Laboratory investigation on reactivated residual strength

Recent research has outlined that mobilised in situ strength at the reactivation of displacements along a pre-existing sliding surface may be slightly greater than laboratory residual strength. Although geotechnical literature explains this difference in various ways - for instance, strength gain due to soil ageing - little work has been devoted to this topic. Bromhead ring shear tests were carried out on remoulded samples of the Cormons flysch, a typical geological formation in north-eastern Friuli-Venezia Giulia (Italy). The specimens came from the shallow weathered crust of the Cormons flysch, near the areas of Montona and Rosazzo. This experimental study aimed at highlighting the effect of a limited number of parameters on strength gain at the reactivation of displacements along aged sliding surfaces. The results of this research confirm that ageing may cause a rising of reactivated shear strength above the residual value, depending on time of ageing and vertical stress. The longest ageing period - about a month - showed a 20% strength increase for the Rosazzo flysch, and a 30% increase for the Montona flysch. Reactivated strength increases according to a power law: almost 50% develops over a short time, i.e., after 24 hours of ageing. In the long term, strength growth is characterised by a deviatoric creep rate gradually approaching zero.