N2PC-Salt and N2PC-MCT - PLAXIS UDSM

Rock, especially rock salt, exhibits time-dependent behaviour. Creep of rock and rock salt is related to various applications in civil engineering, the mining industry, or petroleum engineering, such as energy storages, wellbore stability in oil and gas extraction, radioactive waste facilities design, etc. (Cornet, Dabrowski & Schmida, 2018).

For instance, drilling a deep borehole (around 5000m of depth) for oil extraction could face a serious instability problem when going through a creeping salt layer (see Dusseault, Maury, Sanfilippo & Santarelli (2004)). Most of the underground applications are concerned with large time scales (tens to thousands of years or even longer) and thermal effects at great depth (hundreds to thousands of meters of depth). The Norton power creep law (Norton, 1929) is widely used for describing the steady-state creep behaviour, which is relevant for those large time scales.

In this context, an enriched version of the Norton law, the Norton-based Double Power Creep (N2PC) model for Salt Rock, including temperature effects, was implemented in PLAXIS software. Although the model is particularly adapted for creep of rock salt, it can be used also for other types of rock that exhibit similar creep behaviour.

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The N2PC model is particularly useful for studying the time-dependent (long-term) deformation of rock structures (for example subsidence analysis for solution mining). In some situations, not only deformation but stress analysis is also important to predict failure and damage of the structures. To better capture this aspect, the model is further extended by including a Mohr-Coulomb with Tension cut-off (MCT) failure mechanism. The extended model, called N2PC-MCTmodel, allows modeling more adequately both instantaneous plastic deformation and stress evolution related to rock plastic strength parameters.