Sieve analysis, commonly known as a gradation test, is a basic test for non-cohesive soils such as sand and aggregate to determine the percentage of different particle sizes.

A good thermal backfill is a non-porous, well-graded material passing a ¼-inch (6.4mm) sieve size and containing about 12-18% fines (material passing #200 sieve).

Geotherm performs the ASTM D6913: Standard Test Method for Particle-Size Distribution Test of Soil.