The stability of a slope in a geological environment can differ based on a number of factors that include material properties, geometry of slopes, hydrology, as well as the structure of a geological environment.
Material properties
The type and strength of rock that overlies and underlies a slope are of great significance. Strong rocks like basalt and granite are more stable rocks than others. Less stable rocks like mudstone and schist can produce instabilities. Faults and joints within rocks are planes that can produce failure if they are parallel to a slope and have a dip angle of nearly zero degrees. The strength of soil can decrease in a saturated state. The strength of soil can increase if it is saturated and has a high surface tension. A saturated but porous soil can have that strength.
Slope geometry
The inclination and elevation of a slope are mainly controlling in stability. The steeper the slope, the more will be shear stress and chances of failure. The critical angle is related to a natural angle of repose for a given material (about 30°-35° for a granular substance). The convex/concave nature of a slope is a controlling factor in concentrating and dispersing water over slopes. The greater the elevation of a slope, the greater is its potential energy.
Hydrological factors
Water content and groundwater flow within the material of a slope can have a notable effect on strength. The greater the saturation ratio in a slope’s material, the higher its pore pressure and resulting lower shear strength. Groundwater level and rain infiltration are fundamental in this aspect. In other words, well-drained slopes are stable while poorly drained slopes contribute to a reduction in a slope’s material strength over time.
Geology structure
The presence of discontinuities in a rock like joints, faults, and bedding planes influences the stability of a slope as they provide a preferred path for failure. Geological history in terms of past landslide and tectonic activity can influence a slope under investigation. The orientation of features to a slope surface is a key factor.
External and temporal factors
External factors include earthquakes, human activity in excavation and construction, vegetative factors, and climatic factors. Earthquakes can produce ground motions and/or liquefaction. Vegetation can produce forces of stability through its roots and contribute to forces of gravity through its weight as well as affect moisture through evaporative cooling. Climatic changes and weathering can produce stability through freeze/thaw actions.
In a nutshell, the stability of a slope in a geological environment is a function of a number of factors that either contribute to a probable failure of a given slope or its stability.
To achieve a successful design and risk evaluation and subsequent mitigation in slope engineering, a high level of understanding is required as mentioned above.
