Geotechnical Engineering & Risk Assessment Flashcards

A saturated soil element has a total vertical stress of 200 kPa and a pore water pressure of 80 kPa. What is the vertical effective stress?

Answer: 120 kPa

Terzaghi's effective stress principle gives σ' = σ − u = 200 − 80 = 120 kPa.

Which deposit is MOST susceptible to earthquake-induced liquefaction?

Answer: Loose, saturated, clean fine sand

Loose, saturated, cohesionless soils build excess pore pressure under cyclic loading and lose strength.

In an unconsolidated-undrained (UU) triaxial test on saturated clay, the Mohr failure envelope is typically:

Answer: Horizontal, giving φu = 0 and strength equal to su

Saturated clay tested undrained shows the same deviator stress regardless of confining pressure, so φu = 0 and strength equals su.

A clay has a preconsolidation pressure of 300 kPa and a current vertical effective stress of 100 kPa. Its overconsolidation ratio (OCR) and state are:

Answer: 3.0, overconsolidated

OCR = σ'p / σ'v0 = 300/100 = 3, indicating an overconsolidated clay.

If the drainage path length of a clay layer is doubled, the time to reach the same degree of primary consolidation will:

Answer: Increase about fourfold

Since t = Tv·Hdr²/cv, time scales with the square of the drainage path.

Why are field SPT blow counts commonly corrected to N60?

Answer: To normalize for hammer energy delivered relative to 60% of theoretical free-fall energy

Hammer systems deliver different energy ratios, so N is normalized to a 60% energy efficiency standard.

A 'quick' (boiling) condition in sand occurs when the upward hydraulic gradient:

Answer: Equals or exceeds the critical gradient, roughly γ'/γw ≈ 1

At the critical gradient, upward seepage force cancels buoyant weight and effective stress drops to zero.