Figure below shows an element of clay located at a depth 8m from the surface. Explain the use of the Principle of Effect

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Figure below shows an element of clay located at a depth 8m from the surface.
Explain the use of the Principle of Effective Stress when determining the stresses.
Compute σyyσyy, σ′yyσyy′ and u at depths 0m, -1m, -4m and -8m.
Assuming that σ′xx=0.5σ′yyσxx′=0.5σyy′, compute the total horizontal stress and the effective horizontal stress at depth -8m.
Plot the variations in the above three parameters with depth. Show their principal values.
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