Geotechnical and Foundation Engineering Flashcards

The water table is at the ground surface in a soil with saturated unit weight 20 kN/m³. Using γw = 9.81 kN/m³, what is the vertical effective stress at 5 m depth?

Answer: about 51 kPa

σ' = 5 × (20 - 9.81) ≈ 51 kPa.

For a cohesionless backfill with φ' = 30° behind a smooth vertical wall with a horizontal surface, the Rankine active earth pressure coefficient Ka is:

Answer: 0.33

Ka = tan²(45° - φ/2) = tan²(30°) = 1/3.

For the same backfill with φ' = 30°, the Rankine passive earth pressure coefficient Kp is:

Answer: 3.00

Kp = tan²(45° + φ/2) = tan²(60°) = 3.0.

An unconsolidated-undrained (UU) triaxial test on a saturated clay typically yields a failure envelope with:

Answer: φ = 0 and strength equal to the undrained shear strength cu

Saturated UU tests give equal deviator stresses regardless of confining pressure, producing a horizontal φ = 0 envelope.

A sand has Gs = 2.65 and void ratio e = 0.65. What is the critical hydraulic gradient that would cause a quick condition?

Answer: 1.00

ic = (Gs - 1)/(1 + e) = 1.65/1.65 = 1.00.

Using Jaky's equation, the at-rest earth pressure coefficient K0 for a normally consolidated sand with φ' = 30° is:

Answer: 0.50

K0 = 1 - sin φ' = 1 - 0.5 = 0.50.

During drained shearing, a dense sand typically exhibits which behavior?

Answer: Dilation with a peak strength followed by softening toward critical state

Dense sand dilates as particles ride over each other, giving a peak followed by strain softening.