Introduction:
In the sieve analysis of granular material, different sieve sizes are used to separate particles of different sizes. The ratio of the size of the sieve opening that retains 60% of the material (D60) to the size of the sieve opening that retains 10% of the material (D10) is known as the uniformity coefficient.

Explanation:
Sieve analysis is a commonly used method for determining the particle size distribution of granular materials. It involves passing a sample of the material through a series of sieves with different opening sizes. The material retained on each sieve is weighed, and the results are used to calculate the percentage of material retained on each sieve.

Uniformity coefficient:
The uniformity coefficient is a measure of the range of particle sizes in a granular material. It is calculated by dividing the size of the sieve opening that retains 60% of the material (D60) by the size of the sieve opening that retains 10% of the material (D10). The formula for calculating the uniformity coefficient is as follows:

Uniformity coefficient (UC) = D60 / D10

The uniformity coefficient provides an indication of the range of particle sizes present in the material. A high uniformity coefficient indicates a wide range of particle sizes, whereas a low uniformity coefficient indicates a narrow range of particle sizes.

Other options:
The other options mentioned in the question are not correct in the context of sieve analysis of granular material. Let's briefly explain what each option refers to:

a) Well coefficient: The well coefficient refers to the efficiency of a well in drawing water from an aquifer.

c) Recharge coefficient: The recharge coefficient refers to the rate at which water enters an aquifer through percolation.

d) Curvature coefficient: The curvature coefficient is a mathematical term used in curve fitting and data analysis, unrelated to sieve analysis.

Conclusion:
The ratio of D60 and D10 sieve sizes in the sieve analysis of granular material is referred to as the uniformity coefficient. It provides a measure of the range of particle sizes in the material and is calculated by dividing D60 by D10. The uniformity coefficient is an important parameter in characterizing the particle size distribution and can be used to classify and compare different granular materials.