Civil Engineering PE Shallow and Deep Foundations 2 — Questions and Answers
Question 1: A 10-ft square footing carries a 400-kip column load. The soil below has a unit weight of 120 pcf and an undrained shear strength of 1,500 psf. What is the net ultimate bearing capacity using Terzaghi's general shear failure equation (Nc = 5.7, Nq = 1, Nγ = 0)?
- 8,550 psf (Correct answer)
- 7,200 psf
- 9,120 psf
- 6,840 psf
Correct answer: 8,550 psf
For a square footing in clay: q_ult = 1.3·c·Nc = 1.3 × 1,500 × 5.7 − (1)(1) × depth term ≈ 8,550 psf net.
Question 2: Which of the following best describes the 'critical depth' concept used in Meyerhof's method for pile capacity in sand?
- Effective stress increases linearly with depth up to a critical depth, then remains constant (Correct answer)
- The pile tip resistance becomes zero below the critical depth
- Skin friction reverses direction below the critical depth
- The pile must extend below the critical depth to develop any capacity
Correct answer: Effective stress increases linearly with depth up to a critical depth, then remains constant
Meyerhof observed that effective overburden stress and unit skin friction in sand reach limiting values at a critical depth (typically 15–20 pile diameters).
Question 3: A mat foundation is used instead of individual spread footings primarily when:
- Individual footing areas would overlap or cover more than 50% of the building footprint (Correct answer)
- The soil is highly permeable and drainage is needed
- Column loads exceed 500 kips each
- Frost depth is greater than 3 ft
Correct answer: Individual footing areas would overlap or cover more than 50% of the building footprint
When spread footings would cover more than about 50% of the plan area, a mat foundation becomes more economical and provides better load distribution.
Question 4: The factor of safety against overturning for a retaining wall footing is typically required to be at least:
- 2.0 (Correct answer)
- 1.5
- 3.0
- 1.0
Correct answer: 2.0
Standard practice requires a minimum factor of safety of 2.0 against overturning for retaining wall foundations.
Question 5: Negative skin friction (downdrag) on a pile is most critical when:
- A soft compressible layer overlies the bearing stratum and is subject to consolidation settlement (Correct answer)
- The pile is installed in dense sand below the water table
- The pile cap is cast directly on the ground surface
- Uplift loads govern the pile design
Correct answer: A soft compressible layer overlies the bearing stratum and is subject to consolidation settlement
Downdrag develops when surrounding soil settles relative to the pile, dragging it downward — most severe in soft, consolidating clays above a firm bearing layer.
Question 6: For a drilled shaft, the O'Neill and Reese method computes tip resistance in clay as:
- q_tip = 9 × Su (Correct answer)
- q_tip = 6 × Su
- q_tip = Nc × Su where Nc = 5.14
- q_tip = Su × Kp
Correct answer: q_tip = 9 × Su
For drilled shafts in clay, the unit tip resistance is taken as 9Su, reflecting the deep bearing capacity factor for circular shafts.
Question 7: When checking a combined footing for structural design, the net upward soil pressure used for beam shear calculations is based on:
- Factored column loads divided by the footing area (Correct answer)
- Unfactored column loads with a 20% increase
- Allowable bearing capacity of the soil
- Total building dead load only
Correct answer: Factored column loads divided by the footing area
Structural design of footings uses factored (LRFD) or increased (ASD) loads divided by footing area to find the net upward pressure for shear and moment calculations.
Civil Engineering PE Shallow and Deep Foundations 2
A 10-ft square footing carries a 400-kip column load.
The soil below has a unit weight of 120 pcf and an undrained shear strength of 1,500 psf.
What is the net ultimate bearing capacity using Terzaghi's general shear failure equation (Nc = 5.7, Nq = 1, Nγ = 0)?