When calculating the settlement of a shallow footing on granular soil, what specific soil parameter is primarily used to estimate immediate settlement, and how does it differ from the parameter used for consolidation settlement in clay?

When calculating the immediate settlement of a shallow footing on granular soil, the specific soil parameter primarily used is the Elastic Modulus (often denoted as E or E_s). The Elastic Modulus is a measure of the soil's stiffness or resistance to elastic deformation under stress. Elastic deformation refers to the instantaneous compression of soil particles and air voids without significant expulsion of pore water, and it is largely recoverable if the load is removed. For granular soils like sand and gravel, which have high permeability, water drains very quickly, meaning that almost all settlement occurs immediately due to this elastic compression and particle rearrangement. A higher Elastic Modulus indicates a stiffer soil, resulting in less immediate settlement for a given load. The calculation typically involves elastic theory, considering the applied stress, footing geometry, and the soil's Elastic Modulus and Poisson's ratio.

This parameter differs significantly from the one used for consolidation settlement in clay. For consolidation settlement in clay, the primary soil parameter used is the Compression Index (C_c) or, for previously loaded soils, the Recompression Index (C_r or C_s). Consolidation settlement is a time-dependent process unique to fine-grained, low-permeability soils like clay. It occurs as pore water is slowly expelled from the soil's void spaces under a sustained load, leading to a gradual reduction in the soil's volume over months or even years. The Compression Index quantifies the compressibility of the clay, specifically describing the change in the soil's void ratio (the ratio of the volume of voids to the volume of solid particles) for a logarithmic increase in effective stress during virgin compression. A higher Compression Index indicates a more compressible clay, leading to greater consolidation settlement. Unlike the Elastic Modulus, which describes instantaneous elastic deformation, the Compression Index describes an inelastic, permanent deformation process driven by the slow dissipation of excess pore water pressure.