Test: Soil Mechanics- 6 - Civil Engineering (CE) MCQ

In flexible footing, settlement is non-uniform but contact pressure is more or less uniform. In case of sands, modulus of elasticity (E) increases with the confining pressure and therefore increases with the depth. Further ‘E’ is greater near the centre than at edges, As a result, the distribution of deformation below a flexible footing will be of the invested dish pattern while the contact pressure again be more or less uniform.

The stress – strain curves for dense sands and loose sands are similar in shape to the stress – strain curve for over consolidated and normally consolidated clays respectively.

The relationship between critical void ratio and confining pressure is shown in figure. As confining pressure increases, the critical void ratio decreases. With this knowledge of relationship it is possible predict the behaviour of sand at any ratio and at any given confining pressure.

All other statements are also true.

Here, σ₃ = 250 kN/m²

σ₁ = σ₃ + σ_d

Where, σ_d is the deviator stress

Using plastic equilibrium condition,

As sandy soil, C = 0

 

⇒ 250 + σ_d = 962.96

⇒ σ_d = 712.95 kN/m²

The purpose of a UU test is to determine the undrained shear strength of a saturated soil. The UU test consists of applying a cell pressure to the soil sample without drainage of porewater followed by increments of axial stress. The cell pressure is kept constant and the test is completed very quickly because in neither of the two stages—consolidation and shearing—is the excess porewater pressure allowed to drain. 
Suitability of UU test: When construction is very fast such as in case of foundation over fine clay.

Comparing the given equation with equation – (1)

Concept:

Vertical pressure below uniformly loaded circular area is given by:

σ_z = kq

where,

k = 1 – cos³ ∝

cos∝= 

Calculation:

q = 150 kN/m²

z = 8 m

r = 3/2m = 1.5m

cos∝ =  

cos ∝ = 0.983

k = 1 – cos³ ∝ = 1 – (0.983)³

k = 0.050

σ_z = 0.050 × (150)

σ_z = 7.5 kN/m²

The arrangement of the column / footing will be as follows:

Total load = 400 ton.
load in each column, Q = 400/4 = 100 ton
∴ Stress at a depth of 3m under any column

(Two column will be at a distance 3 m & one column will be at a distance 3√2m)

 

= 5.30+ 1.876+ 0.34

= 7.516 ton/m²

Concept:

Pure pressure parameter (B)

σ₃ = cell pressure

U_C = pore pressure

Δσ₃ = change in cell pressure in cell pressure stage

ΔU_C = change in pore pressure in cell pressure stage

Pore pressure parameter (A)

ΔU_d = change in pore in deviator state

Δσ_d = change in deviator stress

Calculation:

 ΔU_c = 5.75 - 4.5 = 1.25 kg/cm²

Δσ₃ = 10 - 8 = 2 kg/cm²