Terzaghi's Principle states that when a rock is subjected to a stress, it is opposed by the fluid pressure of pores in the rock.[1]
More specifically, Karl von Terzaghi's Principle, also known as Terzaghi's theory of one-dimensional consolidation, states that all quantifiable changes in stress to a soil [compression, deformation, shear resistance] are a direct result of a change in effective stress. The effective stress is related to total stress and the pore pressure by the relationship;
reading that total stress is equal to the sum of effective stress and pore water pressure.
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Though the first 5 assumptions are either likely to hold, or deviation will have no discernible effect, experimental results contradict the final 3. Darcy's Law does not seem to hold at low hydraulic gradients, and both the coefficients of permeability and volume compressibility decrease during consolidation. This is due to the non-linearity of the relationship between void ratio and effective stress, although for small stress increments assumption 7 is reasonable. Finally, the relationship between void ratio and effective stress is not independent on time, again proven by experimental results.