Dynamic Load Disturbances Research Articles

Dynamic mechanical characteristics of coal in front of the mining face under different mining layouts

To study the dynamic mechanical characteristics of coal in the area affected by mining in front of the mining face under different mining methods, an improved split Hopkinson pressure bar (SHPB) was used to apply different static loads to different positions. Then, dynamic mechanical tests were conducted on coal at different positions on the mining face, analyzing the dynamic response under strong dynamic load disturbances, under three different mining layouts. Within the range of static water pressure to the peak support pressure, the dynamic strength of coal gradually increases with increasing distance from the mining face. The dynamic strength is the smallest at the peak support pressure stress, and under strong external disturbances, instability and failure are increasingly likely to occur at the peak stress. Under the same loading rate conditions, the dynamic strength of the peak stress in coal is as follows: Protective coal-seam mining (PCM) > Top-coal caving mining (TCM) > Nonpillar mining (NM).

The mechanism and prevention of rockburst induced by the instability of the composite hard-roof coal structure and roof fractures

Despite the implementation of prevention and control measures, rockburst still occur. The characteristics of composite coal were analyzed by combining monitoring and early warning means. Two types of composite coal in the hard roof were distinguished based on their distinct initiation and destruction processes despite experiencing the same rockburst event. Rockburst caused by structural instability was studied in different mining stages, and the following conclusions were considered. 1) The mechanical model of the instability of composite coal in hard roof was established for different mining stages to reveal rockburst mechanisms. The high-speed advance of working faces changed the fractured structure of upper hard rocks in the pressure-relief protection range. The sudden fracture of long cantilever structures caused dynamic load disturbance to high-stress coal, which resulted in rockburst in the insufficient mining stage. The failure and deformation of coal under high-stress static loads created conditions for the instability and fractures of roof. The disturbance of low hard rock strata aggravated the deformations of damaged coal. Therefore, rockburst appeared in the full mining stage. 2) Structural instability and roof fracture caused by different factors were the main causes of rockburst for composite coal in hard roof in different mining stages. Relevant prevention and control measures were formulated to ensure the safety of working faces, which provided references for the prevention and control of rockburst in working faces under similar conditions.

Mechanism and Prevention of Rock Burst in a Wide Coal Pillar under the Superposition of Dynamic and Static Loads

To address the frequent occurrence of rock burst disasters in areas with wide coal pillars during mining in the western mining area of China, the wide coal pillar area of the Tingnan coal mine in Shanxi Province was used as the research background. Theoretical analysis, numerical simulation, and field tests were used to establish the mechanical criterion and the energy criterion for the dynamic instability of wide coal pillars. The process and mechanism of wide coal pillar dynamic instability under dynamic and static load disturbances were revealed, and a wide coal pillar rock burst prevention and control scheme was proposed. The results indicated that when the load above a coal pillar reached the stress failure index and the energy failure index was met, the coal pillar reached the critical conditions for rock burst. With increasing static load, the stress, energy, and range of the plastic zone all showed increasing trends on both sides of the coal pillar. Under a given dynamic load, the stress and plastic zone range of the coal pillar significantly increased compared to those without a dynamic load. Under a given static load, the greater the dynamic load, the more likely the coal pillar was to undergo dynamic instability. The evolution of coal pillar dynamic instability was divided into three stages: energy accumulation, local instability, and dynamic instability. When the critical stress and energy conditions for coal pillar dynamic instability are exceeded, rock burst will occur. To reduce the static and dynamic loads of coal pillars, a rock burst prevention and control scheme of energy release and load reduction was proposed and applied onsite. The monitoring results showed that this control plan effectively reduced the stress of the coal pillar and the dynamic load generated by the fracture of the overlying rock layer, indicating safe mining in this area of wide coal pillars.

Study on the Impact Failure Rules of Roadway Floor Induced by Dynamic Load Disturbance under Extremely Thick Conglomerate

Study on the Impact Failure Rules of Roadway Floor Induced by Dynamic Load Disturbance under Extremely Thick Conglomerate

Study on Shear Slip Characteristics of Sandstone Plane Joints under Normal Dynamic Load Disturbance

Abstract Rock joints are susceptible to slip instability due to dynamic load disturbances such as blasting, earthquakes, and fracturation. A series of direct shear tests under the dynamic load were conducted on sandstone plane joints using the RDS-200xl. The work investigated the effects of normal static loads and normal dynamic-load frequencies and amplitudes on plane joints. Besides, the following items were proposed, that is, the peak-to-valley response rate, shear velocity vibration dominant frequency, shear-stress reduction coefficient, and discrete element numerical simulation method for plane-joint direct shear tests. The results were as follows: (1) The normal dynamic load frequency played a role in attenuating the shear stress amplitude with a threshold value of 0.5 Hz. (2) The shear velocity of the plane joint was completely controlled by the high normal dynamic load frequency. Their vibrational dominant frequencies were identical. (3) The amplitude of shear stress increased, and the median stress decreased with the increased normal dynamic load amplitude. The reduction-coefficient equation for sandstone plane joints was proposed to evaluate the shear stress under the normal dynamic load disturbance. (4) The shear-stress hysteresis phenomenon existed in the plane joints under the normal dynamic load, which required excessive shear displacements to reach peak shear strength. The peak shear displacement increased with the increased normal static load. Numerical simulations and indoor tests showed that high- and low-shear-velocity regions were the main reason for shear-stress hysteresis. The findings are conducive to revealing the shear destabilization mechanism of rock joints under dynamic load disturbance.

Numerical study on the characteristics of roadway failure and instability in coal seam with rock parting

In order to explore the mechanism of rockburst in coal seam with rock parting, a combination of on-site and numerical experiment is used to study the failure and instability process, crack propagation mechanism, and influencing factors. The following four points were addressed: (1) the instability is a process that roadway in coal seam with rock parting go through from stable locking in the initial stress unloading stage to slipping unlocking, and then to spatter ejection in slipping dynamic load disturbance stage. (2) The fracture development caused by unloading excavation of coal seam with rock parting will change from shear crack to tensile crack. In this process, coal-rock contact surface slip and coal-rock fracture are coupled with each other. (3) The greater the mining depth is, the greater the lateral pressure coefficient is, and the higher the rockburst risk is. On the contrary, the lower the risk of rockburst. (4) When choosing the support form of roadway in coal seam with rock parting, the two supporting forms of bolting (cable) and supplementary masonry support should be preferred. The results enrich the theory of the dynamics of surrounding rock fracture in coal mine, further clarify the potential dangers to mining-area roadways and working faces, and provide technical information to ensure the safe and efficient mining of bifurcated coal seam.

Graphene oxide coated fly ash for reinforcing dynamic tensile behaviours of cementitious composites

By virtue of its low cost, high workability, wide availability and suitability for large-volume fabrication, cement-based grouting is the most applied material in underground projects. However, due to the weak tensile strength and high-frequency impact dynamic load disturbance in underground engineering, cement-based grouting is vulnerable to tensile damage, seriously influencing the stability and safety of underground projects. Hence, in this work, the graphene oxide (GO) coated fly ash was adopted to reinforce the dynamic tensile behaviour of the cement-based matrixes to fabricate high-performance, cost-effective and environmentally friendly cementitious composites. The test results show that the GO-coated fly ash could significantly enhance the dynamic tensile performance of the cementitious composites. With only 0.08 wt% GO mixing, the dynamic tensile properties can be strengthened by 19.5–88.5%. After GO nanosheets accelerate the cement hydration process and improve the chemical composition of hydration products, GO nanosheets exhibit macroscopic nucleation and "crack-bridging" effects on the cement matrixes, and the cracks propagation of the hardened specimens would be suppressed under dynamic loading, thus enhancing the tensile resistance of the specimen. The fractal dimension calculation results prove that the GO-coated fly ash-modified cementitious composites have higher integrity after tensile impact, about 2.8–11.9% higher than plain cement-based slurry. Finally, the theoretical analysis reveals that GO nanosheets can enhance the tensile behaviours of the cement matrix by optimizing the dynamic load coefficient. Nevertheless, with the crack propagation, the reinforcing efficiency of GO would be gradually weakened. The findings of this study can promote an understanding of the enhancing mechanisms of GO on the dynamic tensile behaviour of cementitious composites and inspire the potential application of GO-coated fly ash to reduce cement usage in grouting engineering.

Study on Mechanism and Main Influencing Factors of Rockburst under Complex Conditions of Hard and Deep Overburden

Xinzhuang Coal Mine is a typical kilometer-deep rockburst mine in the Ningzheng mining area of China Huaneng Group, which is still in the capital construction period. At present, there are a few studies on the rockburst mechanism and prevention of this mine. In order to explore the occurrence mechanism and main influencing factors of rockburst under the complex geological conditions of large buried depth, thick topsoil, and hard overburden in Xinzhuang Coal Mine, this paper, based on the analysis of the basic geological data of the mine, deeply explores the comprehensive disaster factors of Xinzhuang Coal Mine; then, uses the analytic hierarchy process to carry out quantitative analysis of each disaster factor, and finally, it obtains the impact type and occurrence mechanism of the mine. Through research, it is found that the main disaster factors of Xinzhuang Coal Mine are mining depth, coal seam thickness, coal seam thickness change, tectonic stress field, hard overburden, roadway layout, bottom coal reservation, and tunneling activities. The key to the process of the rockburst disaster in Xinzhuang Coal Mine is the superimposed effect of static and dynamic loads on the roadway surrounding the rock system. The potential rockburst type during excavation and mining is the “high static and dynamic load disturbance” type.

Experimental study on the slip evolution of planar fractures subjected to cyclic normal stress

The frictional rupture mechanisms of rock discontinuities considering the dynamic load disturbance still remain unclear. This paper investigates the transitional behaviors of slip events happened on a planar granite fracture under cyclic normal stress with different oscillation amplitudes. The experimental results show that the activations of fast slips always correlate with unloading of normal stress. Besides, the intensive normal stress oscillation can weaken the shear strength which is recoverable when the normal stress return to constant. The rupture patterns are quantified by stress drop, slip length and slip velocity. With the effect of small oscillation amplitudes, the slip events show chaotic shapes, compared to the regular and predictable style under constant normal stress. When the amplitude is large enough, the big and small slip events emerge alternately, showing a compound slip style. Large amplitude of the cyclic normal stress also widens the interval differences of the slip events. This work provides experimental supports for a convincible link between the dynamic stress disturbance and the slip behavior of rock fractures.

Seismic Mitigation Effect of Overlying Weakening Strata in Underground Coal Mines

Artificial construction of a weakening zone over the roadway is an essential method for preventing coal bursts and rock bursts caused by strong mining tremors. However, concerning the seismic absorption and load reduction capabilities of an artificial structural weakening zone, the degree of rock mass damage to the roadway under weakening zone protection remains unclear. This study employed principles of elasticity and UDEC (Universal Distinct Element Code) to explore the seismic attenuation and load reduction capabilities of the weakening zone. The results indicate that the absorbing ability of the weakening zone increases exponentially with its weakening coefficient. Under the same dynamic load disturbance, when the weakening coefficient rises from 0.00 to 0.99, the sidewall displacement from the elastic wave source side changes from 0.400 m to 0.228 m. The total number of cracks in the roadway-surrounding rock, and the ranges of overstressed zones decreased linearly. The critical threshold of the roadway resisting the mining tremor disturbance increased. In particular, when the mining tremor is located directly above the roadway, the initial deformation of the roof is the largest, and the cumulative deformation of the rib is greater than the roof. By creating a weakening zone with a coefficient exceeding 0.95, the roadway remains unaffected by the 20 MPa dynamic loading. The study provides a theoretical basis for controlling coal burst that is triggered by mining tremors.

Study on Mechanical Behavior and Mechanism of Sandstone under the Coupling Effect of Water Content and Dynamic Load

In the process of underground engineering construction, rock mass often faces the dual influence of dynamic load disturbance and groundwater, it is therefore essential to investigate the mechanical response of the rock mass under the coupling effect of dynamic load disturbance and water content. In this paper, dynamic load impact tests were carried out on sandstone with bullet velocities of 5 m/s, 10 m/s, and 15 m/s and water content of 0, 0.3, 0.6, and 0.9, and the mechanical behavior and mechanism response of water content to sandstone were investigated. The research findings indicate that this study has made significant contributions in quantifying the along grain and trans-grain fractures of microcracks. It has explored the influence of water content and dynamic loading on the strength mechanism of sandstone. It was discovered that the dynamic loading and water content significantly affect the ratio of along grain and trans-grain fractures, thereby influencing the dynamic behavior of sandstone. The findings suggest a negative association between rock strength and water content and that its peak strength rises as the bullet velocity rises. The fracture characteristics of rock are influenced by water content and bullet velocity. The sample’s fracture degree increases with an increase in water content, its particle size distribution map is evident, and there is a positive relation between bullet velocity and fractal dimension. The energy conversion mechanism of the rock is influenced by the water content, as the bullet velocity increases, the absorbed energy density of the rock becomes higher. Furthermore, the correlation between the absorbed energy intensity and density and its fractal dimension is quantified. It is found that energy density and strength are positively correlated. The greater the fractal dimension, the higher the energy density absorbed.

Evolution Laws of Stress–Energy and Progressive Damage Mechanisms of Surrounding Rock Induced by Mining Disturbance

The rock burst induced by the mutual disturbance of mining and excavation is significantly influenced by high static load stress and external dynamic load disturbance. In this paper, the evolution characteristics and progressive damage mechanism of surrounding rock in the process of mutual disturbance of mining and excavation are systematically studied. The results show that the evolution of surrounding rock stress can be roughly divided into three stages: rapid rise in the early stage, continuous rise and step-like decline in the middle stage, and slow rise in the late stage. In the process of parallel mining, the overlying rock movement above the goaf shows the sequence of horizontal penetration of tiny fissures—fracture intensification transition to stratification—non-coordinated caving of middle–low overlying rock—obvious horizontal cracks in the upper key layer. Only under the quasi-static loading mining action does the upper key layer not reach the breaking condition. The wave side of the heading face which is close to the focal point is affected by the dynamic load disturbance, the acceleration duration is short, and the attenuation is relatively fast, so it is the area prone to the earliest impact failure in the face of mining disturbance. The conclusion is helpful to deepen the understanding of the coal burst mechanism of mutual disturbance of mining and excavation.

Dynamic Pressure Manifestation Mechanism and Control Techniques for Roadways with Large Mining Heights and Intense Mining: A Case Study

Responding to severe surrounding rock deformation failures and other problems in roadways in western China with large mining heights and intense mining, the work presented in this paper studied the mechanism of surrounding rock deformation failures in roadways with dynamic pressure through field investigations, theoretical analysis, and numerical simulation. According to the research findings, mining roadway deformation failures are affected by roadway layout orientation, working face mining intensity, and dynamic load disturbances from roof breakage. Coal pillars, as bridges connecting the roof and floor, constitute the energy transfer path near roadways surrounding rock, and an unreasonable coal pillar size and lateral overhanging roof structure may aggravate static load energy accumulation in the roadway surrounding rock. Roadway protection with small or large coal pillars may increase elastic energy loss in the energy transfer path; a reasonable size of small and large coal pillars is 15 m and 35 m, respectively. Using roof cutting for pressure relief may reduce the elastic energy of roadway surrounding rock by 14.35-26.33% during primary mining and 21.57-29.31% during secondary mining, thereby reducing the static load elastic energy in the surrounding rock and improving the stability of roadway surrounding rock.

Mechanism and Control of Asymmetric Floor Heave in Deep Roadway Disturbed by Roof Fracture

In view of the serious problem of bottom-drum damage in deep mining along empty roadways, the asymmetric bottom-drum damage characteristics and control mechanisms of deep mining along an empty roadway were studied using the trackway of the 11060 working face in Zhao Gu II mine as the research background. Based on the slip-line theory, support-pressure distribution law, and Griffith’s damage-criterion theory, the mechanism of asymmetric bottom drums and the maximum fracture-development depth of the bottom plate in a deep roadway under top-plate fracture perturbation were analyzed. The 3DEC discrete-element software was used to simulate and analyze the characteristics and evolution of the asymmetric bottom bulge of the roadway under dynamic-load disturbance, and the asymmetric control scheme of “slurry anchor reinforcement + top cutting and pressure relief” was proposed. The results show that, under the influence of static load of deep high-abutment pressure and the dynamic-load impact of the instability of the masonry-beam structure under periodic pressure of the adjacent working face, the deep-mining goaf roadway was prone to producing asymmetric floor heave. The floor-heave degree and maximum fracture-development range of the roadway in the affected area under the influence of dynamic load > those in goaf roadway > those in the roadway in the stable area affected by tunneling. The distribution of stress, displacement, and maximum floor heave was skewed to the side of the coal pillar in the goaf, showing an inverted right oblique V shape. The asymmetric floor heave of a roadway can be effectively controlled by grouting anchor-cable reinforcement (increasing the anti-damage limit) and roof-cutting pressure relief (cutting off the dynamic-load source). The research results can provide an important reference for the control of roadway floors under similar geological conditions.

Study on Characteristics of Acoustic Emission b Value of Coal Rock with Outburst-Proneness under Coupled Static and Dynamic Loads

In this paper, a rock dynamic and static triaxial mechanical test system is adopted to conduct triaxial compression failure acoustic emission test on coal rock with outburst-proneness. The purpose is to study the evolution law of the acoustic emission (AE) b value of coal rock under static and dynamic load of varying frequencies. The test results show that the dynamic load disturbance will accelerate the instability failure process of coal rock, that the average stress level/ultimate strain of coal rock decline/rises correspondingly with the increasing dynamic load frequency. Moreover, the overall AE b value after dynamic load disturbance is relatively low, and the higher the dynamic load frequency, the larger the decrease in the b value. In addition, as the dynamic load frequency increases, the AE ringing count and AE energy show a trend of first increasing and then decreasing, and they are negatively correlated with the evolution characteristics of the acoustic emission b value. Furthermore, the change rule of AE b value is closely related to the instability and failure process of coal rock. A second-stage of the overall b value of acoustic emission shows a “cliff jumping type” decrease dip angle or “fault cliff type” decrease dip angle indicates the imminent failure of coal rock, which can be used as an evaluation index for real-time monitoring of dynamic disasters of coal rock with outburst-proneness.

Experimental investigation on strength-weakening effect and fracture behavior of red sandstone under coupled static and dynamic cyclic loadings

Underground rock masses are often exposed to dynamic disturbances due to seismic waves and blasting excavation disturbances, resulting in significant changes in their physical and mechanical properties. To reveal the strength-weakening effect of rock under the coupled stress, the Brazilian disc tests of red sandstone were carried out by coupling pre-static and dynamic disturbing. The impacts of preloading, loading wave amplitude, dynamic disturbance frequency, and cycle number were studied on the weakening of Brazilian disc splitting strength. Furthermore, the fracture behaviors were characterized and discussed from the fracture morphology and fractal dimension. The results show that the loss ratios are positive under dynamic disturbance loading, indicating that the Brazilian splitting strength is weakened. For the same load wave amplitude, the loss of strength and P-wave velocity under 70 % preloading is more than twice that under 55 % preloading. At high preloading, the maximum strength loss and wave velocity losses are 13.7 % and 19.2 %, respectively. Comparing the disturbance degree of the sample with the increase of frequency by 1 Hz, cycle number by one time, and load wave amplitude by 1 % PS, it is concluded that the decline of wave velocity for the rock sample with the increase by 1 % PS in load wave amplitude is the largest, followed by the frequency and cycle. Only micro-cracks appear on the fracture surface under pure static loading. However, when the dynamic load disturbance is applied, the cement wrapped with debris falls off, besides micro-cracks. And with the increase of amplitude, the area of bare debris also progressively intensifies. Meanwhile, the width and number of micro-cracks gradually increased, with the broadest micro-crack reaching 18.2 μm. Preloading, loading wave amplitude, loading frequency, and cycles are positively correlated with fractal dimension D. The higher the D value, the more broken the sample.

Mechanical Properties and Failure Behavior of Mortar-Encased Coal Bodies under Impact Loads: Insights from Experimental Investigation

Abstract Failure of residual coal pillars under dynamic load disturbances can induce goaf collapse, ground subsidence, or coalbursts. Encasing the residual coal pillar in mortar is an effective method for reinforcing the residual coal pillar. However, the mechanical behaviors of mortar-encased coal bodies under impact loads remain poorly investigated. In this study, impact tests were conducted on coal, mortar, and mortar-encased coal specimens using a split Hopkinson pressure bar (SHPB) system. The mechanical properties and failure behavior of the mortar-encased coal specimens under impact loading were systematically investigated in terms of several metrics including dynamic stress-strain curves, failure patterns, strength change characteristics, and energy consumption laws. Results show that, owing to the different mechanical properties of the coal and mortar elements in the composite specimens, the mortar-encased specimen has a nonlinear deformation characteristic. The mortar has a higher energy absorption rate compared to the coal. Additionally, increasing the thickness of the external mortar body is helpful for absorbing more stress wave energy and increasing the dynamic strength of the mortar-encased coal specimens. Furthermore, under low strain rate loading, the external mortar body of the composite specimen initially experienced axial splitting failure. With increasing strain rate, axial splitting failure occurred in both the external mortar body and inner coal body. This study provides useful guidelines for reinforcing residual coal pillars in underground engineering.

Marginal Cost-Based Reactive Power Reinforcement Using Dynamic and Static Compensators

The main philosophy of this article is to develop customer-persuaded electric power tariff scheme, based on the composition of additionally required services. Ancillary services may be categorized into desirable and essential electric power market services and so independent system operator (ISO) looks into the rights of all the utilities involved in the electric power market for getting the most acceptable rate of electrical energy in favor of all. Reactive power compensation, one of the 12 ancillary services underlined by North American Electric Reliability Council (NREC) along with Electric Power Research Institute (EPRI), is necessarily important to attend the voltage profile at load terminals. STATCOM, a dynamic compensator, has more advanced technologies over a fixed capacitor (FC), which is a static compensator; however, its involvement results in more financial burden to utilities. This article reinforces an adequate reactive power compensation scheme, for attending static and dynamic load disturbances, using ISE criterion-based proposed current-controlled STATCOM with FC in a grid-tied system. The composition of reactive power supplied by static and dynamic compensation is optimized using marginal cost, based on base and peak reactive power demand concept, keeping in mind the system frequency regulation. The results in terms of compensation scheme are compared for step changes in three load conditions and with and without static reactive power compensators.

Mechanism and Control of Rockburst Induced by Draining Spatial Islands and Squaring

A small-pillar gob-side roadway showed rockburst appearance during the mining of a gob-side working face located in the Shaanxi-Inner Mongolia mining area. This study examines the 2202 gob-side working face of a coal mine in Inner Mongolia as a case study. A stress evolution model was built for the static-stress spatial islands formed by drainage regions and goafs based on the spatial relationships between drainage regions and goafs. The average microseismic frequency and energy of the high-stress zone of spatial islands were at least 1.37 times of those of other zones, validating the presence of spatial islands. The dynamic and static load effects of working face squaring were obtained based on the evolution of the stope roof as well as changes in microseismic data. Microseismically active zones were advanced to 200 m–300 m on working faces. The rockbursts induced by high static loads and dynamic and static loads formed by spatial islands and squaring were calculated. According to calculation results, the critical stress concentration value under high static loads was 3.27; the critical static stress concentration value under dynamic and static loads was 1.81. The superposition of drainage boundary stress, goaf lateral stress, and lead stress might reach the critical stress concentration under dynamic load disturbances, causing rockbursts. A stress adjustment scheme was established, including overall hydraulic fracturing of the external roof of the drainage region, reduction of stoping speed, and pressure relief of large-diameter boreholes. The stress adjustment scheme was implemented on-site and supplemented by monitoring and early warning methods to safely advance by the first squaring region.

Effects of Acidic Environment on Dynamic Mechanical Properties and Porosity Evolution Characteristics of Coal

Coal pillars left in coal mines are often subjected to long-term submersion by groundwater and chemical solutions and are susceptible to deterioration and even destabilization damage under dynamic load disturbance. In order to investigate the effects of acidic environment on dynamic mechanical properties and porosity evolution characteristics of coal, a split Hopkinson bar (SHPB) was used to test the dynamic compressive strength and tensile strength of coal samples under different acid environment. The results showed that the sample density gradually decreased with the increase of the number of wet and dry cycles, but the decrease was significantly related to the pH value. Longitudinal wave velocity of coal sample decreases gradually with the increase of drying and wetting cycles, and the decreasing speed is first fast and then slow. The stronger the acidity of the solution, the more times the dry-wet cycle, and the higher the water absorption of the sample. In the early stage of dry-wet cycle, the coal is significantly affected, and the average deterioration degree is large. After that, the influence of cyclic action is reduced, and the average degradation degree is small. Porosity of coal increases continuously under the action of dry-wet cycle. The stronger the acidity, the greater the change in initial porosity. In the 20th cycle, the porosity of the acidic environment increases significantly at once and then decreases continuously.