Uniformity coefficient, abbreviated as \(C_u\) , also called coefficient of uniformity, a dimensionless number, is used in soil mechanics to describe the spread of particle sizes in a coarse-grained soil based on its particle-size distribution curve.
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
Cu Close to 1 (e.g., 1–2) - Particles are nearly all the same size, a uniformly graded (poorly graded in USCS terms) soil. Picture beach sand or uniform fine gravel, the grain-size distribution curve is nearly vertical/steep.
Cu Large (e.g., > 6–10) - A wide range of particle sizes is present, meaning smaller particles can fill voids between larger ones, a well-graded soil (assuming the shape of the gradation curve is also smooth, see limitation below). The grain-size curve is flatter and spread out.
Engineering Significance
Well Graded Soils (high Cu, proper Cc)
Generally achieve higher density and lower void ratio when compacted, because finer particles occupy voids left by coarser ones.
Higher shear strength and bearing capacity, lower compressibility.
Preferred for structural fills, road bases, backfill, and dam cores/filters where strength and low permeability (in fine-graded contexts) or controlled permeability (in filter design) are needed.
Poorly/Uniformly Graded Soils (low Cc)
Tend to have higher void ratios, are more prone to segregation and erosion (piping), and can be more susceptible to liquefaction under cyclic/seismic loading (loose uniform sands are classic liquefaction-prone materials).
Often more permeable in a predictable, uniform way, actually useful for filter/drainage material design (uniform sands/gravels are often intentionally selected for filters precisely because their pore structure is predictable).
Cu is therefore used both to flag soils that compact well as engineered fill and to identify soils that need special attention for liquefaction potential or piping/internal erosion.
