Detecting Problems With Mine Slope Stability

Slope stability accidents are one of the leading causes of fatalities at U.S. surface mining operations. The Spokane Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) is currently conducting research to reduce the fatalities associated with slope failures. The purpose of this paper to discuss some of this research and to present potential new technologies for slope monitoring and design. The paper also briefly discusses various warning signs of slope instability, introduces the most common slope monitoring methods, describes the limitations of various slope monitoring systems, and presents some field results using some of this new technology. CONSEQUENCES OF SLOPE FAILURES Whether on the surface or underground, unanticipated movement of the ground can pose hazardous conditions which may lead to endangerment of lives, demolition of equipment, and the loss of property. In the five years since 1995, 33 miners have lost their lives as a result of surface ground control accidents (see Figure 1). Figure 1. Map of 33 fatalities occurring from surface ground control problems January 1995 -June 2000. As such, the National Institute for Occupational Safety and Health’s (NIOSH) Office of Mine Safety and Health Research in Spokane, Washington has initiated a research program with the goal of reducing the number of injuries and fatalities resulting from slope failures at mines. There are several ways to reduce the chances of surface ground control failures: 1) safe geotechnical designs; 2) secondary supports or rock fall catchment systems; or 3) monitoring devices for advance warning of impending failures. While it is important to note that geotechnical designs can be improved to increase factors of safety, proper bench designs can be improved to minimize rock fall hazards, and certain support systems may enhance overall rock mass strength, diligent monitoring and examination of slopes for failure warning signs is the most important means of protecting exposed mine workers. Even the most carefully designed slopes may experience failure from unknown geologic structures, unexpected weather patterns, or seismic shock (Figure 2). Figure 2. Consequences of unexpected slope failures. SLOPE MONITORING SYSTEMS Relative displacement measurements are the most common type of monitoring, complemented by monitoring of groundwater. The most important purpose of a slope monitoring program is to: 1) maintain safe operational practices; 2) provide advance notice of instability; and 3) provide additional geotechnical information regarding slope behavior (Sjöberg, 1996). The following is a list of the most common monitoring systems currently in use and is not intended to be an all-inclusive list of monitoring equipment. Readers interested in a more comprehensive list are referred to Szwedzicki, 1993. Surface Measurements Survey Network: A survey network consists of target prisms placed on and around areas of anticipated instability on the slopes, and one or more non-moving control points for survey stations. The angles and distances from the survey station to the prisms are measured on a regular basis to establish a history of movement on the slope. It is extremely important to place the permanent control points for the survey stations on stable ground. The surveys can be done manually by a survey crew or can be automated. Tension Crack Mapping: The formation of cracks at the top of a slope is an obvious sign of instability. Measuring and monitoring the changes in crack width and direction of crack propagation is required to establish the extent of the unstable area. Existing cracks should be painted or flagged so that new cracks can be easily identified on subsequent inspections. Measurements of tension cracks may be as simple as driving two stakes on either side of the crack and using a survey tape or rod to measure the separations. Another common method for monitoring movement across tension cracks is with a portable wire-line extensometer (Figure 3). Figure 3. Portable wire-line extensometer for monitoring a tension crack. The most common setup is comprised of a wire anchored in the unstable portion of the ground, with the monitor and pulley station located on a stable portion of the ground behind the last tension crack. The wire runs over the top of a pulley and is tensioned by a weight suspended from the other end. As the unstable portion of the ground moves away from the pulley stand, the weight will move and the displacements can be recorded either electronically or manually. Long lengths of wire can lead to errors due to sag or to thermal expansion, so readjustments and corrections are often necessary. The length of the extensometer wire should be limited to approximately 60 m (197 ft) to keep the errors due to line sag at a minimum (Call and Savely, 1990). Subsurface Measurements Inclinometers: An inclinometer (figure 4) consists of a casing that is placed in the ground through the area of expected movements. Figure 4. Cross-sectional schematic of typical traverse-probe inclinometer system. The end of the casing is assumed to be fixed so that the lateral profile of displacement can be calculated. The casing has grooves cut on the sides that serve as tracks for the sensing unit. The deflection of the casing, and hence the surrounding rock mass, are measured by determining the inclination of the sensing unit at various points along the length of the installations. The information collected from the inclinometers is important to slope stability studies for the following reasons (Kliche, 1999): •= to locate shear zone(s); •= to determine whether the shear along the zone(s) is planar or rotational; •= to measure the movement along the shear zone(s) and determine whether the movement is constant, accelerating, or decelerating. Time Domain Reflectometry (TDR): Time Domain Reflectometry is a technique in which electronic pulses are sent down a length of a coaxial cable. When deformation or a break in the cable is encountered, a signal is reflected giving information on the subsurface rock mass deformation. While inclinometers are more common for monitoring subsurface displacements, TDR cables are gaining popularity and have several advantages over traditional inclinometers (Kane, 1998): •= Lower cost of installation. •= Deeper hole depths possible. •= Rapid and remote monitoring possible. •= Immediate deformation determinations. •= Complex installations possible. Recent advances have also been made in the use of TDR for monitoring ground water levels and piezometric pressures (Dowding, et al. 1996). A summary of applications of TDR in the mining industry is provided by O’Connor and Wade (1994). Borehole Extensometers: An extensometer consists of tensioned rods anchored at different points in a borehole (figure 5). Changes in the distance between the anchor and the rod head provide the displacement information for the rock mass. Figure 5. Multi-point borehole extensometer. Piezometer: Piezometers are used to measure pore pressures and are valuable tools for evaluating the effectiveness of mine dewatering programs and the effects of seasonal variations. Excessive pore pressures, especially water infiltration at geologic boundaries, are responsible for many slope failures. Data on water pressure is essential for maintaining safe slopes since water behind a rock slope will decrease the resisting forces and will increase the driving forces on potentially unstable rock masses. Highwalls should be visually examined for new seeps or changes in flow rates as these are sometimes precursors to highwall failure. Additionally, pit slopes should be thoroughly examined for new zones of movement after heavy rains or snowmelts. RESEARCH AND TECHNOLOGY Stress, gravity loading, rock mass strength, geology, pore pressure, the presence of unknown underground workings, and many other factors contribute to slope failures. Because of the enormous surface area of many large open-pit mines, several varieties and scales of instabilities can occur. Complete vigilance to monitor each and every potential failure block is neither feasible, nor economical, and is certainly not attainable using today’s most common point displacement monitoring techniques. Many of the current monitoring methods are also difficult to implement at quarries and surface coal mines, where near-vertical faces and lack of benching limit access to areas along the highwall. Additionally, as mining progresses, it is necessary to monitor different sections of the pit walls. Continually relocating devices is not only a costly and time consuming operation, but can also be dangerous -especially with an unstable slope. In an effort to make up for the shortcomings of point monitoring systems, NIOSH is examining new technology for slope monitoring that will look at the entire surface of the mine highwall for rock mass displacement and rock mass composition (Girard, 1998). Additionally, software has been created under a NIOSH contract to assist geotechnical engineers with bench designs to minimize rock fall hazards. A discussion of each technology follows. Highwall Monitoring Using Radar Systems Synthetic aperture radar (SAR) is a type of ground-mapping radar originally designed to be used from aircraft and satellites. SAR can be used to generate high quality digital elevation maps (DEM’s) and to detect disturbances of the earth’s surface. A variation of SAR – Interferometric Synthetic Aperture Radar (IFSAR) – uses differences in time-lapsed SAR images to generate maps of displacements (Fruneau and Achache, 1996). This technique has been successfully applied to produce displacement maps of ground movement caused by earthquakes, volcanic activity, and mine subsidence (Massonet, 1997; Canec, 1996). IFSAR can also be used to monitor displacement of unstable slopes or landslides (Reeves et al., 1997; Sabine et al., 1999). IFSAR’s have many advantages over current types of