Small Strain Shear Modulus of Undisturbed Gravelly Soils During Undrained Cyclic Triaxial Tests

The determination of the small strain shear modulus G₀ of gravels is a very important issue, both under monotonic or cyclic loading conditions. In the paper, the results of a series of triaxial tests carried out in a large apparatus on frozen (undisturbed) specimens of gravel are presented, along with the description of a new experimental device developed to measure the velocity of the shear waves V_s. During undrained cyclic tests, V_s values were measured before and after liquefaction, to analyse the effect of this peculiar stress history on the small strain stiffness G₀ of coarse grained soils. The small strain shear stiffness decreases as pore pressure in the specimen builds up. However, even in tests in which liquefaction was attained during the cyclic loading phase, G₀ showed to depend only on the current value of the effective stress: its values become smaller than the initial one (before cyclic loading was applied) but not nil, depending on the transient value of the pore pressure and therefore of the effective stress. Furthermore, the experimental results showed that, since there was no sudden drop of G₀ upon liquefaction triggering, the gravel did not have a structure which significantly affected its mechanical behaviour.