Missoula's development follows the Clark Fork River and the ancient lakebed of Glacial Lake Missoula. That history left a mess of interbedded silts, clays, and gravels across the valley. We see it in every sample. A project off Broadway might hit clean alluvium, while three blocks south the auger brings up fat clay. Textural variation here is extreme. The grain size analysis is not a routine check—it is the first filter for drainage design, frost depth, and liquefaction screening. We run the full ASTM D6913 sieve stack plus ASTM D7928 hydrometer on every sample from the Bitterroot to Rattlesnake Creek. Combined with an Atterberg limits run, we classify the material under USCS and flag anything that will hold water or lose strength when worked. In Missoula, that flag goes up often.
A 12% fines content in Missoula gravel can reduce permeability by three orders of magnitude—enough to trap water behind a wall and trigger failure.
Local considerations
The classic Missoula mistake is assuming a gravel is free-draining because it looks clean in the bucket. We see it on retrofit jobs all the time: a contractor backfills a wall with pit-run material, skips the gradation test, and two freeze-thaw seasons later the wall is leaning. The 200-wash catches the silt. The hydrometer catches the clay. Without those numbers, you cannot design a filter, size a subdrain, or predict frost heave. Missoula's frost depth reaches 42 inches per IBC Table 301.2. Silty soils in that zone will heave. A gradation curve with a flat slope in the fines range is a red flag. We flag it hard.
Questions and answers
What does a grain size analysis cost in Missoula?
A combined sieve and hydrometer analysis per ASTM D6913/D7928 runs between US$100 and US$200 per sample, depending on the number of samples and whether we handle sample preparation. Wash-only sieve analysis falls at the lower end of that range.
How much sample material do you need for a full gradation test?
For material with gravel up to 19 mm, we need roughly 500 grams. For material with cobbles up to 75 mm, the standard requires 5 kilograms or more. We provide sampling containers and guidance before fieldwork starts.
Why is the hydrometer step necessary if I only care about the sand and gravel fractions?
Because Missoula's glacial deposits often carry a silt and clay fraction that the No. 200 sieve alone cannot characterize. The hydrometer quantifies clay content, which controls frost susceptibility, consolidation settlement, and drainage behavior. A wash-only test misses that entirely.
Can you classify the soil from the gradation curve alone?
For coarse-grained soils where fines are less than 12%, yes—we classify by ASTM D2487 from the sieve data. For soils with more than 12% fines, we need Atterberg limits in addition to the gradation curve to assign a proper USCS group symbol.
