Rockburst mechanism caused by the mining-induced drainage of confined water in deep extra-thick aquifer

Extra-thick sandstone confined aquifer is widely spread in the overburden strata in Shaanxi and Inner Mongolia provinces. During coal mining, the water inflow in the working face is severe and the confined water level in the roof aquifer decreases significantly. The monitoring of rockbursts and high-energy seismicity shows that the mining-induced drainage of the confined water in the extra-thick sandstone confined aquifer has a certain effect on inducing rockburst. For a better understanding of rockburst mechanism as a result of the drainage of the roof confined water, the fluid-solid coupling numerical simulations were carried out. The mechanical behavior of rock mass with different pore water pressures and the evolution of stress-energy field in surrounding rock under the condition of mining-induced drainage of the confined water in the overburden aquifer were analyzed, and the effect of the mining-induced drainage of the confined water in aquifer on the stress field in surrounding rock was determined. On this basis, considering the effect of extra-thick strata on the dynamic and static stress in surrounding rock, the rockburst mechanism caused by the mining-induced drainage of the confined water in aquifer was proposed. The results show that the dynamic behaviors under extra-thick confined aquifer, including rockbursts and high energy seismicity, occur during the rapid drop of water level in overburden confined aquifer, and the mining-induced drainage of confined water causes stress disturbance to surrounding rock. Under the condition of mining-induced drainage of the confined water, the pore water pressure of the confined aquifer decreases, the strength and bearing capacity of the sandstone strata increase, and the overburden load is transferred to both sides of the goaf, which results in the increase of the stress and elastic energy in the surrounding rock. The effect of the mining-induced drainage of the confined water in aquifer on the stress-energy field is positively correlated with goaf scale and drainage degree, and negatively correlated with the distance between coal seam and confined aquifer, the thickness and strength of confined aquifer. The large-scale roof cantilever-hinged structures under the extra-thick confined aquifer lead to high static stress and strong dynamic stress in the surrounding rock. Under the effect of the mining-induced drainage of the confined water in the aquifer, the superimposed stress in coal and rock mass exceeds its critical load and the total energy released exceeds the energy consumed by its failure, which induces rockburst. In the process of rockburst, high static stress is the stress basis, and the stress disturbances caused by seismicity and the mining-induced drainage of the confined water are important inducements. In view of the influence of the mining-induced drainage of the confined water and the extra-thick strata structure on the rockburst risk, the confined water plugging of the extra-thick aquifer by ground grouting and optimizing the panel size were put forward. Field application indicates the two measures inferred above can reduce the effect of the mining-induced drainage of the confined water on the stress-energy field, and avoid the formation of the large-scale cantilever-hinged structures, which can reduce the rockburst risk significantly

Key strata of mining-induced seismicity in overburden rocks in coal mines and the energy-releasing mechanism of its fracture

Mining-induced seismicity in strata has become one of the main dynamic phenomena in some coal mines. The theoretical identification of the main controlling rock stratum where mining-induced seismicity occurs is the basis for accurate prevention and control of mining-induced seismicity. In this paper, theoretical analysis, numerical simulation and on-site observation are used. The connotation and type of mining seismic events and mining-induced seismicity are sorted out, and the relationship between mining seismic events and mining-induced seismicity is clarified. The concept of the Key Strata of Mining-induced Seismicity (KSMIS) in overburden rocks is put forward, and the characteristics of the KSMIS are summarized, and the difference between the KSMIS and key strata is presented. The identification method of the KSMIS is proposed, the crack development and energy evolution during the fracture process of the KSMIS are analyzed, and the energy release mechanism for the fracture of the KSMIS is revealed. The results show that: ① Mining seismic events can be classified into micro-seismic events, high-energy seismic events and mining-induced seismicity, and mining-induced seismicity can be classified into overburden rocks, faults, coal pillars, floor, folds and composite types; ② The KSMIS in overburden rocks refers to a layer or group of layers in the roof that control the occurrence and distribution of the mining-induced seismicity, and is classified into two types of high-level and low-level according to the location of the KSMIS; ③ Considering the failure criteria and energy characteristics of thick and hard rock layers, a method of identifying the KSMIS is proposed, and the method is verified by case study; ④ In addition to the crack zones on both sides of the mined area, horizontal shear cracks appeared between the KSMIS and the roof in low position. The strain energy and shear dissipation energy of the interlayer joints were concentrated, and the shear slip between the rock layers occurred. There are strain energy and shear dissipation energy accumulation zones in some areas of the KSMIS, and layering damage exists within the KSMIS; ⑤ When the actual maximum stress of the rock exceeds the strength limit of the rock layer or structural contact surface, the KSMIS will be broken or unstable, resulting in the formation of mining-induced seismicity in overburden rocks. In the process, some of the elastic strain energy and gravitational potential energy is converted into mining-induced seismicity energy as well as various types of dissipation energy, etc. The results of the study can provide theoretical guidance for the prevention and control of mining-induced seismicity in overburden rocks

Harness of room-temperature polar skyrmion bag in oxide superlattice.

Skyrmion bag is a higher-order skyrmion-based topological structure that can be used to reduce the risk of data loss in memory devices. It consists of an outer skyrmion wall and several inner skyrmions with opposite polarities. While skyrmion bags have been observed in ferromagnetic materials and liquid crystals, creating a polar skyrmion bag has proven challenging. Herein, we report the creation, evolution, and destruction of a polar skyrmion bag in a ferroelectric/dielectric oxide superlattice. The creation of a polar skyrmion bag is facilitated by applying an electric field pulse through a charged tip, which could be eliminated by changing the field direction. We also discover that the topological charge inside a polar skyrmion bag can be manipulated in situ by adjusting the magnitude of the applied potential. Through combined experimental observations and theoretical calculations, we have gained new insights into multipolar boundaries and identified promising prospects for future high-density, scalable, robust memory and low-loss nanoelectronics devices

Order-disorder transitions in a polar vortex lattice

Order-disorder transitions are widely explored in various vortex structures in condensed matter physics, that is, in the type-II superconductors and Bose-Einstein condensates. In this study, the ordering of the polar vortex phase in [Pb(Zr0.4Ti0.6)O3]n/(SrTiO3)n (PZT/STO) superlattices is investigated through phase-field simulations. With a large tensile substrate strain, an antiorder vortex state (where the rotation direction of the vortex arrays in the neighboring ferroelectric layers are flipped) is discovered for short-period PZT/STO superlattice. The driving force is the induced in-plane polarization in the STO layers due to the large tensile epitaxial strain. Increasing the periodicity leads to antiorder to disorder transition, resulting from the high energy of the head-to-head/tail-to-tail domain structure in the STO layer. On the other hand, when the periodicity is kept constant in short-period superlattices, the order-disorder-antiorder transition can be engineered by mediating the substrate strain, due to the competition between the induction of out-of-plane (due to interfacial depolarization effect) and in-plane (due to strain) polarization in the STO layer. The 3D ordering of such polar vortices is still a topic of significant current interest and it is envisioned that this study will spur further interest toward the understanding of order?disorder transitions in ferroelectric topological structuresThis work was supported by the Joint Funds of the National Natural Science Foundation of China under grant U21A2067 (Y.W.), and the Fundamental Research Funds for the Central Universities (No. 2021FZZX003-02-03, Z.H.). Z.H. also gratefully acknowledge a start-up grant from Zhejiang University. The financial support from Grant PGC2018-096955-B-C41 funded by MCIN/AEI/10.13039/501100011033 is acknowledged (J.J., P.G.-F., F.G.-O.). F.G.-O. acknowledge financial support from Grant No. FPU18/04661 funded by Spanish Ministry of Universities. The phase-field simulation was performed on the MoFang III cluster on Shanghai Supercomputing Center (SSC). S. D. is currently at Materials Research Centre, Indian Institute of Science, Bangalore, India

Examining Nostalgia in Old Life: Antecedence and Outcome

Abstract Nostalgia is a self-conscious, bittersweet but predominantly positive and fundamentally social emotion. The regulatory model of nostalgia suggests that experiencing nostalgia can buffer against social threat (e.g. social exclusion) by providing individuals with sense of social connectedness (Sedikides, et al., 2015). In the current research, we propose that this salutary effect of nostalgia may be stronger among older adults compared to younger adults because older adults value social meaningfulness to a greater extent. Fifty-nine younger adults (Mage = 20.15, SD = 0.215) and 56 older adults (Mage = 71.02, SD = 0.679) completed daily questionnaires three times a day for ten consecutive days, and reported their emotional experience and social activities. Results showed that perceiving social threat was positively correlated with nostalgia experience reported at the subsequent time point, and this association was stronger among older adults. In addition, nostalgia positively correlated with subsequent social activities among the older participants but not among the younger participants. These findings highlight that nostalgia brings beneficial psychological (sense of social connectedness) and behavioral (social engagement) outcomes to older adults.</jats:p

Privacy Protection of Task in Crowdsourcing: Policy-Hiding and Attribute Updating Attribute-Based Access Control Based on Blockchain

Crowdsourcing is a new way to solve complex problems by using human intelligence. However, the tasks and user information privacy concerned in crowdsourcing have not been adequately addressed. It is necessary to design a privacy protection mechanism for tasks that need to be restricted to specific user groups. Ciphertext-policy attribute-based encryption (CP-ABE) is an efficient and feasible cryptographic tool, particularly for crowdsourcing systems. The encryptor can choose the access policy independently, which limits the scope of decryption users. At present, most CP-ABE schemes adopt a centralized management platform, which poses problems such as high trust-building costs, DDoS attacks, and single point of failure. In this paper, we propose a new access control scheme based on CP-ABE and blockchain, which has the properties of policy hiding and attribute updating. To protect the privacy of worker’s attributes, we adopt a test algorithm based on a fully homomorphic cryptosystem to confidentially judge whether the worker’s attribute lists match the hidden attributes policy in ciphertext or not before the decryption. Experiment results and comprehensive comparisons show that our mechanism is more flexible, private, and scalable than existing schemes.</jats:p

Using elastic wave velocity anomaly to predict rockburst hazard in coal mines

For the prevention and control of rockburst in underground coal mines, a detailed assessment of a rockburst hazard area is crucial. In this study, the dependence between stress and elastic wave velocity of axially-loaded coal and rock samples was tested in a laboratory. The results show that P-wave velocity in coal and rock is positively related to axial stress and can be expressed by a power function. The relationship showed that high stress and a potential rockburst area in coal mines can be determined by the elastic wave velocity anomaly assessment with passive seismic velocity tomography. The principle and implementation procedure of passive seismic velocity tomography for elastic wave velocity were introduced, and the assessment model of rockburst hazard using elastic wave velocity anomaly was built. A case study of a deep longwall panel affected by rockbursts was introduced to demonstrate the effectiveness of tomography. The rockburst prediction results by passive velocity tomography closely match the dynamic phenomenon in the field, which indicates the feasibility of elastic wave velocity anomaly for rockburst hazard prediction in coal mines