Angle of repose: definition, factors affecting it and why is it important in geotechnical studies?
The angle of repose is defined as the angle that granular solid makes in relation to the horizontal area when it rests by sliding and rolling itself in a free or loose form. Or, in simpler terms, the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. It’s an important parameter to evaluate interparticle force. In geotechnical studies, the soil that is being constructed needs to be studied to measure its load bearing capacity, and other factors that finally determine the kind of construction that must occur. Slopes with angles greater than the angle of repose for the soil are prone to failure, especially when subjected to additional loads or environmental factors such as rainfall. In the context of subgrades, it means subgrades with steeper slopes often require additional stabilization measures to prevent slope failure such as reinforcement through geogrids, geocells, or other reinforcement measures.
Therefore, it’s one of the most critical parameters to consider that influences the design and stability of structures involving soil, such as retaining walls, embankments, and slopes. The angle of repose is the maximum angle at which a pile of granular material remains stable without collapsing. Here, factors such as the roughness of the base, density of the soil, mean grain size, grain shape, and dilatancy come into play. Each of these influences the angle of repose.
Roughness of the Base: A rougher base provides interlocking between particles, which may result in an increased angle of repose. Smooth bases allow particles to slide more easily, thereby increasing the chances of slippage or collapse.
Density of Soil: Higher-density soils typically have stronger interparticle forces, leading to a higher angle of repose. Water content and moisture retention are also considerations that come into play in how the final angle of repose is affected.
Mean Grain Size: Larger grain sizes tend to have a higher angle of repose because larger grains can interlock more effectively, increasing the stability of the pile. The positive correlation between the two doesn’t however mean that the particle shape is independent.
Grain Shape: The shape of the grains correlates with interlocking. Angular grains tend to interlock more effectively, leading to a higher angle of repose. The angle of repose is influenced by both the physical state of the assembly (including the arrangement of particles, skeleton, and porosity) and the particle interactions within the granular material.
Dilatancy: Dilatancy is an increase in the volume of particles that occur when soil is sheared. It impacts the angle of repose by altering the arrangement of particles. For instance, dilatancy can cause particles to arrange more loosely, leading to a lower angle of repose. This is relevant both in subgrade applications and the choice of reinforcement applied.
This is among the many scenarios where geogrids are used since its usage directly impacts three aspects of soil– particle movement, skeleton formation, and porosity. Geogrids affect particle movement by restricting lateral displacement, promote skeleton formation by interlocking with soil particles, and influence porosity by maintaining void spaces within the soil mass. If the angle of repose of a material is low, it means the material is prone to slumping or settling under load—and here the properties of geogrids in improving load transfer is key to improving subgrades and working with soft soils, and maintains the required levels of load distribution in a variety of applications.