The correct answer is: C. equal to or less than 1.
The uniformity coefficient (Cu) is a measure of the uniformity of the particle size distribution of a soil. It is defined as the ratio of the D60 (the particle size that is 60% finer by weight) to the D10 (the particle size that is 10% finer by weight).
A soil with a high Cu value has a wide range of particle sizes, while a soil with a low Cu value has a narrow range of particle sizes. Soils with a high Cu value are more likely to be unstable and to segregate during construction.
The uniformity coefficient is always equal to or less than 1. This is because the D60 must always be greater than or equal to the D10. Therefore, the ratio of the D60 to the D10 must always be less than or equal to 1.
Here are some additional details about each option:
- Option A: always less than 1. This is incorrect because the uniformity coefficient can be equal to 1. For example, a soil with a particle size distribution of 100% sand would have a Cu value of 1.
- Option B: always equal to 1. This is incorrect because the uniformity coefficient can be less than 1. For example, a soil with a particle size distribution of 10% sand and 90% clay would have a Cu value of 0.1.
- Option C: equal to or less than 1. This is the correct answer because the uniformity coefficient is always equal to or less than 1.
- Option D: equal to or greater than 1. This is incorrect because the uniformity coefficient can never be greater than 1.