Question

The critical height of an unsupported vertical cut in a cohesive soil is given by

Answer 1

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Rankine’s Earth Pressure in Cohesive Soil for Active Case

AUGUST 29, 2013 BY ENGINEEERING PROJECTS 0 COMMENTS

Figure 1 shows the Mohr’s circle in which point B indicates the vertical stress and point E represents the active pressure. The circle is tangent to the failure envelope. For this case the relation between Pa and \sigma_v is given by:

 

P_a=K_a\sigma_v-2c\sqrt{K_a}———-(1)

P_a=K_a\gamma z-2c\sqrt{K_a}  ———-(2)  

Where, K_a=\dfrac{1-sin\phi}{1+sin\phi}=tan^245^0-\dfrac{\phi}{2}

In the above equation (2) where z = 0

P_a=-2c\sqrt{K_a}

The negative sign indicates that the pressure is negative and tensile, As a result there would be gap between backfill and wall. The tensile stress decreases with the increasing value of z. When z = zc say Pa=0. Then,

0=K_a\gamma z_c-2c\sqrt{K_a}

Or, 0=\gamma z_c\sqrt{K_a}-2c

2c=\gamma z_c\sqrt{K_a}

Z_c=\dfrac{2c}{\gamma\sqrt{K_a}}———-(3)

Answer 2

The maximum height of the surface at which a vertical cut in a cohesive soil will be able to stand unsupported is known as the critical height of the cohesive soil. It is the lowest height which is in relation to an aerodrome which is specified level below which an approach procedure cannot be continued in a safe manner with any of the instruments. The experimental person who is conducting the experiment must commence the missed-approach procedure of the soil to conduct the experiment.