Arch Mechanism Research Articles

DEM investigation on the dynamic characteristics of wall normal stress and particle flow during silo discharge

Violent fluctuations of wall normal stress and particle velocity during silo discharge have been frequently reported by researchers, yet a detailed particle-scale description of these phenomena is still lacking. In this work, the discharge of granular assembly from a flat-bottomed silo was investigated by performing three-dimensional (3D) Discrete Element Method (DEM) simulations. It is found that arc-shaped rupture surfaces, characterized by the lower solid volume fraction, stronger shear interaction between particles and higher particle rotating velocity, developed continuously in the bottom converging part of flowing zone. And the temporal evolution of particle velocity field in this converging part presented a swinging feature manifested by the alternate appearance of large particle velocity magnitude in the right- and left- side regions. Discrete Fourier transform results showed that the fluctuation of wall normal stress at the transition point contained two dominate frequencies. The lower frequency matched well with the dynamic evolution of the rupture surface joining the transition point, and the higher frequency was nearly the same as the fluctuating frequency of particle vertical velocity. Thus, our DEM simulation results suggested that for the silo discharge considered in this work, the periodic fluctuation of wall normal stress at the transition point was contributed by both the alternating flow pattern mechanism and the free-fall arch mechanism.

Evaluating the mechanisms and performance of Geosynthetic-Reinforced Load Transfer Platform of pile-supported embankments design methods

This study evaluates the existing design methods of Geosynthetic-Reinforced Load Transfer Platform for Pile-Supported Embankments (GLTP-PSE) through comprehensive 3D Finite Element (FE) analyses. It scrutinizes the assumed arching mechanisms, methodologies, design criteria (arching height, maximum strain, differential settlement, and T in geosynthetics), and overall performance of these methods. The 3D FE analysis results and measurements from two case studies were compared with six established GLTP-PSE design methods based on the four design criteria. Key findings include the identification of a progressive concentrated ellipsoid as the developed soil arching formation, with arching height dependent on the embankment equivalent height (including embankment and traffic load), pile spacing, maximum strain along the geosynthetics, and the number of geosynthetic layers. The load distribution on geosynthetic reinforcement was observed to align more closely with a non-linear inverse triangle. These insights led to recommendations for updating existing design methods, enhancing the accuracy and reliability of GLTP-PSE designs. The study's outcomes contribute significantly to advancing and refining GLTP-PSE design practices by providing a deeper understanding of soil arching mechanisms and the performance of geosynthetic reinforcements.

Effect of Aligning Forces by Two Preadjusted Edgewise Techniques on a Buccally Positioned Maxillary Canine at Varying Vertical Displacements: A Finite Element Study.

To evaluate the effect of continuous arch and piggyback mechanics in a straight wire appliance (SWA) for the alignment of buccal and variably vertically positioned maxillary canines. A three-dimensional finite element model with near-normal occlusion and buccal and vertically displaced maxillary canines was used. Two groups were created to simulate two commonly used SWAs techniques, continuous archwire (Group 1) and piggyback models (Group 2). Each group had three subgroups with varying vertical displacement of the canine from 2 to 6 mm from the occlusal plane. The displacement and stress distribution were noted in each group. As the vertical displacement increased in Group 1, the concentration of von Mises stress increased progressively at the incisal third (0.36, 0.41 and 0.44 MPa) at 2, 4, and 6 mm, respectively, with decreased maximum occlusal movement in the vertical plane with respect to the canine. Group 2 exhibited a similar pattern but greater occlusal movement of the canine compared with Group 1. A vertical displacement of 4 mm is the optimal level at which continuous arch mechanics should be considered. For displacements beyond 4 mm, the piggyback wire technique is a suitable alternative.

Large-scale model test and numerical analysis of load-bearing arch characteristics of large cross-section tunnel under high geostress

Understanding the failure process and the load-bearing arch effect of the surrounding rock is crucial for the stability control and support design of tunnels under high geostresses. In this study, the progressive collapse behavior and the formation mechanism of the load-bearing arch of a soft rock tunnel under high geostress were investigated by a model test and numerical simulations. First, a large-scale model test was conducted to study the stress redistribution rules and the failure process of the soft rock under different overloads. According to the model test results, the formation of the final load-bearing arch was divided into four stages including initial local load-bearing arch formation, rock failure inside the local arch, overall load-bearing arch formation and load-bearing arch outward transition. Both the progressive failure and arch mechanism were related to the stress transfer effect. Then, numerical simulations were carried out to verify the stress response results from the model test. The determination method for the inner and outer boundaries of the load-bearing arch was put forward based on the circumferential stress distribution, which was validated by the model test results. Furthermore, the relationship between the radial displacement of the tunnel boundary and the arch position was illustrated. It was found that the load-bearing arch showed an oval shape with a big top and a small bottom. The arch position was linearly correlated with the radial displacement at the tunnel wall. This study provides a comprehensive understanding of the load-bearing arch characteristics of large cross-section tunnels in soft rocks under high geostress conditions, which can be referred to in construction control and support design during tunnel construction.

Effective Elastic Properties and Micro-mechanical Damage Evolution of Composite Granular Rocks: Insights from Particulate Discrete Element Modelling

The mechanical behavior of composite granular rocks is a multifaceted phenomenon with broad relevance in various geomechanical applications. Traditional homogenization models and continuum mechanics-based numerical methods often fall short of accurately capturing the intricacies of granular materials. Granular materials exhibit heterogeneity and arching mechanisms governing the force networks that ensure system stability. Unlike continuum-based approaches, discrete element methods (DEM) have an advantage in assessing effective material properties by considering material heterogeneity and grain-level physical interactions. This study evaluates effective elastic properties using DEM with flat-joint contact law for composite binary mixtures with a stiff inclusion embedded within a matrix material. We examine variations in the elastic properties across different structural and laminated geometrical distributions of inclusion materials. Our findings closely adhere to the Voigt-Reuss and Hashin–Strikman bounds within their specific conditions, demonstrating the promising application of DEM in the analysis of composite materials. In addition, our research provides an in-depth analysis of the distinctive stress-evolution and damage-evolution characteristics exhibited by various geometrical configurations of inclusions under unconfined compressive loading. These results offer invaluable insights into the mechanical behavior of composite granular rocks and underscore the potential applications of DEM in addressing rock physics modeling problems encountered in petroleum engineering.

A study on the arch mechanism of pedestrian evacuation and congestion alleviation strategies at building exits

The pedestrian flow capacity of building exits has always been a focus of research in evacuation field. This paper focuses on the congestion mechanism of pedestrian flow at exit bottlenecks through simulation and crowd experiments, and tries to propose strategies to alleviate evacuation congestion. Firstly, pedestrian flow evacuation experiments were conducted by setting up scenarios with exits in the middle and corners, and corresponding exit structure models were established using Massmotion software to analyze the congestion mechanisms of pedestrian flow at the exits through simulations coupled with the social force model. Secondly, the severity of congestion in the exit structure was quantified using the example of the middle exit. The width range of the pedestrian evacuation trajectory within 1 m before the exit was defined as the average chord length of the arch structure, and the area enclosed by the semicircular arch with this chord length was defined as the area with a relatively high instantaneous density of pedestrian flow. The results indicate that corner exits are more conducive to evacuation as they reduce the expected velocity of pedestrians near the center of congested areas. Both building width and the inflow rate of pedestrian flow, along with the number of pedestrians, demonstrate significant effects on the average chord length of the arch. The research results can be used to further supplement and improve the dynamic theory of pedestrian flow evacuation in buildings and provide a scientific reference for crowd congestion control at building exits.

Arching mechanism for large span flexible culverts with shallow soil covers

Soil-steel composite bridges (SSCBs) use the surrounding soil and the culvert’s flexible corrugated steel thin plates to support vertical loads above the culvert. Existing codes use empirical equations based on field measurements and full-scale tests for small and ordinary span SSCBs to estimate the arching process and how it distributes vertical stresses on the culvert walls. The empirical equations then compute the design straining actions. Recent developments in construction technology and urbanization have led to a significant increase in typical spans of SSCBs. This paper investigates large span SSCBs and the associated induced arching action using three-dimensional finite element analysis (FEA). The study compares the FEA findings of straining actions for large span SSCB case studies to the calculations of valid design codes. The comparison demonstrates that existing codes fail to predict the real arching mechanism and, consequently, the resulting straining actions. In addition, the FEA results illustrate how the arching mechanism varies as the span changes. Arching action is typically positive for relatively small spans but becomes negative as the span increases. Finally, results prove that current codes cannot accurately predict real arching, leading to inappropriate design straining actions. For large span SSCBs, these codes require modifications for the arching factors and the profile aspect ratio factors.

Orthodontic Correction of an Anterior Open Bite with Periodontally Compromised Upper Incisors Using Miniscrews

This case report describes the camouflage treatment of a patient with an anterior open bite, bimaxillary skeletal protrusion with a mild class II skeletal pattern, reduced periodontium with generalized slight bone loss, localized severe bone loss at the maxillary upper incisors, and a tongue thrust habit. Orthodontic treatment included modifying the resting posture of the tongue and sectionally intruding maxillary posterior teeth while controlling the extrusion of mandibular posterior teeth using temporary anchorage devices (TADs), followed by sectional mechanics at the maxillary arch and continuous arch mechanics at the mandibular arch, along with interpromximal reduction at the mandibular teeth and inter-arch elastics from the mandibular teeth to the maxillary TADs to minimize orthodontic force magnitude and duration specifically applied to periodontally compromised maxillary incisors. The patient’s chief complaint was addressed, and treatment objectives were achieved, with good esthetic and occlusal outcomes. Stability was shown over a 56-month follow-up period.

Passive Soil Arching Effect in Aeolian Sand Backfills for Grillage Foundation.

The passive soil arching effect exists in many soil-grille interaction systems. Increasing mental grillage foundations are used for transmission lines in aeolian sand areas; thus, exploring the evolution mechanism of passive soil arching is crucial. This study investigates the evolution and influencing factors of passive soil arching through a series of tests using a trapdoor device and particle image velocimetry (PIV). The test results show that the evolution of the arching structure causes the aeolian sand deformation to gradually extend to the backfill surface and stationary zone, generating two triangular arching surfaces between the movable beams and sliding surface at the junction of the active and stationary zones. Cracks in the arching and sliding surfaces were connected to form a W-shaped shear band. The development of the soil pressure was divided into four arching structure stages. The different stages of the inner and outer arches of the bearing characteristics had strong differences. Taking the appearance of the first arch surface as the time point, the soil pressure changes abruptly and the inner and outer arches alternate to bear the as a major role. The beam spacing significantly affected the arching evolution. A smaller beam spacing formed an initial bending configuration with an inconspicuous arching structure and incomplete shear band. As the beam spacing increased, the arching shape changed from triangular to parabolic, sudden changes in the soil pressure were more pronounced, and the arch height increased. The relative density and water content had little impact on the arch shape and shear zone but significantly affected the arching strength, soil pressure transfer, and arching height. The medium and high relative densities and low water contents resulted in a stronger arching structure and greater arching height, while low relative densities and high water contents weakened the soil pressure transfer. The range values for the optimum beam spacing, relative density, and water contents are given based on the variation characteristics of the evaluated parameters (E, n) under different conditions.

Novel Spatial Soil Arching Effect on a Steep-Slope High Embankment in a Double-V Narrowing Gully

A novel spatial soil arching (SSA) effect and the arching mechanisms and characteristics of a steep-slope high embankment in a double-V narrowing gully (DVNG) were investigated. First, the consistency between the results obtained via a lab-scale experiment and the three-dimensional finite-element model (FEM3D) confirmed the existence of SSA in the DVNG (DVNG-SSA) and validated the numerical model feasibility. The enlarged FEM3D revealed that the DVNG-SSA was generated by stress redistribution caused by uneven soil displacement. Moreover, the DVNG-SSA could generally be divided into horizontal (H-) and vertical (V-) DVNG-SSAs, which could interact with each other to form large-scale spatial soil arches at the front and bottom of the embankment. Parameter sensitivity analysis revealed that increasing the Poisson’s ratio of the soil negatively affected both arches. The H-DVNG-SSA only had side banks acting as arch feet, while the V-DVNG-SSA had an additional ditch bottom. Compared with H-DVNG-SSA, V-DVNG-SSA had a broader range, flatter shape, longer length, deeper thickness, lower height, and smaller maximum rise–span ratio. Finally, the strength of the DVNG-SSA was lower than that of the conventional pile–soil SSA despite its extensive coverage, which could considerably impact the overall embankment stability.

Experimental investigation on the in-plane and out-of-plane interaction of isolated infills in RC frames

This paper presents the findings of a series of bidirectional quasi-static tests conducted on isolated infilled frames to investigate the in-plane and out-of-plane interaction of the infills with different boundary constraints. The effects of prior in-plane damage on the out-of-plane behavior of isolated infill walls were assessed. It was found that the presence of existing cracks that developed during the in-plane loading stage had an effect on the out-of-plane cracking patterns, decreased the integrity of the isolated infills, and weakened the infill-frame boundary restraints. The behavior of isolated infill walls under combined in-plane and out-of-plane loading was also affected by the effectiveness of boundary constraints. The experimental results demonstrated that reliable boundary constraints were necessary for forming arching mechanisms and preventing rigid body motion.

Study of Stress Paths in Arching Effect Using Frictional Strain Hardening and Softening in Fine Sand

Arching is one of the most common phenomena that occur in most geotechnical structures. To determine the properties and quality of this phenomenon, a physical model has been designed and constructed. The apparatus comprises rectangular trapdoors with different widths that can yield downward while stresses and deformations are recorded simultaneously. As the trapdoor starts to fail, the whole soil mass deforms elastically. However, after an immediately specified displacement, depending on the width of the trapdoor, the soil mass behaves plastically. This behavior of sand occurs due to the flow phenomenon and continues until the stress on the trapdoor is minimized. Then the failure process develops in the sand, and the measured stress on the trapdoor shows an ascending trend. This indicates a gradual separation of the yielding mass from the whole soil body. Finally, the flow process leads to the establishment of a stable vault of sand called the arching mechanism or progressive collapse of the soil body. To simulate this phenomenon with continuum mechanics, the experimental procedure is modeled in ABAQUS software using stress- dependent hardening in an elastic state and plastic strain-dependent frictional hardening-softening with Mohr Coulomb failure criterion applying user sub-routine. The results show that the experimental data have an acceptable corresponding to the numerical analysis data. So the selected soil behavior could indicate the main aspects of the arching effect, such as the flow that occurs in specific periods of strains. In the following, the stress path in p, q, and p, ν space was extracted from numerical analysis, and the results have been discussed.

Dynamic progressive collapse response of multi-storey frame structures with masonry infills

The response of reinforced concrete (RC) frame building structures to exceptional events like explosions or impacts causing the loss of a primary structural element depends on the robustness of the structural configuration. In the case of a sudden column loss, the resisting mechanism of the involved spans substantially changes, while a progressive collapse mechanism can develop under the arising dynamic loads. The presence of masonry infill walls can substantially modify the overall response to a sudden column loss scenario. In this context, this paper investigates the response of reinforced concrete frame structures undergoing instantaneous column losses in order to assess the influence of masonry infills on the progressive collapse response under dynamic load demands. A recently formulated equivalent-strut macro-modelling approach is employed to reproduce the mechanical interaction between infill walls and the frames. A ten-storey, six-bays 2D reinforced concrete frame is selected as a case study structure considering different reinforcement layouts (seismic or non-seismic design) and column loss scenarios (central column or corner column loss). The simulations are carried out using the fiber-section beam/column elements available in the OpenSees software platform, as they can account for the arching mechanism developing in the post-cracked regime. The dynamic responses of the case-study tests to the sudden column loss scenarios are assessed with and without the inclusion of masonry infills within the structural models. Results demonstrate that masonry infills introduce a substantial modification of the resisting mechanism and of the dynamic response, limiting the propagation of progressive collapse in most of the considered cases.

Arch model of roof and optimization of roof-contacted filling rate in two-step mining

To investigate the reasonable roof-contacted filling rate (RCFR) of the backfill in the goaf of a deep mine, the three-hinged arch mechanics model was applied to the two-step mine, and the stress expressions of the goaf roof with different roof-contacted filling rates were derived. Then, the influence of RCFR on the stress state of the roof was analyzed combined with numerical simulation. The results indicated that vertical stress changed the stress state of the roof in the room-stopes and pillar-stopes, while horizontal stress mainly affected the stress state of the roof in the room-stopes. The RCFR in the room-stopes had a greater influence on the formation of the self-stable arch of the roof in pillar-stopes. During the mining process of the Dongguashan Copper Mine, the actual RCFR at the site was about 80%, which achieved a safe and stable uninterrupted recovery of mineral resources.

A 3D torus-slice model for limit equilibrium analysis of shield tunnel face under soil arching effect

This paper proposes a torus-slice model to estimate the tunnel face critical support pressure, (σcrit) from the shield machine under the influence of soil arching. The failure envelope of the tunnel face is created by a cluster of discrete points using a spatial discretization technique. The vertical earth pressure loaded on the failure soil mass in front of the face is determined by an arching mechanism based on the rotation trajectory of the major principal stress. The tunnel face support pressure distribution is derived from the stress interaction using a stepwise method between the horizontal slices divided from the torus. The comparisons between the centrifuge tests and analytical solutions demonstrate the accuracy of the proposed model. The pressure distribution is further validated by the results of a FLAC3D model and analytical solutions. The failure envelope is compared to the geometry of the shear band from the numerical model, indicating the rationality of the proposed method. With an improved geometry considered, the proposed torus-slice model demonstrates the advantages over the classical silo-wedge model. Finally, the effects of material and geometrical parameters on a required tunnel face pressure are discussed. The results show that the soil internal friction angle φ’, cohesion c’, the soil-shield interface friction angle δ, and the geometric shape factor Tm all have significant effects on the tunnel face stability.

SHEAR STRENGTH OF THE STRENGTH–DUCTILITY TYPE SEISMIC RETROFITTING TECHNIQUE FOR EXISTING RC COLUMNS USING PRESTRESSING BAR AND STEEL PLATES

This study explores a seismic retrofit method for existing RC columns using a cast-in-site partial thick hybrid wing-wall (THW) technique. The THW technique plays two important roles: (1) to enable easy and cost-effective construction and (2) to considerably increase lateral strength and ductility. The aim of this study is to experimentally determine the shear resistance (truss and arch mechanism) and shear strength of the columns retrofitted using the THW technique. Based on the analytical assessment of the THW technique, design equations of ultimate shear strength and steel plate thickness are proposed.

Failure characteristics of reinforced concrete circular slabs subsequently subjected to fire exposure and static load: An experimental study

AbstractThis study investigated the effect of fire on the ultimate load‐bearing capacity of reinforced concrete (RC) slabs. The structural response of RC circular specimens subjected to static load conditions after exposure to a hydrocarbon fire on one side of the specimen was examined. Two fire exposure times were considered (60 and 120 min) in addition to reference non‐exposed specimens. The static response was evaluated in the damaged specimens in residual conditions after natural cooling from the elevated temperatures. The temperature distribution across the thickness of the slabs and their load–displacement response was measured. The decrease in the stiffness of the slabs due to the thermal exposure was studied by means of direct ultrasonic pulse velocity (UPV) measurements made before and after the fire tests. The decrease in global stiffness was partially accounted for by UPV measurements. The experimental results showed two peaks in the load‐deflection response of the slabs. The first peak was related to an arching mechanism introduced by the specific set‐up used. The second peak, corresponding to the ultimate load, occurred due to tensile membrane action at large deflections. While the former was strongly affected by the fire exposure, with the load being halved after the 120‐min exposure, the latter was not greatly affected by either the presence of fire or the exposure time. Simplified mechanical models were used to explain the behavior of the RC slabs during the tests.

Effects of joint stiffness degradation on vertical stability of CRTSII slab ballastless track

Joint damage of the China Railway Track System (CRTS)II slab track leads to typically non-uniformity of the longitudinal stiffness of the track slab, which tends to produce different degrees of arching at the joint. The objective of the present study was to investigate the arching mechanism of the non-uniform track slab. The buckling eigenvalues and arch curves of the non-uniform track slab were obtained using the Newton iterative eigenvalue algorithm. An analytical expression for the equilibrium path of the slab with an initial arch was obtained using the energy method. Then, the arching law and stability of the non-uniform track slab were analysed. The following results were obtained. 1) The reduction of the joint bending stiffness significantly affects the arching deformation and buckling mode of track slab, thus, it is necessary to consider the non-uniform characteristics of the track slab when analysing its stability. 2) The most unfavorable wavelength of track slabs is between 6 m, and 7.5 m considering the different joint bending stiffnesses and elastic moduli, and the reduction of the bending stiffness of the joint is the main factor for reducing the most unfavorable wavelength. 3) The reduction of the joint bending stiffness is the most important factor for causing the instability of the track slab, the maximum allowable temperature rise decreased from 83.97 to 49.12 ℃ when the bending stiffness of the joint was reduced by a factor of 100. 4) For improving the stability of the track slab, releasing part of the temperature force in the track slab by reducing the elastic modulus of the joints is a feasible method, the maximum temperature rise of track slabs can be rapidly increased when the elastic modulus of the joints is < 5000 MPa.

Laboratory experiments and discrete element modeling on the surface failure induced by EPB tunneling: The effects of cutterhead open ratio and relative tunneling depth

This paper presents laboratory tests and high-fidelity numerical simulations to examine the ground failure induced by Earth Pressure Balance (EPB) Shield Tunneling in granular soils. Specifically, we designed and conducted laboratory tests using a reduced-scale EPB Shield Tunnel machine, and built corresponding three-dimensional (3D) discrete-element method (DEM) models to simulate the dynamic excavation process and to gain further understandings of the mechanisms of the ground movement. The study is particularly focused on the combined influences of the cutterhead open ratio and the relative tunnel depth on the ground motions and stress perturbations during the EPB tunneling, which remain understudied in existing studies. The results show that the open ratio of the cutterhead has significant effects on the ground failure in granular soils, especially in cases of relatively shallow tunnels. The larger the open ratio of the cutterhead, the wider the surface and subsurface settlement trough and the greater the soil pressure in the chamber. The pressure distribution in the soil chamber with a larger cutterhead open ratio is significantly more non-linear than that of the case with a smaller cutterhead open ratio. For a given cover-to-diameter ratio (C/D) of the tunnel, the width parameter (k) is generally constant regardless of the depth for the case with a large cutterhead open ratio, especially in the relatively shallow tunnels, while it increases non-linearly with the depth for the case with a small cutterhead open ratio, especially in relatively deep tunnels. Furthermore, soil arching mechanisms are analyzed qualitatively with respect to different cutterhead open ratios and C/Ds. The study demonstrates quantitatively the response of ground surface to the cutterhead open ratios and C/Ds as well as the associated ground failure mechanisms through a combination of laboratory experiments and 3D DEM simulations, providing further insights into the ground behaviors during the dynamic EPB tunneling process.

Segmented arch technique for the correction of crowding in Angle s Class I malocclusion – A case report

A 13year old male patient presented with buccally erupted maxillary and mandibular canines, upper and lower anterior proclination, and Class I molar relationship. First premolar extraction with maximum anchorage protocol was planned to correct the crowding. Segmental arch mechanics with T-loop (0.017 × 0.025" TMA) were used to correct the buccally erupted canines. At the end of treatment patient’s soft tissue profile and smile improved significantly. Crowding and proclination of maxillary and mandibular arch were corrected and class I molar relationship was maintained.