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Vibrating boundaries are widely encountered, for example, between soil and bedrock during earthquake shaking. We understand that vibration of such boundaries can lead to instabilities in granular media with many applications to geological hazards, such as liquefaction and landslides, and during geological engineering applications. Although numerous studies have been dedicated to revealing the behavior of granular flows under various flowing regimes, the significance of vibrating boundaries remains an open problem. To fill this gap, we introduce a vibrating base boundary into the collapse of a granular column with a numerical scheme. To understand the role of fluids, we contrast the behavior of granular flows under dry and fluid-saturated conditions. From the simulations, the development of anisotropy in spatial inter-grain contact force distribution is studied. The fluid-saturated condition is achieved via a two-way coupled CFD-DEM method. From these simulations, a scaling law of granular flow is derived for vibrating boundaries. We illustrate for the first time the energy evolution of the granular system with vibrating boundaries. This work demonstrates the role of vibration in increasing the runout distance and the maximum kinetic energy of granular flows, this suggests a link between the mesoscale inter-grain responses and macro-scale dynamics of granular geological hazards triggered by earthquakes. Additionally, the spatial distribution of inter-grain contact forces is presented under dry and fluid-saturated conditions to indicate the anisotropic development inside the granular assembly.

The acoustic emission (AE) technique can be used to locate failure-induced AE sources and provides an alternative to continuously visualise the development of failures inside stressed materials; however, a literature review reveals almost no studies regarding its application for source location in granular materials. Yet the visualisation of ubiquitously observed strain localisation and shear banding is critical to an in-depth understanding of the progressive failure mechanism of granular materials. In this paper, an original AE source three-dimensional (3D) tracing code based on the idea of ‘time difference of arrival’ (TDOA) was developed using Matlab programming, for the first time, to continuously visualise particle-scale interactions involved in saturated granular soils. After validating its feasibility through a series of pencil lead break (PLB) tests, the developed AE source 3D tracing code was applied to saturated granular soils subjected to drained triaxial shearing. Both the results of PLB tests and drained triaxial shearing tests demonstrate a good consistency between the traced AE source and actual PLB/particle interaction-induced sources. This suggests that the developed AE source 3D tracing code could be used to visualise the initiation and evolution of strain localisation and shear banding in saturated granular soils subjected to drained triaxial shearing.

Since the mechanical failure of granular soils is the macro-manifestation of complex internal particle interactions, studying this relevant issue from micro-scale perspective is of fundamental importance. Acoustic Emission (AE) has become a promising approach for this purpose; however, its synthetic and validating study on granular soils with various ground conditions is far from substantive. In an attempt to verify and correlate the relationship between AE behaviors and mechanical behaviors of granular soils with various conditions, results of drained triaxial compression tests along with AE measurements recorded during testing are discussed. These results revealed intrinsically different propagation properties of acoustic waves between saturated and unsaturated/dry sands. The water in saturated sands facilitates acoustic waves to propagate with less attenuation, whereas discontinuities (i.e. particle-particle contacts and particle interfaces with water) in dry/unsaturated sands lead acoustic waves to propagate with larger attenuation. The AE manifestation showed its high sensitivity to different saturation degrees, package densities and confining stress levels, which allows the evaluation of ground conditions to be made by AE. The congruous AE evolution in saturated sands was considered closely related to particle interactions during drained triaxial compression; whereas the changeable AE evolutions in accordance with different failure patterns in dry sands were considered relating to soil structure behaviors.

Particle breakage is a common occurrence in granular systems when the external stress exceeds the individual particle strength. A large number of experimental evidences suggested that particle breakage may significantly influence the soil behavior. In the case of pile foundations, the subsoil below the pile tip experiences considerable high stress and consequently prone to break. Due to the lack of sufficient understanding on particle breakage mechanism, there is currently no consentaneous theoretical background for particle breakage analysis during the pile penetration process. This study aims to clarify the location of particle breakage and its evolving characteristics with the aid of acoustic emission (AE) source location method. The spatial distribution of AE hypocenters is interpreted to be associated with the mechanism of particle breakage. Results showed that the AE sources were not uniformly distributed, but concentrated within certain zones below the pile tip. This AE concentration zone was pushed downward with the advancing pile tip, and its distance from the real time pile tip position decreased after certain depth of pile penetration. The location of particle breakage interpreted from AE source location was verified with posttest excavations and the insights on the particle breakage evolution zone were further discussed. Keywords: Acoustic emission, Source location, Sand, Particle breakage, Pile

Acoustic emission (AE) technique that is capable of diagnosing the failure process of stressed materials has rarely reported its application to sandy soils subjected to triaxial compression. In this paper, drained triaxial compression tests incorporating with a high-performance AE measurement system were conducted for dry sands with different confining stresses and initial relative densities. Generally, an increased confining stress or initial relative density generates more acoustic emissions, while there also exist exceptions due to different failure patterns. A good resemblance between stress–strain and AE hit rate–strain relations was observed, and power functions between the mechanical parameters and AE hit rate were well established regardless of different confining stresses and initial relative densities. Besides, the behavioral state of yield and peak during compression could be also evaluated by AE hit rate, compared with conventional stress–strain determination. Particularly, the peak AE hit rate is found not always synchronous to but fluctuating at around the peak stress depending on different failure patterns, which might provide beneficial insights into the incipient failure of sands. The present good consistencies suggest that AE characteristics could be used as alternative parameters to evaluate and even predict the mechanical behavior of dry sands.

There is increasing concern regarding local concentrated erosion scenarios such as piping and underground cavities due to their catastrophic consequences. In this study, local concentrated erosion was reproduced by placing shaped erodible columns within the triaxial specimens during sample preparation. The preplaced erodible columns (made of glucose) were set similar in regard to their volume fraction but different in configurations. Bender element measurements were performed in dry, the erosion on-going, and post erosion states to evaluate the mechanical behavior of the samples subjected to concentrate particle removal. The influence of concentrated particle erosion on the shear wave velocity, shear strength, dilatancy and peak angle of shearing resistance was revealed and discussed. It is generally concluded that the resultant effect on the mechanical behavior of soils is greater when erodible particles are more localized within a limited region, especially for densely packed soils.

The study of failure mechanisms from a microperspective demands a comprehensive understanding of the initiation and evolution of shear banding within granular materials. However, this topic is not fully understood due to the technical constraints in continuous quantification of the failure degree and visualization of particle interactions within soil specimens due to their inherent opacity. This paper reports the possibility of acoustic emission (AE) technique in characterizing the micromechanical behavior of saturated coral sands subjected to drained triaxial shearing. Results show that coral sand is less emissive than silica sands, while its AE rate forms concrete mathematic relations with soil mechanical parameters, which agrees with those revealed for silica sands. This allows soil mechanical parameters of coral sands that are difficult to be assessed in the field to be back-evaluated by acoustic measuring. The traced AE source locations confirm their spatiotemporal correspondence with the photographed appearance of deformed specimens, in which complex shear banding process, regarding the initiation and evolution, is traced. Three acoustic precursors are observed before the complete soil failure and are considered resulting from the structural variation that affects the emission and propagation of AEs, which hence could be used as indicators for early warning of soil’s instability.

Shallow foundation is a common foundation type that is usually used for small to medium size structures. The bearing ability and the failure mechanism of shallow foundation are the fundamental concerns for geotechnical engineers, and the demand for new insights into the relevant issue is still increasing. This paper presents an acoustic emission (AE) characterization of the failure process of shallow foundation, with the aim of revealing the fundamental information on AE signals associated with shallow foundation loading as well as its connection with the ground bearing behavior. Experiments were carried out to model the failure process of shallow foundation resting on sandy ground with different densities (i.e. loose and dense) and subjected to different loading conditions (i.e. monotonic and cyclic loading). Comparisons between AE activities and ground bearing behavior are presented. The feasibility of using AE for stability monitoring of shallow foundation is revealed and discussed.

Understanding the subsoil behavior subjected to group pile loading is of foundation important to clarify the group pile behavior. The acoustic emission (AE) signals produced by the stressed subsoil subjected to group pile loading are expected to contain useful information about the behavior of group pile. In this study, measurements of AE signals generated during group pile loading were performed. To evaluate the effect of pile spacing, group pile loading tests were performed with two different pile spacing to represent strong and weak group effects. Results showed that the group pile in case of narrow spacing might significantly reduce the apparent stiffness of the ground, which was evidenced by lower values of ground secant modulus and the released AE energy. In addition, center pile carried more load and emitted higher level of AE compared with other pile positions when the pile spacing was narrow. However, it is observed that such group pile effect was affected by the loading history as well. Considering the high consistency between the results of conventional load-settlement measurement and the AE monitoring, it is therefore suggested that the AE monitoring can be potentially used as an alternative method for evaluation of the group pile behavior.

The application of the acoustic emission (AE) method for granular media has been considered to be challenging and remains limited. Nevertheless, recent successful explorations from different aspects have greatly encouraged people's attempts in this filed. Typical applications concerning geotechnical element testing, the soil-structure systems, as well as the AE source location suggest that the recent studies aim to obtain direct insights into the mechanical behaviors of the granular soils based on AE features, and emphasize on the quantitative relations between them. Future development of the AE method demands further advances in the following aspects: higher performance of AE instrumentation technology enabling high signal-to-noise ratio; full waveform and broadband signal data acquisition; new insights into the source mechanisms of AE by incorporating more comprehensive and in-depth signal analysis and emerging new technologies; higher levels of correlation study between AE signals and the associated physical processes.

Particle breakage is a common phenomenon in granular soils and has been well observed during laboratory and in-situ pile studies. In this paper, an image analysis approach was developed to ...

Acoustic emission (AE) testing is regarded as an effective non-destructive technique and is capable of detecting micro-level defects inside a material. In the field of civil engineering, the acoustic emission technique has been widely applied to the studies of steelwork, concrete, and composite materials. However, the geophysical investigations of the application of acoustic emission, dealing with porous granular media, are comparatively limited. In this study, the acoustic emission testing is implemented in a model pile system to investigate subsoil behaviour subjected to pile penetration. The results reveal that the tendency of acoustic emission settlement and load settlement shows high similarity. In addition, the detected acoustic emission signals are studied in the frequency domain using fast Fourier transformation. Higher frequency acoustic emission signals (>100 kHz) are interpreted to be associated with sand particle crushing, which provides a new insight to evaluate the feature of sand grain crushing. Furthermore, the distinction of acoustic emission characteristics observed among different pile penetration sequences demonstrates the effect of ground density on the subsoil behaviour. The results obtained in this paper are beneficial to further clarify the bearing mechanism of pile foundations and also to provide useful information on the fundamental characteristics of acoustic emission signals originating from stressed granular soils, that can be extended to other acoustic emission-based field investigations.

Iron ore fines (IOFs) are indispensable material for industries with over million tons transported annually by sea. The transportation involves serious risk of marine accidents due to cargo liquefaction caused by the loss of shear strength under cyclic loads. In this work, the effect of moisture content on the unsaturated shear strength of the IOFs was analyzed, including the matric suction and the effective normal stress. The cyclic strength of the IOFs as a function of moisture content and number of cycles was presented, as well as a correlation between the cyclic strength ratios in the field and laboratory tests. Moreover, dynamic geotechnical centrifuge model tests were conducted to mimic the field condition and verify the method for liquefaction potential evaluation of the IOFs. The results showed that moisture content was the critical factor affecting the shear strength of the IOFs. The zones with high liquefaction potential were identified according to the cyclic strength ratio and the cyclic shear stress ratio along the depth. The evaluation results agreed well with those from the dynamic geotechnical centrifuge model tests. This work provided an approach to accurately evaluate the liquefaction potential of the IOFs utilizing the variation of moisture content during shipping.

Pile-anchor structures are now widely used in major slope reinforcement projects, but research on the seismic response of slopes reinforced by these structures has been very limited. This paper presents the results of a series of 50g dynamic centrifuge tests that reproduce the seismic response of a slope reinforced by a pile-anchor structure exposed to an earthquake-induced stress field. A silica gel model of a slope is used to conduct multiple tests. Long-period wave amplitude amplification and frequency filtering are observed after spectral analysis. Pile bending moments and anchor tensions were recorded before the earthquake, at the moment of peak ground acceleration, and after the earthquake and the data show that the pile is the main load-bearing structure. The anchor aids in keeping the pile from tilting outward to improve the durability of the pile. Pile-anchor structure failure is caused by anchor relaxation, pile bending, and slope displacement after the passage of multiple seismic waves. A suggestion for pile-anchor structure design is provided. This paper provides a useful reference for future research on the interactions between support structures and a slope during and after earthquakes. The conclusions provided by these tests lay the foundation for designing pile-anchor reinforcement structures for better dynamic performance.

The pile-anchor structure is one of the most commonly used and effective methods in slope treatment. However, the dynamic behavior of slope reinforcement using a pile-anchor structure remains poorly understood. Performance assessment of the reinforcement system under seismic loading is therefore critical for reinforcement design. In this study, a numerical model of slope reinforced by pile-anchor structure, together with the analysis procedure, is briefly discussed. We consider four different reinforcement schemes to examine the effectiveness of pile-anchor reinforcement under seismic loading. Finite element analysis is used to derive the dynamic response, and Newmark sliding block analysis is employed to examine the stability of the reinforced slope. The performance assessment results indicate that the pile-anchor structure presents significant advantages of slope deformation reduction with a more reasonable internal force distribution and increased slope stability.

Particle breakage has been recognized as a crucial factor affecting the mechanical behavior of stressed granular assemblages. To understand such underlying micro-mechanical behavior, Acoustic Emission (AE) technique that is capable of continuously diagnosing the deterioration and failure process of stressed materials was employed into single particle compression tests on silica sands. Regardless of different particle sizes, the fracturing process could be highly featured by AE characteristics, in which AE hit rate and peak frequency characteristics were analyzed to evaluate the intensity and mode of micro-mechanical behaviors, respectively. "Early warning omens" regarding the impending failure of the stressed particle is revealed in terms of the initiation and rapid increase of high-frequency AE components, as well as the rapid increase of AE hit rate. The effect of "prehistory of failure" on the stressed particle is sensitively featured by the highly emitted AE events after the catastrophic failure. Furthermore, a frequency-based method is suggested to distinguish different modes of micro-mechanical behaviors associated with particle readjustment, asperity abrasion, and microcracking. Further employment of the present result is expected to continuously evaluate the intensity and mode of particle interactions in stressed granular assemblages.

The response of the subsoil subjected to pile loading is crucial to clarify the bearing mechanism of pile foundations. This study presents a novel acoustic emission (AE) method to monitor the subsoil behavior in a model pile testing system. The AE testing aims to capture the “micro-noises” released from sand grain dislocation and crushing around the pile shaft during penetration. The correlations between the pile settlement and the AE characteristics including count, amplitude and energy are revealed and discussed, highlighting that the ground density and the shear zone formed during pile penetration mainly affect the AE behavior. The results also suggest that the yielding of ground can be determined based on the development of the AE activity. The technique shows promise as an in-situ methodology for monitoring of subsoil behavior during the process of pile loading.

The fragmentation of single particle is one of the common observations in stressed soils. Individual particles are generally randomly shaped and exhibit twisted and discrepant fracture features. Conventional descriptions based on either strength prediction or comminution energy estimation are more or less statistical, which inevitably brings difficulties to understand the physical mechanism of fragmentation. As an attempt to overcome such limitations, this study presents an Acoustic Emission (AE) testing method to monitor the process of the single-particle fragmentation. AE testing is considered as an effective non-destructive technique and is capable of detecting micro-level defects inside a material. In the tests, the activity and the frequency characteristics of the AE signals were recorded and analyzed. It was found that the process of particle fragmentation was highly distinguished by AE characteristics. The AE activity was relatively low at first, followed by an inactive period. Finally, the AE signals tended to be significant active and exhibited a clustering of high frequency AEs before the impending fracture failure. This feature provides a useful insight to clarify the inner crack initiation and growth during the process of particle fragmentation. The technique is also promising for further applications such as particle fracture issues in granular systems which consist of numerous single grained particles.

It is known that bearing behavior of group pile is substantially different from the superposition of the single-pile loading in both clay and sandy grounds when the pile spacing is narrower. However the mechanism which causes the difference, called soil–pile–soil interaction, is less known in sand than in clay. This is because the sandy ground that bears the group pile has not been investigated well in previous studies. In order to obtain more direct information on the subsoil behavior, several advanced procedures including visualizing test and image analysis (PIV) and tactile sensors were adopted to discuss the behavior of the ground in this study. The clear difference of the ground deformation between wider and narrower spacing of piles in the group pile exists only before the bearing load yielded as revealed from visualizing test and PIV. It corresponded to the lower secant modulus of the tip resistance in narrower-spacing group pile at the same stage. After the ground yielding, the bearing behavior does not show significant difference among the piles. The bearing mechanism after yielding was investigated by PIV as well.

In previous studies of granular flow, it has been found that gravity plays a compacting role, causing convection and stratification by density. However, there is a lack of research and analysis of the characteristics of different particles’ motion under normal gravity contrary to microgravity. In this paper, we conduct model experiments on sand flow using a model test system based on a drop tower under microgravity, within which the characteristics and development processes of granular flow under microgravity are captured by high-speed cameras. The configurations of granular flow are simulated using a modified MPS (moving particle simulation), which is a mesh-free, pure Lagrangian method. Moreover, liquid-gas-like phase transitions in the sand flow under microgravity, including the transitions to “escaped”, “jumping”, and “scattered” particles are highlighted, and their effects on the weakening of shear resistance, enhancement of fluidization, and changes in particle-wall and particle-particle contact mode are analyzed. This study could help explain the surface geology evolution of small solar bodies and elucidate the nature of granular interaction.

The understanding of internal response of particle dislocations is vital to clarify the progressive failure in granular materials. This paper proposes a non-destructive testing method, Acoustic Emission (AE) technology, to characterize the mechanical behavior associated with particle-to-particle sliding and asperity/particle breakage in sand subjected to drained-triaxial compression. Particle dislocations during compression is accompanied by a sudden release of stored strain energy, which could be detected by AE sensors and characterized as elastic waves with different frequency properties. Insights into the correlations of stress-strain and frequency response of AE activities in terms of total, high frequency and low frequency AE event rates are offered, demonstrating that the mechanical behavior of particle dislocations and soil density could be highly characterized by AE activities. Besides, particle dislocations associated with particle-to-particle sliding and asperity/particle breakage is distinguished by high frequency and low frequency AE activities. The result suggested that the frequency response of AE activities is closely related to the failure mode, degree and rate of sand particle dislocations under drained triaxial compression. This technology seems promising as an alternative means to clarify the inter-particle mechanism during progressive failure in sand.

Slope failures caused by rainfall are common occurrences in many parts of the world and are severe in Japan where there are significant numbers of slopes susceptible to failure. To understand the mechanisms and conditions leading to such failures, constant shear stress tests that mimic the field stress path were conducted on a sandy soil taken from a natural sliding slope whose surface soils were moving. Water was infiltrated from the bottom and top of an initially unsaturated cuboid soil specimen at constant shear stress until failure. The effects of cyclic wetting and drying on the deformation characteristics and four parameters reflecting the initial conditions, i.e., the dry density, overburden pressure, rainfall intensity, and slope angle, as well as various parameter combinations, on the time and moisture content allowing for failure initiation were investigated. The experimental results exhibited a unique relationship between the deformation and moisture content and suggested a relationship for predicting the time and moisture content of onsets of landslides based on the four parameters. The results obtained can serve as guidelines for the design of landslide hazard detection systems and mitigation measures against rainfall.

Liquefaction-induced lateral ground deformation has been a common observation during many past earthquakes. Piles are widely used in liquefaction-prone areas, and their performance under such circumstances has become an important issue for researchers and engineers. Previous studies on this issue have focused primarily on piles located behind quay walls or in level ground conditions, which have considerably different boundary conditions compared to those of an inclined sloping ground condition that is more common for waterfront structures. This study presents a series of shaking table tests modeling the performance of piles subjected to liquefaction-induced slope deformation. Different pile configurations, including single piles, piles aligned in one row parallel to the direction of the slope, piles aligned in one row perpendicular to the direction of the slope, and piles in a 3 × 3 configuration, were tested. Several aspects regarding the behaviors of excess generation of pore water pressure, pile bending moment and lateral slope deformation under different pile configurations were revealed and discussed. The results obtained in this study can be used for validation of related numerical approaches, or as benchmarks that can further facilitate performance-based pile designs in liquefaction-prone areas.

Because of the low-gravity on the Moon and Mars, landslides there have characteristics that are very different from those observed in a conventional gravity environment. These include highly marked dynamic characteristics, evidence of fierce movement at high speed and on a large scale. One of the key problems in extra-planetary exploration is understanding the behavior of granular material flows under the influence of low gravity. A drop-tower testing system situated in Beijing has been developed and used to investigate granular flow in a microgravity environment. A set of granular flow tests was performed in both normal and microgravity environments, during which the configurations of sand flows were captured by the monitoring system. Preliminary results provide fundamental information for the future exploration of planetary landscapes.

Current experimental techniques used to understand the shear banding process in sands provide little insight into the internal micro-structure evolution. To this end, Acoustic Emission (AE), as a non-destructive testing technique, was proposed in this paper with great interest in characterizing the internal micro-structure response leading to the evolution of shear bands formed in laboratory triaxial compression. Silica sand was used to conduct consolidated-drained triaxial compression tests at a constant axial strain rate under an effective confining pressure of 100 kPa. AE events were collected and analyzed. Insights regarding relations of the deviatoric stress, source rates and dissipated energy rates of AE events with the increasing global axial strain are offered. The result indicated that with the increase of relative densities, the evolution envelope of AE source rates transits from a steep shape to a flat shape, and total amount of AE source events decreases gradually. According to the evolution of AE energy rate, shear banding process can be divided into four stages in terms of O-A, A-B, B-C and C-D, corresponding to the strain hardening regime, incipient strain softening regime, highest rate of strain softening regime and residual stress regime. From which point A could be considered as an omen of the initiation of strain localization, point B as the initiation of visible shear band and point C as the completion of shear banding. AE technologies can be provided as an alternative means to clarify and indicate the initiation and evolution of shear banding in sand.

Liquefaction-induced ground deformation is a major cause of structural damage during earthquakes. However, a better understanding of seismic liquefaction is needed to improve earthquake hazard analyses and mitigate structural damage. In this paper, a dynamic triaxial test apparatus was employed to investigate the fluidic characteristics of post-liquefaction sand. The specimens were vibrated to the point of liquefaction by dynamic loading, and then the liquefied sand was further sheared by triaxial compression in an undrained manner. It was found that a non-Newtonian fluid model can accurately describe the shear stress and the shear strain rate of post-liquefaction sand during undrained triaxial compression. The apparent viscosity, a major parameter in a constitutive model of a non-Newtonian fluid, decreases with an increase in the shear strain rate.

Flow deformation of post-liquefaction soil during an earthquake can cause serious damage to engineering structures. To overcome the limitations of conventional deformation analysis methods based on solid mechanics for extremely large systems, a computational fluid dynamics (CFD) method is proposed to numerically simulate the flow behavior of post-liquefaction soil. The liquefied soil is assumed to be a viscous fluid, and the volume of fluid (VOF) model is used for interface tracking in the numerical scheme. The results of a modeling test conducted on liquefaction-induced ground flow to verify the validity of the method showed good agreement, indicating the proposed method is capable of being used to reproduce the flow behavior of post-liquefaction soil.

Soil flow and induced air blasts are of great harm to humanity, and historically they have caused a lot of damage to infrastructure. However, these phenomena cannot be described by traditional analog modeling methods that limit their use in disaster prevention efforts. Computational fluid dynamics (CFD) is an applied technique commonly used in a range of fields including the chemical industry, and aircraft and automobile manufacturing, but little is reported on the use of this method to simulate flowing soil in geotechnical engineering applications. The CFD method can effectively make up for the deficiency of normal calculation methods in the analysis of soil flow and air blasts. This paper uses the FLUENT (version 6.3) CFD calculation software to simulate the processes of soil flow and induced air blast changes during soil flow with an Eulerian air–soil two-phase model included in a standard k-e turbulence model. Velocity vectors of air blasts at different times during soil flow are obtained, and the characteristics of turbulent flow can be found based on the velocity vectors. The numerical simulation techniques adopted in this paper captured precise configurations of soil flow. The results show that the CFD method is especially suitable for simulating the process of soil flow; hazard assessments can be implemented, and the performance of structures involved with disaster prevention can be improved based on the numerical simulation of changing air blasts.