Geotechnical Engineering and Foundations — Practice

Question 1

A saturated soil has w = 30 percent and G = 2.70. Find the void ratio.

Accepted Answer

e = wG/S = 0.30 x 2.70 = 0.81.

Question 2

Find Ka for phi = 30 degrees.

Accepted Answer

Ka = (1 - 0.5)/(1 + 0.5) = 1/3.

Question 3

A 4 m clay layer has Cc = 0.3, e0 = 1.0, initial effective stress 100 kPa and an increase of 100 kPa. Find the settlement.

Accepted Answer

s = (0.3 x 4/2) log10(2) = 0.6 x 0.301 = 0.18 m.

Question 4

Find the active thrust on a 6 m wall, level dry sand backfill, gamma = 18 kN/m3 and phi = 30 degrees.

Accepted Answer

Pa = 0.5 x (1/3) x 18 x 36 = 108 kN/m, acting 2 m above the base.

Question 5

If the drainage path of a clay layer doubles, how does the time for the same degree of consolidation change?

Accepted Answer

It increases four times, because t is proportional to the square of the path.

Question 6

Find the critical hydraulic gradient for a soil with G = 2.65 and e = 0.65.

Accepted Answer

ic = (G - 1)/(1 + e) = 1.65/1.65 = 1.0.

Question 7

Why is horizontal permeability of a layered deposit at least as large as the vertical?

Accepted Answer

Horizontal flow is dominated by the most permeable layer, as a weighted mean, while vertical flow is limited by the least permeable layer, as a harmonic mean, so kh is at least kv.

Question 8

Define the plasticity index and the liquidity index.

Accepted Answer

PI = LL - PL. LI = (w - PL)/PI.

Practice — Geotechnical Engineering and Foundations

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