D-shaped Hole Research Articles

Experimental investigation on rockburst characteristics of highly stressed D-shape tunnel subjected to impact load

Rockburst has always been a challenge for the safe construction of deep underground engineering. This study investigated the rockburst characteristics in highly-stressed D-shape tunnels under impact loads from rock blasting and other mining-related dynamics disturbances. The biaxial Hopkinson pressure bar was utilized to apply varying biaxial prestress and the same impact loads to cube specimens with D-shape hole. High-speed camera and digital image correlation (DIC) were used to capture the failure process and strain field of specimen. The test results demonstrate that the D-shape hole specimen experience rockburst under coupled static stress and impact load. Under this circumstance, the rockburst mechanism of the D-shaped hole specimens involves spalling in sidewall induced by impact load, indicating dynamic tensile failure. The high static prestress provides the initial stress field, while the impact load disrupts the stress equilibrium, result in the stress or strain concentration in the sidewall of the D-shape hole, inducing rockburst. Moreover, the rockburst process can be divided into (1) calm stage, (2) crack initiation, propagation, and coalesce stage, (3) spalling stage and (4) rock fragments ejection stage. Impact load triggers rockburst occurrence, while vertical stress further determines the rockburst characteristics. The influence range and magnitude of strain concentration zone and displacement deformation of the tunnel surrounding rock increases with increasing vertical stress, thus inducing more severe rockburst.

Influence of maximum principal stress direction on the failure process and characteristics of D-shaped tunnels

To investigate the failure process and characteristics of D-shaped tunnels under different maximum principal stress directions θ, true-triaxial tests were conducted on cubic sandstone samples with a through D-shaped hole. The test results show that the failure process can be divided into 4 periods: calm, buckling deformation, gradual buckling and exfoliation of rock fragment, and formation of a V-shaped notch. With an increase in θ from 0° to 90°, the size of the rock fragments first decreases and then increases, whereas the fractal dimension of the rock fragments first increases and then decreases. Meanwhile, the failure position at the left side shifts from the sidewall to the corner and finally to the floor, whereas the failure position at the right side moves from the sidewall to the spandrel and finally to the roof, which is consistent with the failure position in underground engineering. In addition, the initial vertical failure stress first decreases and then increases. By comparing the results, the failure severities at different maximum principal stress directions can be ranked from high to low in the following order: 90°>60°>30°>45°>0°.

Experimental Investigation of Spalling Failure of D-Shaped Tunnel Under Three-Dimensional High-Stress Conditions in Hard Rock

The D-shaped cross section is a commonly used tunnel cross section in underground engineering. To simulate the failure process of a D-shaped hole under deep three-dimensional (3D) high-stress conditions, true-triaxial tests were conducted on cubic granite specimens with a through D-shaped hole, and the failure process of the hole sidewall was recorded in real time. Results show that the spalling failure process of the D-shaped hole sidewall can be divided into four periods: calm, fine particle ejection, crack generation and propagation, and rock slab gradually buckling and spalling. Afterwards, symmetrical V-shaped notches were formed on both sidewalls between the corner and arch springing. The spalling failure shows tensile failure characteristics. Under high vertical stress and constant horizontal axial stress, increasing the lateral stress reduces the severity of the spalling failure and the depth of the V-shaped notch. The initial failure vertical stress of the D-shaped hole sidewall is higher than that of circular hole sidewall, and the failure of D-shaped hole sidewall is mainly characterized by static failure. The failure of the circular hole sidewall is a more severe dynamic failure. When the vertical applied stress is the maximum principal stress, the position of the V-shaped notch tip is 0.20–0.25 h (h is the height of the D-shaped tunnel) from the tunnel floor, whereas that in the circular tunnel is 0.5 d (d is the diameter of the circular tunnel) from the tunnel floor. Specific support schemes should therefore be designed for tunnels with different cross sections according to the damage location, depth of failure zone, and severity of failure.

Influence of sidewall size on spalling in deep D-shaped tunnels: An experimental simulation

A series of true triaxial compression tests was performed herein on cubic red sandstone samples containing a D-shaped hole with three sidewall sizes to study the influence of sidewall size on spalling in D-shaped tunnels. The initial horizontal stress of the tests refers to the in-situ stress at 500 m depth. The hole axis was arranged along the direction of the maximum horizontal principal stress. The vertical stress was increased to simulate the increasing surrounding rock stress after tunnel excavation. During the test, a self-made monitoring device was used to monitor and record the failure process of the tunnel models in real time. Furthermore, the spalling process of the D-shaped tunnels with different sidewall sizes was reproduced indoors. The failure process and the stress and fracture characteristics of the tunnel models with three different sidewall sizes were then analyzed and summarized. The effect of the sidewall size on spalling in D-shaped tunnels will be discussed in this paper. The results show that the failure of the tunnel models with different sidewall sizes first occurred on the sidewall near the corner then gradually developed upward and axial along the sidewall with the increase of the vertical stress. The width and the depth of the failure zone gradually increased, and a V-shaped notch was finally formed on the sidewall. No failure occurred on the roof and the floor of the tunnel models. The dominant failure modes were tensile failure. The failure forms were mainly spalling occasionally accompanied by slab ejection. The slabs were shaped in thin plates. The vertical stress that caused the initial failure of the tunnel models gradually decreased with the increasing sidewall size. Furthermore, the ability of the surrounding rock to store elastic strain energy decreased. The tensile characteristics became more prominent, and the size of the plate-shaped slabs increased. In the same stress environment, the greater the sidewall size, the larger the failure width and depth on the sidewall, and the more visible the V-shaped failure zone. The stability of deep D-shaped tunnels can be improved, and spalling can be mitigated by reducing the sidewall size. However, the risk of a rockburst will increase. This finding has an important engineering significance for the optimization design of a tunneling cross-section and the prevention of spalling and rockburst in D-shaped tunnels.

A Novel Photonic Crystal Fiber Sensor with Three D-shaped Holes Based on Surface Plasmon Resonance

A novel photonic crystal fiber (PCF) sensor with three D-shaped holes based on surface plasmon resonance (SPR) is analyzed in this paper. Three D-shaped holes are filled with the analyte, and the gold film is deposited on the side of three planes. The design of D-shaped holes with outward expansion can effectively solve the uniformity problem of metallized nano-coating, it is beneficial to the filling of the analyte and is convenient for real-time measurement of the analyte. Compared with the hexagonal lattice structure, the triangular arrangement of the clad air holes can significantly reduce the transmission loss of light and improve the sensitivity of the sensor. The influences of the air hole diameter, the distance between D-shaped holes and core, and the counterclockwise rotation angle of D-shaped holes on sensing performance are studied. The simulation results show that the wavelength sensitivity of the designed sensor can be as high as 10100 nm/RIU and the resolution can reach 9.9 × 10−6 RIU.

Experimental simulation analysis of the process and failure characteristics of spalling in D-shaped tunnels under true-triaxial loading conditions

Spalling is a typical failure mode encountered on the sidewalls after the tunnel face excavation of deep D-shaped tunnels or caverns. It significantly affects the safe and efficient construction and long-term stability of tunnels. To study the process and failure characteristics of sidewall spalling in D-shaped tunnels, four different three-dimensional initial stress states were established to perform a series of true-triaxial tests on cube specimens with a D-shaped hole. During the tests, a wireless micro camera was used to monitor and record the failure process of the sidewall in real time. Based on the test results, the process and failure characteristics of sidewall spalling were summarised, and the influence of cross-section shape on tunnel failure characteristics and tunnel stability were discussed. The results indicate that both sidewalls perpendicular to the maximum principal stress are cut into approximately parallel rock slabs, demonstrating typical spalling features. Failure zones on both sidewalls are located primarily between the corner and spandrel, and their outlines are “V” shaped (i.e., V-shaped notch). Spalling slabs are thin in both wings, and thick in the middle; they are cut into multilayer thin slices that are not completely detached from one another. With the increase in failure depth, the shape of the cracks first evolved from a straight shape into an arced shape, and subsequently evolved into a “V” shape. When the maximum principal stress is perpendicular to the tunnel axis, tunnel excavation along the minimum principal stress is conducive to the prevention of spalling. Compared with the circular tunnels, the D-shaped tunnels can effectively reduce the severity of rock failure and transform the rockburst into a static spalling. In a relatively low stress environment, the D-shaped cross section can improve the stability of the tunnel, while it is not conducive to the stability of the tunnel in a high stress environment. The result of this study benefits the design and prevention of spalling and rockburst of hard rock tunnels.

Double D-shaped hole optical fiber coated with graphene as a polarizer.

A double D-shaped hole optical fiber coated with graphene is proposed as a polarizer at the wavelength of 1.55μm. As the planar surfaces of D-shaped holes are both coated with graphene, the interaction between the core and graphene can be doubled. Moreover, the interaction can be further improved by introducing functional materials into the holes. The proposed fiber provides a high extinction ratio (ER) and low insertion loss, and it operates in the single polarization mode. The ER of 42.5dB with a 2.5-mm-long optical fiber can be achieved for a transverse-electric-pass polarizer, and the insertion loss is approximately 1.08dB. Specifically, the proposed fiber can achieve simultaneously dual-band polarization at 1.55μm and 1.31μm. The proposed fiber is feasible for seamless integration in existing fiber systems. We hope our work benefits high-efficiency polarizers, and we believe that the proposed fiber has some potential applications in photonic integrated circuits.

High sensitivity D-shaped hole fiber temperature sensor based on surface plasmon resonance with liquid filling

A high sensitivity D-shaped hole double-cladding fiber temperature sensor based on surface plasmon resonance (SPR) is designed and investigated by a full-vector finite element method. Within the D-shaped hole double-cladding fiber, the hollow D-section is coated with gold film and then injected in a high thermo-optic coefficient liquid to realize the high temperature sensitivity for the fiber SPR temperature sensor. The numerical simulation results show that the peaking loss of the D-shaped hole double-cladding fiber SPR is hugely influenced by the distance between the D-shaped hole and fiber core and by the thickness of the gold film, but the temperature sensitivity is almost insensitive to the above parameters. When the thermo-optic coefficient is −2.8×10−4/°C, the thickness of the gold film is 47 nm, and the distance between the D-shaped hole and fiber core is 5 μm, the temperature sensitivity of the D-shaped hole fiber SPR sensor can reach to −3.635 nm/°C.

Noncircular Tool Fabrication by WEDG Using Electrode Jump Motion and Noncircular Array Holes Drilling

Electrode jump motion strategy was adopted to fabricate noncircular tool using wire electrical discharge grinding (WEDG). The machining procedure and electrode jump motion strategy for noncircular tool fabrication were determined through experiments. This approach has been verified by successfully machining of rectangular electrode (38μm×38μm, 700μm length) and D-shaped electrode (diameter 45μm, height 42μm, 500μm length). D-shaped array holes of 6×6 matrix were drilled using corresponding single noncircular tool which was manufactured by the adopted machining approach. The array holes have tight diameter tolerance and height tolerance which are both less than 2μm, and positioning errors are both less than 1μm in the direction of the X, Y-axes.

Double-clad Yb3+-doped fiber lasers with virtually non-circular cladding geometry

We report on cladding pumped Yb3+-doped fiber lasers with novel virtual-non-circular cladding geometries. The geometry of the inner-cladding is in fact circular with D-shaped holes, that form virtual-D (one hole), and virtual-square (four holes) structures for the inner-cladding. The laser performance of this structure is assessed at several wavelengths and lengths, showing efficiencies as high as those obtained with other geometries for the inner-cladding.

Efficient operation of double-clad Yb3+-doped fiber lasers with a novel circular cladding geometry.

A highly efficient double-clad Yb3+-doped fiber laser with a novel cladding geometry is described. A round double-clad fiber with a small D-shaped hole for breaking a circular symmetry in a cladding combines the advantages of ease of manufacture and handling round fibers with efficient absorption of pump light. Fiber lasers with a double-D cladding shape and a D-hole cladding are compared. We report what are to our knowledge the highest slope efficiencies of 73% and 69%, respectively, pumping at 915 nm. Output powers in excess of 13 W are demonstrated.

Flatheaded Borer in White Alder Landscape Trees

Flight of a flatheaded borer (Agrilus burkei) begins in late March or the beginning of April and lasts for 35 to 62 days. After emergence through D-shaped holes in the bark, the adults feed for one week on the foliage; they live about 12 days. The first egg mass is oviposited 7 days after emergence. Hatched larvae penetrate the bark surface directly into the phloem tissues. The larval stage lasts from April to February of the next year. In March a pupa is formed beneath the bark and the new imago emerges. The apparent propensity of A. burkei to attack poorly growing alder trees suggests that white alders, Alnus rhombifolia, should be planted only in landscapes where moisture stress can be avoided or minimized.

Simple method for calibration of stress-induced birefringence in fibres with arbitrary structures

A simple method is presented for the calculation of stress-induced birefringence of optical fibres. The method is applicable to fibres with arbitrary cross-sectional structures. Numerical results for fibres with D-shaped hole and elliptic core are presented.

Geometrical birefringence in fibres with D-shaped hole

Geometrical birefringence in fibres with a D-shaped hole has been investigated in detail by vectorial wave analysis using the improved finite-element method (FEM) in terms of all three components of the magnetic field. A dependence of the modal birefringence on normalised frequency and geometrical parameters is obtained. The highest modal birefringence is found to be about 4.5 × 10-5. This fibre can therefore maintain a single polarisation state to some degree.