Loading history effect on time-dependent deformations after unloading – reversible creep of soft rock (marl)

Abstract

Practical problem in geotechnical design of support systems for tunnels, slopes, and excavation pits requires numerical modeling of different phases of excavation and support installation. The applied constitutive model should therefore include aspects of the influence of the loading history on the deformation response of the rock material during both loading and unloading. Notwithstanding the practical significance, researches into this subject-matter with a view to soft rocks are rare. Some aspects of deformational behavior of soft rock after unloading given in this paper are based on results of time-cycling loading and stepwise unloading long-term creep experiments conducted by the authors on the marl specimens. The results of the experiment indicate that the time of unloading $t_{\mathrm{c}}$ measured from the beginning of a specific load history is a basic parameter, which can describe the influence of history on the deformation response of marl because it implicitly reflects the process of preconsolidation (hardening) of the material. The deformation response after unloading is described by determining the dependence of the parameters of the modified Wallner rheological model of $t_{\mathrm{c}}$ . This approach requires a larger volume of experimental data, so a semiempirical approach has been applied as an alternative, which can include the influence of magnitude of creep stress, the time of maintaining such stress before unloading, and the duration of the last period in which rock material was completely unloaded during load history.