Phenomenological behavior of rock salt:On the influence of laboratory conditions on the dilatancy onset

Recently,stress-based dilatancy criteria have become essential tools to design underground facilities in salt formations such as gas storage caverns.However,these criteria can depend critically on the volumetric strain measurements used to deduce the dilatancy onset.Results from conventional triaxial compression tests can show different volumetric behavior depending on the loading conditions,as well as on the measurement techniques.In order to obtain a quantitative understanding of this problem,an experimental program was carried out and the testing procedure was investigated numerically under homogeneous and heterogeneous stress states.The experimental results showed that the deviatoric stress corresponding to the dilatancy onset was significantly dependent on the measurement techniques.With a heterogeneous stress state,the simulation results revealed that the strain measurements at different scales(referred to as local,hybrid or global)can provide different volumetric results with moderate to significant deviations from the idealized behavior,and hence different onsets of dilatancy.They also proved that,under low confinement,tensile stresses can take place within the compressed specimen,leading to a great deviation of the dilatancy onset from the idealized behavior.From both experimental and numerical investigations,the difference in sensitivity to the measurement techniques between the deviatoric and the volumetric behaviors is explained by the relatively small values of the volumetric strain.The non-ideal laboratory conditions have more impact on this strain than on the deviatoric one.These findings can have implications for the interpretation of the dilatancy behavior of rock salt,and hence on the geomechanical design aspects in salt formations.     

Effect of insoluble materials on the volumetric behavior of rock salt

This paper focuses on the presence of nodules of insoluble materials within salt specimens,and their effect on the volumetric strain measurements and the dilatancy phenomenon.We analyzed experimental results of over 120 conventional triaxial compression tests,and found that in 20%of the cases,the volumetric strain measurements were atypical.We also noted that the natural variability of the specimens can lead to a non-negligible data scattering in the volumetric strain measurements when different specimens are subjected to the same test.This is expected given the small magnitude of those strains,but it occasionally implies that the corresponding specimens are not representative of the volumetric behavior of the studied rock.In order to understand these results,we numerically investigated salt specimens modeled as halite matrices with inclusions of impurities.Simulations of triaxial compression tests on these structures proved that such heterogeneities can induce dilatancy,and their presence can lead to the appearance of tensile zones which is physically translated into a micro-cracking activity.The modeling approach is validated as the patterns displayed in the numerical results are identical to that in the laboratory.It was then employed to explain the observed irregularities in experimental results.We studied the natural variability effect as well and proposed a methodology to overcome the issue of specimen representativity from both deviatoric and volumetric perspectives.     

Investigation of a Near-Perfect Quad-Band Polarization-Insensitive Metamaterial Absorber Based on Dual-T Circular Shaped Resonator Array Designed on a Silicon Substrate for C-, X- and Ku-bands Applications

Silicon - Multi-band metamaterial absorbers (MMAs) are increasingly becoming main devices for many electrical systems. Optimizing the electromagnetic qualities of this type of absorber in the...