Uniformity coefficient, abbreviated as \(C_u\), also called coefficient of uniformity, a dimensionless number, classifies a soil as well graded or poorly graded. It is a measure used in geotechnical and civil engineering to describe the particle size distribution of a granular material such as soil, sand, or gravel. It is an important parameter in soil mechanics and plays a significant role in various engineering applications, including the design of foundations, embankments, and drainage systems.
Uniformity Coefficient Formula |
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\( C_u \;=\; \dfrac{ D_{60} }{ D_{10} }\) (Uniformity Coefficient) \( D_{60} \;=\; C_u \cdot D_{10} \) \( D_{10} \;=\; \dfrac{ D_{60} }{ C_u }\) |
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| Symbol | English | Metric |
| \( C_u \) = uniformity coefficient | \(dimensionless\) | \(dimensionless\) |
| \( D_{10} \) = is the sieve diameter (grain size) which there are 10% of particles go through. | \(in\) | \(mm\) |
| \( D_{60} \) = is the sieve diameter (grain size) which there are 60% of particles go through. | \(in\) | \(mm\) |
Uniformity Coefficient Interpretation
- Uniformity Coefficient ( \(C_u\) ) is interpreted as an index of the spread of particle sizes within a granular soil, based on the ratio \(D60/D10\) obtained from the grain-size distribution curve.
- \(C_u \approx 1\) - Indicates that the soil particles are nearly the same size, meaning the soil is uniformly graded (poorly graded) with a very narrow range of particle diameters.
- Low \(C_u\) (close to 1) - Signifies a steep grain-size distribution curve, reflecting minimal variation in particle size and limited packing efficiency.
- High \(C_u\) Indicates a wide range of particle sizes, meaning the soil is well-graded, with smaller particles filling the voids between larger particles.
- For gravels, a commonly accepted interpretation is:
- Indicative of well-graded gravel (subject to meeting additional gradation criteria such as coefficient of curvature).
- Indicative of poorly graded gravel.
- For sands, a commonly accepted interpretation is:
- \(C_u < 6 \rightarrow\) - Indicative of well-graded sand (again, contingent on additional criteria).
- ndicative of poorly graded sand.
- Higher \(C_u\) - Generally correlates with better packing characteristics, allowing denser configurations due to the filling of voids by finer particles.
- Soils with High \(C_u\) Typically exhibit lower permeability than uniformly graded soils of similar composition, because smaller particles reduce void connectivity.
- Soils with Low \(C_u\) Tend to have higher permeability, as the relatively uniform particle size creates larger, more continuous void spaces.
- The uniformity coefficient alone is not sufficient for full gradation characterization, it must be interpreted alongside other parameters (e.g., curvature coefficient) and the overall shape of the grain-size distribution curve.
- The interpretation applies primarily to coarse-grained soils (sands and gravels), its use for fine-grained soils is limited and generally not meaningful due to different governing mechanisms of behavior.
Engineers use the uniformity coefficient to assess the suitability of a granular material for various construction purposes. Materials with a high \(C_u\) may have a wide range of particle sizes, which can affect their compaction properties, permeability, and stability in construction projects. In contrast, materials with a lower \(C_u\) are often preferred for certain applications because they are more uniform and predictable in their behavior.
