Geotechnical and Foundation Engineering Flashcards

A clay has a natural water content of 35%, liquid limit of 45%, and plastic limit of 25%. What is its liquidity index?

Answer: 0.50

LI = (w - PL)/PI = (35 - 25)/(45 - 25) = 0.50.

The activity of a clay soil is defined as which ratio?

Answer: Plasticity index divided by percent clay-size fraction finer than 2 µm

Skempton's activity A = PI / (% finer than 2 µm) indicates clay mineral swelling potential.

Under USCS, a soil with 70% passing the No. 200 sieve, LL = 62, and Atterberg limits plotting above the A-line is classified as:

Answer: CH

Fine-grained soil with LL ≥ 50 plotting above the A-line is a high-plasticity clay, CH.

The overconsolidation ratio (OCR) of a clay is defined as:

Answer: Preconsolidation pressure divided by current effective vertical stress

OCR = σ'p / σ'v0, so normally consolidated clay has OCR = 1.

Secondary compression of a clay layer is best described as settlement that:

Answer: Continues after excess pore pressure has essentially dissipated, due to creep of the soil skeleton

Secondary compression, governed by Cα, is creep that follows primary consolidation.

In Terzaghi's one-dimensional consolidation theory, the time factor Tv corresponding to 90% average degree of consolidation is approximately:

Answer: 0.848

Tv ≈ 0.848 for U = 90%, while Tv ≈ 0.197 corresponds to U = 50%.

A 6-m-thick clay layer is sandwiched between two sand layers. What drainage path length should be used in consolidation time calculations?

Answer: 3 m

With double drainage, Hdr equals half the layer thickness, 6/2 = 3 m.