Philipp Braun

Fracture in fluid-saturated porous solids is governed not only by solid mechanics but also by fluid flow within the pore space. In systems ranging from geomaterials to biological tissues, deformation in the near-tip region perturbs pore pressure and thereby alters crack growth. Yet these pore-fluid effects have largely been inferred rather than directly observed. Here, we perform in situ pore-pressure measurements during controlled tensile rupture of a water-saturated microporous cement. We demonstrate that a propagating crack generates a localized underpressure ahead of its tip, providing a direct experimental signature of poromechanical coupling. The amplitude of this transient pressure drop scales with the square root of crack velocity, in agreement with poroelastic theory. This underpressure reduces the effective stress near the crack tip, leading to a coupled mechanical response: the apparent fracture energy doubles across the tested velocity range while the fracture process zone shrinks by half. Our findings reveal poromechanical coupling as a rate-dependent control on fracture in saturated porous materials, governed by effective stress changes induced by competing fluid diffusion and crack advance.

Callovo-Oxfordian (COx) claystone is the candidate host rock for the future French deep geological repository for radioactive waste, designed to experience thermal loads of up to 90 °C during operation. In this context, this study characterizes cores extracted after an in-situ heating test where the maximum temperature reached 95°C. A multistep isotropic test was performed on a sample resaturated close to the in-situ stress state. Drained and repeated mechanical unloading-reloading cycles were carried out. Reversible elastic strain and irreversible plastic strain were decomposed from specially designed loading steps to characterize the evolution of plastic yield mechanisms. Results indicate that the sample exhibited plastic swelling during the unloading paths, when the effective mean stress decreased below 8 MPa. The unloading also seemed to have reduced the compression yield limit, since during subsequent reloading, we observed plastic compression at stress levels below the initial state. Furthermore, unloading after loading to high confining pressure revealed that the swelling yield limit increased, exceeding the initial stress level, thereby demonstrating a hardening effect. These observations could be represented by a swelling yield limit at a low effective mean stress, which is strongly coupled with the compression yield limit during the hardening process. The observed swelling during mechanical unloading could be relevant for in-situ stress paths involving decreasing mean effective stress, namely tunnel excavation, fracture self-sealing, thermal pressurization and retrieval of laboratory specimens.

Underground hydrogen storage in porous formations (UHSP) subjects reservoir rocks to long‐term cyclic effective stress variations, potentially altering storage capacity and injectivity. In this study, we conduct hydrostatic loading tests on 18 Saint‐Maximin limestone (SML) samples (connected porosity 31.9%–37.2%) under various loading paths, complemented by measurements of permeability and acoustic wave velocities. A specific creep‐fatigue interaction test, combining stabilized creep and subsequent cyclic loading at identical peak stress, is conducted to distinguish cycle‐dependent ratcheting deformation from time‐dependent creep. Experimental results reveal that cyclic behavior of SML is governed by a dual stress‐threshold mechanism. The peak stress ratio governs initiation and evolution of damage, while the valley stress ratio describes the reopening of generated fractures during unloading. > 1 leads to enhanced creep and onset of ratcheting deformation and < 1 induces 1.2% additional porosity reduction within 50 cycles after creep stabilization. Across all loading paths, porosity change serves as a robust metric for mechanical hardening and permeability evolution within the experimental range of up to a 12.8% reduction in porosity and an order‐of‐magnitude decrease in permeability. These findings establish a reference testing approach for quantitatively partitioning different deformation mechanisms, providing an essential basis for more reliable predictions of long‐term UHSP performance.

We aim to explore how heterogeneous porosity and grain size contribute to strain localization patterns in highly porous rocks based on phase‐field simulations. The strain localization patterns, including shear bands, dilatation bands, and compaction bands, are commonly observed in geological field studies and laboratory experiments. An elastoplastic phase‐field model is developed, where the driving forces are formulated considering both the brittle breakage and ductile dissipation to capture the evolution of strain localization patterns. We distinguish between brittle‐like fracture released energy (i.e., grain breakage and crushing) and plastic‐like dissipation energy (i.e., grain friction and rearrangement) during the strain localization process. The heterogeneity in porosity and grain size is represented by a heterogeneous distribution of consolidation pressure, which is mathematically related to the porosity collapse, resulting in the constitutive responses during strain localization. We proceed to investigate the effects of confining pressure and the degree of heterogeneity on the formation of strain localization patterns, including the simultaneous occurrence of shear bands, dilatation bands, and compaction bands, through a series of simulations.

Underground hydrogen storage in porous formations (UHSP) provides a large-scale and long-term solution to balance the seasonal supply-demand mismatch of renewable energy. Assessing the long-term safety and performance of UHSP requires understanding how reservoir rocks respond to cyclic pressure variations. In this study, we conduct hydrostatic loading tests on Saint-Maximin limestone (SML) samples under various loading paths, complemented by measurements of permeability and acoustic wave velocities. The resulting evolutions of porosity, permeability and the damage are systematically investigated. Specific loading paths are applied to distinguish cycle-dependent ratcheting deformation from time-dependent creep. Experimental results reveal that for cyclic loading, mechanical behavior of SML is governed by a dual stress-threshold mechanism. The peak stress ratio (PR) governs initiation and evolution of damage, as supported by ultrasonic velocity measurements and a post-loading microstructural observation, and leads to enhanced creep and onset of ratcheting deformation when PR > 1. The valley stress ratio (VR) describes the reopening of existing fractures during unloading and induces an amplified ratcheting rate when VR < 1. Across all loading paths, porosity change serves as a robust metric for mechanical hardening and permeability evolution. These findings offer new insights into the coupled hydro-mechanical processes governing reservoir rocks under long-term cyclic operation and contribute to more reliable predictions of UHSP performance.

Some (saturated) claystones, considered as host rocks for deep radioactive waste disposal, are sensitive to drying and wetting. Wetting may result in swelling and damage during the resaturation of specimens for mechanical testing. Given that, due to evaporation, specimens may have lost some pore water during coring, transport, storage and trimming prior to lab testing, it is required to re-saturate them by reinjecting the evaporated water content, compressing the pore space under constant water content, or by using a combination of both. It is important to characterize the hydromechanical processes involving suction changes under total stress loading in order to adapt preparation methods and minimize sample disturbances. We present a novel experimental setup to test claystone specimens under isotropic compression at constant water content, while measuring suction, and we discuss the data of three specimens testedat different initial suctions and degrees of saturation. Interestingly, the path followed in the (decreased) suction vs (increased) total confining stress diagram was observed to be close to the Gens-Alonso “Neutral Line” (a line inclined of 45° in a suction/mean total stress diagram, illustrating similar effects of changes of both variables). In other words, even in close to saturated conditions, changes in confining stress resulted in comparable changes in suction. The data obtained indicate that applying a total stress of 12.2 MPa during resaturation keeps the material on the compressive side of the Neutral Line, limiting swelling-induced sample disturbance.