Settlement of Soil Calculator

Immediate (elastic) settlement occurs instantly upon load application due to elastic deformation of soil without volume change. Consolidation settlement is time-dependent volume change due to expulsion of pore water, primarily in clay soils.

Isolated footings: 25 mm (1 in). Raft foundations: 50 mm (2 in). Differential settlement should be limited to 1/300 to 1/500 of span length. IS 8009 provides detailed limits.

Schmertmann method is preferred for sands and includes strain influence factors. Simple elastic theory (Boussinesq) is suitable for clays and homogeneous soils.

Cc is the slope of the virgin compression curve (e vs log σ). Typical values: 0.2–0.5 for clays, 0.01–0.2 for silts, 0.5–1.5 for highly plastic clays. It can be estimated as Cc = 0.009(LL-10) for clays.

Overconsolidated soils have been preloaded in the past. If the applied load does not exceed the preconsolidation stress σ'p, settlement uses the recompression index Cr (≈0.1 to 0.2Cc) instead of Cc, resulting in much smaller settlement.

The influence factor I depends on footing shape and rigidity. For a flexible footing at the center: I ≈ 1.0. For average settlement: I ≈ 0.85 for strip footings, with adjustments for rectangular footings based on L/B ratio.

Loose sand: 10–20 MPa, Dense sand: 30–80 MPa, Soft clay: 2–15 MPa, Stiff clay: 15–50 MPa, Silt: 5–25 MPa. Best determined from plate load tests or correlations with SPT N-values.

IS 8009 recommends: total settlement ≤ 25 mm for isolated footings on sand, ≤ 40 mm for clay. Differential settlement ≤ 0.0015L for sand, ≤ 0.002L for clay. Angular distortion ≤ 1/500.