Modulus of Subgrade Reaction

The modulus of subgrade reaction (k) is the ratio of bearing pressure to the corresponding settlement of the soil, expressed in kN/m³ (or kN/m²/m), representing the soil idealised as a bed of independent elastic springs (the Winkler model). It is the key input for the structural analysis of rafts, mat foundations, grade slabs and rigid pavements, where the soil is modelled as a continuous elastic support rather than a uniform pressure.

The value is determined from the plate load test per IS 9214 (and used in IRC 58 rigid-pavement design via the effective k on the subgrade/sub-base), then corrected for plate size, saturation and the actual loaded area, because k is not a fundamental soil property — it depends on footing size, shape, embedment and stress level. Using an unadjusted small-plate k for a large raft significantly overestimates stiffness and underestimates settlement, a classic design error; for important structures, soil-structure interaction or elastic-continuum analysis is preferred over a single Winkler k.