Investigation of Percolation-Driven Fluid Transport in Rock Salt under Repository-Relevant Conditions (PeTroS)

According to the state of the art in mining and repository research, undisturbed rock salt is impermeable to fluids. Hence, rock salt formations are considered as host rock for nuclear waste repositories. Viscous, polycrystalline salt rock with low humidity contains no connected pore spaces. Two mechanisms are known for fluid transport: (a) damage due to large deviatoric and tensile stresses generates dilatancy, and hence permeability. (b) Fluid pressure exceeding the minor principal stress can open pathways (pressure-driven percolation, Minkley et al., 2013). To assess barrier integrity of rock salt barriers, the dilatancy and minimal stress criteria have been derived. Recently (Ghanbarzadeh et al., 2015; Lewis and Holness, 1996), high permeabilities in rock salt have been postulated under certain conditions. In particular, at high stresses and temperatures, including possible repository conditions, rock salt is claimed to develop a connected, thus permeable, pore space. In the PeTroS project (Minkley et al., 2020), we investigated fluid transport in the supposedly permeable region. Five points in pressure-temperature space were defined – pressures of 18 and 36 MPa, temperatures of 140, 160, and 180 ∘C. At each point, experiments with both nitrogen and saturated NaCl solution (brine) were performed. Samples were prepared from natural rock salt of German Zechstein formations, both bedded and domal salt. Sample material was generally relatively pure rock salt with minor impurities. Cylindrical samples (diameter 100 mm, length 200 mm) were loaded in a triaxial (Kármán) cell. Fluid pressure was applied to a central pressure chamber; any transmitted fluid was collected and extracted at the secondary side. The entire cell was heated to the specified temperature. Experiments generally comprised an isotropic phase (several stages of fluid pressure almost up to the confining stress) and a fluid breakthrough phase (lowering of axial stress by strain-controlled extension). After the test, a coloured tracer fluid was injected to visualise fluid discharge points. Fluid breakthroughs with fluid pressure above the minor principal stress were observed at all five pressure-temperature conditions. Some samples showed an approximately Darcian flow at fluid pressure below the minor principal stress, with permeabilities in the order of 10−22 m2, as is regularly observed due to the small size and initial damage from sample preparation (Popp et al., 2007). Tests consistently showed a gradual decrease of flow rate, i.e. reduction of the initial damage. A stable permeability over longer times, as would be expected due to the formation of a connected pore space network, was not observed in any of the experiments. Intriguingly, experiments with brine showed no initial permeability even though the wetting fluid should plausibly favour the formation of a stable connected pore network. Predictions of the static pore scale theory (Ghanbarzadeh et al., 2015) could thus not be confirmed. Regarding repositories for heat-generating waste, it can be concluded that from a geomechanical point of view, the dilatancy and minimal stress criteria are the relevant criteria for barrier integrity even at higher pressure and temperature.

Experimental Investigation of the Static and Dynamic Compression Characteristics of Limestone Based on Its Initial Damage

In the current work a new equation for initial damage assessment of limestone based on plane strain theory is proposed. Detailed investigations of the static and dynamic characteristics of limestone with different initial damage degree, using longitudinal wave speed, and static-dynamic compression tests are performed. This study investigated the static and dynamic characteristics of limestone with different initial damage degree, using longitudinal wave speed, and static-dynamic compression tests. Experimental results show that the degree of initial damage decreases with increasing longitudinal wave speed, which reaches the minimum when the longitudinal wave speed is approximately 6000 m/s, and the smaller the longitudinal wave velocity, the greater the degree of initial damage. The static and dynamic compressive strengths of limestone increase with the longitudinal wave velocity and strain rate, but the elastic modulus and Poisson’s ratio do not change significantly. Finally, based on the experimental results, the definitions of damage threshold value and strain softening are proposed, which further verify the influence of strain rate and initial damage on rock compression characteristics. The present study sheds light on the importance of initial damage for the mechanical state of rock in underground engineering.

Study on mechanical behavior and damage process of concrete with initial damage under eccentric load

In order to analyze the influence of eccentric load on mechanical properties and damage process of concrete with initial damage, the eccentric load compression tests of concrete under different confining pressures were carried out with the help of PFC particle flow program. The results show that: the eccentric load does not change the relationship between peak stress, crack initiation stress and confining pressure of concrete under uniform load, but decrease the value of them. The peak stress increasing coefficient under uniform load is higher than that under eccentric load, and the peak stress increasing coefficient increases in a linear function with the confining pressure, and the increasing rate is approximately the same. Under uniaxial compression of eccentric load, a type I shear crack approximately parallel to the loading direction is formed, while under biaxial compression, a bending type shear crack with the lower tip of the initial crack as the inflection point is formed. The number of microcracks in concrete under uniform load and eccentric load can be divided into three stages: the calm period at the initial loading stage, the pre-peak expansion period from crack initiation point to peak point, and the rapid increase period after the peak.

Study on Dynamic Behavior and Energy Dissipation of Rock considering Initial Damage Effect

To explore the influence of initial damage on the dynamic characteristics of rock mass, the Φ 50 mm split Hopkinson pressure bar (SHPB) test system was used, and the uniaxial impact compression tests on yellow sandstone specimens with different damage degrees were conducted, and then the variation law of mechanical properties of rock specimens with the initial damage was determined. The test results show that the dynamic stress-strain curve of rock specimens with initial damage can be roughly divided into compaction stage, elastic deformation stage, crack evolution stage, and strain-softening stage; the higher the initial damage degree of rock mass, the more significant the compaction stage. With the increase of the initial damage degree, the dynamic elastic modulus and peak stress of rock mass decrease gradually in a power number, while the peak strain of rock mass increases exponentially. With the increase of the initial damage degree, both the reflected energy ratio and the dissipated energy ratio decrease linearly, while the transmitted energy ratio increases linearly; the increasing rate of the transmitted energy ratio is greater than the decreasing rate of the reflected energy ratio.

Experimental Study on Initial Damage Point of Deep Granite under Step Cyclic Loading Method

In deep engineering, the initial damage point of host rock has become an important concern. Nowadays, there are many methods which can work out the initial damage point. However, each existing method introduces some significant subjective or parametric errors. In this paper, a new determination method of initial damage point employing step cyclic loading tests was designed and a series of tests were conducted on deep granite. After analyzing the peak strength and effect of cyclic loading, the turning point of initial damage was confirmed. The testing results show that the turning stress of initial damage was about 45% peak strength and will present a little decrease with the confining pressure increasing. These calculated damage points are much more scientific, accurate, and intuitive, which provides a new method for the study on rock mechanics in deep mining.

Fatigue Reliability Assessment for Orthotropic Steel Bridge Decks Considering Load Sequence Effects

Fatigue damage accumulations would dramatically reduce the reliability and service life of the orthotropic steel decks. Incorrect fatigue assessment results may be obtained when load sequence effects are omitted. In the present study, fatigue reliability assessments of rib-to-deck weld joints in orthotropic steel bridge decks are conducted with the consideration of load sequence effects. The method, which judiciously considers the fatigue loading history and is derived from the sequential law and the whole-range S-N curve, is first proposed for fatigue reliability calculation. And then, the whole-range S-N curve describing the fatigue propagating process of the rib-to-deck weld joint is introduced. Finally, the developed method is applied to evaluate the fatigue reliability of two rib-to-deck weld joints in an orthotropic steel deck based on long-term measured strain histories. The influence of traffic growth and initial damage on the fatigue reliability is discussed. The results indicate that it is advisable to consider load sequence effects when assessing the fatigue reliability of orthotropic steel decks equipped with long-term strain monitoring systems and the initial damage significantly reduces the fatigue reliability of orthotropic steel decks.