Laboratory investigation on reactivated residual strength

Recent research has outlined that mobilized in situ strength at the reactivation of displacements along a preexisting 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 aging-little work has been devoted to this topic. Brombead ring shear tests were carried out on remolded samples of the Cormons flysch, a typical geological formation in northeastern 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 aging may cause a rising of reactivated shear strength above the residual value, depending on time of aging and vertical stress. The longest aging 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 h of aging. In the long term, strength growth is characterized by a deviatoric creep rate gradually approaching zero.