The basic objective is to maintain slope stability, which is related to steepness, height, the orientation of weakness planes with respect to the slope angle, and cleft-water pressures.
Failure in rock slopes usually occurs suddenly, with rapid movement.
Slope Stability Problems
Natural slopes may become unstable from weathering effects, erosion, frost wedging, or the development of high cleft-water pressures.
Open-pit mines are excavated with the steepest possible stable slope, and low safety factors are accepted for economic reasons. Instrumentation monitoring is generally accepted as standard procedure for deep pits.
Hillside cuts for roadways and other constructions are usually less steep than open-pit mine slopes in the same geologic conditions, and require a higher safety factor against failure.
Evaluations are usually based on experience and empirical relationships, and instrumentation is normally used only in critical situations.
Instrumentation (Figure 1)
Internal lateral movements are monitored in terms of displacement vs. time with inclinometers (L) and deflectometers (m) which can be attached to alarm systems, or with borehole extensometers (o). Shear-strip indicators (n) and the acoustical-emissions device (q) provide indications of mass movements.
External lateral movements are monitored with the convergence meter (g) or tiltmeter (e) on benches, strain meters (h) on tension cracks, optical surveys (a) with the laser geodimeter that provide rapid measurements of the entire slope, and terrestrial stereophotography (i) that provides a periodic record of the entire slope face. GPS will find increasing use for long-term monitoring.
Bottom heave is monitored by the optical survey of monuments or “settlement points”; it often precedes a major slope failure.
Vibrations from blasting and traffic, which may affect stability, are monitored (j) and cleft-water pressures are monitored with piezometers (r).
Inclinometers, deflectometers, piezometers, and extensometers are often limited to early excavation stages to obtain data for the determinations of stable slope inclinations. They are expensive instruments to install and monitor, and not only will they be lost if failure occurs, but in mining operations many will be lost as excavation proceeds. In addition, they monitor only limited areas by section.
Tiltmeters, convergence meters, and optical surveys are both economical and “retrievable,” and provide for observations of the entire slope rather than a few sections; therefore, they provide the basic monitoring systems. In critical areas, where structures or workers are endangered if a collapse occurs during mining operations, MPBX extensometers and the acoustical-emissions device are used as early warning systems.
In soil slopes, the objectives are to detect movements when they first occur since in many instances slope failures develop gradually; and, when movement occurs, to monitor the rate of movement and accelerations, locate the failure surface, and monitor pore pressures. These data provide the basis for anticipation of total and perhaps sudden failure.
Slope Stability Problems
In potentially unstable natural or cut slopes, failure is usually preceded by the development of high pore-water pressures, an increase in the rate of slope movement, and the occurrence of tension cracks. Slope movement is not necessarily indicative of total failure, however, since movements are often progressive, continuing for many years. Stability evaluations require information on the failure surface location, pore pressures, and rates of movement as well as the geologic and climatic factors.
Surface movements of the natural slope shown in Figure 2 are monitored by precise leveling with the laser geodimeter (a), convergence meters (g), and strain meters (h). The meters may be attached to alarm systems. GPS will find these meter to be increasingly useful for long-term monitoring.
Subsurface deformations are monitored with the inclinometer (L), shear strip indicator (n), or steel-wire sensor to locate the failure surface. Acoustical emissions (q) may indicate approaching failure.
Pore-water pressures are monitored with piezometers (r). One can estimate the pore pressures required for total failure by knowing the failure surface location and soil shear strength.
Piezometers, inclinometers, extensometers, and other devices used in boreholes may not monitor the most critical areas, especially in slopes in a stable condition. Therefore, optical surveys, which provide information over the entire study area, are always important, although, because of terrain conditions, perhaps not always possible.