Large-scale Collapse Research Articles

Experimental study on progressive collapse mechanism in braced and tied-back retaining systems of deep excavations

Collapses of braced or tied-back excavations have occurred frequently. However, the influence of the failure of some retaining structure members on the overall safety performance of a retaining system has not been studied. Model tests of failures of retaining piles, struts or anchors were conducted in this study and the load-transfer mechanisms underlying these conditions were analysed. When failures or large deformations occurred in certain piles, the increasing ratios of the bending moments in adjacent piles were much larger in the braced retaining system than in the cantilever system and more easily triggered progressive failure. When the strut elevation was lower or the excavation depth was greater, the degree of influence and range of pile failures became larger. When certain struts or anchors failed, their loads transferred to a few adjacent struts or anchors, possibly leading to further strut or anchor failure. The influence mechanisms of strut or anchor failure on piles were different from those of pile failure. As the number of failed struts or anchors increases, the bending moments of the piles in the failure zone first decrease and then increase to very high values. Therefore, the progressive failure path extends from struts or anchors to piles and will lead to large-scale collapse.

Application of Structure-from-Motion Terrestrial Photogrammetry to the Assessment of Coastal Cliff Erosion Processes in SW Spain

Del Río, L.; Posanski, D.; Gracia, F.J. and Pérez-Romero, A.M., 2020. Application of structure-from-motion terrestrial photogrammetry to the assessment of coastal cliff erosion processes in SW Spain. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1057–1061. Coconut Creek (Florida), ISSN 0749-0208.The episodic and irregular nature of cliff changes makes their assessment a challenging task, essential in order to cope with risks related to cliff erosion. In this work, structure-from-motion terrestrial photogrammetry was used to measure erosion processes in the Torre Bermeja cliff in SW Spain, where cliff instability poses a threat to people walking along the fronting beach. Two sites were investigated on 14 photogrammetric surveys conducted along a 9-month period in order to detect morphological changes in relation to external forces. The erosive events at both sites showed a strong episodic character, with some short periods reflecting intensive erosion and long periods of stability. Average figures for the observation period indicate a mean retreat of 2.8 mm/day at one of the sites and 0.5 mm/day at the other site. The differences were attributed to variations in the lithology and a higher wave exposure of the first site, due to a smaller fronting beach. A comparison of erosion and meteorological data of the survey period showed a positive correlation between mean retreat and maximum rainfall intensity. However, the storms with highest wave heights did not significantly affect the cliff foot, as most wave-related erosion occurred when a modal storm coincided with a high spring tidal range. Therefore, rainfall can be considered as the trigger for most of the erosion events at the study site, although the unstable conditions required for large-scale collapses are generated by wave attack in the first place. In this respect, it is assumed that climate change will lead to higher retreat rates in the investigated cliff, due to a rising mean sea level and a higher frequency of heavy rainfall events.

Stability evaluation of flat large-span cavern in jointed rock mass

The excavation of the rock mass will have a great influence on the characteristics of the caverns under extremely bad geological conditions. According to the characteristics of the engineering geology, several groups of the caverns are designed to be excavated in the rock mass with three sets of through joints. Based on the summary of the failure mechanism and the failure pattern, the classification method of the failure modes is done for the flat large-span underground cavern with three sets of through joints. The classification method takes into account the control factors of the surrounding rock instability, failure mechanism, distribution of the disturbed zone, and characteristics of the rock weakness, etc. The failure modes of the flat large-span caverns contain six types: the relatively stable, the partial area falling, and the partial collapse of vault, the zigzag failure of rock stratum, the V-shaped failure, and the large-scale collapse. The change law of the vertical stress is first increasing and then decreasing for the surrounding rock in the disturbed zone. The horizontal stress shows an exponential change trend with an increase in the distance from the vault. Influenced by the failure degree of the caverns, the change law of the vertical displacement for the vault can be divided into two types. The horizontal displacement decreases with an increase in the distance from the sidewall, and there is an exponential relationship between them.

Predicting Hard Rock Pillar Stability Using GBDT, XGBoost, and LightGBM Algorithms

Predicting pillar stability is a vital task in hard rock mines as pillar instability can cause large-scale collapse hazards. However, it is challenging because the pillar stability is affected by many factors. With the accumulation of pillar stability cases, machine learning (ML) has shown great potential to predict pillar stability. This study aims to predict hard rock pillar stability using gradient boosting decision tree (GBDT), extreme gradient boosting (XGBoost), and light gradient boosting machine (LightGBM) algorithms. First, 236 cases with five indicators were collected from seven hard rock mines. Afterwards, the hyperparameters of each model were tuned using a five-fold cross validation (CV) approach. Based on the optimal hyperparameters configuration, prediction models were constructed using training set (70% of the data). Finally, the test set (30% of the data) was adopted to evaluate the performance of each model. The precision, recall, and F1 indexes were utilized to analyze prediction results of each level, and the accuracy and their macro average values were used to assess the overall prediction performance. Based on the sensitivity analysis of indicators, the relative importance of each indicator was obtained. In addition, the safety factor approach and other ML algorithms were adopted as comparisons. The results showed that GBDT, XGBoost, and LightGBM algorithms achieved a better comprehensive performance, and their prediction accuracies were 0.8310, 0.8310, and 0.8169, respectively. The average pillar stress and ratio of pillar width to pillar height had the most important influences on prediction results. The proposed methodology can provide a reliable reference for pillar design and stability risk management.

How do bound star clusters form?

ABSTRACT Gravitationally bound clusters that survive gas removal represent an unusual mode of star formation in the Milky Way and similar spiral galaxies. While forming, they can be distinguished observationally from unbound star formation by their high densities, virialized velocity structures, and star formation histories that accelerate towards the present, but extend multiple free-fall times into the past. In this paper, we examine several proposed scenarios for how such structures might form and evolve, and carry out a Bayesian analysis to test these models against observed distributions of protostellar age, counts of young stellar objects relative to gas, and the overall star formation rate of the Milky Way. We show that models in which the acceleration of star formation is due either to a large-scale collapse or a time-dependent increase in star formation efficiency are unable to satisfy the combined set of observational constraints. In contrast, models in which clusters form in a ‘conveyor belt’ mode where gas accretion and star formation occur simultaneously, but the star formation rate per free-fall time is low, can match the observations.

Simulation of Alfvénic avalanche onset in NSTX

In some tokamak discharges, a number of Alfvén eigenmodes (AEs) have been observed to cause a large-scale collapse of the high energy particle distribution, a phenomenon referred to as an avalanche. We examine the necessary conditions for an avalanche using the available experimental information from NSTX on the equilibrium and mode properties for two cases, one with a measurable but benign AE activity and one with an AE activity leading up to an avalanche. To produce an avalanche, the modes present in the discharge must possess resonances that can overlap with a modest increase in instability magnitude, providing a path to global particle stochastic motion. We find that the modes present in the avalanche-free discharge do not provide such a path even at a very large amplitude. During the discharge which subsequently produces an avalanche, the high energy population is growing and the Alfvén frequency is dropping due to increasing density, and we find that both these changes, producing a small increased drive or an increased resonance width for the Alfvén modes, can lead in this case to uncontrolled mode growth and large-scale beam particle loss.

Performance evaluation on effects of all types of infill against the progressive collapse of reinforced concrete frames

Progressive collapse in a building has caused local damage, so it spreads across the system and causes large-scale collapse of the entire building. Progressive collapse is usually due to fire, gas explosion, terrorist attack, vehicle collisions, and misplaced design and construction. Therefore, it is necessary to study and investigate the effect of this phenomenon on structures and to rebuild the building against it. In recent years, much research has been conducted on the effect of infills against progressive collapse, but, in this study, the effect of all types of infill performance is investigated and which infills has best performance against progressive collapse. In this research, we examined and evaluated some proposed solutions in technical literature for four simple frame modes and considering the intermediate brick conditions, as well as the use of a plate and wall of a 3D panel and their comparison. In the present study, a five-story frame using two-dimensional Opensees software is retrofitted using the above techniques, and the results show that it reduces vertical displacement and also improves the re-distribution of forces.

Magmatically folded and faulted schlieren zones formed by magma avalanching in the Sonora Pass Intrusive Suite, Sierra Nevada, California

AbstractThe southwestern margin of the Late Cretaceous Sonora Pass Intrusive Suite, northern Sierra Nevada, California (USA), preserves a densely populated zone of magmatic structures that record dynamic magmatic layer formation and deformation (faulting and folding) within a solidifying upper-crustal magma mush. This zone consists largely of coupled melanocratic (or schlieren) and leucocratic bands hosted within the 95.6 ± 1.5 Ma Kinney Lakes granodiorite (Leopold, 2016), with orientations approximately parallel to the intrusive margin and with inward younging directions. Schlieren consist of a high modal abundance of medium-grained ferromagnesian minerals (hornblende + biotite), zircon, sphene, apatite, opaque minerals, and minor plagioclase and interstitial quartz. Leucocratic bands are dominated by coarse-grained feldspar + quartz with minor ferromagnesian and accessory minerals. Whole-rock geochemical and Sr and Nd isotopic data indicate that the schlieren are derived from the Kinney Lakes granodiorite by effective mechanical separation of mafic minerals and accessory phases.We interpret that the schlieren zone at the margin of the Kinney Lakes granodiorite formed by large-scale collapse of crystal mush by “magma avalanching,” facilitated by gravity, local convection, and possibly by host-rock stoping at the margin. This process eroded a significant portion of the solidifying margin of the chamber and resulted in the formation of magmatically deformed layered structures, which experienced further mingling, re-intrusion, magmatic erosion, and recycling processes. We envisage that magma avalanching of magma mushes in plutons can be achieved by any unstable process (e.g., tectonic, fluid-assisted, stoping, or gravity-driven) in large, long-lived magma-mush chambers.

A Comprehensive Evaluation Approach for Static Voltage Stability Analysis in Electric Power Grids

Large-scale voltage collapse incidences, which result in power outages over large regions and extensive economic losses, are presently common occurrences worldwide. Therefore, the voltage stability analysis of power systems has become a topic of increasing interest. This article presents a comprehensive method for conducting static voltage stability analysis in electric power systems. To overcome the limitations associated with single-index systems in the evaluation of static voltage stability, the analysis approach employs a multi-index system with three primary criteria based on separate analysis methods with six sub-criteria based on individual indices. In addition, we propose a comprehensive method for establishing index weights, which combines the subjective analytic hierarchy process weighting method and the objective entropy weighting method. An innovative index-weight optimization method based on the Lagrange Conditioned Extreme Value (LCEV) is also presented and sensitive analysis is applied to test the robust of the proposed method. Finally, Fuzzy Order Preference by Similarity to Ideal Solution (Fuzzy-TOPSIS) method is employed to rank the voltage buses of a power system as the final results, considering system functionality and proportionality. The results obtained for the wind farms of an actual power grid in Hami City, China demonstrate that the proposed method represents an innovative approach for determining the weakest bus in static voltage stability analysis of power systems.

Failure pattern of large-scale goaf collapse and a controlled roof caving method used in gypsum mine

Physical model tests were first performed to investigate the failure pattern of multiple pillar-roof support system. It was observed in the physical model tests, pillars were design with the same mechanical parameters in model #1, cracking occurred simultaneously in panel pillars and the roof above barrier pillars. When pillars 2 to 5 lost bearing capacity, collapse of the roof supported by those pillars occurred. Physical model #2 was design with a relatively weaker pillar (pillar 3) among six pillars. It was found that the whole pillar-roof system was divided into two independent systems by a roof crack, and two pillars collapse and roof subsidence events occurred during the loading process, the first failure event was induced by the pillars failure, and the second was caused by the roof crack. Then, for a multiple pillar-roof support system, three types of failure patterns were analysed based on the condition of pillar and roof. It can be concluded that any failure of a bearing component would cause a subsidence event. However, the barrier pillar could bear the transferred load during the stress redistribution process, mitigating the propagation of collapse or cutting the roof to insulate the collapse area. Importantly, some effective methods were suggested to decrease the risk of catastrophic collapse, and the deep-hole-blasting was employed to improve the stability of the pillar and roof support system in a room and pillar mine.

Aseismic Mining Subsidence in an Abandoned Mine: Influence Factors and Consequences for Post-Mining Risk Management

In the Lorraine area of eastern France, decades of iron-ore mining from 1850 to 1997 have left vast underground cavities beneath or in the vicinity of urban areas. Several major collapses occurred in the southern part of this iron-ore basin in the 1990s, after the mine closure and the flooding of underground mine workings. Following these large-scale collapses, the French government initiated a strategy of post-mining risk management to prevent and control risks associated with these ground failures. The high-risk zones are secured either by reducing the vulnerability while the moderate risk zones are monitored for public safety purposes by using in situ monitoring. This monitoring relies mainly on real-time microseismic systems, to detect precursors to a rapid large-scale collapse. After the progressive closing and then flooding of the northern iron basin ending in 2008, subsidence was observed in a town of the Lorraine basin in autumn of 2009. However, this local subsidence, with a low velocity of few centimeters per month, was not clearly detected by the borehole microseismic monitoring station located nearby. Only some microseismic events were recorded, which could not be unambiguously related to the beginning of the subsidence event. To better understand this lack of microseismic precursor a geophysical investigation was launched. A calibration blast experiment was carried out from a remaining old underground access in order to answer to the following questions: (1) what is the seismic wave attenuation field?; (2) what is the minimum source power that can be detected by the sensors?; (3) what is the impact of the geology, the faults corridor and the integral pillar extraction zone on the wave propagation field? The results of this study show strong anelastic attenuation of the seismic waves though the monitored overburden most likely related to the extensive fault system intersecting the study site. Strong attenuation might explain the lack of detected microseismicity during the subsidence event. In order to clarify this issue, a mobile GPS monitoring system was designed and tested to address this type of situation in future.

Анализ развития системной аварии в ОЭС Востока 1 августа 2017 г.

The large-scale collapse occurred in the Russia’s UES East Power Pool caused the power pool to separate into two isolated sub-areas at the cut set “Transition through the Amur River”. Generally, the collapse was mainly attributed to poorly coordinated operation of the power pool elements, including the set of closed-loop and automatic control devices. The key factors and events involved in the course of this system collapse are analyzed. Explanations of certain events and processes specific to this collapse are suggested. Conclusions and recommendations derived from the analysis of this system collapse are formulated. To ensure coordinated development of different components of the East Power Pool and their coordinated operation under the conditions when there are many entities participating in these processes, it is necessary to develop and adopt a number of regulatory documents, one of which is the “Regulations for Process Operation of Electric Power Systems”.

Slow Recovery from Local Disturbances as an Indicator for Loss of Ecosystem Resilience

A range of indicators have been proposed for identifying the elevated risk of critical transitions in ecosystems. Most indicators are based on the idea that critical slowing down can be inferred from changes in statistical properties of natural fluctuations and spatial patterns. However, identifying these signals in nature has remained challenging. An alternative approach is to infer changes in resilience from differences in standardized experimental perturbations. However, system-wide experimental perturbations are rarely feasible. Here we evaluate the potential to infer the risk of large-scale systemic transitions from local experimental or natural perturbations. We use models of spatially explicit landscapes to illustrate how recovery rates upon small-scale perturbations decrease as an ecosystem approaches a tipping point for a large-scale collapse. We show that the recovery trajectory depends on: (1) the resilience of the ecosystem at large scale, (2) the dispersal rate of organisms, and (3) the scale of the perturbation. In addition, we show that recovery of natural disturbances in a heterogeneous environment can potentially function as an indicator of resilience of a large-scale ecosystem. Our analyses reveal fundamental differences between large-scale weak and local-scale strong perturbations, leading to an overview of opportunities and limitations of the use of local disturbance-recovery experiments.

Sea Level Fingerprints in a Region of Complex Earth Structure: The Case of WAIS

Abstract Sea level fingerprints associated with rapid melting of the West Antarctic Ice Sheet (WAIS) have generally been computed under the assumption of a purely elastic response of the solid Earth. The authors investigate the impact of viscous effects on these fingerprints by computing gravitationally self-consistent sea level changes that adopt a 3D viscoelastic Earth model in the Antarctic region consistent with available geological and geophysical constraints. In West Antarctica, the model is characterized by a thin (~65 km) elastic lithosphere and sublithospheric viscosities that span three orders of magnitude, reaching values as low as approximately 4 × 1018 Pa s beneath WAIS. Calculations indicate that sea level predictions in the near field of WAIS will depart significantly from elastic fingerprints in as little as a few decades. For example, when viscous effects are included, the peak sea level fall predicted in the vicinity of WAIS during a melt event will increase by about 20% and about 50%, relative to the elastic case, for events of duration 25 and 100 yr, respectively. The results have implications for studies of sea level change due to both ongoing mass loss from WAIS over the next century and future, large-scale collapse of WAIS on centennial-to-millennial time scales.

COLLAPSE RISK PREDICTION OF BUILDINGS ON FIRE USING KEY ELEMENT INDEX

It is difficult to estimate the redundant strength remaining in a building on fire, since the fire conditions such as their ranges and locations may be different in each case. A large-scale fire occurred on the New York World Trade Center 7 (WTC-7) after the 9.11 terrorist attacks in 2001. The building continued to burn for 7 hours, and ended in a total collapse. The official report on the collapse investigation of WTC-7, released by the National Institute of Standards and Technology (NIST) in 2005, had suggested that the total collapse was triggered by a severe damage of an important column of the building called the key element. On the other hand, there is also an example of a high-rise building on fire that avoided a total collapse, regardless of its main structure continuously heated by fire for a long time; the Windsor building of Madrid in 2005. From the viewpoint of preventing the fire-induced collapse of buildings, it is necessary to clarify the relationship between various structural parameters of buildings, fire locations, their ranges, and the scale of collapse. The purpose of this study is to predict collapse risks of buildings on fire using a key element index (KI). The KI is defined as the ratio of the ultimate yield strengths of the structure with one column eliminated and the initial, undamaged structure. It indicates the contribution of a structural column to the vertical capacity of the structure. We investigated the relationship between the sum of KI values in various cases of fire range and the sum of the height of remains after the collapse. We applied an adaptively shifted integration (ASI) - Gauss code utilizing linear Timoshenko beam elements to investigate the collapse behaviors of a ten-story steel framed building model with various fire patterns. Fracture contact, contact release and re-contact algorithms were implemented in the code. The reduction curves of elastic modulus and yield strength of steel related to elevated temperature, shown by NIST, were adopted to consider the heat effect of fire. Thermal expansion of materials was also considered. From the numerical results of the fire-induced collapse analyses, it is found that the scale of collapse highly depends on fire ranges and locations. The risk of total collapse tends to increase when a large range of fire occurs in the lower layer. In addition, the risk increases much higher in those cases when fire occurs at the peripheral area of the building, compared to those cases at the inner area. The sum of heights of remains seems to depend on the sum of KI values, and there is a specific threshold of the sum of KI values that relates to the initiation of a large scale collapse. Therefore, the sum of KI values may be used to predict and take measures to avoid the risk of collapse under various fire conditions.

Stability Analysis of Steel Lining at Pressure Diversion Tunnel Collapse Zone during Operating Period

At the collapse zone, the effects of the thickness of the consolidation grouting layer and the water pressure on the steel lining are vital to the stability of steel-lined pressure diversion tunnels. In this paper, a joint element and the load-sharing ratio of the consolidation layer are introduced to investigate the joint load-bearing characteristics of the steel lining and the consolidation layer and to determine a suitable consolidation layer thickness; a coupling method for simulating the hydromechanical interaction of the reinforced concrete lining is adopted to investigate the effect of internal water exosmosis on the seepage field at the collapse zone and to determine the external water pressure on the steel lining. In the case of a steel-lined pressure diversion tunnel, a numerical simulation is implemented to analyse the effect of the thickness of the consolidation layer and the distribution of the seepage field under the influence of internal water exosmosis. The results show that a 10 m thick consolidation layer and the adopted antiseepage measures ensure the stability of the steel lining at the collapse zone under internal and external water pressure. These research results provide a reference for the design of treatment measures for large-scale collapses in steel-lined pressure tunnels.

Hierarchical Star Cluster Assembly in Globally Collapsing Molecular Clouds

We discuss the mechanism of cluster formation in a numerical simulation of a molecular cloud (MC) undergoing global hierarchical collapse (GHC). The global nature of the collapse implies that the SFR increases over time. The hierarchical nature of the collapse consists of small-scale collapses within larger-scale ones. The large-scale collapses culminate a few Myr later than the small-scale ones and consist of filamentary flows that accrete onto massive central clumps. The small-scale collapses form clumps that are embedded in the filaments and falling onto the large-scale collapse centers. The stars formed in the early, small-scale collapses share the infall motion of their parent clumps. Thus, the filaments feed both gaseous and stellar material to the massive central clump. This leads to the presence of a few older stars in a region where new protostars are forming, and also to a self-similar structure, in which each unit is composed of smaller-scale sub-units that approach each other and may merge. Because the older stars formed in the filaments share the infall motion of the gas onto the central clump, they tend to have larger velocities and to be distributed over larger areas than the younger stars formed in the central clump. Finally, interpreting the IMF at face-value as a probability distribution implies that massive stars only form once the {\it local} SFR is large enough to sample the IMF up to high masses. In combination with the increase of the SFR, this implies that massive stars tend to appear late in the evolution of the MC, and only in the central massive clumps. We discuss the correspondence of these features with observed properties of young stellar clusters, finding very good qualitative agreement, thus providing support to the scenario of global, hierarchical collapse of MCs, while explaining the origin of the observed cluster structure.

Assessing Democratic Governance in the Nigerian Fourth Republic, 1999 -2014

The paper assesses democratic governance in the Nigerian fourth republic. Most Nigerians expected the return to democratic governance on May, 29 1999 to set the stage for the socio-economic and political transformation of the country. The paper contends that the democratic experience has been problematic for Nigerians. The paper, underpinned by the elite theory, is descriptive, historical and analytical in nature and employs the secondary source in the gathering of data. The paper argues that the uninterrupted character of the democratic transition, improvement in civil and political rights and some economic development dividends are positive outcome of the democratic experience. Yet, the successes associated with the democratic experience seem to have been overshadowed by poor governance, the problems of poverty, de-industrialization, widespread unemployment, large scale collapse of infrastructure, illiteracy, insecurity of lives and property, political corruption, and weak governance institutions. Consequently, public trust in the democratic process, institutions and actors is in decline. The paper suggests amongst others, responsible and effective political leadership and civil society as critical to a productive democratic performance in Nigeria.

Abrupt onset of tongue deformation and phase space response of ions in magnetically-confined plasmas.

An abrupt onset of the new tongue-shaped deformation of magnetic surface in magnetized plasmas, which was conjectured in since the 1960s but has not been observed, is experimentally identified just before an abrupt onset of a large-scale collapse event. Two novel properties of the event are identified. First, the transition of symmetry of perturbation (rather than a growth of linearly unstable MHD modes) was found to be a key for the onset of abrupt collapse, i.e., the transition of symmetry gives a new route to the collapse from stable state. Second, as a phase-space response of ions, the distortion from Maxwell-Boltzmann distribution of epithermal ions was observed for the first time.

Nonlinear Interaction of Edge-Localized Modes and Turbulent Eddies in Toroidal Plasma under n=1 Magnetic Perturbation.

The effect of static n=1 resonant magnetic perturbation (RMP) on the spatial structure and temporal dynamics of edge-localized modes (ELMs) and edge turbulence in tokamak plasma has been investigated. Two-dimensional images measured by a millimeter-wave camera on the KSTAR tokamak revealed that the coherent filamentary modes (i.e., ELMs) are still present in the edge region when the usual large scale collapse of the edge confinement, i.e., the ELM crash, is completely suppressed by n=1 RMP. Cross-correlation analyses on the 2D images show that (1)the RMP enhances turbulent fluctuations in the edge toward the ELM-crash-suppression phase, (2)the induced turbulence has a clear dispersion relation for wide ranges of wave number and frequency, and (3)the turbulence involves a net radially outward energy transport. Nonlinear interactions of the turbulent eddies with the coexisting ELMs are clearly observed by bispectral analysis, which implies that the exchange of energy between them may be the key to the prevention of large scale crashes.