Soil Mechanics and Lab Testing Flashcards
7 cards from real Civil Engineering PE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Soil Mechanics and Lab Testing flashcards as text
A soil has a shrinkage limit (SL) of 12%, plastic limit (PL) of 22%, and liquid limit (LL) of 48%. What is the shrinkage ratio if the soil volume at the shrinkage limit is 24.5 cm³ and dry mass is 36 g, using γw = 1 g/cm³?
Answer: 1.47
Shrinkage ratio SR = Ms/(Vs × γw) = 36/(24.5 × 1.0) = 1.47 g/cm³.
In geotechnical engineering, what is the significance of the preconsolidation pressure (σ'c) determined from a consolidation test?
Answer: It marks the stress boundary between recompression and virgin compression behavior
Preconsolidation pressure separates the lightly (elastic) recompression range from the normally consolidated (plastic) compression range on the e-log p curve.
A saturated soil specimen in an unconsolidated-undrained (UU) triaxial test shows zero friction angle (φ = 0). If the unconfined compressive strength (qu) is 80 kPa, what is the undrained shear strength (su)?
Answer: 40 kPa
For φu = 0, su = cu = qu/2 = 80/2 = 40 kPa; the Mohr circle radius equals the shear strength.
A sand has a maximum void ratio of 0.82 and minimum void ratio of 0.45. A field sample has a void ratio of 0.60. What is the relative density (Dr)?
Answer: 59.5%
Dr = (emax − e)/(emax − emin) × 100 = (0.82 − 0.60)/(0.82 − 0.45) × 100 = 0.22/0.37 × 100 ≈ 59.5%.
Which index property best distinguishes between a silty soil (ML or MH) and a clayey soil (CL or CH) in the USCS classification system?
Answer: Position relative to the A-line on the plasticity chart
The A-line (U = 0.73(LL − 20)) on the Casagrande plasticity chart separates clays (above) from silts and organic soils (below).
In a sieve analysis, what percentage of soil passes the No. 200 sieve if the retained amounts are: No. 4 = 15 g, No. 10 = 25 g, No. 40 = 35 g, No. 200 = 45 g, and pan = 30 g from a total dry mass of 150 g?
Answer: 20%
Mass passing No. 200 = pan fraction = 30 g; percent passing = 30/150 × 100 = 20%.
A compaction curve shows optimum moisture content (OMC) at 15%. A contractor compacts soil at 12% moisture. Compared to the dry side of optimum, what is the most significant engineering implication?
Answer: Higher strength and stiffness but more brittle and susceptible to collapse upon wetting
Compaction dry of optimum produces a flocculated structure with higher strength but more collapse potential when saturated, compared to the dispersed, more ductile wet-side structure.