The ability of materials to self-heal from mechanical and thermally induced damage is explored in this paper and has significance in the field of fracture and fatigue. The history and evolution of several self-repair systems is examined including nano-beam healing elements, passive self-healing, autonomic self-healing and ballistic self-repair. Self-healing mechanisms utilized in the design of these unusual materials draw much information from the related field of polymer–polymer interfaces and crack healing. The relationship of material damage to material healing is examined in a manner to provide an understanding of the kinetics and damage reversal processes necessary to impart self-healing characteristics. In self-healing systems, there are transitions from hard-to-soft matter in ballistic impact and solvent bonding and conversely, soft-to-hard matter transitions in high rate yielding materials and shear-thickening fluids. These transitions are examined in terms of a new theory of the glass transition and yielding, viz., the twinkling fractal theory of the hard-to-soft matter transition. Success in the design of self-healing materials has important consequences for material safety, product performance and enhanced fatigue lifetime.

Self-healing materials: a review

Large earthquakes initiate chains of surface processes that last much longer than the brief moments of strong shaking. Most moderate‐ and large‐magnitude earthquakes trigger landslides, ranging from small failures in the soil cover to massive, devastating rock avalanches. Some landslides dam rivers and impound lakes, which can collapse days to centuries later, and flood mountain valleys for hundreds of kilometers downstream. Landslide deposits on slopes can remobilize during heavy rainfall and evolve into debris flows. Cracks and fractures can form and widen on mountain crests and flanks, promoting increased frequency of landslides that lasts for decades. More gradual impacts involve the flushing of excess debris downstream by rivers, which can generate bank erosion and floodplain accretion as well as channel avulsions that affect flooding frequency, settlements, ecosystems, and infrastructure. Ultimately, earthquake sequences and their geomorphic consequences alter mountain landscapes over both human and geologic time scales. Two recent events have attracted intense research into earthquake‐induced landslides and their consequences: the magnitude M 7.6 Chi‐Chi, Taiwan earthquake of 1999, and the M 7.9 Wenchuan, China earthquake of 2008. Using data and insights from these and several other earthquakes, we analyze how such events initiate processes that change mountain landscapes, highlight research gaps, and suggest pathways toward a more complete understanding of the seismic effects on the Earth's surface.

Earthquake-Induced Chains of Geologic Hazards: Patterns, Mechanisms, and Impacts

Clay formations in their natural state exhibit very favourable conditions for disposal of radioactive waste. One concern regarding waste disposal is that due to the necessary underground excavations and the associated disturbance and damage in the area close to these excavations, the favourable properties of such formations could change and the host rock could lose part of its barrier function. Stress redistribution will lead to the creation of a so-called excavation damaged zone (EDZ) which will be controlled by the initial stress field, the material properties (e.g., material anisotropy), the existence of natural fracture zones or local inhomogeneities of the rock mass and the geometry of the tunnel. Comprehensive investigations at different sites (e.g., HADES, Belgium, Mont Terri, Switzerland, Tournemire, France) have shown that an EDZ occurs in soft or plastic clays as well as in indurated and more brittle claystones. The short-term excavation-induced reaction of the rock during tunnelling, which leads to the initial EDZ, cannot be avoided but is reasonably well understood and the associated processes can be adequately modelled. The long-term behaviour of the tunnel near-field can be significantly influenced by adequate support measures and the time-dependent evolution of the EDZ before the emplacement of the waste and the backfilling of the tunnel can be controlled. The properties of the initial EDZ alter significantly during the transient phase, when the buffer and rock mass are heated by the heat-producing waste and become saturated due to the flow of formation water from the host rock. Experimental results in the laboratory and in-situ clearly show that (self-)sealing leads to a significant reduction in the effective hydraulic conductivity of the EDZ with time, thus reducing the potential flow along underground excavations. Expected long-term conductivities within the EDZ are in the range of 10 −10 to 10 −12  m/s. Performance assessment calculations for different repository designs in different clay host rock formations show that the influence of the EDZ on radionuclide release is quite limited. It has been shown that even for very conservative, so-called “what if?” cases the very stringent regulatory guidelines can be met.

The excavation damaged zone in clay formations time-dependent behaviour and influence on performance assessment

The performance and safety assessment and technology demonstration are the main objectives of research programs for feasibility studies for deep geological repository of radioactive waste. In this context, the French national radioactive waste management agency (ANDRA) started to develop the Meuse/Haute-Marne underground research laboratory (URL) at Bure, nearly 300 km East of Paris. The host formation consists of a Callovo-Oxfordian claystone found between 420 and 550 m below ground, overlain and underlain by poorly permeable carbonate formations. One of the major concerns related to performance assessment is the excavation-induced fractures which may provide groundwater preferential pathway for radionuclide migration. The extent of the fractures possibly acting significantly in the radionuclide migration is known as the excavation damaged zone (EDZ). A scientific study on the EDZ characterization is performed at the main level of the URL (−490 m). Observations such as structural analysis on core, overcored resin-filled samples, geological survey of the drift face and sidewalls, were made to better understand the fracture network characteristics, extent and its generation. Pulse and constant head test hydraulic conductivity measurements were performed with multi packer system to estimate the extension of the EDZ hydraulic conductivity. Fractures exhibited high transmissivity near the excavation walls, but farther from the exaction walls, shear fractures showed hydraulic conductivity values reflecting values of undisturbed or slightly disturbed rock mass condition. The major findings in terms of geometry and properties of excavation-induced fractures are discussed in detail in this paper. For example, it is observed that the shape of the fracture network depends on the orientation of the drift in relation to the orientation of the in situ stress field.

Geometry and Properties of the Excavation-Induced Fractures at the Meuse/Haute-Marne URL Drifts

Natural tracer profiles across argillaceous formations

In assessing the performance of a deep HLW repository, the evolution of the excavated damaged zone with time is a key issue. In the framework of SELFRAC fracturing, self-sealing and self-healing processes of Opalinus and Boom clay were studied in laboratory and in situ experiments. Definitions for the terms excavation damaged zone (EDZ), excavation disturbed zone (EdZ), sealing and healing are presented. It is shown that sealing and partial healing occur and the consequences of the results for performance assessment of HLW disposal in argillaceous rocks are discussed. The results of several in situ experiments and observations at the HADES underground research facility are detailed. The origin and extent of excavation induced fractures are discussed and sealing and (partial) healing of these fractures is demonstrated. In the description of the hydraulic features of the EdZ, the anisotropic pore pressure distribution around HADES and its evolution with time are discussed. Pore pressure is influenced several tens of metres into the host rock and its evolution is influenced by the anisotropic in situ stress state and the anisotropic hydraulic conductivity of Boom clay. Around the connecting gallery, an increase of hydraulic conductivity is measured up to about 6–8 m into the host rock, outside this influenced zone values between 4 × 10 −12  m/s and 6 × 10 −12  m/s were obtained. The highest value measured (close to the gallery) was of the order of magnitude of 10 −11  m/s. The observed increase is caused by lower effective stress levels close to the gallery rather than by excavation induced fractures. Self-boring pressuremeter tests show that total stress is influenced up to 6–8 m into the host rock and material parameters such as undrained shear strength and shear modulus are influenced up to 2–3 m into the host rock. In situ seismic transmission measurements showed that the closure of a borehole influences the seismic parameters of the surrounding host rock: a decrease in seismic velocity is measured and higher frequencies disappear from the transmitted signals. After closure of the borehole, sealing of the damaged zone around it occurs, this is observed by the recovery of seismic velocity and the reappearance of higher frequencies. Fracture sealing is also demonstrated by seismic and hydraulic measurements on a reinstalled fractured clay core.

SELFRAC: Experiments and conclusions on fracturing, self-healing and self-sealing processes in clays

Among the various laboratory studies to investigate the Thermo-Hydro-Mechanical (THM) behaviour of Boom clay, relatively few were devoted to the time dependent behaviour, limiting any relevant analysis of the long-term behaviour of the disposal facility. The present work aims at investigating the time-dependent behaviour of Boom clay under both thermal and mechanical loading. High-pressure triaxial tests at controlled temperatures were carried out for this purpose. The tests started with constant-rate thermal and/or mechanical consolidation and ended with isobar heating and/or isothermal compression at a constant stress rate or by step loading. Significant effects of temperature as well as of compression and heating rates were observed on the volume change behaviour. After being loaded to a stress lower than the pre-consolidation pressure (5 MPa) at a low temperature of 25\degree C and at a rate lower than 0.2 kPa/min, the sample volume changes seemed to be quite small, suggesting a full dissipation of pore water pressure. By contrast, after being subjected to high loading and heating rates (including step loading or step heating), the volume changes appeared to be significant, particularly in the case of stresses much higher than the pre-consolidation pressure. Due to low permeability, full consolidation of Boom clay required a long period of time and it was difficult to distinguish consolidation and creep from the total volume change with time.

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Investigating the time-dependent behaviour of Boom clay under thermo-mechanical loading

https://hal-enpc.archives-ouvertes.fr/hal-00926883/document

Effects of pore water chemical composition on the hydro-mechanical behavior of natural stiff clays

ATLAS III in situ heating test in boom clay: Field data, observation and interpretation

https://hal-enpc.archives-ouvertes.fr/hal-00926893/document

Xiangling Li

Papers

SELFRAC: Experiments and conclusions on fracturing, self-healing and self-sealing processes in clays

Investigating the time-dependent behaviour of Boom clay under thermo-mechanical loading

Effects of pore water chemical composition on the hydro-mechanical behavior of natural stiff clays

ATLAS III in situ heating test in boom clay: Field data, observation and interpretation

An insight into the unloading/reloading loops on the compression curve of natural stiff clays