Behavior Of Ballast Research Articles

Discrete elements method to model ballast behaviour under railway turnouts

Discrete elements method to model ballast behaviour under railway turnouts

Use of Geogrid for Improved Performance of Ballasted Tracks: Experimental and DEM Approaches

This paper presents a study on the enhanced performance of ballasted tracks through the implementation of geogrids. A series of large-scale direct shear tests and impact tests was conducted with three distinct types of geogrids. The behavior of ballast was evaluated in terms of shear stress–strain responses and stress concentration using stress sensing sheets. Additionally, a micromechanical analysis utilizing the discrete element method was simulated on ballast assemblies with different geogrid reinforcements. The shear stress–strain responses of ballast simulated from DEM are comparable with those measured from large-scale direct shear tests, indicating that the inclusion of geogrid can enhance the performance of ballast by increasing its shear strength, as well as reducing the vertical displacement and the load distribution with depth. Micromechanical analysis was performed to investigate the influences of geogrids on contact force distribution, coordination number and orientation of contact which could not be captured in a laboratory environment. The use of geogrids in ballasted tracks certainly shows promise for sustainable and efficient railway infrastructure, as evidenced by the experimental and DEM-based findings, offering valuable insights into optimizing track stability and longevity.

Rig test of light railway transport ballast vibration

This article presents findings from rig tests focusing on the oscillatory behavior of ballast in light rail transportation. Tests were carried out using an Amsler HB-250 pulsator, applying a load of 50 kN and operating at a frequency of 8 Hz. Accelerometers were positioned at depths of 10, 30, and 50 cm beneath the sleeper within the ballast shoulder to record vibrations of the crushed stone particles. The study determined the amplitudes of particle vibration accelerations in three directions: vertically, perpendicular to the track axis, and along the track axis. Results indicate that the lowest vibration accelerations were consistently observed across the track axis (Y-axis) at all sensor locations. At a depth of 10 cm beneath the sleeper, contiguous values of vibration accelerations were recorded in the vertical plane (Z-axis) and horizontal accelerations perpendicular to the track (X-axis). However, at depths of 30 and 50 cm below the sleeper, vertical vibrations registered the highest accelerations, while horizontal vibrations along the X-axis were 30-36% lower.

Investigating the effect of ballast contamination in vertical and shear interlocking stiffness: Experimental and numerical study

The maintenance of railway tracks faces a significant challenge due to ballast contamination. This issue arises from the inevitable ingress of fine particles into the track structure through various means, such as fine sands, coal infiltration, aggregate breakage, and substructure pumping. Climate change exacerbates these conditions, leading to increased external debris contamination and the accelerated degradation of ballast aggregates. This contamination has major consequences, impacting track quality in terms of drainage capacity and dynamic behavior. As a response to this, the study focuses on assessing ballast contamination from sand infiltration, specifically evaluating vertical and shear interlocking stiffness through a dedicated experimental setup for ballast behavior analysis. Using materials from an actual track, contamination levels from clean to 30 % were simulated, and an empirical formulation of contaminated ballast stiffness was derived based on laboratory tests. Moreover, numerical sensitivity analyses were conducted to explore the impacts of ballast contamination on the dynamic behavior of track components. The findings indicate a significant increase in dynamic track responses up to a 30 % contamination level, based on gravimetric measurements, leading to substantial increases in maximum absolute accelerations of sleeper and ballast layer as well as exerted forces on the ballast layer by about 28 %, 21 %, and 46 %, respectively.

The discrete and continuum behaviour of railway ballast

The mechanics of coarse-grained geomaterials have traditionally been examined from a continuum mechanics standpoint, but this has led to ever more complex constitutive models as an increasing number of novel aspects of behaviour are investigated. For this reason, there is a tendency to move towards discrete approaches. Such approaches require accurate definition and modelling of the particle shapes, for which considerable progress has been made, and also the contact mechanics between particles, about which little is known. For very coarse materials such as railway ballast, our understanding at the continuum scale has been hampered further by the technical difficulties in the laboratory testing so that the very small strain behaviour has been difficult to define with accuracy. New apparatus and techniques will be described to test ballast at both the continuum and particulate scales. These allow the small strain stiffness to be defined with accuracy in triaxial tests and to carry out precise particle-to-particle contact tests. At the particle contact scale, the behaviour is dominated by the roughness of the particles and their relatively sharp contacts, so that plasticity and particle wear predominate, which both change the contact properties significantly. It is the contact plasticity that then controls the decay in continuum stiffness with strain. The work emphasises the inadequacy of using simple linear and/or elastic DEM models calibrated from macro-scale tests rather than deriving the correct contact behaviour directly.

Minimizing breakage and optimizing ballast reusability during excavation for ballast cleaning: A computationally efficient model and operational parameter recommendations

During the excavation for ballast cleaning, intense interactions result in ballast breakage and damage. This reduces both the reusable mass and the mechanical properties of excavated ballast. Given the high rate of ballast breakage, existing breakable ballast models suffer from low computational efficiency and struggle to account for the accumulation of internal damage in particles. Thus, we introduced a computationally efficient breakage model that considers cumulative damage. This model was tested through experiments to simulate ballast behavior. The discrete element and multibody dynamics coupling methods were employed to develop a model that integrates an excavation device with a ballasted track. Utilizing this coupled model, we investigated the behavior of ballast breakage, the reusability of excavated ballast, and the impact of various operational parameters. These findings revealed that approximately 60 % of the ballast experienced breakage. However, such breakage can be mitigated by controlling the mass of excavated ballast. Theoretical excavation and rake-chain velocities were recommended to optimize excavation efficiency while minimizing breakage and improving the reusability of excavated ballast. This study can provide insights into enhancing the quality of ballast cleaning and reducing maintenance costs.

Behaviour of ballast stabilised with recycled rubber mat under impact loading

During the passage of trains, dynamic impact loads caused by wheel imperfections or rail abnormalities cause significant ballast degradation. In this study, the use of rubber mats manufactured from recycled tyres placed underneath a ballast layer is investigated to mitigate the adverse effects of impact loads. Based on a series of tests conducted using a high-capacity drop-weight facility to evaluate the dynamic impact responses, the experimental results show that the inclusion of a rubber mat beneath the ballast assembly significantly reduces particle breakage. This study also describes a numerical analysis following a coupled discrete–continuum modelling approach to examine the complex interaction of discrete ballast grains with the recycled rubber mat. In particular, a mathematical framework coupling the discrete and continuum domains is developed to facilitate the exchange of forces and displacements at the ballast–mat interface. Laboratory data measured from large-scale impact tests are used to calibrate and validate this coupled model. Subsequently, the model is used to predict the deformation and breakage of ballast, contact force distributions, impact forces, coordination numbers and the evolution of energy components during impact testing. The energy-absorbing properties of the rubber mat are captured in terms of reducing particle breakage from a micromechanical perspective.

Study on the splashing behavior of high-speed railway ballast in ice and snow environment: Analysis of the influence of the train speed, ice mass and striking position

The snow flight in severe cold regions has occurred on many high-speed railway lines in China, directly impacting the safety of high-speed railway operations. However, the research on this problem has not been carried out systematically at present, and the establishment of the theoretical simulation model remains blank. In this paper, the DEM-MBD-CFD coupling analysis method is used to build a refined simulation model of snow flight on high-speed train-ballasted track, which reproduces the actual scenes of three typical cases where the snow flight hits the train body and verifies the reliability of the model with field test results. The results show that the wind pressure amplitude at the center of the ballast bed is the largest, the bottom of the rail is the second largest, and the shoulder of the ballast is the smallest. After the ballast particles fly away from the ballast bed, the main power source is the negative pressure generated by the train’s airflow field. The number of flying ballast particles and the impact force on the train floor grow with the increase of train speed. When the train speed increases to 400 km/h, the average impact force increases to 12.76 kN, which is approximately 8.7 times the impact force at 250 km/h. With the increase of ice mass, the impact stress first increases rapidly and then slowly. When the ice mass is 7 kg, the impact stress has exceeded the yield strength of the train body, seriously affecting the safety of the operation. Many prevention measures, such as reducing the operation speed, implementing ice and snow removal, installing anti-splash coating and grille, and optimizing the structure design, can be taken to reduce the damage of snow flight.

Effects of Particle Size and Grading on the Breakage of Railway Ballast: Laboratory Testing and Numerical Modeling

Ballast is coarse aggregate with particle size normally ranging from 10 mm to 65 mm. Upon repeated train loading, ballast deteriorates in the form of either continuous abrasion of sharp corners or size degradation, which have been reported as the fundamental cause for the instability of railway tracks. In this study, the splitting behavior of ballast grain with varying particle sizes under diametrical compression was examined to investigate the size effect and the Weibull characteristics of ballast tensile strength; a Weibull modulus of 2.35 was measured for the tested granite ballast. A series of large-scale monotonic triaxial tests on ballast aggregates having various size gradings was performed to study the effect of particle gradation on the mechanical behavior of ballast. The results show that compared to mono-sized uniformly distributed aggregates, non-uniformly distributed aggregates generally have greater shear strength, larger peak friction angle, 50% strength modulus, and greater volumetric dilation. The ballast aggregate conforming to the recommended PSD as per current standards exhibited the most superior mechanical performance, possessing the greatest shear strength, peak friction angle, and 50% strength modulus. Micromechanical analysis showed that aggregates with larger d50 values have higher coordination numbers, inter-particle contact forces, and higher anisotropy level of contact normals, thus causing a greater possibility of particle breakage during shearing.

Discrete element modelling of the effects of particle angularity on the deformation and degradation behaviour of railway ballast

Railroad ballast exhibits distinct morphological characteristics represented by shape irregularity, corner angularity, and surface texture. Upon repeated train loading, the morphology of ballast undergoes inevitable degradation, particularly in terms of its corner sharpness, which can affect track performance and even pose a substantial threat to operational safety. These aspects have rarely been captured insightfully in most DEM studies on ballast. In contrast, this study examines the influence of particle angularity on the deformation and degradation behaviour of railway ballast upon repeated loading using the discrete element method (DEM). The angularity of ballast particles is captured and quantified using the CT scanning technology in conjunction with an image-based processing strategy, after which the irregularly shaped particles are reconstructed in the DEM. In this numerical procedure, aggregates with varying angularities are created by incorporating a particle degradation subroutine to capture corner abrasion and surface attrition of ballast to mimick real-life field processes. The macro-response of a typical ballasted track subjected to cyclic rail loading is investigated, and the results show that as the angularity increases, the permanent deformation of the track corresponds to a lower permanent strain rate and a higher resilient modulus. However, the opposite behaviour is observed if excessive breakage of the aggregates occurs that reduces the angularity of the individual particles. In this study, detailed microscopic analysis based on DEM in terms of interparticle interaction and associated vibration velocity has also been performed. The results offer distinct clarity to the essential micro-mechanisms embracing particle angularity, and the accompanying influence on the deformation and degradation characteristics of ballast is elucidated with greater insight.

Formulation and Evaluation of the Ballast Shear Interlocking Coefficient based on Analytical, Experimental, and numerical Analyses

The interaction among ballast particles, known as ballast shear interlocking (BSI), significantly influences the dynamic responses of railway track. This research presents a meticulously derived mathematical formulation for calculating BSI stiffness, which is then validated through experimental data obtained from a novel laboratory test procedure. The accuracy of the BSI formulation is demonstrated by its successful prediction of BSI properties in laboratory tests. Additionally, a numerical layered model is introduced to evaluate the sensitivity of the proposed formula to the mechanical and dynamic attributes of the ballasted track. This model highlights the resilience of tracks and emphasizes the critical role of BSI effects in railway track design, offering insights into parameters affecting track behavior. The study comprehensively explores ballast behavior, emphasizing the importance of BSI in track resiliency and advocating for its precise consideration in railway track design, modeling, and maintenance to ensure operational safety and efficiency. The developed methodology not only enhances the modeling of ballasted railway tracks as multi-layer systems but also advances the assessment of shear interlocking effects through laboratory testing.

Performance of Geogrid-Reinforced Rubber-Coated Ballast and Natural Ballast Mix under Direct Shear Conditions

Rubber-coated ballast (RCB) is a new innovative ballast aggregate obtained by coating natural aggregates with recycled crumb rubber by using polyurethane binder. The objective of RCB is to reduce the ballast degradation, thereby reducing the deterioration of rail track geometry and also maintenance costs. Natural ballast; crumb rubber of sizes 0.4, 0.8, and 2 mm; Elastan polyurethane; and geogrid (triangular, 69 mm aperture size) were used in this study. The results of abrasion, impact value, and soundness tests show that RCB has a higher resistance to abrasion, impact, and weathering compared to natural ballast. Subsequently, the shear behavior and particle breakage of natural ballast, RCB, unreinforced, and geogrid-reinforced RCB–natural ballast mix was explored using the large-scale direct shear apparatus at different normal stresses and shearing rates (Sr). The test results indicate that due to rubber coating the peak friction angle (φ) of RCB samples reduces as compared to natural ballast. The reduction in φ of RCB samples is attributed to a change in surface texture. Moreover, 0.4-RCB (RCB made with rubber crumbs of size 0.4 mm) shows the least reduction in φ compared to 0.8-RCB and 2-RCB (RCB made with rubber crumbs of size 0.8 and 2 mm). It was further observed that with the increase in proportion of 0.4-RCB in geogrid-reinforced natural ballast both the peak friction angle and particle breakage (Bg) of ballast decreased significantly. The current study established the optimum content of 0.4-RCB to be mixed with geogrid-reinforced natural ballast to be in the range of 44%–53%.Practical ApplicationsThe problem of excessive degradation of ballast particles and the associated changes in the track geometry owing to the repeated application of heavy axle loads is addressed by coating the natural ballast with crumb rubber generated from waste tires. The rubber-coated ballast (RCB) is found to offer better abrasion, impact, and weathering resistance when compared to natural ballast. Moreover, it undergoes negligible particle breakage under direct shear conditions. The decrease in the extent of particle breakage helps in reducing the frequency of ballast replacement operations and hence tracks maintenance costs. Furthermore, RCB, when reinforced with geogrid, offers better shear strength than that of natural ballast alone. These observations imply that the tracks with geogrid-reinforced RCB can effectively reduce the risk of track instability.

Investigation of Railway Ballast Breakage Through Large-Scale Triaxial Tests and a New Particle Breakage Approach in Discrete Element Modeling

Crushed aggregates with irregular shapes and sharp corners are preferred ballast materials to ensure proper track bearing capacity because of good contact between the particles. Meanwhile, crushed aggregates are prone to deterioration and breakage under repeated train loads. The effect of particle degradation on the mechanical behavior of ballast has been investigated before, but previous research rarely separates ballast breakage during sample compaction from breakage during shearing. This study investigates ballast particle breakage at different stages through large-scale triaxial tests and discrete element modeling. The results show that corner breakage is the primary particle breakage mode and mostly happens during compaction instead of during the shearing process. The results also indicate ballast corner breakage during sample compaction would increase the peak strength of the ballast but would not change Young’s modulus. A rubber pad is strongly recommended to reduce ballast breakage during the triaxial test sample compaction.

Modelling Time-Dependent Flow through Railway Ballast

In this study, a numerical simulation of fluid flow through railway ballast in the time domain is presented, providing a model for unsteady-state flow. It is demonstrated that the position of the free surface with respect to time can also be used to solve the steady flow case. The effect of ballast fouling is included in the model to capture the realistic behavior of railway ballast, which is critical to understanding the impact of flooding. A thorough comparison with a range of previous studies, including theoretical and experimental approaches, is made, and very close agreement is obtained. The significant impact of ballast fouling on fluid flow and its potential consequences for railway infrastructure are highlighted by the simulation. Valuable insights into the behavior of water flow through porous media and its relevance to railway ballast management are offered by this study.

Application of Elastomeric Polyurethane in Performance Improvement of Rail Ballast Subjected to Cyclic Loading

The deformation and degradation behavior of unstabilized ballast and that stabilized with elastomer and geogrid was evaluated using process simulation test (PST) apparatus at various loading frequencies (f). The results indicated that elastomer has significantly reduced the extent of both vertical and lateral deformations of ballast. For example, elastomer reduced the extent of vertical settlement (Sv) in ballast from 22.76 to 9.83 mm, and the lateral deformation (ld) from 8.14 to 1.95 mm (f=15 Hz). It was also seen that the deformation and degradation of both stabilized and unstabilized ballast increased nonlinearly with the increase in the applied loading frequency (f). Further, the beneficial effect of elastomer in restraining the lateral flow of ballast was seen over the entire depth of treated ballast, unlike the geogrid, whose efficiency was maximum at its placement location and then reduced at farther locations. Moreover, elastomer has significantly enhanced the resilient modulus (Mr) and damping ratio (D) of ballast when compared with geogrid-reinforced ballast. For example, the elastomer increased the Mr and D of ballast by 25.9% and 66.6%, respectively, in comparison with an increment of only 15.1% and 25.3% in the case of geogrids (f:15 Hz). The Mr and D of ballast were found to be influenced by the effectiveness of the ballast stabilization technique in reducing lateral displacements. Additionally, elastomer has significantly reduced the vertical stress (σv) at the ballast–subballast interface by 34%. Further, elastomer was found to be more efficient than the geogrid in reducing the dynamic amplification factor (DAF) at any loading frequency. Similarly, elastomer reduced the breakage of ballast by 71% compared with the 40% of geogrid reinforcement (f=15 Hz).

DEM analysis of the cyclic behavior of ballast mixed with crumb rubber

This study presents a numerical investigation of the cyclic behavior of ballast mixed with crumb rubber (CR) using the discrete element method (DEM). To this aim, a series of box tests on ballast mixed with different percentages (0%, 5%, 10% and 20% by volume) of CR were simulated by a three-dimensional DEM code, PFC3D. First, the macroscopic mechanical behavior, i.e., the sleeper displacement, the dissipated energy, and the vertical stiffness of the samples, were compared with previous experimental results to validate the proposed DEM model. Further, the microscopic response, including the coordination number, sliding contact ratio, particle displacement and rotation, contact force ratio, contact force and its distribution, and energy evolution were analyzed to mainly reveal the mechanism underlying the sample deformation and ballast degradation. The results show that the simulation results can well capture the effect of CR on the macro-mechanical behavior of ballast with different CR percentages. On the micro-scale, the sliding contact ratio and particle rotation velocity increase with increasing CR percentage, leading to an increase in sleeper settlement. In addition, as the CR percentage increases, the coordination number for the ballast-ballast contacts decreases while the occurrence of larger contact forces increases. The effect of CR on reducing the coordination number for the ballast-ballast contacts outweighs increasing the occurrence of larger contact forces. At the same time, the contact force distribution for the ballast with a higher CR percentage tends to be more homogeneous. These will reduce the risk of ballast breakage; however, as the CR percentage increases, the frictional energy dissipated by the ballast-ballast contacts increases, indicating an increase in ballast abrasion.

Mesoscale Mechanism of Asphalt Track Bed in Reducing Cyclic Settlement of Ballast Layer under High-Speed Train Traffic Loads

Asphalt track bed, in which an asphalt mixture is inserted underneath the ballast in a conventional granular track bed, has been experimentally proven to have better safety, stability, and economic efficiency. However, most existing theoretical studies were based on continuous mechanics theory, mainly focusing on the impact of the asphalt layer on the macroscale response of the track bed or subgrade, and rarely on the particle-scale mechanical behavior of ballast due to the limitations of the method. Hence, aiming at the improved performance of asphalt track bed and its mesoscale mechanisms, the discrete element method (DEM) model of asphalt track bed is established in this study and then validated with the measurements obtained from on-site tests. Subsequently, the validated model is used to investigate the effect of asphalt layer on the dynamic response and cyclic settlement of ballasted track bed under high-speed train moving loading. Results reveal that the asphalt layer has an ability to reduce vibration of the track bed, which contributes to less active ballast and more stable interparticle structure, thereby decreasing the accumulative settlement of the ballast layer. Besides, the asphalt layer can improve the uniformity of the force chain network inside the ballast layer. The well distributed and less intense force chain network in the ballast results in a slower degradation rate. A series of parameter analyses is performed in the end, concluding that the required thickness of the ballast layer in the asphalt track bed could be reduced by approximately one-third without changing the bearing capacity of the subgrade, and the asphalt track bed structure is a more economical and effective track form for high-speed or heavy-haul railway lines. This study discusses the improvement of ballasted railway track by insertion of asphalt layer and its mesoscale working mechanisms, and the results provide guidance for the design of asphalt track bed in high-speed railways or heavy-haul railways.

Laboratory investigation of the cyclic behavior of rock ballast mixed with slag ballast for use in railway tracks

In this paper, the combined behavior of igneous (andesite) rock ballast and EAF slag ballast has been investigated. In the first step, to accurately identify the phases that make up the materials, the properties of both materials have been evaluated and compared using X-ray fluorescence (XRF) and X-ray diffraction (XRD) tests and also physical tests. Then, five samples containing combinations of rock and slag ballast, including 0 %, 25 %, 50 %, 75 %, and 100 % slag, were prepared. Finally, the abrasion resistance of samples was investigated by performing the Los Angeles and micro-Deval abrasion tests, and by conducting ballast box tests, settlement, breakage, stiffness, and damping ratio of each sample were determined under cyclic load. According to the Los Angeles abrasion test results, by increasing the percentage of using slag ballast in the samples, the abrasion increased from 19.38 % to 23.91 %. While in the same samples, the micro-Deval abrasion decreased from 13.4 to 9.3. According to obtained results of the ballast box test, the settlement of slag ballast was 33 % less than rock ballast, but the ballast breakage index of slag ballast was 146 % more than rock ballast. The stiffness and damping ratio of samples were calculated according to the hysteresis loop. The results illustrated that the use of slag ballast increased the stiffness of the track by 35 % compared to rock ballast. Furthermore, increasing the percentage of slag in the sample brought about a rise in the damping ratio as the damping ratio of the 100 %SB increased by 76 % compared to the 0 %SB. Eventually, by examining the obtained results and comparing the stiffness and damping of the samples with the 0 %SB, in the 75 %SB, the increase in damping compared to the 0 %SB was very significant; however, the increase in stiffness did not change significantly. Thus, the 75 %SB as a suitable combination of slag and rock ballast was suggested.

Behaviour of coal-fouled elastomeric polyurethane-stabilized ballast under cyclic loading

The current study assesses the deformation and degradation behaviour of geogrid and elastomer-stabilized coal fouled ballast under cyclic loading with frequencies ( f) ranging from 15 to 35 Hz. Different percentages of coal were added to ballast to have void contamination index (VCI) in the range of 0–45%. Test results confirmed that the deformation and degradation behaviour of ballast was highly affected by the presence of coal fines. The elastomer-stabilized ballast (ESB) exhibited lower deformations and degradation when compared to geogrid-stabilized (GSB) and unstabilized ballast (USB). The effectiveness of geogrid/elastomer in arresting the spread of ballast along its depth is evaluated in terms of the stabilization efficiency factor ( S ef, cyc). The S ef, cyc of clean GSB is computed to be 0.107, whereas the S ef, cyc of clean ESB is 0.811. The S ef, cyc of GSB decreased significantly with the increase in VCI but in case of ESB the reduction is only marginal. Further, the resilient modulus ( M r) and damping ratio ( D) of ballast reduced due to coal fouling. The elastomer has significantly improved resilient modulus ( M r) and damping ratio ( D) of ballast when compared to GSB. Moreover, the extent of lateral and vertical deformations was found to be influenced by the interface shear strength as obtained under direct shear conditions. In addition, empirical models were developed to predict deformation, breakage and resilient modulus of ballast as functions of loading frequencies and VCI.

Influence of Type of Sleeper–Ballast Interface on the Shear Behaviour of Railway Ballast: An Experimental and Numerical Study

The shear resistance at the sleeper–ballast interface of a ballasted track is an important contributor in maintaining track stability under faster and heavier axle loads where the ballast undergoes significant lateral sliding. Different types of sleeper–ballast interfaces based on the type of sleeper arrangements, such as concrete sleepers, timber sleepers, and under sleeper pads (USPs) attached to the concrete sleepers influence the lateral stability of railway tracks. Therefore, in this study the shear and degradation behaviour of ballast at concrete–ballast, timber–ballast, and USP–ballast interfaces were examined in the laboratory using large-scale direct shear tests under 60 kPa normal stress. The use of waste materials in the construction of civil infrastructure is gaining a lot of interest in the engineering community. Therefore, in addition to commercial USPs manufactured using raw materials, recycled USPs manufactured from granulates of end-of-life rubber tyres were also tested in this study. The discrete element modelling (DEM) approach was used to predict the shear behaviour of ballast at 30, 90, 120, 150, and 180 kPa normal stresses. The bonded particle model (BPM) was adopted in the DEM to simulate the effects of particle breakage during shearing. The results exhibited that both commercial and recycled USPs significantly improve the shear resistance at the sleeper–ballast interface while reducing particle degradation compared to concrete and timber sleeper interfaces.