What are the two possible two-letter USCS classification symbols for gravel 10. What lab test would you need to run to classify a gravel soil layer? 11. What would you use - split-spoon sampler or Shelby tube - to obtain an undisturbed sample? Why? 12. How is the N value for a soil determined? What is an N, value? 13. In the USCS classification system, why is grain size analysis not used to classify the fine grained fraction of the soil? What is used instead? 14.In the USCS classification system what sieve number separates coarse from fine grained soils?