Which two factors are needed for the determination of primary consolidation settlement?

The determination of primary consolidation settlement is primarily influenced by the net increase of vertical stress in the soil. This factor is crucial as it reflects the change in stress that the soil experiences due to loading conditions, which directly influences the volume change and resulting settlement of soil over time.

When a load is applied to a saturated soil layer, it leads to an increase in vertical stress, which initially causes porewater pressure to rise. However, during primary consolidation, the water within the soil pores is gradually expelled, allowing the soil particles to rearrange and reduce the void ratio. This process continues until equilibrium is reached, which is fundamentally dictated by the net increase of vertical stress. This stress change directly correlates to the amount of consolidation that will occur, thus establishing the relationship between stress and resultant settlement.

While changes in porewater pressure and the void ratio are important aspects of the consolidation process, they are more of responses to the net vertical stress change rather than independent factors needed for settlement determination. Horizontal stress in soil, although relevant in certain analyses, does not directly influence the vertical consolidation settlement in the same way that vertical stress does.