Experimental assessment of the small-strain response of residual soil under monotonic and cyclic loading

Residual soil from the Passo Fundo region (Brazil), presents natural cementation, responsible for a peculiar behaviour that is clearly distinguishable from that of soils with other origins. Understanding this behaviour and the effects of the fabric, with an influence on the cementation bonding, is essential to characterise these materials. Therefore, the aim of the present study is the mechanical characterisation of this soil, mainly by analysis and testing of remoulded and undisturbed samples. The experimental programme included uniaxial compressive strength tests, oedometer tests, consolidated drained monotonic triaxial tests and consolidated undrained cyclic triaxial tests. Based on the data collected, a clear influence of interparticle bonding, provided by the natural cementation, was detected on the soil's intrinsic behaviour; the undisturbed material showed a higher stiffness modulus than the remoulded material, mainly due to the bond breakage to which the latter was submitted, losing its natural cementation. A uniaxial confining stress of 89.4 kPa was obtained for the undisturbed material, assumed to be its yield stress. This value was corroborated by the oedometric (87.5 kPa), isotropic (80 kPa) and triaxial compression (90 kPa). The natural cementation was influential in every type of loading and should be considered during the design process.