Effect of secondary consolidation on long-term settlements of large-area raft foundations

Introduction. This paper evaluates the effect of creep (secondary consolidation) on long-term settlements of large-area raft foundations. The study extends the authors’ previous work on the role of permeability anisotropy in two-dimensional primary consolidation and focuses on quantifying the contribution of skeleton creep to total settlement.

Materials and methods. A parametric finite element study was carried out using Plaxis 2D for an axisymmetric raft foundation model on a clay stratum. The Soft Soil (SS, no creep) and Soft Soil Creep (SSC) models were compared while varying R/H (0.5; 2.0; 6.0), the modified creep index μ* (0.005; 0.01) and permeability anisotropy kx/ky (1; 5). The effect of horizontal sand interlayers was also investigated.

Results. Creep can increase the final settlement by a factor of 1.5–5.4 depending on R/H and μ* (coefficient η). Sand interlayers accelerate primary consolidation by a factor of 3–11 but reduce the creep-related final settlement by only 5–16 %. Permeability anisotropy has a negligible effect on final settlement (difference below 2 %).

Conclusions. Secondary consolidation must be accounted for when μ* ≥ 0.005 (Cα/Cc ≥ 0.10), typical of soft organomineral clays, peaty soils, and silts. Horizontal drainage layers do not eliminate the problem of long-term secondary settlements.

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