Introduction:
In soil engineering, the grading of soil particles refers to the distribution of different particle sizes within a soil sample. A well-graded soil sample is one that contains particles of various sizes in good representation. This means that the soil sample has a balanced distribution of particles, without an excess of certain particle sizes.

Explanation:
A soil sample may be well graded if it meets the following criteria:

1. Good representation of particles of all sizes:
A well-graded soil sample should contain particles of all sizes, ranging from large to small. This means that there is a balanced distribution of coarse, medium, and fine particles within the sample. It is important to have a good representation of all particle sizes to achieve the desired engineering properties of the soil.

2. Absence of excess of certain particles:
A well-graded soil sample should not have an excess of certain particle sizes. If a soil sample contains an excessive amount of particles of a specific size, it is considered poorly graded. This can result in the formation of voids and gaps, leading to poor compaction and reduced strength of the soil.

3. Optimal packing arrangement:
A well-graded soil sample allows for optimal packing arrangement of particles. This means that particles of different sizes can interlock and fill the void spaces between each other. The presence of particles of various sizes helps to achieve a more compact and stable soil structure.

Importance of well-graded soil:
A well-graded soil sample is desirable in many civil engineering applications due to its improved engineering properties. Some of the advantages of well-graded soil include:

1. Enhanced stability: A well-graded soil sample provides better stability and shear strength compared to poorly graded soils. This is because the interlocking of particles of different sizes helps to resist shear forces and prevent excessive settlement.

2. Improved drainage: The presence of particles of various sizes in a well-graded soil allows for better drainage. The larger particles create pathways for water to flow, while the smaller particles help to retain some moisture. This balanced drainage can prevent excessive water accumulation and potential issues such as erosion or soil instability.

3. Better compaction: Well-graded soils are easier to compact due to the optimal arrangement of particles. The interlocking of particles helps to achieve greater density and reduced void spaces, resulting in improved compaction characteristics.

Conclusion:
A soil sample is considered well graded if it has a good representation of particles of all sizes and does not contain an excess of certain particle sizes. Well-graded soils offer improved stability, drainage, and compaction characteristics, making them desirable for various civil engineering applications.