BCE Geotechnical 5 โ Questions and Answers
Question 1: What is the primary purpose of a geosynthetic reinforcement layer placed within a compacted fill embankment?
- Reduce permeability of the fill material
- Provide tensile resistance to improve embankment stability and reduce lateral spreading (Correct answer)
- Increase the unit weight of the fill to improve bearing capacity
- Act as a drainage blanket to accelerate consolidation
Correct answer: Provide tensile resistance to improve embankment stability and reduce lateral spreading
Geogrid or geotextile reinforcement carries tensile stress that soil cannot, improving overall embankment stability and controlling lateral deformation at the base.
Question 2: Which slope stability method satisfies both force and moment equilibrium for each slice and is considered the most rigorous?
- Ordinary (Fellenius) Method
- Bishop's Simplified Method
- Spencer's Method (Correct answer)
- Infinite Slope Method
Correct answer: Spencer's Method
Spencer's method assumes a constant interslice force inclination and satisfies both overall moment and force equilibrium, making it one of the most complete methods.
Question 3: A soil profile shows SPT N-values of 2-4 in the upper 5 m followed by N > 50 below. What foundation type is most appropriate for a heavily loaded column?
- Shallow spread footing bearing on the soft upper layer
- Mat (raft) foundation on the soft layer
- Deep pile foundation driven to the dense layer (Correct answer)
- Grillage foundation on the soft clay
Correct answer: Deep pile foundation driven to the dense layer
N = 2โ4 indicates very soft to soft ground incapable of supporting heavy loads; deep piles driven to the dense bearing stratum (N > 50) are required.
Question 4: What is the definition of the overconsolidation ratio (OCR)?
- Ratio of current void ratio to initial void ratio
- Ratio of preconsolidation pressure to current vertical effective stress (Correct answer)
- Ratio of undrained to drained shear strength
- Ratio of horizontal to vertical effective stress
Correct answer: Ratio of preconsolidation pressure to current vertical effective stress
OCR = ฯ'p / ฯ'v0; OCR = 1 means normally consolidated, OCR > 1 means overconsolidated, indicating the soil has experienced higher stresses in its past.
Question 5: During a triaxial CD test on dense sand, what typically occurs to pore pressure and volume as shear strain increases past peak?
- Positive pore pressure builds and volume decreases (contractive)
- Negative pore pressure develops and volume increases (dilative) (Correct answer)
- Pore pressure remains zero and volume remains constant
- Volume decreases then increases with no pore pressure change
Correct answer: Negative pore pressure develops and volume increases (dilative)
Dense sand dilates (expands) during shear in a drained test; in undrained conditions this would manifest as negative (suction) pore pressures.
Question 6: What is the role of the compression index (Cc) in settlement calculations for normally consolidated clay?
- Determines rate of consolidation settlement over time
- Quantifies the slope of the e-log ฯ'v curve used to calculate primary consolidation settlement magnitude (Correct answer)
- Defines the shear strength at the preconsolidation pressure
- Measures volume change under undrained loading conditions
Correct answer: Quantifies the slope of the e-log ฯ'v curve used to calculate primary consolidation settlement magnitude
Cc is the slope of the virgin compression line on an e vs. log ฯ'v plot; it is used in ฮH = CcยทH/(1+e0)ยทlog(ฯ'f/ฯ'0) to compute consolidation settlement.
Question 7: Which of the following ground improvement techniques uses vertical drains to accelerate consolidation of soft clay under an embankment?
- Dynamic compaction
- Stone columns with prefabricated vertical drains (PVDs) (Correct answer)
- Jet grouting
- Soil nailing
Correct answer: Stone columns with prefabricated vertical drains (PVDs)
Prefabricated vertical drains (band drains) shorten the drainage path from the full layer thickness to the drain spacing, dramatically reducing time for consolidation under surcharge loading.
BCE Geotechnical 5
What is the primary purpose of a geosynthetic reinforcement layer placed within a compacted fill embankment?