Which condition is assumed in the Unconsolidated Undrained test?

In the Unconsolidated Undrained (UU) test, it is assumed that the angle of internal friction is effectively zero. This assumption stems from the nature of the test, which occurs rapidly enough that there is no time for pore water pressure to dissipate during loading. Therefore, the soil sample does not have the opportunity to consolidate, and the effective stress cannot develop as it would under drained conditions. This leads to a scenario where the failure of the soil occurs primarily due to cohesion, with the contribution from friction being negligible, hence the basis for considering the angle of internal friction as effectively zero. Importantly, this concept is crucial for understanding short-term stability conditions in saturated soils where rapid loading or sudden changes occur.

In contrast, the assumption about soil draining freely during loading does not hold true because the test specifically conditions the sample to remain undrained. The idea that the soil is partially saturated would not be applicable since the test is typically performed on fully saturated samples. Finally, while effective stress theories are fundamental in soil mechanics, in this test, the effective stress doesn't increase with loading as it would in a drained condition; rather, the pore pressure increases while the effective stress remains constant until failure occurs.