Mechanical Behavior Of Rock Salt Research Articles

Microfabrics impact on rock salt geomechanical diversity – an experimental investigation

Geomechanical studies of rock salt serve as the foundation for designing and ensuring the long-term stability of storage caverns. Research conducted in recent years has shown that rock salt exhibit highly variable mechanical parameters. Even macroscopically similar rock salt samples from the same formation have very different mechanical parameters in standard strength tests. In this paper, the effect of microfabrics on the mechanical behaviour of rock salt was studied using a uniaxial compression device with simultaneous observation under a microscope. The tests were carried out on rock salt samples, previously tested on standard uniaxial compression (UCS). Tested samples showed different strength values despite macroscopic similarity and similar anhydrite content. The tests revealed that the microfabrics, such as fluid inclusions and anhydrite crystals distributed along the halite grain boundaries, play a significant role in the strength of a rock salt sample. Four types of grain boundaries were recognized in both group of samples: (1) dominated by anhydrite crystals with a small number of fluid inclusions (FIs), (2) with FIs prevailing over anhydrite crystals, (3) only with FIs, and (4) without anhydrite crystals or FIs. The experimental uniaxial compression tests in microscale (mUCS) showed that the damage process is initiated by FIs activity at the halite grain boundaries and consists of three main stages. The samples dominated by anhydrite crystals at halite grain boundaries were characterised by the highest mUCS. Conversely, if FIs prevail at the grain boundaries the lowest mUCS values were registered. Therefore, anhydrite crystals located along halite grain boundaries influenced the mechanical behaviour and deformation process. On the contrary, anhydrite crystals dispersed within halite grains had no impact on failure and mechanical parameters. The experiment showed that apart from impurity content, the distribution of these impurities within halite grains should be examined during geomechanical evaluation of rock salt samples. Thus, the overall strength of rock salt samples determined in standard test results from the local variations of petrological features in micro- and macro scale.

Failure transition of shear-to-dilation band of rock salt under triaxial stresses

Great potential of underground gas/energy storage in salt caverns seems to be a promising solution to support renewable energy. In the underground storage method, the operating cycle unfortunately may reach up to daily or even hourly, which generates complicated pressures on the salt cavern. Furthermore, the mechanical behavior of rock salt may change and present distinct failure characteristics under different stress states, which affects the performance of salt cavern during the time period of full service. To reproduce a similar loading condition on the cavern surrounding rock mass, the cyclic triaxial loading/unloading tests are performed on the rock salt to explore the mechanical transition behavior and failure characteristics under different confinement. Experimental results show that the rock salt samples present a diffused shear failure band with significant bulges at certain locations in low confining pressure conditions (e.g. 5 MPa, 10 MPa and 15 MPa), which is closely related to crystal misorientation and grain boundary sliding. Under the elevated confinement (e.g. 20 MPa, 30 MPa and 40 MPa), the dilation band dominates the failure mechanism, where the large-size halite crystals are crushed to be smaller size and new pores are developing. The failure transition mechanism revealed in the paper provides additional insight into the mechanical performance of salt caverns influenced by complicated stress states.

A thermodynamic modeling of creep in rock salt

A number of constitutive models have been proposed along time in order to describe the mechanical behavior of rock salt. The basic formulation of these models relies, in general, on the description of deformation mechanisms in the macro or the micro scale. The present note aims at providing a framework for the development of models to describe the creep behavior of rock salts based upon continuum thermodynamics principles. This note provides the theoretical basis and, based on the proposed framework, presents the development of a two-mechanism model that describes transient and steady-state creep of rock salt. Results obtained with this model are presented as well as a discussion on the main features of the proposed formulation.

The Effects of Cycle Frequency on Mechanical Behavior of Rock Salt for Energy Storage

The Effects of Cycle Frequency on Mechanical Behavior of Rock Salt for Energy Storage

Effects of grain size distributions on the macro-mechanical behavior of rock salt using micro-based multiscale methods

Although the traditional phenomenological mechanical model of rock salt based on macro-mechanical experiments adequately describes its macroscopic mechanical behavior, the effect of the microstructure of this mineral on its macro-mechanical properties and intrinsic physical deformation and failure processes remains unclear. Therefore, in this study, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) were used to investigate the grain boundaries and grain size distribution, and a nanoindentation technique was employed to analyze the elastic characteristics of the grains, to better understand the structural and mechanical characteristics of rock salt on a microscopic scale. Furthermore, a multiscale numerical model of polycrystalline rock salt based on the microstructure was established, using the specified particle size Voronoi polyhedron modeling method and the discrete element method. In addition, numerical models were constructed with different embedded grain size distribution characteristics, enabling the effects of the distribution parameters on the mechanical behavior to be clarified in detail. The results indicated that the macroscopic mechanical behavior of rock salt is simultaneously determined by the physical and mechanical properties of the grains and grain boundaries. In addition, the resulting simulation results based on the numerical models indicate that the strength and ductility of rock salt are affected by the average diameter, heterogeneity index and skewness coefficient.

The mechanical behavior of rock salt under different confining pressure unloading rates during compressed air energy storage (CAES)

Compressed air energy storage (CAES) is a promising energy conversion storage technology and underground salt caverns are recognized as the appropriate storage places for it. The reduction of air during the withdrawal periods leads to the surrounding rock being unloaded, which threatens the stability of the salt cavern. To investigate the mechanical behavior of surrounding rock under different unloading rates, the confining pressure unloading tests in six unloading rates have been conducted on rock salt. Results show the rock salt undergoes volume expansion at the beginning of unloading due to the reduction of elastic compression strain. The volume strain expands rapidly beyond a threshold, i.e. accelerated strain point. The stresses at failure point and accelerated strain point show that a low unloading rate is beneficial to enhance rock strength. Nevertheless, larger inelastic strain appears in a lower unloading rate. The effect of the unloading rate on rock salt failure mode is that rapid unloading will promote crack growth. Moreover, the relationship between the unloading rate of surrounding rock and the air withdrawal rate is established. The outcome derived from the research can provide some instruction and inspiration for air withdrawal rate design in CAES.

A Modified Creep Model for Cyclic Characterization of Rock Salt Considering the Effects of the Mean Stress, Half-Amplitude and Cycle Period

The fluctuation of the operation pressure during injection and withdrawal is a key factor affecting the long-term stability of surrounding rock in a compressed air energy storage (CAES) cavern. To assess the mechanical behavior of rock salt under different pressure fluctuations and to optimize the operating parameters of CAES caverns, uniaxial cyclic loading tests under the sine condition were carried out by changing three cycle parameters, namely the mean stress, half-amplitude and cycle period. The test results show that the deformation behavior during cyclic loading tests can be divided into transient, steady, and accelerated stages, which is similar to that of creep tests. The strain and steady strain rates increase with increasing cycle parameters, showing a nonlinear deformation property of rock salt under cyclic loading. To describe the nonlinear response that cannot be captured by the Burgers model, a stiffness scaling factor is proposed based on the test results. A modified cyclic creep model of rock salt is established by introducing this stiffness scaling factor into the conventional Burgers creep model. The agreement with the experimental data indicates that the modified cyclic creep model can accurately reflect the influence of the three cyclic parameters on the deformation properties of rock salt.

Error‐controlled implicit time integration of elasto‐visco‐plastic constitutive models for rock salt

SummaryThe mechanical behavior of rock salt is rate‐dependent at different time scales. Using caverns in rock salt formations for renewable energy storage implies that the underground structures are subjected to both short‐term and long‐term loads, increasing robustness and flexibility requirements for numerical simulators used to assess the safety of such structures. So far, explicit time integration with model‐specific heuristics for time‐step size determination dominate in application studies. In this paper, the suitability of error‐controlled adaptive time‐integration schemes of the diagonally implicit Runge‐Kutta type is investigated in comparison with the Backward Euler and Crank‐Nicolson schemes when applied to the integration of typical elasto‐visco‐plastic constitutive models of rock salt. The comparison is made both for monotonic and for cyclic loads as well as taking account of thermo‐mechanical coupling. Analyses of the time‐integration errors and the time step‐size evolution show the suitability of the integration scheme for these material models. The automatic adjustment of the time‐step size was found to be robust across all material models and boundary conditions as well as for non‐isothermal situations for a single algorithmic parameter set.

The influence of a grain-shape fabric on the mechanical behaviour of rock salt: Results from deformation experiments

In order to assess the effect of shape preferred orientation (SPO) of halite grains on deformation of rock salt, ductile plane strain deformation experiments have been conducted. Natural halite cubes with and without an SPO from the Asse mine of northern Germany were deformed at 345 °C, atmospheric pressure and a strain rate of ~10−7 s−1. The SPO was oriented at different angles (ß = 0°, 45° and 90°) to the main shortening direction, Z. The sample is confined in the Y direction and can freely move to the principal stretching direction, X. Most of the strain was accommodated by dislocation creep. The qualitative analysis of the stress-strain data shows the clear influence of the SPO on the mechanical data. At a shortening strain of ca. −10%, the samples with β = 0° and 45° are weaker than the samples without an SPO. In contrast, the sample with β = 90° is stronger than the samples without an SPO. A new SPO is formed in all samples with the long axis of the grain shape fabric being subparallel to the principal stretching axis, X. At these larger shortening strains, the samples show similar strength, with an exception of the 90° oriented sample that is still stronger than the samples without an SPO. Even though the experimental set up has to be improved to obtain quantitative results on the influence of SPO on mechanical strength of rock salt, the results show that the SPO of rock salt likely has to be taken into account when building a repository for nuclear waste and could have an effect on the stability of caverns.

Plastic Strain Energy Model for Rock Salt Under Fatigue Loading

The fatigue test for rock salt is conducted to investigate the effects of stress amplitude, loading frequency and loading rate on the plastic strain energy, from which the evaluation rule of the plastic strain energy is analyzed, which is divided into three stages: cyclic hardening, saturation and cyclic softening. The total accumulated plastic strain energy only depends on the mechanical behavior of rock salt, but is immune to the loading conditions. A novel model for fatigue life prediction is proposed based on the invariance of the total plastic dissipation energy and the stability of the plastic energy per cycle.

Modeling deformation and damage of rock salt using the discrete element method

Rock salt is considered to be a suitable host rock formation for radioactive waste repositories. However, local stress changes caused by drift construction lead to the evolution of an excavation damaged zone (EDZ). Such an EDZ can have a major impact on the safety of a radioactive waste repository because it can violate the integral sealing function due to the development of a permeable microcrack network. The objective of this investigation is the development of a modeling strategy that can be used to simulate the damage behavior of rock salt at grain scale. For this purpose, discrete element modeling techniques were used in combination with polyhedral-shaped elements (Voronoi polyhedra) in order to duplicate the shape and arrangement of grains. To calibrate the material parameters used in the constitutive models and to validate the relevant damage and fracture processes, uniaxial compression tests were combined with acoustic emission measurements. Overall deformation behavior and crack evolution was reproduced by modeling and validated by lab experiments. The new proposed modeling strategy based on DEM and Voronoi polyhedra shows new possibilities to simulate the mechanical behavior of rock salt in a realistic manner by explicitly considering the fracture processes at grain scale.

Creep and Dilatancy Behavior of Rock Salt around Underground Caverns

In the present paper, the time-dependent behavior of a salt cavern was analyzed using finite element method. A viscoplastic model considering inelastic volume changes was utilized to describe the time-dependent mechanical behavior of rock salt. The model was then implemented in a finite element procedure to analyze the stress and deformation of rock salt around an underground cavern using an axisymmetric representation. Finally, finite element analyses results were compared with those obtained by a viscoplastic model neglecting inelastic volume changes.

A thermo-plastic/viscoplastic damage model for geomaterials

A thermo-plastic/viscoplastic damage coupled model was formulated to describe the time independent and time dependent behaviors of geomaterials under temperature effect. The plastic strain was divided into instantaneous plastic strain and creep plastic strain. To take temperature effect into account, a temperature variable was introduced into the instantaneous and creep plastic behavior descriptions and damage characterization, and a linear thermal expansion law was used in constitutive equation formulation. According to the mechanical behavior of rock salt, a specific model was proposed based on the previous model and applied to Avery rock salt, in which the numerical results obtained from our model had a good agreement with the data from experiments.

Development of damage and permeability in deforming rock salt

Permeability of undisturbed rock salt, or of rock salt which has been sufficiently compacted during laboratory testing, is very low (less than 10−20m2). Therefore, rock salt structures are used as host rocks for storage caverns (oil, hydrocarbons) and are considered for the long-term storage of radioactive waste. Rock salt deforms plastically without the formation and propagation of dilating cracks as long as the state of stresses remains within the non-dilatant stress domain. This paper presents results of deformation experiments focusing on the transport properties of rock salt and describes the conditions for the transition from non-dilatant to dilatant deformation at the so-called dilatancy boundary. Combined measurements of ultrasonic wave velocities (Vp, Vs) and permeability are used to determine the state of stresses at the dilatancy boundary, in addition to the more usual measurement of the volumetric strain. The results confirm the dilatancy boundary as described by the equation of Cristescu and Hunsche (1998). Loading in the dilatant stress domain gives rise to the generation and propagation of cracks and to the increase of damage with strain. The evolution of damage is continuously monitored by combined ultrasonic and permeability measurements. Damage (porosity) and permeability are controlled by several parameters. Most important are minimal principle stress and loading geometry. Pore pressure is found to affect only the mechanical properties of damaged and dilated rock salt. Implications for the large-scale mechanical behavior of rock salt are discussed.

Rock salt — the mechanical properties of the host rock material for a radioactive waste repository

For the long-term prediction of deformation, stress and permeability of a repository in a salt formation, one needs a reliable extrapolation of the mechanical behaviour of rock salt. This is only possible by means of material laws with a physical basis. A detailed description of the so-called composite model for transient and steady state creep is given, which is based on micromechanisms. This model is not only able to model deformation in a wide range of stresses and temperatures, but also after stress decrease, as well as the large differences in creep caused by variations in the distribution of impurities. As a result of this sound physical and experimental basis, the modelling results can be reliably extrapolated. Dilatancy, healing, damage, failure, and deformation of rock salt is described by an elasto–viscoplastic constitutive equation. An important feature is the stress dependent dilatancy boundary which separates the dilatant domain from the compressible one and forms a kind of safety boundary. Finally, the relation between deformation, dilatancy, and permeability is addressed. This publication shows that far advanced tools exist for the prediction of the long-term as well as the short-term mechanical behaviour of a repository in a salt formation.

Mechanical behaviour of rock salt: phenomenology and micromechanisms : Senseny, P E; Hansen, F D; Russel, J E; Carter, N L; Handin, J W Int J Rock Mech Min SciV29, N4, July 1992, P363–378

Mechanical behaviour of rock salt: phenomenology and micromechanisms : Senseny, P E; Hansen, F D; Russel, J E; Carter, N L; Handin, J W Int J Rock Mech Min SciV29, N4, July 1992, P363–378

Mechanical behaviour of rock salt: Phenomenology and micromechanisms

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.

Experimental study of mechanical behaviour of rock salt in extension (in French) : Bergues, J; Charpentier, J P; Dao, M Proc 6th International Conference on Rock Mechanics, Montreal, 30 Aug–10 Sept 1987V2, P805–808. Publ Rotterdam: A A Balkema, 1987

Experimental study of mechanical behaviour of rock salt in extension (in French) : Bergues, J; Charpentier, J P; Dao, M Proc 6th International Conference on Rock Mechanics, Montreal, 30 Aug–10 Sept 1987V2, P805–808. Publ Rotterdam: A A Balkema, 1987

Der Einfluß radioaktiver Strahlung auf das mechanische Verhalten von Steinsalz

Der Einfluß radioaktiver Strahlung auf das mechanische Verhalten von Steinsalz

Geotechnical investigation methods for rock salt

The need for storage caverns (oil, gas.) and depositories (radioactive waste, toxic waste) is rising world-wide. Rock salt (halite) formations are exceptionally suitable for the construction of such cavities. However, accurate dimensioning can only be carried out if the mechanical behaviour of rock salt is known with sufficient accurary.