Geotechnical and Foundation Engineering Flashcards

In the alpha (α) method for driven piles in clay, unit skin friction is calculated as:

Answer: fs = α × cu

The α method is a total stress approach where fs = α·cu, with α decreasing as cu increases.

Negative skin friction (downdrag) on a pile is most likely to develop when:

Answer: Recently placed fill causes the surrounding soft clay to consolidate and settle more than the pile

Downdrag occurs when surrounding soil settles relative to the pile, dragging it downward.

For an infinite slope of dry cohesionless soil with friction angle φ inclined at angle β, the factor of safety is:

Answer: tan φ / tan β

For dry cohesionless infinite slopes, FS = tan φ / tan β, independent of depth.

A field SPT blow count N = 20 was recorded with a hammer having 75% energy efficiency. What is N60?

Answer: 25

N60 = N × ER/60 = 20 × 75/60 = 25.

A 60-in rock core run recovers pieces of 6, 3, 8, 2, 5, and 12 in. What is the Rock Quality Designation (RQD)?

Answer: 52%

Only pieces ≥ 4 in count: (6 + 8 + 5 + 12)/60 = 31/60 ≈ 52%.

Prefabricated vertical drains (wick drains) are installed in soft clay primarily to:

Answer: Shorten the drainage path and accelerate primary consolidation

Wick drains create short radial drainage paths, greatly reducing consolidation time, usually combined with preloading.

Compared with the standard Proctor test, the modified Proctor test (ASTM D1557) for the same soil typically gives:

Answer: Higher maximum dry unit weight and lower optimum moisture content

Modified Proctor uses about 56,000 ft-lbf/ft³ versus 12,400, so the curve shifts up and to the left.