Soil Mechanics and Lab Testing Flashcards

A standard Proctor compaction test yields a maximum dry unit weight of 112 pcf at an optimum moisture content of 14%. If the field dry unit weight is 107 pcf at 13% moisture, what is the relative compaction?

Answer: 95.5%

Relative compaction = (field dry unit weight / max dry unit weight) × 100 = (107/112) × 100 = 95.5%.

In a triaxial consolidated-undrained (CU) test, pore water pressure is measured. If the total minor principal stress is 100 kPa and the pore pressure at failure is 40 kPa, what is the effective minor principal stress?

Answer: 60 kPa

Effective stress = total stress − pore pressure = 100 − 40 = 60 kPa.

A consolidation test on a 0.75-inch thick sample (double drainage) reaches 50% consolidation in 8 minutes. What is the coefficient of consolidation (cv) in in²/min?

Answer: 0.00527 in²/min

cv = T50 × Hdr² / t50 = 0.197 × (0.375)² / 8 = 0.00527 in²/min, where Hdr = half thickness for double drainage.

Which ASTM standard governs the Atterberg Limits test (liquid limit and plastic limit) for soils?

Answer: ASTM D4318

ASTM D4318 is the standard test method for liquid limit, plastic limit, and plasticity index of soils.

A soil has a liquid limit of 52 and a plastic limit of 26. What is the plasticity index, and how is it classified on the plasticity chart?

Answer: PI = 26, plots in CH zone

PI = LL − PL = 52 − 26 = 26; with LL > 50 and plotting above the A-line, the soil classifies as CH (fat clay).

In a constant-head permeability test, a sample 10 cm long and 50 cm² in cross-section passes 320 cm³ in 4 minutes under a head of 25 cm. What is the hydraulic conductivity?

Answer: 0.0107 cm/s

k = QL/(Aht) = (320 × 10)/(50 × 25 × 240) = 3200/300000 ≈ 0.0107 cm/s.

A sand layer has D10 = 0.15 mm and D60 = 0.75 mm. What is the coefficient of uniformity (Cu)?

Answer: 5.0

Cu = D60/D10 = 0.75/0.15 = 5.0; a value ≥ 6 is needed for well-graded sand per USCS.