Relationship of precompression stress with elasticity and plasticity indexes from uniaxial cyclic loading test

The precompression stress (σp) has been considered as the soil load bearing capacity, because σp theoretically separates predominantly elastic (loads lower than σp) from predominantly plastic (loads greater than σp) deformations. However, the relationship of σp and soil elasticity and plasticity indexes has not been widely investigated. In this study we determined σp and elasticity and plasticity indexes in cyclic loading tests (17 cycles per load) with a series of increasing σ (12.5, 25, 50, 100, 200, 400 and 800 kPa) of undisturbed samples of horizons A and B of eight soils (48 samples) from southern Brazil. The σp was also determined from static load test. The objective was to verify if σp match the load in the transition from predominantly elastic to predominantly plastic deformation. Two elasticity indexes were calculated from the logσ-ε diagram, which relates the void ratio (ε) to the log of the loads (σ): SI, as the slope of rebound line of the logσ-ε diagram; and SI', being the ratio of recovered ε with soil decompression in relation to the change in ε by compression. Two plasticity indexes were also calculated: CI, as the slope of virgin compression line of log σ-ε diagram; and CI', which is 1-SI'. All these four indexes were calculated at 25, 50, 100, 200, 400 and 800 kPa loads. Both SI and SI' decreased during the repetitions of a given load, but neither SI nor SI' have a same behavior throughout the entire range of loads. In some samples there was increase in SI with increased loading, but the opposite occurred in other samples. SI' also behaved like SI, but not at exactly for the same samples. Surprisingly, both SI and CI augmented with increased loading, suggesting that the soil becomes more elastic and more plastic simultaneously, which seems physically incoherent. This physical inconsistency does not result from SI' and CI' interpretation, because SI' + CI' = 1 is perfectly complementary. Therefore, SI' and CI' are more coherent indexes of soil elasticity and plasticity. Another critical finding of this study is that σp does not match the load where SI and CI or SI' and CI' change from predominantly elastic to predominantly plastic deformation. Furthermore, SI and CI or SI' and CI' do not support the assumption that σp define a transitions from elastic to plastic behavior.