Influence of joints on creep processes involving rock masses. Results from physical-analogue laboratory tests

Viscoplastic behavior of jointed rock masses still represents a main research topic due to the reduced possibilities of carrying on long-term tests on site as well as to the reduced representativeness of laboratory samples if compared to the scale of the study-case. Rock masses are often involved in creep processes as in the case of gravity-induced deformations affecting relevant portions of natural slopes. These processes can interact with main infrastructures, among which dams, pipelines, aqueducts, tunnels, highways and railways. This study is focused on laboratory tests performed to deduce the rheological behavior of a rock masses at a reduced scale for defining the influence of joint attitude and properties on viscoplasticity. A physics of reduction scaling of mechanical behavior (down-scaling) was preliminary applied to select the most adapt material for laboratory tests; the obtained results were then reported to the effective slope scale (up-scaling). The tested specimens were realized by an artificial moisture whose mechanical properties were scaled respect to limestones outcropping in a mountain ridge involved in already documented gravity-induced slope deformation that caused an intense rock mass jointing. Viscoplasticity was analyzed based on uniaxial compressive strength (UCS) and creep tests. The test results were then up-scaled according to an “equivalent continuum approach” and an equivalent viscosity for the outcropping rock mass was derived. The following performed up-scaling is representative for a creep process acting over a time interval of about 200 years. The obtained viscosity values for the equivalent rock range from 4.23⋅1018 up to 1.56⋅1019 Pa∙s and are in good agreement with the values derived by back-analysis through stress-strain numerical modeling of the gravity-induced slope deformation involving Mt. Rocchetta. The obtained results demonstrate that properties of pervasive joints are not negligible on rock mass creep acting at slope scale which can drive gravity-induced processes toward paroxysmal failures.