Which factor contributes to secondary consolidation being slower than primary?

Secondary consolidation, also known as creep, is the gradual deformation of soil over time under constant load after primary consolidation has occurred. The correct choice highlights that alterations in soil structure play a significant role in this phenomenon.

During primary consolidation, the expulsion of pore water from the soil voids is the primary mechanism for achieving a decrease in volume, which occurs relatively quickly due to the application of effective stress. In contrast, secondary consolidation involves a slower process where soil particles rearrange and reorient themselves under the existing load, leading to additional deformation. This coordination of soil particles is influenced by factors such as bonding, inter-particle friction, and the internal structure of the soil, which are not instantaneously corrected during loading.

The other factors mentioned do impact soil behavior, but they do not specifically address the delayed nature of secondary consolidation relative to primary. For instance, while changes in pore water pressure are crucial during the initial consolidation phase, in secondary consolidation, the pore water pressures have already dissipated, and the effective stress remains constant. Similarly, a reduction in effective stress is typically more applicable during the primary phase rather than during the later stages of secondary consolidation.

Thus, the alterations in soil structure are fundamental to understanding why secondary consolidation progresses at a slower rate