Abstract:
Application of downhole geophysical logging techniques, specifically Acoustic Televiewer (ATV) and sonic logging, is used to significantly enhance pit slope stability assessment for proposed or expanding open pit and quarry mining operations. Acquisition of discontinuity orientation data required for slope stability assessment using downhole geophysical techniques is faster, more accurate, and significantly less expensive than acquisition via oriented core drilling. As a result of the cost effectiveness, additional data can be collected to provide a more thorough understanding of spatial variations in discontinuity patterns throughout a proposed open pit mine area. Discontinuity data can be collected utilizing rotary and reverse circulation drilling methods, reducing the need for expensive, time-consuming core drilling. The speed of the data acquisition using down-hole geophysical methods in rotary holes, as compared to acquisition via oriented core, can eliminate interruptions to mining operations. Data collected via ATV logging is coupled with in-pit observations and, if available, core data resulting in fast, cost-effective stability evaluation. Field data collection involves drilling and geophysical logging of holes, geologic and discontinuity mapping by geologists, and collection of core or rock samples for laboratory analysis. The data collected is utilized to conduct both deterministic and probabilistic assessment of kinematic and overall slope stability conditions.
Introduction
Relatively recent slope failures have increased attention, including that of regulatory agencies, to slope stability assessment in open pit mines. Generally speaking, drilling efforts often understandably focus on exploration of an ore body. Such practices often produce geologic models that define the ore body very well, but that contain very little to no geotechnical (and sometimes even geological) data with regard to the expected pit wall rock. Geotechnical data can be collected via mapping by geologists in excavated areas, but open pit mine mapping is often limited due to ongoing operations and potentially unsafe access conditions. Additionally, the only way to assess the geotechnical aspects of proposed pit walls in unmined areas (expansion or new mining) is via drilling.
Downhole geophysical logging tools have been used for geotechnical characterization of rock and subsurface conditions associated with mining and non-mining applications for many decades. Examples include road cut slope stability assessment [1], fractured rock aquifer flow assessment [2], and fracture delineation for tunneling [3]. Specific to mining, the technology has been successfully utilized to collect geotechnical data in numerous hard rock, coal, and aggregate mining scenarios in both the eastern and western United States.