If 10% of grain sizes pass the #200 sieve, what should be done with the material used as filter sand?

• A. Approve as filter sand
• B. Not okay as filter sand
• C. Reclassify as fine gravel
• D. Use as backfill

Answer: (B)

The main idea here is that filter sand must have very limited fines to keep enough open space for water to flow. The #200 sieve tests for the finest particles; if 10% of the grains pass this sieve, that means a relatively high fines content for a sand intended to filter water. Those fines can clog the pore spaces in the filtration material, reducing hydraulic conductivity and hindering drainage, which compromises the effectiveness of the septic system’s filter bed. Because of that, the material does not meet the requirements for filter sand and should not be approved for that use. If possible, you’d look to source a coarser material or adjust the material (for example, washing or blending with sands that have fewer fines) so it meets the specification before it’s used as filter sand.