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Derive the expression for Rankine's Active earth pressure for cohesive back fill.

written 3.9 years ago by | modified 9 months ago by |

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written 9 months ago by |

Consider a retaining wall of height $\mathrm{H}$ with a smooth vertical back, retaining a cohesive backfill.
The relationship between the major principal stress $\sigma_{1}$ and minor principal stress $\sigma_{3}$ at failure (Plastic equilibrium) can be expressed in the form
$$
\sigma_{1}=\sigma_{3}\left(\frac{1+\sin \phi}{1-\sin \phi}\right)+2 c \sqrt{\frac{1+\sin \phi}{1-\sin \phi}}
$$
**Active State**
In active state, lateral stress $\sigma_{h}$ reduces to its minimum value i.e., $p_{a}$ while the vertical stress $\sigma_{v}$ remains unchanged.
Since,
$$
\sigma_{v}\gt\sigma_{h}
$$
Hence,
$$
\begin{array}{l}
\sigma_{1}=\sigma_{v} \\
\sigma_{3}=\sigma_{h}=p_{a}
\end{array}
$$
Substituting the values of $\sigma_{1}$ and $\sigma_{3}$ in above eq., we get
$\sigma_{v}=\sigma_{h}\left(\frac{1+\sin \phi}{1-\sin \phi}\right)+2 c \sqrt{\frac{1+\sin \phi}{1-\sin \phi}}$
$\sigma_{v}=p_{a}\left(\frac{1+\sin \phi}{1-\sin \phi}\right)+2 c \sqrt{\frac{1+\sin \phi}{1-\sin \phi}}$
$p_{a}=\left(\frac{1-\sin \phi}{1+\sin \phi}\right) \sigma_{v}-2 c \sqrt{\frac{1-\sin \phi}{1+\sin \phi}}$
$p_{a}=k_{a} \sigma_{v}-2 c \sqrt{k_{a}} \quad\left[\because k_{a}=\frac{1-\sin \phi}{1+\sin \phi}\right]$

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