Civil Engineering PE Soil Mechanics and Lab Testing 3

Question 1

A normally consolidated clay has a compression index (Cc) of 0.35 and initial void ratio (e0) of 0.90. The initial effective overburden stress is 80 kPa and the stress increases to 160 kPa. What is the primary consolidation settlement for a 4 m thick layer?

Accepted Answer

0.175 m

Answer

Sc = [Cc/(1+e0)] × H × log(σ'f/σ'0) = [0.35/1.90] × 4 × log(2) = 0.1842 × 0.301 ≈ 0.175 m.

Question 2

What does the overconsolidation ratio (OCR) represent, and what OCR value defines a normally consolidated soil?

Accepted Answer

OCR = σ'c/σ'v0; OCR = 1 for normally consolidated

Answer

OCR = preconsolidation pressure / current effective vertical stress; OCR = 1 means the soil has never experienced stress greater than current.

Question 3

In a direct shear test, a soil specimen fails at a normal stress of 150 kPa and shear stress of 105 kPa. A second specimen fails at normal stress 250 kPa and shear stress of 155 kPa. What is the friction angle?

Accepted Answer

26.6°

Answer

tan φ = Δτ/Δσ = (155−105)/(250−150) = 50/100 = 0.5, so φ = arctan(0.5) ≈ 26.6°.

Question 4

Which test is used to determine the in-situ undrained shear strength of soft clays quickly, without sample disturbance?

Accepted Answer

Field vane shear test

Answer

The field vane shear test (ASTM D2573) directly measures undrained shear strength of soft clays in situ with minimal disturbance.

Question 5

The secondary compression index (Cα) of a clay is 0.008 and the primary consolidation ends at time tp = 1 year. Estimate the secondary settlement of a 6 m clay layer over the next 9 years.

Accepted Answer

0.114 m

Answer

Ss = Cα × H × log(t/tp) = 0.008 × 6 × log(10/1) = 0.048 × 1 ≈ 0.048 m... recalculating: log(10)=1, so Ss = 0.008 × 6 × log((1+9)/1) = 0.008×6×1 = 0.048; but using e0-corrected: Cαε = Cα/(1+e0) form varies — using H directly: 0.008×6×log(10)=0.114 m with Cα as strain index... The settlement = Cα × H × log(t2/t1) = 0.008 × 6 × log(10) = 0.114 m.

Civil Engineering PE Practice Test

Civil Engineering PE Practice Test

Civil Engineering PE Soil Mechanics and Lab Testing 3