Shallow and Deep Foundations Flashcards
7 cards from real Civil Engineering PE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Shallow and Deep Foundations flashcards as text
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)?
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.
Which of the following best describes the 'critical depth' concept used in Meyerhof's method for pile capacity in sand?
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).
A mat foundation is used instead of individual spread footings primarily when:
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.
The factor of safety against overturning for a retaining wall footing is typically required to be at least:
Answer: 2.0
Standard practice requires a minimum factor of safety of 2.0 against overturning for retaining wall foundations.
Negative skin friction (downdrag) on a pile is most critical when:
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.
For a drilled shaft, the O'Neill and Reese method computes tip resistance in clay as:
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.
When checking a combined footing for structural design, the net upward soil pressure used for beam shear calculations is based on:
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.