Influence of loading history

The loading history has a substantial influence on the distribution of deformation curve and therefore also on the values of deformation characteristics. The following figure displays the deformation curve (Δe = f(Δσ_ef) diagram) derived from oedometric loading test corresponding, e.g. to natural dense sandy soil.

Load history
a) Deformation curve for clayey soils from oedometric test

b) Simplified interpretation of deformation curve

The soil sample was gradually loaded to reach the stress level σ_bef, the stress-strain relationship (σ_bef -ε) within the section a-b is linear and is denoted as primary or virgin (i.e., relative compression is encountered). Upon exceeding the stress level σ_bef the sample was elastically unloaded and the soil moved up the b-c section of the deformation curve. Upon reloading the soil moved down the b-c section till reaching the original stress σ_bef prior to unloading. When loading beyond σ_bef the deformation curve aproaches asymptotically within the d-e section the primary line accompanied by inelastic deformation of a soil sample.  Such a complex stress-strain curve is often simplified by the idealized deformation curve (fig. b). Such a curve characterizes so called overconsolidated soils, which were in the past subjected large stresses and subsequently unloded. The overconsolidation ratio (OCR) then represents the ratio between the maximum preconsolidation stress the soil has ever experienced and the current vertical stress. Overconsolidated soils typicaly follow the deformation curve given by points c-d-e. The change in slope along this line (given app. by point d) corresponds either to the vertical geostatic stress σ_o (normally consolidated soils) or to preconsolidation pressure σ_c (overconsolidated soils). This point influences the soil deformation, which is smaller within the c-d section when compared to the d-e section (where for the large degree of overconsolidation the soil deformation increases). Additional deformation characteristics such as deformation modulus upon unloading E_e, one-dimensional swelling index C_e, recompression index C_r, etc. were introduced to describe such a complex soil behavior.  Currently the most often used parameter is the recompression index C_r suitable for the computation of settlement of overconsolidated soils.