ASBOG Engineering Geology 4 — Questions and Answers
Question 1: Karst terrain presents which primary engineering hazard for foundation design?
- High lateral earth pressures
- Sudden sinkhole collapse due to dissolution cavities (Correct answer)
- Excessive frost heave
- Slope instability from high pore pressures
Correct answer: Sudden sinkhole collapse due to dissolution cavities
Dissolution of soluble rock (limestone, dolomite) by groundwater creates subsurface cavities that can collapse suddenly, causing sinkholes beneath structures.
Question 2: The Mohr-Coulomb failure criterion for a rock joint is expressed as:
- τ = c + σn tan φ (Correct answer)
- τ = c – σn tan φ
- σ1 = σ3 + 2c
- τ = σn / tan φ
Correct answer: τ = c + σn tan φ
The Mohr-Coulomb criterion states that shear stress at failure (τ) equals cohesion (c) plus normal stress (σn) times the tangent of friction angle (φ).
Question 3: Which condition best describes an artesian aquifer?
- Water table coincides with the top of the saturated zone
- Groundwater is confined under pressure above the top of the aquifer (Correct answer)
- Aquifer is unconfined and recharged directly by surface infiltration
- Groundwater flows only through fractures, not matrix porosity
Correct answer: Groundwater is confined under pressure above the top of the aquifer
An artesian (confined) aquifer is bounded above by an aquitard and contains water under pressure greater than atmospheric, so the piezometric level rises above the top of the aquifer.
Question 4: A geologist maps a fault plane with orientation 060/45 NW (strike/dip). The apparent dip seen in a north-south cross section is:
- Equal to the true dip of 45°
- Less than the true dip of 45° (Correct answer)
- Greater than the true dip of 45°
- Zero because the cross section is parallel to strike
Correct answer: Less than the true dip of 45°
Apparent dip is always less than or equal to true dip; it equals true dip only when the cross section is perpendicular to strike, and decreases as the section obliquity increases.
Question 5: The primary purpose of a piezometer installed in a slope is to:
- Measure the total vertical stress in the soil
- Monitor pore water pressure to assess stability (Correct answer)
- Determine the grain size distribution
- Sample groundwater for chemical analysis
Correct answer: Monitor pore water pressure to assess stability
Piezometers measure pore water pressure, which is a critical parameter in the Mohr-Coulomb effective stress equation used for slope stability calculations.
Question 6: Which characteristic distinguishes a debris avalanche from a rockfall in terms of failure mechanics?
- Debris avalanche involves free-fall of individual rock fragments
- Debris avalanche is a rapid, flowing mass movement of a heterogeneous mixture (Correct answer)
- Rockfall generates higher peak velocities than a debris avalanche
- Debris avalanche occurs only on submarine slopes
Correct answer: Debris avalanche is a rapid, flowing mass movement of a heterogeneous mixture
A debris avalanche is a rapid, large-scale flow involving mixed rock, soil, and vegetation that travels long distances, whereas rockfall involves the free fall or bouncing of discrete rock blocks.
Question 7: In highway cut slope design through bedded sedimentary rock, the most hazardous discontinuity orientation is when bedding planes:
- Dip into the slope at any angle
- Strike parallel to the road and dip out of the slope toward the cut face (Correct answer)
- Are horizontal
- Dip perpendicular to the road alignment
Correct answer: Strike parallel to the road and dip out of the slope toward the cut face
When bedding planes dip out of the slope (daylight into the cut face) parallel to the road, planar sliding can occur along those surfaces without lateral confinement.
ASBOG Engineering Geology 4
Karst terrain presents which primary engineering hazard for foundation design?