F.D. Hansen

Mechanical behaviour of rock salt: phenomenology and micromechanisms

Abstract The mechanical behaviour of rock salt is very complex over the ranges of stress and temperature usually encountered in geoengineering. During the past decade substantial progress has been made in measuring and understanding this behaviour, primarily because of the studies that support proposed disposal of nuclear wastes in salt formations. Safe nuclear waste disposal in salt requires a fundamental understanding of its mechanical behaviour to predict performance of nuclear waste repositories for times much longer than those usually encountered in conventional geoengineering practice. This understanding of the mechanical behaviour relies on accurate measurements of the macroscopic phenomenology and interpretation of the role of micromechanisms in producing the observed phenomena.

Dynamic compaction properties of bentonite

Abstract Bentonite has been proposed as a primary material for sealing shafts of nuclear waste repositories. It possesses low permeability, chemical and physical stability, and compatibility with most host rock masses and groundwater chemistries. This paper investigates the construction properties of bentonite and evaluates the use of dynamic compaction in constructing an effective bentonite shaft seal. Extensive laboratory tests of dynamic compaction have been conducted to study the densification of granular bentonite mixed with distilled deionized water or with brine from the Waste Isolation Pilot Plant (WIPP). Results of the dynamic compaction investigations delineate the influence of moisture content, compactive energy, mixed brine content, lift thickness, and rammer weight on the achievable dry density. Dynamic compaction can densify bentonite to a dry density of 1.86 Mg/m 3 when mixed with WIPP brine and 1.74 Mg/m 3 when mixed with distilled deionized water. At these densities bentonite exhibits permeabilities on the order of 1.0×10 −19 m 2 .

Reconsolidating salt: Compaction, constitutive modeling, and physical processes

Abstract The proposed use of crushed salt as a functional seal element in the Waste Isolation Pilot Plant (WIPP) shafts has given rise to several technological advances. A large-scale dynamic compaction demonstration using mine-run salt produced a compacted mass having a fractional density of 0.9 and a permeability of 9×10 −14 m 2 . To model evolution of crushed salt from these initial conditions to a high-density, low-permeability seal requires implementation of a suitable constitutive model. Candidate models were evaluated, generalized, and fit to limited data. Refinement of model parameters and definition of the relationship between permeability and density necessitate further experimental work as discussed in this paper. Implementation of a proper constitutive model, inclusion of a reliable permeability/density relationship for reconsolidating crushed salt, and a fundamental understanding of micromechanical processes assure credible design and analyses of the WIPP shaft seal system.

Physical and mechanical properties of degraded waste surrogate material

This paper discusses rock mechanics testing of surrogate materials to provide failure criteria for compacted, degraded nuclear waste. This daunting proposition was approached by first assembling all known parameters such as the initial waste inventory and rock mechanics response of the underground setting after the waste is stored. Conservative assumptions allowing for extensive degradation processes helped quantify the lowest possible strength conditions of the future state of the waste. In the larger conceptual setting, computations involve degraded waste behavior in transient pressure gradients as gas exits the waste horizon into a wellbore. Therefore, a defensible evaluation of tensile strength is paramount for successful analyses and intentionally provided maximal failed volumes. The very conservative approach assumes rampant degradation to define waste surrogate composition. Specimens prepared from derivative degradation product were consolidated into simple geometries for rock mechanics testing. Tensile strength thus derived helped convince a skeptical peer review panel that drilling into the Waste Isolation Pilot Plant (WIPP) would not likely expel appreciable solids via the drill string.

Case studies of sealing methods and materials used in the salt and potash mining industries

Sealing methods and materials currently used in salt and potash industries were surveyed to determine if systems analogous to the shaft seal design proposed for the Waste Isolation Pilot Plant (WIPP) exist. Emphasis was first given to concrete and then expanded to include other materials. Representative case studies could provide useful design, construction, and performance information for development of the WIPP shaft seal system design. This report contains a summary of engineering and construction details of various sealing methods used by mining industries for bulkheads and shaft liners. Industrial experience, as determined from site visits and literature reviews, provides few examples of bulkheads built in salt and potash mines for control of water. Sealing experiences representing site-specific conditions often have little engineering design to back up the methods employed and even less quantitative evaluation of seal performance. Cases examined include successes and failures, and both contribute to a database of experiences. Mass salt-saturated concrete placement under ground was accomplished under several varied conditions. Information derived from this database has been used to assess the performance of concrete as a seal material. Concrete appears to be a robust material with successes in several case studies. 42 refs.