ASBOG Engineering Geology 3 — Questions and Answers
Question 1: Which type of fault movement is most likely to produce a pressure ridge in alluvium along a strike-slip fault?
- Extensional releasing bend
- Compressional restraining bend (Correct answer)
- Normal dip-slip segment
- Oblique thrust segment
Correct answer: Compressional restraining bend
A compressional restraining bend on a strike-slip fault forces material to converge and buckle upward, forming a pressure ridge in the overlying alluvium.
Question 2: The Atterberg limit most relevant to the shrinkage and cracking potential of an expansive clay is the:
- Liquid limit
- Plastic limit
- Shrinkage limit (Correct answer)
- Activity index
Correct answer: Shrinkage limit
The shrinkage limit marks the water content below which further drying causes no further volume change, and low shrinkage limits correlate with high swell-shrink potential.
Question 3: In a triaxial compression test, if the deviator stress at failure is 200 kPa and the cell pressure is 100 kPa, the major principal stress σ1 at failure is:
- 100 kPa
- 200 kPa
- 300 kPa (Correct answer)
- 400 kPa
Correct answer: 300 kPa
σ1 = cell pressure (σ3) + deviator stress = 100 + 200 = 300 kPa.
Question 4: Swelling pressure in an expansive soil is best measured using which laboratory test?
- Unconfined compression test
- Constant volume oedometer test (Correct answer)
- Direct shear test
- Consolidated undrained triaxial test
Correct answer: Constant volume oedometer test
In a constant volume oedometer test, load is applied to prevent any swell, and the pressure required to maintain zero volume change equals the swelling pressure.
Question 5: Which drainage pattern typically indicates a structurally controlled, folded terrain with alternating resistant and weak rock units?
- Radial
- Trellis (Correct answer)
- Dendritic
- Parallel
Correct answer: Trellis
A trellis drainage pattern forms where main streams follow weak rock layers (e.g., valleys) with short tributaries cutting perpendicular through resistant ridges in folded rocks.
Question 6: The critical height of an unsupported vertical excavation in a purely cohesive soil (φ = 0) is given by:
- Hc = 2c / γ
- Hc = 4c / γ (Correct answer)
- Hc = c / γ
- Hc = 6c / γ
Correct answer: Hc = 4c / γ
Terzaghi's stability formula gives the critical height Hc = 4Su/γ for a vertical cut in undrained clay, where Su is undrained shear strength and γ is unit weight.
Question 7: Which rock texture is most indicative of explosive volcanic eruption and rapid magma vesiculation?
- Phaneritic
- Pyroclastic (scoriaceous or pumiceous) (Correct answer)
- Foliated
- Porphyritic
Correct answer: Pyroclastic (scoriaceous or pumiceous)
Pyroclastic textures with abundant vesicles (scoria, pumice) form when gas-rich magma erupts explosively and chills rapidly, trapping gas bubbles as void space.
ASBOG Engineering Geology 3
Which type of fault movement is most likely to produce a pressure ridge in alluvium along a strike-slip fault?