BCE Geotechnical 3 — Questions and Answers
Question 1: A retaining wall has a saturated backfill with a water table at the surface. How does the presence of water affect the total lateral pressure on the wall?
- Reduces total pressure because water lubricates soil particles
- Has no effect if drainage is provided
- Increases total pressure significantly due to hydrostatic water pressure added to earth pressure (Correct answer)
- Reduces effective earth pressure to zero
Correct answer: Increases total pressure significantly due to hydrostatic water pressure added to earth pressure
With a high water table, both the effective earth pressure (using submerged unit weight) and full hydrostatic water pressure act on the wall, greatly increasing total lateral force.
Question 2: What is the significance of the preconsolidation pressure (σ'p) in soil mechanics?
- Maximum past effective stress the soil has experienced (Correct answer)
- Current effective overburden stress
- Stress at which shear failure occurs
- Effective stress at the liquid limit
Correct answer: Maximum past effective stress the soil has experienced
σ'p is the maximum effective stress the soil has ever been subjected to; stresses below it cause small elastic (recompression) settlements, while stresses above cause larger virgin consolidation settlements.
Question 3: Which method is most appropriate to determine the in-situ undrained shear strength of a soft sensitive clay?
- Standard Penetration Test (SPT)
- Field Vane Shear Test (VST) (Correct answer)
- Plate Load Test
- Cone Penetration Test with piezocone (CPTU)
Correct answer: Field Vane Shear Test (VST)
The field vane shear test directly measures peak and remolded undrained shear strength of soft clays without disturbing the sample.
Question 4: What is critical hydraulic gradient (ic) in the context of piping failure?
- Gradient causing 50% consolidation
- Upward hydraulic gradient at which seepage forces reduce effective stress to zero (Correct answer)
- Minimum gradient needed for Darcy's Law to apply
- Gradient causing liquefaction in loose sand
Correct answer: Upward hydraulic gradient at which seepage forces reduce effective stress to zero
ic = (Gs − 1)/(1 + e) ≈ 1 for most soils; when the upward seepage gradient reaches ic, effective stress becomes zero and the soil loses stability (quick condition or piping).
Question 5: In a direct shear test, peak shear stress occurs before residual shear stress. What does the residual shear strength represent?
- Strength at zero normal stress
- Long-term strength after large displacement when soil structure is fully remolded (Correct answer)
- Strength at the liquid limit water content
- Undrained shear strength under rapid loading
Correct answer: Long-term strength after large displacement when soil structure is fully remolded
Residual strength corresponds to the minimum, post-peak strength after clay platelets have aligned parallel to the shear surface following large displacement.
Question 6: What does the term 'sensitivity' (St) of clay describe?
- Ratio of undrained strength at liquid limit to plastic limit
- Ratio of peak undrained shear strength to remolded undrained shear strength (Correct answer)
- Rate of pore pressure dissipation after loading
- Sensitivity of clay to changes in pH
Correct answer: Ratio of peak undrained shear strength to remolded undrained shear strength
St = su(peak)/su(remolded); high sensitivity (>8) indicates quick clays that lose most strength upon disturbance.
Question 7: Which compaction curve characteristic is most directly affected by increasing compaction energy in a Standard Proctor test?
- Optimum moisture content increases and maximum dry density decreases
- Both optimum moisture content and maximum dry density increase
- Optimum moisture content decreases and maximum dry density increases (Correct answer)
- No change occurs in the compaction curve
Correct answer: Optimum moisture content decreases and maximum dry density increases
Higher compaction energy shifts the curve upward and to the left: maximum dry density increases and optimum moisture content decreases.
BCE Geotechnical 3
A retaining wall has a saturated backfill with a water table at the surface.
How does the presence of water affect the total lateral pressure on the wall?