Similarly, strains on other two planes are: !-!(„+„, ~E ~ E

Now it can be shown that, no matter what the stresses on the faces of the cube, the volumetric strain is equal to the sum of the strains on each face.

Compressibility of a material is the volumetric strain per unit pressure, i.e. for a soil skeleton,

Average pressure increase = j(cti + ct2 + 03). Therefore, for a perfectly elastic soil:

Consider a sample of saturated soil subjected to an undrained triaxial test. The applied stress system for this test has already been discussed (Fig. 3.15). The pore water pressure, u, produced during the test will be made up of two parts corresponding to the application of the cell pressure and the deviator stress. Let ua = pore pressure due to <73

If we consider the effects of small total pressure increments A<73 and A<T] then Ac3 will cause a pore pressure change Aua and Actj — Acr3 will cause a pore pressure change Aud.

Effect of Actj

When an all-around pressure is applied to a saturated soil and drainage is prevented the proportions of the applied stress carried by the pore water and by the soil skeleton depend upon their relative compressibilities:

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