Secondary Compression in Soil Mechanics

Secondary compression is the time-dependent deformation of saturated cohesive soils occurring after primary consolidation, under a constant effective stress. It results from plastic adjustment and creep of soil particles, causing ongoing soil volume reduction not accounted for by initial pore water expulsion. This behavior is most prominent in fine-grained soils like clays and peats. The degree of secondary compression is quantified by the secondary compression index $C_\alpha$, which typically remains constant and defines the rate of creep settlement. The strain from secondary compression, $\varepsilon_{sc}$, can be expressed as $\displaystyle \varepsilon_{sc} = C_\alpha \log \frac{t}{t_p}$, where marks the end of primary consolidation. Accurately predicting long-term settlement requires incorporating secondary compression effects, as neglecting it may lead to underestimated deformations, risking structural safety and serviceability. Therefore, foundation design, maintenance scheduling, and lifecycle cost estimation for infrastructure on soft, cohesive soils must consider this phenomenon.