Soils Laboratory Testing and Classification
Scope
This domain tests whether you can choose the correct AASHTO/ASTM procedure for a given soil and turn the results into a classification and a compaction target.
Index tests
- Gradation - sieve and hydrometer analysis, AASHTO T 88. Material must be soaked and dispersed or the fines content is under-reported.
- Atterberg limits - liquid limit T 89, plastic limit T 90; PI = LL − PL. Run both on material passing the 425-µm (No. 40) sieve.
- Specific gravity - T 100; needed for hydrometer calculations.
Moisture–density and strength
T 99 uses a 2.5-kg rammer with a 305-mm drop; T 180 uses a 4.54-kg rammer with a 457-mm drop. Methods A–D are selected by the largest particle size in the sample (passing 4.75 mm up to passing 19.0 mm). If the field soil is coarser than the tested fraction, apply the oversize correction in T 224. T 272 one-point Proctors are verified against a family of curves. CBR (T 193) is run at a specified density and moisture, usually soaked.
Classification
AASHTO M 145 assigns groups A-1 to A-7 plus a group index. USCS (ASTM D 2487) uses gradation, LL, PI, and the A-line.
Worked example
Fines (passing No. 200) = 60%, LL = 45, PI = 20. A-7 needs PI > 10 and LL > 40. Test PI against LL − 30 = 15; 20 > 15, so the group is A-7-6. GI = (60−35)[0.2 + 0.005(45−40)] + 0.01(60−15)(20−10) = 25(0.225) + 4.5 = 10.
Traps
Judging a T 99 specification against a T 180 curve; skipping the oversize correction; running limits on the whole sample; reporting a negative PI without explanation; reporting a negative group index instead of zero.