Civil Engineering PE Shallow and Deep Foundations 3 — Questions and Answers
Question 1: A 16-in diameter closed-end steel pipe pile is driven 40 ft into medium-dense sand (φ = 32°, γ = 115 pcf). Using the β method with β = 0.30, what is the total unit skin friction at a depth of 20 ft?
- 690 psf (Correct answer)
- 575 psf
- 460 psf
- 805 psf
Correct answer: 690 psf
At 20 ft depth, effective stress = 115 × 20 = 2,300 psf; unit skin friction = β × σ'v = 0.30 × 2,300 = 690 psf.
Question 2: Punching shear failure in a shallow foundation is most likely when:
- The foundation is on loose sand or soft clay and the depth-to-width ratio is small (Correct answer)
- The foundation is on dense gravel with high confinement
- The footing is very wide relative to its depth
- The footing is loaded by eccentric moments only
Correct answer: The foundation is on loose sand or soft clay and the depth-to-width ratio is small
Punching shear (local shear) failure occurs in weak, compressible soils where soil near the footing compresses without developing a distinct failure wedge.
Question 3: A pile group efficiency factor less than 1.0 indicates that:
- The group capacity is less than the sum of individual pile capacities due to overlapping stress zones (Correct answer)
- Each pile in the group carries more load than a single isolated pile
- The piles are too closely spaced for driving equipment
- The pile cap reduces lateral capacity
Correct answer: The group capacity is less than the sum of individual pile capacities due to overlapping stress zones
Group efficiency < 1.0 means overlapping failure surfaces or stress zones reduce the average capacity per pile below that of an isolated pile.
Question 4: Which of the following parameters is NOT directly used in Skempton's bearing capacity equation for saturated clay (φ = 0)?
- Angle of internal friction φ (Correct answer)
- Undrained shear strength Su
- Shape factor sc
- Depth factor dc
Correct answer: Angle of internal friction φ
For saturated clay under undrained conditions (φ = 0), Nq = 1 and Nγ = 0, so internal friction angle is irrelevant; only Su, shape, and depth factors matter.
Question 5: The primary purpose of a pile load test (ASTM D1143) is to:
- Verify pile capacity and measure load-settlement behavior at the specific site (Correct answer)
- Determine soil stratification and SPT N-values
- Measure pile installation energy during driving
- Evaluate corrosion potential of the pile material
Correct answer: Verify pile capacity and measure load-settlement behavior at the specific site
Static pile load tests confirm that the installed pile achieves the required capacity and document the load-settlement curve for design verification.
Question 6: Heave failure at the bottom of a deep excavation in soft clay is evaluated using the factor of safety against:
- Basal heave, using Terzaghi's or Bjerrum-Eide's method with Su and excavation geometry (Correct answer)
- Slope stability with Bishop's circular arc method
- Bearing capacity with surcharge correction
- Consolidation settlement with Cv and drainage path
Correct answer: Basal heave, using Terzaghi's or Bjerrum-Eide's method with Su and excavation geometry
Basal heave FS is computed by dividing the soil's resistance to upward movement (based on Su and geometry) by the driving stress from the excavation surcharge.
Question 7: According to ASCE 7, the minimum depth of a foundation in frost-susceptible soils must be:
- Below the local frost depth as established by the local building code or authority (Correct answer)
- At least 12 inches below finished grade
- A minimum of 3 ft regardless of climate
- Below the seasonal groundwater table
Correct answer: Below the local frost depth as established by the local building code or authority
Foundations must extend below the local frost depth to prevent frost heave; this depth varies by location and is specified in local codes.
Civil Engineering PE Shallow and Deep Foundations 3
A 16-in diameter closed-end steel pipe pile is driven 40 ft into medium-dense sand (φ = 32°, γ = 115 pcf).
Using the β method with β = 0.30, what is the total unit skin friction at a depth of 20 ft?