How is the percentage of sand in a soil sample calculated?

• A. Sand (%) = passing #4 (%) - passing #10 (%)
• B. Sand (%) = passing #4 (%) + passing #200 (%)
• C. Sand (%) = passing #4 (%) - passing #200 (%)
• D. Sand (%) = 100 - gravel (%)

Answer: (C)

To calculate the percentage of sand in a soil sample, one must understand the composition of soil as it relates to different size particles. The primary method for determining sand content involves utilizing a sieve analysis, where the soil sample is passed through a series of sieves with different opening sizes.

The correct method is to take the percentage of soil that passes through the #4 sieve, which captures all particles smaller than 4.75 mm, and subtract the percentage of soil that passes through the #200 sieve, which captures particles smaller than 0.075 mm. This is because the soil that passes through the #200 sieve consists of silt and clay, which are not considered sand. Therefore, by subtracting the percentage of fine particles (passing #200) from those fine and coarse particles (passing #4), you can isolate the sand fraction.

This reasoning is rooted in understanding soil classification and helps ensure that only the sand-sized particles are accounted for in the calculation. The correct calculation effectively provides a clear picture of the sand content within the soil sample.

The other choices do not accurately represent this method of calculating sand percentage. For instance, combining passing #4 with passing #200 does not yield the sand content, and simply determining gravel content does