predicted volumetric strain goes from compactive to dilatant without the use of a non-associated flow rule. The new model is stable in that Drucker's stability postulates are satisfied.
ABSTRACT: The motivation for this study has been to establish a relation between static and dynamic Poisson's ratio based on sonic measurements. The work was based on laboratory tests where both static moduli and sound velocities were measured simultaneously. The results showed that the static and dynamic Poisson's ratio behaved differently with increasing shear stress. We also found indication of creep effects on the Poisson's ratio. Based on the formalism established by Fjær (1999) a relation between static and dynamic Poisson's ratio was established, which describes the relation during the entire strength range for a standard rock mechanical tests. The relation depends on the key elements used to describe the relation between the static and dynamic values for bulk modulus and Young's modulus, plus an additional term explicitly associated with dilatancy. The equation was found to give a good description of how the Poisson's ratio develops during the test. However, the model occasionally overpredicts the value.
ABSTRACT: The purpose of this study is to investigate the effect of mechanical properties of the impact material, and the shape of boulders on the coefficients of restitution. Four slope platforms with different Young's modulus (granite, shotcrete, plaster and soil) were used in this study. Four different shapes of boulders: spherical, cylinder, hexagonal prism and cube are prepared. Images of each test are captured by using a high-speed camera. Based on the experimental study, it is found that the coefficients of restitution are strongly affected by the elastic modulus of the material of the platform and the shape of boulder. A slope with smaller value of Young's modulus is conducive to permanent deformation during the impact, and, consequently, lower coefficients of restitution will be resulted. For the shape effect of boulders, a more angular boulder, will lead to lower rebound amplitude with rotation. With this induced rotation, the normal rebound velocity is reduced but the tangential velocity is increased. Three empirical formulas are proposed for Rn estimation under the various modulus an different shapes of boulder.
ABSTRACT: An array of ultrasonic transducers was constructed consisting of three identical arrays at various depths in an air intake shaft at the Waste Isolation Pilot Plant (WIPP). Each array consists of transducers permanently installed in three holes arranged in an "L" shape. An active array, created by appropriate arrangement of the transducers and selection of transmitter-receiver pairs, allows the measurement of transmitted signal velocities and amplitudes (for attenuation studies) along 216 paths parallel, perpendicular and tangential to the shaft walls. Transducer positions were carefully surveyed, allowing absolute velocity measurements. Installation occurred over a period of about two years beginning in early 1989, with nearly continuous operation since that time, resulting in a rare, if not unique, record of the spatial and temporal variability of damage development around an underground opening. This paper reports results from the last two years of operation, updating the results reported by Holcomb, 1999. Results will be related to the damage, due to microcracking, required to produce the observed changes. It is expected that the results will be useful to other studies of the long-term deformation characteristics of salt.
ABSTRACT: Under certain conditions of deep drilling with PDC bits, violent stick-slip torsional vibrations take place in the drilling structure. In the classical approach used to analyze this problem, it is assumed that stick-slip events arc triggered by a bit-rock interaction law, characterized by a decrease of the torque with angular velocity (which is formally equivalent to a decreasing apparent friction coefficient with angular velocity). Based on laboratory data, we argue that this relationship is not an intrinsic characteristic of the bit-rock interaction. We then elaborate on the coupling between vertical and torsional vibrations at the bit as a possible primary cause of stick-slip. Preliminary results obtained with this model indicate that such coupling is sufficient to generate stick-slip without the need to assume a decreasing friction coefficient.
Cracks develop in brittle materials under compressive load as a result of growth of microcracks causing irreversible damage and produce acoustic emission (AE). At high applied load, this damage causes strain localization leading to the ultimate fracture plane. Laboratory studies are used to investigate the fracturing process in a brittle rock to identify the precursory sequences of a rockburst or an earthquake. In this study, the objective was to develop a methodology to predict the ultimate fracture in laboratory-scale rock specimens of Apache Leap tuff by monitoring the clustering phenomenon of the AEs. Multifractal analysis of spatial distribution of the AEs, collected in laboratory experiments under uniaxial compression, indicates that the time for development of an ultimate failure plane can be identified through a sharp decrease in the fractal dimension. Clustering of seismic events at a particular location indicates the location of the failure plane.