The behavior of anhydrite layers deforming in a bedded salt medium is examined. Applications of structural analyses for the Waste Isolation Pilot Plant include rock mechanics calculations of room closure and porosity surfaces for performance assessment. The calculation scheme includes salt creep and large-strain deformation involved in room closure. Salt crystal plasticity is well established and room-closure calculations have reasonably approximated full-scale measurements during the experimental phase and continue to adequately predict closure after ten years of operational experience. Natural bedded salt formations comprise sequences of anhydrite stringers which are stiffer and stronger than salt. The rheological contrast gives rise to combinations of fracture and deformation features of both the salt and the anhydrite. Although room closure is dominated by salt deformation, the physical and mechanical properties of anhydrite stringers above and below the disposal horizon have important performance implications. Finite element analyses of anhydrite shear failure within the creeping salt medium are augmented with photographic observations from the microscopic scale to the field scale. These results are put in context with a wealth of anhydrite laboratory data and relevant field test observations. This information adds significantly to analyses of mechanical behavior of anhydrite deformation in a creeping salt medium.
This paper develops a series of room closure and porosity surface calculations, which are used to assess performance of the Waste Isolation Pilot Plant (WIPP). In previous papers, Park et al. [1] developed a two-dimensional finite element method (FEM) model to calculate the change in the size and extent of the disturbed rock zone (DRZ) with time in the salt surrounding the openings. The spatial characteristics and temporal changes of the damaged zone are important considerations for WIPP Performance Assessment (PA) calculations, particularly for the groundwater flow analyses and evaluation of various contaminant release scenarios. Park and Hansen [2] provided insight into the structural response of a room full of various wastes, including the influence of the waste in the absence of gas generation, as well the waste’s lack of influence on room closure when gas generation is modeled. Several porosity surfaces were developed to cover a range of possible waste characteristics.
This paper, as the third of the series, describes the behavior of anhydrite layers above and below the disposal room. Although room closure is dominated by salt deformation, the physical and mechanical properties of anhydrite stringers in the vicinity of openings have important performance implications. Finite element analyses of anhydrite shear failure within the creeping salt medium provide a means to evaluate failure and deformation. The FEM results are augmented with photographic observations from the microscopic scale to the field scale. These results are put in context with a wealth of anhydrite laboratory data and relevant field observations. Together, this information adds significantly to analyses of mechanical and hydrological behavior of anhydrite in shaft seal systems, panel closure systems, room closure response and future applications.
ANALYSIS MODEL
The idealized stratigraphy for the WIPP underground, which derives from Munson et al. [3], was used.
