Mechanism of Support Optimization and Confined Blasting of Thick and Hard Rock with a Wedge-Structure Immediate Roof: A Case Study

Based on the occurrence conditions of a thick and hard main roof and wedge-structure immediate roof in the Zhuxianzhuang Coal Mine, the fracture characteristics and instability migration law of a thick and hard roof (THR) were examined via physical simulations. Mining zones were divided with respect to the strata behaviors and roof control difficulty levels, and the principles and methods of zonal control under THR were put forward. This study proposed a coordinated control strategy of using confined blasting in water-filled deep holes, and reasonable support optimization, which could effectively reduce the roof fracture size, increases the supporting intensity and eliminate roof-control disasters. The length of confined blasting blocks and supporting intensity were calculated using a mechanical model for roof control in the strong strata behavior zone and less-strong strata behavior zone. These key parameters were determined as 20–25 m and 1.15–1.28 MPa, respectively, and the mining strategy was successfully applied in working face 880, performing high security and reasonable economical efficiency.

Research on a New Type of Rigid-Flexible Coupling 3-DOF Micro-Positioning Platform

A new type of rigid-flexible coupling three degrees of freedom (3-DOF) micro-positioning platform with high positioning accuracy and high bearing capacity is developed, which consists of flexible drive mechanism and rigid platform. The flexible drive mechanism consists of three sets of symmetrical parallel round flexible hinge structures, each with a wedge structure in the middle of the symmetrical parallel flexible hinge. The rigid platform has an inclined plane with the same angle as the wedge, while the wedge structure is used to achieve the self-locking effect. The flexibility matrix method and ANSYS are used to analyze the statics of the flexible drive mechanism. The first four natural frequencies of the platform are obtained by dynamic simulation analysis. A symmetrical rigid flexible coupling micro-positioning platform experimental system is developed. Output characteristics, positioning accuracy, relationship between frequency and amplitude, and bearing performance of the micro-positioning platform are tested. These experimental results obviously show that the micro-positioning platform has good motion characteristics, high positioning accuracy, large movement distance, and large load bearing capacity performance.

Instability Model of a Coal Wall with Large Mining Height under Excavation Unloading Conditions

On the basis of the stress field characteristics of surrounding rocks at a coal wall on a working face with a large mining height, the theories of unloading rock mass mechanics and fracture mechanics were used to establish a model of the excavation unloading field effect of the coal wall, and its instability mechanism under the action of unloading stress field was analyzed. Results show that the coal mining process is the unloading process of coal and rock masses, and the stress field of surrounding rocks at the coal wall turns into an unloading stress field that consists of original and unloading stresses. Under the action of unloading stress field, cracks in the coal wall will undergo instability, propagation, and combination in the form of composite-type cracks and will gradually evolve into a wedge structure. The wedge stability is inversely related to roof pressure P 0 , unloading force T , and intersection angle φ of structural planes. Elevating the wall-supporting force P h , the initial supporting force of supports on the working face and the cohesion C of coal body can effectively control the occurrence of coal wall caving accidents and contribute to the safe mining of working faces with a large mining height.

Looking at shadows of entanglement wedges

We present a new method of deriving shapes of entanglement wedges directly from conformal field theory (CFT) calculations. We point out that a reduced density matrix in holographic CFTs possesses a sharp wedge structure such that inside the wedge we can distinguish two local excitations, while outside we cannot. We can determine this wedge, which we call a CFT wedge, by computing a distinguishability measure. We find that CFT wedges defined by the fidelity or Bures distance as a distinguishability measure coincide perfectly with shadows of entanglement wedges in anti-de Sitter (AdS)/CFT. We confirm this agreement between CFT wedges and entanglement wedges for two-dimensional holographic CFTs where the subsystem is chosen to be an interval or double intervals, as well as higher-dimensional CFTs with a round ball subsystem. On the other hand, if we consider a free scalar CFT, we find that there are no sharp CFT wedges. This shows that sharp entanglement wedges emerge only for holographic CFTs owing to the large-$N$ factorization. We also generalize our analysis to a time-dependent example and to a holographic boundary conformal field theory (AdS/BCFT). Finally, we study other distinguishability measures to define CFT wedges. We observe that some of the measures lead to CFT wedges which slightly deviate from the entanglement wedges in AdS/CFT, and we give a heuristic explanation for this. This paper is an extended version of our earlier letter (arXiv:1908.09939 [hep-th]) and includes various new observations and examples.

Partial Melts of Intermediate–Felsic Sources in a Wedged Thickened Crust: Insights from Granites in the Sulu Orogen

High-pressure (>15 kbar) melts of intermediate–felsic materials have been well studied by experiments, whereas their existence in nature, especially in orogenic belts, is rarely examined. With the aim of identifying and characterizing high-pressure partial melts of intermediate–felsic continental crusts, this study presents comprehensive geochemical and geochronological data for 47 Jurassic granites (166∼157 Ma) from the Sulu orogen. These Sulu Jurassic granites (SJG) consist of quartz, K-feldspar and plagioclase with minor mineral assemblages of biotite ± muscovite ± garnet ± epidote ± allanite. Their low mafic mineral abundance, high SiO2 and Al2O3, and low FeOt + MgO contents show leucogranite-like affinities. They have low Mg#, low Rb/Sr, and mildly peraluminous features, collectively suggesting an intermediate–felsic orthogneissic source. Whole-rock Zr saturation thermometry and Ti-in-zircon thermometry together suggest initial magma temperatures between 695 ± 32 °C and 751 ± 27 °C (1 standard deviation), indicating derivation from water-present melting. The SJG notably feature high Sr contents (average 792 ppm), high Sr/CaO ratios (average 476) as well as inter-correlated low REE concentrations (average ΣREE 87 ppm), low Th concentrations (average 5·1 ppm) and positive Eu anomalies (Eu/Eu* up to 2·94). These characteristics are best explained by partial melting of intermediate–felsic sources under high pressure (>15 kbar), leaving residuum where feldspar is sparse or absent and allanite is present. Inherited zircon age spectra and Sr–Nd–Pb isotopic compositions suggest that their source components could be mainly the Triassic orthogneisses whose protoliths are from the northern margin of the South China Block, probably in a wedge structure where the exhumed felsic slabs were wedged into the crust of the North China Block in the middle–late Jurassic and formed a stacked thickened crust. The wedge structure was most probably driven by synchronous large-scale strike-slip of the Tanlu fault, as a far-field effect of the oblique subduction of the paleo-Pacific plate. The characteristic chemical features observed in this study may be applied to identifying partial melts with similar petrogenesis elsewhere.

Fracture Analysis for Attaching Fiber Reinforced Composite on V-Notch Wedge Structure

Sharp V-notches with various angles often appear in engineering structures. When being loaded, the high stress at the apex could result in crack propagation on the structure and further fracture. For this reason, safety evaluation should be emphasized for products or engineering structures with such geometric characteristics. Sharp V-notches are regarded as wedge structures that the above situations seriously and often appear on brittle materials. Regarding the stress intensity factor K of the driving force for wedge structure failure, Chen, Dunn, and Seweryn, with numerical analysis for the fracture experiment, explained that the critical stress intensity factor Kc for single isotropic material fracture could be the intensity failure specification for wedge structures. Nevertheless, V-notched brittle materials are likely to receive great stress over the surface elastic energy of the structure when being loaded, causing brittle failure at the apex. When the high-strength and light-weight composite material is attached to reinforce the surface of brittle materials, the energy is reinforced to enhance the critical stress intensity factor of the overall structure, aiming to improve the failure of brittle materials resulted from stress singularity. This paper therefore tends to discuss the effects of the composite attachment, layer, and fiber reinforced direction on the critical stress intensity factor when the structure is being fractured.