Civil Engineering Technology - Soils (CMT-Soils)
This domain, Civil Engineering Technology - Soils (CMT-Soils), tests soil classification, index testing, compaction, and field density. Expect ASTM-based procedures, calculation checks, and specification comparisons.
Core rules and concepts
- Gradation and classification. Sieve analysis (D6913) separates gravel (>4.75 mm, No. 4), sand (0.075–4.75 mm, No. 200–No. 4), and fines (<0.075 mm). Hydrometer (D7928) extends below No. 200. USCS (D2487) uses gradation, LL, and PI; AASHTO uses No. 10/40/200 and group index.
- Atterberg limits (D4318). LL and PL are water contents at the 25-blow and thread-rolling boundaries; PI = LL − PL. Test the No. 40 fraction. The A-line is PI = 0.73(LL − 20); above it with LL < 50 and PI ≥ 7 is CL, below is ML.
- Compaction. Standard Proctor (D698): 3 layers, 5.5-lb rammer, 12-in drop, 25 blows. Modified (D1557): 5 layers, 10-lb rammer, 18-in drop. Each defines OMC and maximum dry unit weight. Relative compaction = field dry unit weight ÷ lab maximum × 100.
- Field density and moisture. Sand cone (D1556), rubber balloon (D2167), and nuclear gauge (D6938) measure in-place density; oven moisture (D2216) uses 110 ± 5°C to constant mass.
Worked example
A sand-cone test gives wet mass 1,850 g, dry mass 1,700 g, and volume 850 cm³. Water mass = 150 g; w = 150/1,700 × 100 = 8.8%. Wet density = 1,850/850 = 2.18 g/cm³; dry density = 1,700/850 = 2.00 g/cm³. Lab maximum dry density is 2.10 g/cm³, so relative compaction = 2.00/2.10 × 100 = 95.2%, passing a 95% spec.
Traps
- Comparing wet density with lab maximum dry density; convert to dry first.
- Using Standard Proctor maximum against a Modified Proctor specification.
- Running Atterberg limits on the whole sample instead of the No. 40 fraction.
- Reporting a USCS symbol from visual description without index tests.
- Ignoring oversize correction when gravel is present.