What equation is used to determine the percentage of gravel in a soil sample?

• A. Gravel (%) = 100 - passing #4 (%)
• B. Gravel (%) = total weight - silt weight
• C. Gravel (%) = passing #200 (%) - passing #4 (%)
• D. Gravel (%) = 100 - passing #10 (%)

Answer: (A)

The correct formulation to determine the percentage of gravel in a soil sample is based on understanding the grading of soil particles. When using sieve analysis, the percentage of material that passes through a specific sieve indicates its size relative to larger particles.

The equation stating that gravel percentage equals 100 minus the percentage of material passing the #4 sieve is accurate because the #4 sieve serves as a critical boundary in soil classification. By knowing how much material passes this sieve, you can deduce the proportion of larger particles retained by it, which includes gravel. Essentially, if you measure the amount of soil particles that are finer than 4.75 mm (the size of the opening in the #4 sieve), the rest of the sample—those retained by the sieve—constitutes the gravel fraction. Thus, the gravel percentage can be calculated by subtracting this finer percentage from 100%.

This relationship is significant in geotechnical and civil engineering, where understanding soil composition is crucial for construction and design purposes. The other choices either do not directly correspond to defining gravel content or mix varying particle sizes inappropriately, making them less suitable for this specific calculation.